177

Chapter 8. Palaeomagnetic Study of Tertiary Lava from Barrington Tops, NSW, Australia

The previous two chapters have dealt with historic lava; the logical next

step is to extend investigations to ancient lava. Tertiary lava from Barrington

volcano, New South Wales, Australia was the chosen study area. The chapter

begins with the reasons for choosing the study area and then describes the

geology and dating of the volcano. Previous palaeomagnetic studies of the

volcano and the construction of Cenozoic Australian APWPs are described. The

background sections are followed by a description of the sampling undertaken for

this study and then the experimental results are discussed. An intensive rock

magnetic investigation, thermal demagnetisation and conventional Thellier

intensity studies have been carried out. These are described and discussed.

Microwave demagnetisation using one sample per flow and a preliminary

microwave intensity study using eight samples from different flows have been

carried out; these results are discussed. The chapter ends with an overall

discussion and summary.

8.1. Introduction

A more comprehensive coverage of palaeomagnetic data in both time and

space is required to gain a more complete understanding of the geomagnetic field.

Australia is one region where palaeomagnetic data, especially field intensity data,

is particularly sparse. This is illustrated in Fig. 8.1, which shows the complete set

of intensity data contained in the 1998 version of the IAGA palaeointensity

database, pint97 (Perrin et al., 1998) for the Australian continent, along with the

recent results obtained from Jurassic intrusions, NSW (Thomas et al., 2000).

There are a total of only 19 field intensity evaluations from six published studies

(Barbetti & McElhinny, 1976; Briden, 1966a; b; McElhinny & Evans, 1976;

Sennayake et al., 1982; Thomas et al., 2000). Directional data are not abundant

either and there are a number of conflicting versions of the Cenozoic Australian

apparent polar wander path (APWP) which are discussed in Section 8.4.2.

178

0

2

4

6

8

10

12

14

0 500 1000 1500 2000

Age Ma

VD

M x

10

22 A

m2

Figure 8.1 All field intensity data for the Australian continent contained in the pint97 database and from Thomas et al. (2000).

0

5

10

15

20

25

30

35

45 50 55 60

Age Ma

VD

M x

10

22 A

m2

Figure 8.2 All field intensity values contained in the pint97 database for the time interval 60 - 45 Ma.

Palaeointensity data are distinctly scarce during the early Tertiary as

illustrated in Fig. 8.2, which contains all the data in the pint97 database for the

time interval from 45-60 Ma, none of which is from Australia. There are ten

palaeointensity evaluations; from Mexico (Urrutia Fucugauchi, 1980), a DSDP

leg at the mid Pacific Ridge (Kono & Tosha, 1980) and from the British Tertiary

Igneous Province (BTIP) (Smith, 1967). The Barrington Volcano, NSW,

Australia is contemporaneous in age with the BTIP and antipodal, thus potentially

allowing the dipolar nature of the magnetic field to be investigated. In addition,

179

this is a key age for investigating the correspondence between field intensity and

the frequency of field reversals as it is an interval in which the frequency of field

reversals is beginning to increase following the Cretaceous Normal Superchron

(c.f. Merrill et al., 1996).

8.2. Geological Setting

Throughout the Cenozoic widespread volcanism took place at sites spread

along the entire length of Eastern Australia (e.g. Johnson, 1989; Wellman &

McDougall, 1974a) with the Barrington province being just one of many lava

fields (Fig. 8.3). The Mount Royal Ranges, situated in northern New South

Wales, 220 km from Sydney, consists of a deeply dissected basalt volcano

(Barrington). The volcanic rocks may have originally formed a low angle shield,

which has subsequently been extensively modified by erosion. The remnants of

the Barrington volcano crop out mainly on the Barrington Tops plateau and its

escarpment. The original total volume of the lava field may have been about 700

km3, but now the volume is about 120 km3 (Johnson, 1989). There is a well

exposed unconformity between the basalt and the underlying deformed

Carboniferous and Devonian sedimentary and volcanogenic rocks. Permian

granites of the Barrington Tops granodiorite are also present in the region (e.g.

Johnson, 1989). Fig. 8.4 illustrates the geology of the Barrington Tops region.

The volcanic pile consists of about 700 m of flat lying flows (Wellman &

McDougall, 1974b) with flow thicknesses of 2-10 m dominating the lava pile.

The majority of the volcanic pile contains alkaline rock types. This includes

transitional basalt, alkali basalt, basanite and nephelinite. Many are porphyritic in

olivine, or clinopyroxene, or both (Johnson, 1989).

180

Figure 8.3 Map of Eastern Australia showing the Cenozoic volcanics (from Wellman et al., 1969).

181

Fig 8.4 Geologic Map of the Barrington Tops Region (from Mason & Kavalieris, 1984).

182

A stratigraphical study has been carried out in the western part of the

Barrington Tops region (Mason, 1982) the main results of which are described in

the following paragraphs. The study concentrated on two areas, around Prospero

Trig and at Semphills Creek. A sketch map of the area is shown in Fig. 8.5 and

the stratigraphic sections are shown in Fig. 8.6. The dominant rock type in the

two sections is alkaline intergranular olivine basalt. Tholeiitic basalt lava is only

present at the Semphills Creek section at the base of the sequence. It was erupted

first and accumulated in topographic lows.

Figure 8.5 Sketch map of Barrington Tops region (from Mason, 1982), stippled area denotes Barrington volcano and the thick black lines denote the location of the two sections studied

by Mason (1982).

183

a l k a l i b a s a l t i c r o c k sol. bas. (intergranular)

ol. bas. (aphitic/subaphitic)

px.-ol. basalts Ankaramite

undifferentiated basaltstholeiitic basalts

lack of exposureCarboniferous sediments

Figure 8.6 Volcanic stratigraphy of Prospero Trig and Semphills Creek sections (from Mason, 1982). Flow thicknesses are approximate.

The volcanic succession in the Prospero Trig area is well exposed in road

cuttings on the Gloucester – Scone road where it traverses the western escarpment

of the Barrington Tops plateau from Moonan Outlook to Moonan Brook. Mason

(1982) states that at least 33 flows are exposed along a ~6 km section of road,

through ~430 m of altitude. The angular unconformity between the folded

Carboniferous sediments and the overlying Tertiary deposits is also well exposed.

The top few tens of metres of Carboniferous sediments are extensively weathered

and the top metre or so comprises conglomeratic material of possible pediment

gravel origin. Over this unconformably lies approximately 2 m of subhorizontal

Tertiary stream bed sediments. On top of this is the oldest basaltic lava flow,

which has an irregular base.

The whole of the volcanic sequence appears to be of lava flow origin with

an average flow thickness of ~10 m (2 – 15 m range). Most of the flows contain a

massive basal section overlain by a vesicular portion. A number of flows, mainly

the top ones, have well developed columnar jointing. White secondary minerals,

including zeolites and calcite cement the clinkery flow tops. These white

secondary minerals also commonly fill vesicles wherever they occur. Some flow

184

tops are very red signifying that they are strongly ferruginised and indicating that

there were significant periods of time between eruptions. Other flows show no

signs of weathering between them suggesting that they were extruded in rapid

succession.

Hand specimens, show that some flows do not contain phenocrysts

whereas others contain large glassy green olivine phenocrysts in a dark greenish

grey groundmass. Some flows contain pyroxene phenocrysts up to 40% in

volume. These rocks are very obvious in the field and are termed ankaramite

(Mason, 1982; 1985). Intergranular olivine basalt dominates the upper half of the

section whereas pyroxene - phyric basalt (including ankaramite) is restricted to

the base of the section.

The second section of the Mason (1982) study is at Semphills Creek. The

section is not as well exposed as that at Prospero Trig but is still good. 27 flows

in 350 m were sampled leaving ~100 m unexposed and unsampled. Rock types

similar to those found at Prospero Trig are also found at Semphills Creek. The

major difference between the sections is the presence of four massive flows

(average 15 m thick) of tholeiitic basalt at the bottom of the section.

For more detailed petrographic results the reader is referred to Mason

(1982; 1985) and Johnson (1989).

8.3. Age of Barrington Volcano

Until the advent of radiometric dating, ages given to the Cenozoic

volcanic rocks of Eastern Australia were mainly based on palaeontological and

physiographic data (Wellman & McDougall, 1974b). Using potassium – argon

(K-Ar) dating the volcanics are now known to range in age from late Palaeocene

to middle Miocene, a larger range than thought from the original studies. The

bulk of the K-Ar dating for Barrington volcano was published in the late 60’s and

early 70’s giving an age for the Barrington volcano of around 52 Ma. When this

age is corrected for currently accepted values of the decay constants, the age

increases to 53 Ma. However, four new determinations have recently been carried

out (Dr. F. L. Sutherland, pers. comm. to N. Thomas) giving the volcano a

slightly wider range of ages.

185

Table 8.1. Whole Rock K-Ar Ages for Barrington Volcano

Sample Calculated age (Ma )

Mean Age (Ma) Location

GA2924

GA2925

GA2949

GA2929

GA2948

51.53 ± 0.7

50.69 ± 0.6

44.76 ± 0.6 44.53 ± 0.6 50.81 ± 0.7

52.41 ± 0.7 52.09 ± 0.9

51.5 ± 0.7

50.7 ± 0.6

44.6 ± 0.6

50.8 ± 0.7

52.2 ± 0.7

Stewarts Brook

GA3465

GA2903

GA2923

GA2902

GA2901

GA2900

GA1961*

47.25 ± 0.6

51.25 ± 0.9

42.57 ± 0.7 43.48 ± 0.6 41.20 ± 0.7 45.60 ± 0.8 52.88 ± 0.9 51.79 ± 0.7 50.02 ± 4.7

48.7 ± 0.5

47.3 ± 0.6

51.3 ± 0.9

43.0 ± 0.7

43.4 ± 0.8

52.3 ± 0.8

50.0 ± 4.7

48.7 ± 0.5

Semphills Creek

GA2946

GA2947

53.3 ± 1.4 53.2 ± 1.4 43.9 ± 1.2 42.3 ± 0.7

53.3 ± 1.4

43.1 ± 1.0

20 km south of Nundle, north of other locations

Stewarts Brook and Semphill Creek determinations from Wellman et al. (1969) apart from * from McDougall & Wilkinson (1967); south of Nundle determinations from Wellman &

McDougall (1974b). Errors for analytical uncertainty are at one standard deviation apart from those from south of Nundle where the uncertainty is given at two standard deviation.

At present a total of 14 published whole rock K-Ar determinations have

been made on samples from Barrington Volcano. McDougall & Wilkinson (1967)

were the first, followed by eleven determinations from Wellman et al. (1969) and

two determinations from Wellman & McDougall (1974b). The results are

summarised in Table 8.1 showing the range in ages from 43 Ma to 53 Ma. An

intruding dyke gave a slightly older plagioclase age of 54.6 ± 2 Ma (Wellman &

McDougall, 1974b) increasing the range of ages to 43 Ma to 55 Ma. The younger

ages have been interpreted as reflecting variable amounts of argon loss by

diffusion, whereas the older ages are more tightly clustered and are thought to

indicate the age of extrusion of the flows. From these results it has been

suggested that the whole of the Barrington Volcano is about 52 Ma (Wellman &

186

McDougall, 1974b). Idnurm (1985a) adjusted the K-Ar ages to conform to the

currently accepted values of the decay constants (Steiger & Jager, 1977)

increasing the age of Barrington volcano to 53 Ma.

From the small dispersion in accepted ages this suggests the flows will

have erupted over a relatively short period of time. Wellman et al. (1969)

estimated the period probably did not exceed 1.4 Ma by taking two standard

deviations of the average age.

Four recent K-Ar whole rock dates have been evaluated (Dr. F. L.

Sutherland of the Australian Museum, Sydney, 2000) and are listed in Table 8.2.

The ages range from 51 to 59 Ma with a mean (and standard deviation around the

mean) of 54 ± 3 Ma. The range of 8 Ma is greater than the range of ages found

previously, however these new samples are all from new locations. This implies

that the lava flows were extruded over a longer period of time than 1.4 Ma. The

mean age using the recent eva luations is consistent with the previous mean age

placing the Barrington volcano as early Eocene.

Table 8.2. New K-Ar whole rock ages for Barrington Volcano

Sample Rock type

Age Ma ± 1 SD Location

DR12662

DR14701

DR16503

DR16517

Basanite, top flow

Basanite flow

Olivine

nephelinite

Alkali basalt flow

58.7 ± 0.5

54.5 ± 0.6

53.3 ± 0.4

50.9 ± 1.5

Thunderbolts lookout, Barrington Tops forest Road, Upper Manning Callemondah Road, Barrington Tops, Ellerston Moppy Lookout Summit, Upper Manning Barrington Tops forest Road, west side Polblue Swamp, Ellerston

Analyses DR12662, 14701, 16503: A. Webb AMDEL Laboratories, Adelaide, South Australia, analysis DR16517: H. Zwingmann, CSIRO Laboratories, North Ryde,

NSW.

187

8.4. Previous Palaeomagnetic Studies

8.4.1. Barrington Tops

The only published palaeomagnetic study of the Barrington volcano was

undertaken by Wellman et al. (1969) in conjunction with their K-Ar dating. In

addition to Barrington they also studied Liverpool and Nandewar volcanoes (Fig.

8.3). The study was purely directional; no rock magnetic analyses were

performed.

At Barrington volcano two sections were studied, 21 km apart, Semphills

Creek and Stewarts Brook (Figs. 8.4 and 8.5). It was due to this study that the

Mason (1982) stratigraphic study chose Semphill Creek as one of their sections

(the Gloucester – Scone road had not been built at the time of the Wellman et al.

(1969) study). Hand samples oriented with a sun compass and level were taken

from a total of 36 flows (13 from Semphill Creek and 23 from Stewarts Brook).

From each hand sample between 3 and 9 cores were taken. An astatic

magnetometer was used to determine the direction and intensity of magnetisation

of the samples after AF cleaning. A test sample from each flow was progressively

AF demagnetised up to a maximum of 60 mT to determine the stable end point.

Thereafter, the other samples were subjected to one AF demagnetisation step at a

field slightly higher than the evaluated stable end point. All flows apart from one

near the base of the Stewarts Brook section were stable to AF demagnetisation.

The flow mean directional results are shown in Fig. 8.7. The Semphill

Creek section is predominantly reversely magnetised with just one flow near the

base of the section being normally magnetised. The magnetisation directions of

the Stewarts Brook section are more complex, with two periods of reversed

polarity separated by flows of normal polarity magnetisation. There are also two

flows that exhibit anomalous directions, which are not included in the overall

directional mean shown in Table 8.3.

188

Figure 8.7 Variation of declination and inclination through the two sections (from Wellman et al., 1969). Dashed line shows overall mean direction.

Table 8.3 Mean directional results of Wellman et al. (1969) for Barrington Volcano

N D I R k α95 s Plat. S Plong. E Α95

33 193.0 +65.5 32.33 48.47 3.6 11.4 70.5 125.6 5.3

Where N is the number of flows, D declination, I inclination, R resultant of N unit vectors, k, precision parameter, α95 radius of circle of 95% confidence about mean direction, s angular

standard deviation, Plat and Plong the palaeomagnetic south pole with associated Α95.

8.4.2. Australian APWP

There are currently differing versions of the Australian Cenozoic APWP,

each with their own uncertainties (e.g. Acton & Kettles, 1996). A key issue is

whether or not the poles derived from the Australian Cenozoic volcanoes, in

particular Barrington and Liverpool, are anomalous.

McElhinny et al. (1974) synthesised all the palaeomagnetic and age data

from igneous bodies that was currently available (including the Barrington results

from Wellman et al. (1969)) to produce an Australian APWP from 60 Ma to

189

recent. Six poles were determined with the Barrington data included in the 60-40

Ma pole (Table 8.4). The derived APWP (Fig. 8.8) “shows a distinct zig-zag with

a westerly excursion around 30 Ma and a knee around 18 Ma” (McElhinny et al.,

1974).

Table 8.4 Different Palaeo South Poles for the early Tertiary

Age Ma Lat Long Α95 ReferenceBasalt pole 60-40 68.5 130.9 5.2 McElhinny et al. , 197450 Ma average 50 65.7 127.6 9.4 Embleton & McElhinny, 1982North Rankin 1 58 61.7 118.4 8.0/3.3/87.0* Idnurm, 1985b

*95% confidence ellipse described by its major axis/minor axis/azimuth of major axis.

Figure 8.8 The McElhinny et al. (1974) Australian APWP (from McElhinny et al., 1974).

Embleton & McElhinny (1982) constructed an alternative APWP using

poorly dated laterite and weathered profile data. The motivation for producing

this new APWP was due to inconsistencies between the basalt derived APWP and

sea floor spreading results. Klootwijk & Pierce (1979) had previously noted that

the Australian APWP did not match the Indian APWP when this was rotated on

to the Australian plate. The accuracy of the pole positions were questioned, in

particular from the Barrington and Liverpool volcanoes (Klootwijk & Pierce,

190

1979; Embleton & McElhinny, 1982) leading to the desirability of determining

the APWP from non igneous bodies. The Embleton & McElhinny (1982) path is

calibrated from estimates based on broad averages of all the available basalt data

(Table 8.4) since no closely constrained ages could be assigned to the laterite and

weathered profile poles. This APWP path fits in better with sea floor spreading

results and exhibits a smoother progression through time. The zig zags of the

McEhinny et al. (1974) path are no longer present and back to about 50 Ma the

path lies near the 120 °E meridian.

Idnurm (1985a; 1994) defined another APWP similar to that of Embleton

& McElhinny (1982) but with a different rate of polar wander. The Idnurm

APWP is derived from dated sedimentary sequences combined with the poorly

dated laterite data. The data from the Barrington, Liverpool, Nandewar and

Tweed volcanoes was all rejected as being anomalous. An azimuthally unoriented

drill core taken from off the north west coast of Australia, North Rankin 1,

provides the pole nearest in age to the Barrington volcano (Idnurm, 1985a) (Table

8.4). The drill core is given a mean depositional age of 58 Ma estimated from

planktonic foraminifera. Remanence of the geomagnetic field at the time of

drilling has been used to azimuthally orient the drill core. All the ancient

directions are of normal polarity indicating that the remanence was acquired

relatively rapidly (thought to be at the time of deposition).

Two reasons have been given for possible anomalous basalt poles, first,

that secular variation has not been averaged out due to episodic extrusion

(Idnurm, 1985a) and second that the magnetisation of the rocks is more

complicated than previously thought (Hoffman, 1984; Embleton & McElhinny,

1982). Musgrave (1989) however, believes that the igneous data may be more

reliable than previously thought. He constructed an APWP based on a polynomial

curve fitted through palaeomagnetic data from the igneous rocks and weathered

profiles. Idnurm believes however that there are problems with the construction

of the Musgrave APWP, which have been outlined in Idnurm (1990).

Fig 8.9 illustrates the Embleton, Idnurm and Musgrave APWPs. In

addition, three poles from averages of global palaeomagnetic data excluding

Australia are shown. The poles were obtained by reconstructing and averaging

poles in to the Australian reference frame (Acton & Gordon, 1994). The

Musgrave APWP is more in line with hot spot derived data than the Idnurm and

191

Embleton APWPs but this discrepancy is believed to be due to true polar wander

(Idnurm, 1985b).

Figure 8.9. Comparison of different Australian Cenozoic APWPs (from Acton & Kettles, 1996). Embleton APWP (region with v pattern), Musgrave APWP (region with dashed line)

and Idnurm APWP (very thick black line). The three triangles are averages from palaeomagnetic data from the rest of the world (excluding Australia) that have been

transposed to the Australian reference frame (Acton & Gordon, 1994). The star, white circle and associated ellipses are the computed pole positions from Acton & Kettles (1996) and is

not of importance here.

8.5. 1997 Field Trip / Sampling

The area chosen for the present palaeomagnetic study was the road section

in the Mason (1982) stratigraphic study near Prospero Trig (Fig. 8.5). This was an

ideal location due to the excellent exposure and the information from the

stratigraphical study. The section is located between the two sections (Semphill

Creek and Stewarts Brook) of the Wellman et al. (1969) study. The start of the

section, EBT (Eocene Barrington Tops) was at the dingo gate at Moonan

Outlook. The section continued west, following the Barrington Tops Forest Road

(Scone – Gloucester Road) down towards Moonan Brook as the road traverses the

western escarpment of the Barrington Tops plateau. The geology and stratigraphy

were as described by Mason (1982) (Section 8.2). Over a distance of about six

192

kilometres we (myself, John Shaw and Tim Rolph) identified 27 distinct lava

flows of which 22 were sampled. Brief field notes are shown in Table 8.5 and

selected field photos in Fig. 8.10.

Table 8.5 Field notes

FLOW (top to bottom)

SITE NOTES

1 2 3 4 5 6 - 8 9 10 11 12 13 14 15 16 17 18 19 20 21

EBT01 EBT02 EBT03 EBT04 EBT05 EBT06 - EBT07 EBT08 EBT09 EBT10 EBT11 EBT12 EBT13 EBT14 EBT15 EBT16 EBT17 EBT19 EBT18 -

Can’t see top of flow. Massive blocks towards bottom of flow. Visible baked contact and clinker of next flow Next flow Small amount exposed Approx 100m down road. Thin, fractured flow. Flow edge exposed. Lots of microfractures in flow. Off road down slope Thin flow missed Down field below EBT06, thin ~3m flow. Back on road. Thin flow, no red contact, think is next flow down but is possibly same as EBT07. More difficult to see contacts between flows now. Next flow down the road Next flow down road. Discrete blocks of lava in flow top rubble. V. thin, badly exposed flow not sampled. By cattle grid. Ankaramite (nodules visible further up road) Discrete blocks in top of flow Same characteristics as EBT13 but flow is horizontal and the road goes down in elevation, so deduce is the next flow. Corner of road. Good exposure of fresh lava, samples taken horizontally along flow. Flow coming towards us – tongues of lava exposed in amongst scoria. Further along road than site EBT18 (road goes up) 1 flow possibly missed

193

22 23 24,25,26 27

EBT21 EBT20 - EBT22

Road goes up from site EBT20. Very thin flow. In soily section, not much exposure. From site EBT19 to site EBT22 minimum of 3 flows missed Along road decrease in altitude ~30m. Blocks in grassy slope, below this are sediments.

An average of seven 2.5 cm diameter cores were taken from each flow

using a petrol powered rock drill. Samples were oriented with a sun compass, or

when there was no sun sited on to a distant point. In the laboratory each core was

cut into, on average, three small cores. This provided one core for thermal

demagnetisation, one for Thellier intensity analysis and one for microwave

studies. Core bottoms were used for rock magnetic analyses.

In addition to the drilled cores, hand samples were also collected from

each flow.

194

Figure 8.10 Field photographs, a) site 2 and 3; b) Tim Rolph drilling site 5, note red top of 6 at base.

a)

b)

195

8.6. Rock Magnetism Two samples per flow (in total 42 samples) were subjected to rock

magnetic analyses.

Table 8.6 Rock magnetic parameters. See text for details of sample type.

sample χlf χlt type RS NRM NRM300°C/NRM Ms Mrs Mrs/Ms Hc Curie Type jh/jc tc1 tc2 TYPE10

-8m

3kg

-110

-5Am

2/kg % Am

2kg

-1Am

2kg

-1mT °C °C

01-02a 1277 1 0.37 1757 7 1.36 0.16 0.12 6.77 4a 0.79 185 525 B01-08a 1141 1 0.17 970 4 1.38 0.15 0.11 4.90 4b 1.07 210 500 B02-04a 1414 1 0.20 61 39 1.46 0.22 0.15 6.91 1a 0.85 278 A02-08a 259 1 0.22 53 43 0.91 0.24 0.26 10.05 1b 0.56 285 A03-01a 937 1/2 0.70 104 91 1.44 0.35 0.24 13.95 4c 1.04 285 565 B03-02a 766 1/2 0.48 14 151 0.87 0.18 0.21 9.68 4c 0.82 278 585 B04-03a 1430 1 0.15 212 4 1.22 0.16 0.13 5.15 1a 0.96 240 A04-07a 1191 1 0.23 2931 3 1.41 0.18 0.13 5.40 4a 1.00 192 495 B05-03a 1682 2 0.74 156 78 2.12 0.47 0.22 12.82 4c 0.94 290 550 B05-04a 1388 1 0.20 53 72 1.96 0.27 0.14 7.54 4a 0.88 290 500 B06-01a 1376 1 0.22 15 260 1.12 0.17 0.15 6.03 1b 0.59 290 A06-06a 826 2 0.81 134 84 1.18 0.19 0.16 19.35 2a 1.05 580 C07-03a 779 2 0.74 101 96 1.33 0.27 0.20 19.73 3 0.97 230 585 C07-06a 1139 2 0.94 107 90 1.46 0.29 0.20 19.60 2b 1.08 558 C08-05a 888 2 0.91 247 90 1.49 0.28 0.19 18.60 2b 1.12 575 C08-07a 894 2 0.94 183 92 1.40 0.29 0.21 21.49 2b 1.10 570 C09-02a 931 1/2 0.42 95 61 1.00 0.19 0.19 9.42 4c 0.93 190 580 B09-03a 841 1 0.17 145 73 0.61 0.10 0.16 5.53 1c 0.66 180 A10-02a 702 3 0.17 395 15 0.07 0.02 0.26 7.79 1c 0.29 190 A10-05a 575 1 0.18 70 20 0.40 0.06 0.16 5.03 1c 0.65 190 A12-06a 417 1 0.24 22 17 0.51 0.07 0.13 4.65 1c 0.82 190 A12-08a 514 2 0.79 63 98 0.99 0.25 0.25 21.49 2b 1.06 555 C13-02a 621 2 1.00 125 75 1.32 0.33 0.25 30.41 2b 1.07 590 C13-07a 448 1 0.25 39 35 0.47 0.08 0.16 5.78 1c 0.54 190 A14-01a 301 1 0.30 51 15 0.21 0.05 0.22 7.26 1c 0.29 185 A14-06a 400 1 0.26 57 20 0.30 0.06 0.19 5.53 1c 0.50 170 A15-01a 412 1 0.19 622 9 0.32 0.07 0.23 6.81 1c 0.31 160 A15-06a 238 1 0.17 405 17 0.22 0.05 0.23 6.79 1c 0.28 150 A16-04a 1174 1 0.09 111 4 1.17 0.08 0.07 3.46 1d 1.09 190 A16-06a 940 1 0.10 95 6 0.88 0.09 0.10 3.64 1d 1.23 170 A17-03a 875 1 0.08 184 4 0.29 0.06 0.22 7.29 1c 0.28 160 A17-04a 828 1 0.12 212 6 0.37 0.08 0.22 7.54 1c 0.24 135 A18-01a 226 3 0.36 116 5 0.14 0.03 0.21 7.54 1c 0.73 240 A18-02a 171 3 0.50 47 9 0.12 0.03 0.21 8.29 1c 0.57 235 A19-01a 956 2 1.03 128 80 1.86 0.52 0.28 28.27 3 0.97 250 580 C19-04a 804 1 0.18 67 34 0.66 0.09 0.13 4.86 4a 0.62 165 520 B20-01a 111 3 0.86 21 34 0.08 0.02 0.26 8.76 1c 0.33 180 A20-05a 102 3 0.87 15 43 0.09 0.02 0.19 9.05 1c 0.32 178 A21-04a 154 3 0.49 28 47 0.09 0.03 0.30 12.19 1c 0.26 190 A21-05a 112 3 0.61 16 54 0.08 0.01 0.15 6.15 1c 0.53 140 A22-04a 1063 1 0.21 235 9 1.16 0.15 0.13 5.65 1a 0.92 270 A22-06a 857 1 0.22 255 3 0.65 0.12 0.19 8.68 1b 0.53 280 A

Table 8.6 lists the main rock magnetic parameters and an assigned sample

classification; type A, B, and C, representing different levels of high temperature

deuteric oxidation. Type A are the least oxidised and type C are the most

oxidised. The classification was initially based on thermomagnetic behaviour

where type A samples contain a single low temperature Curie point; type B

samples contain two discernible Curie points and type C samples are dominated

196

by a single high temperature Curie point. Over half the samples (25) are of type

A, 9 are type B and 8 are type C.

The results from the different rock magnetic experiments are described in

the following sub-sections before a summary of the main results.

8.6.1. Thermomagnetic Experiments Thermomagnetic measurements were carried out using the Curie balance.

The Curie points Tc determined for each sample are listed in Table 8.6 along with

the ratio of the magnetisation on heating and cooling at 100 °C, jh/jc. The curves

have been classified in to different types based on the classification of Mankinen

et al. (1985).

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600

Temperature °C

Mag

net

isat

ion

AU

Type 1b

0

100

200

300

400

500

600

0 100 200 300 400 500 600

Temerpature °C

Mag

net

isat

ion

AU

Type 1c

0

100

200

300

400

500

600

0 100 200 300 400 500 600

Temperature °C

Mag

net

isat

ion

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Type 1d

0

200

400

600

800

1000

1200

0 100 200 300 400 500 600

Temperature °C

Mag

net

isat

ion

AU

Type 1a

Figure 8.11. Type 1 Curie curves. Heating curve is the dark line and cooling curve the lighter line.

Over half (25) of the samples are dominated by a single magnetic phase

with a low Curie temperature ranging from 140 – 290 °C. These low temperature

Curie points are indicative of high Ti titanomagnetite that cooled rapidly so that

no high temperature deuteric oxidation occurred. The curves have been classified

as type 1 Curie curves and are illustrated in Fig. 8.11.

197

There is evidence (type 1a and 1b Curie curves) that low temperature

oxidation has occurred in some flows producing cation deficient titanomagnetite,

titanomaghaemite. On heating, titanomaghaemite inverts to a multiphase

intergrowth of magnetite, ilmenite and other minerals. This is shown in the Curie

curves by the disproportionation peak (type 1b curve) and is diagnostic of highly

maghaemitised samples. The non-reproducibility of the heating curve with an

increase in magnetisation on heating is also an indication of the inversion to a

more highly magnetised magnetite phase. Type 1a curves do not exhibit

disproportionation peaks indicating that the oxidation may be less extreme. Three

samples exhibit type 1a curves, which are similar to the type 1 curve of Mankinen

et al. (1985) and three samples exhibit type 1b behaviour.

The majority (17) of samples exhibiting type 1 Curie curves are of type

1c. A dramatic increase in magnetisation is seen after heating, with magnetisation

values on cooling up to 70% greater at 100 °C. The cooling curve is somewhat

linear with the rise in magnetisation starting at around 520 °C. Disproportionation

peaks are not seen in type 1c curves indicating that low temperature oxidation if

present, is not extreme. The average Curie temperature for type 1c samples is 180

°C, which is lower than the average Curie temperature of 273 °C for types 1a and

1b. As low temperature oxidation is associated with an increase in Curie

temperature, this is further evidence that type 1c samples have not undergone low

temperature oxidation. Type 1c curves are thus indicative of a primary high Ti

titanomagnetite that alters on heating to Ti poor titanomagnetite. The alteration

product contains a range of Curie temperatures indicated by the linear cooling

curves. This is also the case for types 1a and 1b, which also exhibit linear cooling

curves.

The fourth type of type 1 Curie curves; type 1d, are exhibited only by the

two samples from flow 16. These samples exhibit greater reproducibility between

the heating and cooling curves than the other type 1 curves. The cooling curve

crosses the heating curve so at 100 °C the cooling curve is below the heating

curve. These Curie curves are interpreted as being indicative of a primary high Ti

titanomagnetite that has not undergone low temperature oxidation and does not

alter significantly on heating.

198

0

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Temperature °C

Mag

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Type 2a

0

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Temeprature °C

Mag

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Type 3

0

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800

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1200

1400

1600

0 100 200 300 400 500 600

Temperature °C

Mag

net

isat

ion

AU

Type 2b

Figure 8.12. Type 2 and 3 Curie curves. Heating curve is the dark line and cooling curve the lighter line.

Six samples exhibit type 2 Curie curves which are illustrated in Fig. 8.12.

These curves are similar to the type 2 Curie curves of Mankinen et al. (1985). A

single ferrimagnetic phase is present with a Curie temperature between 555 °C

and 590 °C and a decrease in magnetisation after heating. Type 2a curves have

heating and cooling curves that show little or no change in shape and the decrease

in the magnetisation at 100 °C is 10% or less. Only one sample, 06-06, showed

this type of behaviour. The heating and cooling curves show a marked difference

in shape, particularly in the high temperature region in type 2b curves. The

decrease in magnetisation at 100 °C is 12% or less. Five samples exhibited this

behaviour. The magnetic phase producing type 2 curves could be either a primary

low Ti titanomagnetite or may be the result of high temperature deuteric

oxidation of a primary Ti rich titanomagnetite to a Ti poor titanomagnetite

containing ilmenite lamellae. The same magnetic phases are responsible for type

3 curves but with minor variations in magnetic grains or the bulk rock. Type 3

curves differ from type 2 in that they contain a perceptible low Curie temperature

component and the cooling curve crosses the heating curve so that the

magnetisation at 100 °C is higher than before heating. Type 3 behaviour is

199

between type 2 and type 4 behaviour but closer to type 2 than type 4. Two

samples exhibited this behaviour.

0 1 0 0 2 0 03 0 04 0 0

5 0 0 6 0 0

7 0 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 T e m p e r a t u r e - 8 C

M a g n e t i s a t i o n A UT y p e 4 a0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

0

1 0 0

2 0 0

3 0 0

4 0 0

5 0 0

6 0 0

T e m p e r a t u r e ° C

M

a

g

n

e

t

i

s

a

t

i

o

n

A

U

T y p e 4 b

0

2 0 0 4 0 0 6 0 0 8 0 0 10001200

14000 1 0 02 0 0300 400

5 0 06 0 0 T e m p e r a t u r e ° C

M a g n e t i s a t i o n A U

T y p e 4 c F i g u r e 8 1 3 T y p e 4 C u r i e c u r v e s . H e a t i n g c u r v e i s t h e d a r k l i n e a n d c o o l i n g c u r v e t h e l i g h t e r line. Nine samples

e x h i b i t t y p e 4 b e h a v i o u r i n w h i c h t h e r e a r e t w o d i s t i n c t

ferrimagnetic phases (Fig. 8 13). One phase has Curie temperatures in the range

1 6 5 2 6 2 9 0 °

C a n d t h e o t h e r p h a s e i n t h e r a n g e 4 2 0 2 6 5 8 5

6 0

C. Four samples exhibit

t y p e 4 a b e h a v i o u r w h i c h i s s i m i l a r t o type 4 of Mankinen

et al.

(1985). The curve

i s r e p r o d u c i b l e i n t h e h i g h t e m p e r a t u r e r e g i o n b u t e x h i b i t s a n i n c r e a s e i n

magnetisation at lower temperatures. Only sample 01

- 0 8 e x h i b i t s t y p e 4 b

b e h a v i o u r i n w h i c h t h e c o o l i n g a n d h e a t i n g c u r v e s a r e v e r y s i m i l ar. Four samples e x h i b i t t y p e 4 c c u r v e s w h e r e t h e h e a t i n g c u r v e i s t o t a l l y i r r e v e r s i b l e . T h e h i g h

t e m p e r a t u r e C u r i e p o i n t r e d u c e s a f t e r h e a t i n g a n d t h e c o o l i n g c u r v e i s s i m i l a r t o t h e c o o l i n g c u r v e o f t y p e 2 b c u r v e s .

Type 4 curves are commonly seen where h i g h t e m p e r a t u r e o x i d a t i o n h a s c o n v e r t e d o n l y p a r t o f t h e h i g h T i t i t a n o m a g n e t i t e t o a T i p o o r t i t a n o m a g n e t i t e c o n t a i n i n g i l m e n i t e l a m e l l a e . T h e l o w t e m p e r a t u r e p a r t m a y b e e i t h e r a T i r i c h

t i t a n o m a g n e t i t e ( t y p e 4 b ) o r i f i t h a s u n d e r g o n e l o w t e m p e r a t u r e ox i d a t i o n ,

t i t a n o m a g h a e m i t e ( t y p e 4 a ) . I t i s n o t a l w a y s e a s y t o d i s t i n g u i s h b e t w e e n t h e t w o

200

but maghaematisation is associated with an increase of magnetisation on heating.

The low Curie temperature phase of type 4c could be due to either a Ti rich

titanomagnetite or titanomaghaemite.

The thermomagnetic results indicate that type 1 Curie curves have

experienced the least high temperature oxidation and type 2 and 3 potentially the

most. Hence, the samples are classed as being type A if they contain type 1 Curie

curves, type B if they contain type 4 Curie curves and type C if they contain type

2 or 3 curves.

8.6.2. Hysteresis Experiments The hysteresis parameters Ms, Mrs, and Hc were determined using the

VSM and are listed in Table 8.6. The ratio Mrs/Ms can be used to estimate the

bulk magnetic grain size. The only samples to have Mrs/Ms < 0.1 and hence of

bulk MD grain size are from flow 16. This is the only flow to exhibit type 1d

Curie curves. All other samples have PSD bulk magnetic grain size with 0.11 <

Mrs/Ms < 0.30. The values of Hc also indicate bulk grain size, the values

decreasing as bulk grain size increases. Hc varies from 0.07 mT for one sample

from flow 16 up to a maximum of 30.41 mT for a sample exhibiting type 2 Curie

curve behaviour. The hysteresis results are consistent with a reduction in effective

grain size as a response to high temperature oxidation. This is in agreement with

the thermomagnetic results and sample type classification.

8.6.3. Low Temperature Susceptibility Representative low temperature susceptibility, χLT , results are shown in

Fig. 8.14. The results are generally consistent with the thermomagnetic

interpretations.

Type 1 χLT curves have RS values that range from 0.09 to 0.37. These

curves are similar to the group 1 χLT curve of Senanayake & McElhinny (1982)

which they interpret as indicative of mainly MD Ti rich tianomagnetites. The

isotropic point at –150 °C is suppressed for these samples indicating that the

grains have oxidised (Section 2.1.1.3, Özdemir et al., 1993). Type 1 χLT curves

correspond to type 1 and 4 Curie curve which agrees with the interpretation that

Ti rich titanomagnetite and titanomaghaemite is present.

201

Type 2 χLT curves exhibit the isotropic peak at around –150 °C indicative

of magnetite and MD grains (as they are less prone to oxidation). RS values are

between 0.74 and 1.03. These curves are similar to group 3 of Senanayake &

McElhinny (1982) which they interpret as representing MD Ti poor

titanomagnetite. However, Senanayake & McElhinny (1982) did not consider

mixed grain sizes so a preferred interpretation is that type 2 curves contain SD,

CD (or both) grains as well as MD Ti poor titanomagnetite (Radhakrishnamurty

et al., 1977; Radhakrishnamurty, 1990). Type 2 χLT curves correspond to type 2

Curie curves corroborating the interpretation that Ti poor titanomagnetite is

present.

Type 1/2 χLT curves are a combination of type 1 and type 2 curves. They

exhibit an isotropic peak at around –150 °C and have RS values between those of

type 1 and type 2. The four samples that exhibit type 1/2 χLT curves exhibit type 4

Curie curves.

The increase in RS value from type 1, through type 1/2 to type 2 is

consistent with a reduction in effective grain size as a response to high

temperature oxidation.

A third type of χLT curve; type 3 (Fig. 8.14) was exhibited by 7 samples

from flows 18, 20 21 and one sample from flow 10. A Hopkinson peak is present

at temperatures ranging from –33 to –7 °C which is most likely due to the

presence of haemoilmenite. Haemoilmenites with Curie temperatures in this

region have Ti content 0.7 < y < 0.8 (Dunlop & Özdemir, 1997). All these

samples have type 1c Curie curves and are characterised by low values of

magnetisation and susceptibility. These samples also have significantly lower

values of Ms (~0.1 Am2/kg) and Mrs (~0.02 Am2/kg) than the rest of the samples

indicating a lower concentration of ferrimagnetic material at room temperature.

202

0

0.2

0.4

0.6

0.8

1

1.2

-200 -150 -100 -50 0

Temperature °C

/30

Type 1

Type 2

Type 1/2

0

0.5

1

1.5

2

2.5

3

3.5

-200 -150 -100 -50 0

Temperature °C

/30

Type 3

Figure 8.14. Low temperature susceptibi lity curves exhibiting type 1, 1/2, 2 and 3 behaviour.

8.6.4. Room Temperature Susceptibility Room temperature low frequency susceptibility values are listed in Table

8.5. The frequency dependent susceptibility has been determined from the low

and high frequency room temperature susceptibility measurements (Section

2.1.1.2). The frequencies used look at a small window of grain sizes at the SP /

SD boundary so the influence of smaller SP grains will not be detected. The

frequency dependence was found to be less than 1% in all cases showing that the

contribution of SP grains with grain sizes at the SP / SD boundary is negligible.

203

0

0.5

1

1.5

2

2.5

0 100 200 300 400 500 600

Temperature °C

No

rmal

ised

ro

om

tem

per

atu

re s

usc

epti

bili

ty

Type A 14-01

Type B 05-03

Type C 06-06

Figure 8.15 Typical variation in room temperature susceptibility after heating for sample types A, B and C.

Room temperature susceptibility was measured after heating the samples

during thermal demagnetisation experiments. Fig. 8.15 illustrates representative

results for each of the three sample types. The majority of samples are highly

susceptible to thermal alteration as already seen from the thermomagnetic

experiments. Creation of new magnetic material is indicated by the large increase

in susceptibility. Type A and B samples exhibit an increase in susceptibility

starting at 250 or 300 °C, as expected for inversion of titanomaghaemite and the

alteration of titanomagnetite. Type C samples are more stable with an increase in

susceptibility occurring at higher temperatures, in the region 500 °C and above.

This is as expected for samples having undergone more high temperature deuteric

oxidation.

8.6.5. Stability of NRM

NRM values (listed in Table 8.6) range from 14 to 2931 x 10-5 Am2/kg.

Only three samples have NRM values greater than 900 x 10-5 Am2/kg (the next

highest is 622 x 10-5 Am2/kg). The anomalously high NRM values could be

indicative of an IRM induced by lightning strike. However, it is not usually

possible for an IRM to be removed by 300 °C and sample 04-07, which has the

highest NRM value, loses 97% of its NRM by 300 °C. The average NRM

204

(excluding the three anomalously high values) is 130 x 10-5 Am2/kg. The range of

NRM values found in this study are comparable to the range found by Wellman et

al. (1969).

The stability of NRM to heat, defined as the percentage of NRM

remaining at 300 °C, is listed in Table 8.6. This, in general, corresponds with the

thermomagnetic results indicating that the dominant magnetic minerals seen in

the rock magnetic tests are the dominant remanence carriers. As expected type C

samples are the most stable, exhibiting a maximum of 20% reduction in NRM at

300 °C. Type B samples exhibit a range of stability indicating that the remanence

is either dominantly held in the low Curie temperature phase, the high

temperature phase or else is held in both phases. The majority of type A samples

have lost the bulk of their remanence by 300 °C. Some type A samples however,

show more complicated behaviour. This is most likely due to the remanence

being held in finer magnetic grains not visible from the rock magnetic

experiments.

8.6.6. Summary The 42 samples studied have been divided in to three types A, B and C

relating to their level of high temperature deuteric oxidation, with type A the least

oxidised and C the most oxidised. The majority of samples are of type A and

contain a single low temperature Curie point, have type 1 χLT curves and exhibit

the largest bulk grain sizes. The samples are very susceptible to alteration with

the creation of new magnetic material between 250 and 300 °C. There is evidence

that the Ti rich titanomagnetite has undergone varying degrees of low

temperature oxidation. Type C samples contain a single high Curie temperature

ferrimagnetic phase of a low Ti titanomagnetite and experience less dramatic

alteration on heating. Type C samples exhibit type 2 χLT curves and have the

smallest bulk grain size. Type B samples have oxidation states in between A and

C and contain two ferrimagnetic phases.

Samples that have undergone low temperature oxidation will contain a

CRM and not solely a TRM and hence are unsuitable for palaeointensity analysis.

The tendency for the samples to alter on heating is an additional feature that

renders them unsuitable for conventional palaeointensity analysis. Type C

205

samples are the most promising candidates for any palaeointensity experiments as

they have undergone the greatest high temperature deuteric oxidation (assuming it

was at temperatures greater than the final Curie temperature). The evidence for

low temperature oxidation in these samples is less obvious but is still a

possibility. The results of NRM300°C/NRM indicate that the remanence is

generally held in the magnetic phases dominant in the rock magnetic results.

Reflected light microscopy would undoubtedly aid in the identification of

the magnetic minerals present in these samples, and the extent of both low and

high temperature oxidation. As haemoilmenites are visible under the microscope

the interpretation of the Hopkinson peak observed in type 3 χLT curves could be

verified and it would be interesting to see if discrete haemoilmenite is present in

any of the other samples.

Another complementary rock magnetic experiment that would be

beneficial to this study is IRM acquisition with backfield measurements. With the

determination of Hcr, a Day plot could be produced to confirm the general PSD

bulk grain size of the magnetic minerals. IRM acquisition would allow another

means of discriminating between magnetic phases.

8.7. Directional Analysis Using Thermal Demagnetisation Four samples per flow underwent thermal demagnetisation. Two of the

four samples were sister cores to the samples that underwent rock magnetic

analyses. Four samples per flow is the minimum acceptable for directional

analysis (c.f. Butler, 1992). The temperature of demagnetisation was increased in

50° steps up to 450 °C after which 20 or 25 °C steps were used up to a maximum

of 600 °C.

8.7.1. Demagnetisation Behaviour

Stable characteristic remanence directions (listed in Appendix C) have

been obtained for all samples apart from one sample from site 22. Representative

Zijderveld plots (OVPs) are shown in Fig. 8.16.

Secondary low temperature components of magnetisation, where present,

are generally removed by 200 °C. The low temperature component in most cases

is of random orientation but for a few samples is in the direction of the present

Australian field (e.g. Fig 8.16a). For the majority of samples, the secondary and

206

primary components are clearly distinct, but for a few samples there is a region of

overlapping blocking spectra where the secondary component is seen to swing

round to the converging characteristic remanence (Fig. 8.16b). Some samples

consist solely of a single component of magnetisation (Fig. 8.16c; d), albeit noisy

in the low temperature interval in some cases (Fig. 8.16c).

As expected, demagnetisation behaviour is closely linked to rock

magnetic behaviour. The convergent component on the OVP for some samples

remained up to 350 °C after which the signal became too noisy to yield a

direction (Fig 8.16e). This behaviour is seen for rock magnetic type A samples

that contain a low temperature Curie point. Other samples (including those of

type C) yield convergent components up to 550 °C.

The creation of new magnetic material due to alteration on heating (shown

in the non-reversibility of Curie curves and the increase in room temperature

susceptibility after heating) is also evident from thermal demagnetisation. Many

samples exhibit an increase in intensity at around 400 °C, corresponding to the

additional magnetisation of the newly created material. The direction of

magnetisation at this heating step is anomalous indicating that the newly formed

remanence direction, influenced by the ambient field, is not in the same direction

as the original remanence. However, at the next temperature step the newly

formed remanence is removed and the direction returns to that of the original

remanence. This behaviour is seen as a ‘blip’ on the OVP, seen in Figs. 8.16a; c.

Directions of reversed polarity have been obtained for all sites apart from

sites 1, 4 and 10. Sites 1 and 4 have high NRM values and no viscous component

so it is probable these samples have been affected by lightning strikes. Site 10

does not have anomalously high NRM values but does yield OVP plots with no

viscous component and a very smooth trajectory (Fig. 8.16d). It is therefore

concluded that this site has also been affected by a later overprint.

207

N S

W DOWN

E U

Horizontal

Vertical

SAMPLE EBT106-01A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0

5 0 0 5 2 0 5 4 0 5 6 0

0

0.5

1

1.5

2

2.5

3

0 100 200 300 400 500 600Temperature °C

Nor

mal

ised

Inte

nsity

N S

W DOWN

E U

Horizontal

Vertical

SAMPLE EBT107-03A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

5 5 0 5 7 0

0

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Temperature °C

No

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In

ten

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N S

W DOWN

E U

Horizontal

Vertical

SAMPLE EBT106-06A

208

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0

5 0 0 5 2 0 5 4 0 5 6 0

0

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Temperature °C

Nor

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ised

Inte

nsity

0

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0 100 200 300 400 500 600

Temperature °C

Nor

mal

ised

Inte

nsity

N S

W DOWN

E U

Horizontal

Vertical

SAMPLE EBT110-02A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0

N S

W DOWN

E U

Horizontal

Vertical

SAMPLE EBT116-06A

Figure 8.16 Representative Zijderfeld (OVP) plots.

8.7.2. Directional Results

The characteristic remanence directions were deduced from the OVP and

also from stereo projections. The results and associated errors for individual

samples are listed in Appendix C and flow mean directions are listed in Table 8.7.

Some minor variations in the evaluated mean flow directions are seen between

the two approaches. No flow mean could be evaluated for sites 1, 4 and 10 as

after they had been identified as being affected by later overprints they were

discarded, so only two samples per site underwent thermal demagnetisation.

Horizontal

Vertical

d)

e)

100°C

209

Table 8.7 Flow mean directions evaluated using OVP and stereoplot.

OVP stereoplotFlow Site n dec inc α95 k dec inc α95 k

1 12 2 4 243.2 82.6 25.7 13.8 196.2 79.6 17.7 27.83 3 4 206.3 67.7 2.9 1039.5 203.8 67.7 4.1 510.34 45 5 4 163.2 67.6 9.2 101.8 164.2 67.2 10.2 82.96 6 4 186.4 72.1 6.9 179.6 186.0 70.5 6.8 183.278 7 4 200.9 74.5 12.9 51.7 202.2 72.6 11.7 62.99 8 4 174.9 63.5 6.5 200.9 175.7 64.0 5.1 320.910 9 4 199.6 57.7 9.8 89.3 202.8 57.1 8.2 127.411 101213 12 4 167.6 55.4 9.7 90.0 170.2 58.6 6.1 228.914 13 4 182.4 51.7 5.5 279.9 181.5 50.7 5.5 277.015 14 4 188.6 64.5 7.4 157.2 183.4 59.3 4.9 350.816 15 4 190.9 52.6 6.5 202.2 190.7 55.9 5.8 247.817 16 4 187.4 68.4 3.8 571.1 183.7 66.8 6.7 190.718 17 4 189.6 66.0 10.4 79.6 183.0 63.9 9.2 101.519 19 4 189.7 60.8 18.3 26.3 185.9 60.6 16.9 30.420 18 4 205.2 65.3 5.3 305.3 194.3 66.9 9.8 88.12122 21 4 214.4 54.7 9.2 101.1 208.7 60.8 11.0 70.823 20 4 185.9 64.7 19.6 22.9 182.1 65.6 18.7 25.124252627 22 3 226.9 0.1 13.9 81.2 234.4 1.0 15.8 61.7

210

02468

10121416182022242628

0 50 100 150 200 250 300

Declination

Flow

num

ber

02468

10121416182022

242628

-20 0 20 40 60 80 100 120

Inclination

Figure 8.17 Flow mean directions (from OVP). A) declination and inclination through section in stratigraphic order. Solid black line is the overall mean direction (excluding flow 2 and 27), with the dashed line the associated error around the mean. B) Stereo projection of

flow mean directions (circles), triangle is the section mean (excluding flow 2 and 27).

A)

B)

211

Fig. 8.17 plots the flow mean declination and inclination in stratigraphic

order (see Table 8.5) and on a stereo projection. It can be seen that the flow mean

directions are clustered together except for site 22 which has a shallow

inclination. This site direction could be anomalous due to the incomplete removal

of a secondary component, as on the stereo plots the data points tended to lie

along a great circle. This site mean was not included in the evaluation of the

overall section mean. The site 2 mean was not included either in the evaluated

section mean due to the large within flow variation. The mean directional results

for the whole section are given in Table 8.8, with the results of Wellman et al.

(1969) for comparison. The mean direction and palaeo south pole has been

evaluated using the flow means derived from the OVP and from the stereoplots.

Table 8.8 Section mean direction and palaeo south pole determination.

Study N Dec Inc α95 k S Plat Plong Α95 Wellman et al. (1969) 33 193.0 +65.5 3.6 48.47 11.4 -70.5 125.6 5.3

This study (OVP) 16 189.5 +63.6 4.3 76.2 9.3 -74.2 126.4 5.9

This study (stereo projection)

16 187.2 +63.5 3.7 102.4 8.0 -75.2 131.2 5.1

Where k is the precision parameter and S is the angular dispersion.

It can be seen that the three directional evaluations are, within error,

equivalent.

8.7.3 Discussion This study confirms the previous results of Wellman et al. (1969).

However, the two studies differ in the fact that no flows of normal polarity were

found in this study. Only 16 flows were studied compared to the 33 of Wellman

et al. (1969) yet the mean direction and errors are comparable indicating the small

variability between flows.

As this study corroborates the palaeo pole obtained by Wellman et al.

(1969), the question of its validity remains. It has been suggested (Embleton &

McElhinny, 1982) that the direction could be anomalous due to complex

magnetisation of the samples. Hoffman (1984) made a study of the Liverpool

212

volcano, which also gives a questionable palaeo pole. A flow with large internal

variations was investigated using electron microprobe analyses and reflected light

microscopy. It was found that nearly unaltered titanomagnetite extremely rich in

titanium (x > 0.75) was present. Titanomagnetite with x > 0.75 is SP or

paramagnetic at room temperature, so these grains will not obtain a remanence as

the flow cools after extrusion. However, over time, low temperature oxidation of

the extremely high Ti titanomagnetite will produce titanomaghaemite with a

higher Curie temperature. This CRM will be in the direction of the ambient field

at that time, i.e. at an unspecified time after extrusion. This process is the

explanation for samples from one flow that contain reversed, intermediate and

normal polarity directions. The extent of low temperature oxidation is expected to

proceed at varying rates within the flow, resulting in a stable CRM being formed

at different times. In this study of the Barrington volcano there is no comparable

large intra flow variation but there is evidence of low temperature oxidation in

some flows. The majority of flows exhibit type 1c Curie curves which appear to

contain primary Ti rich (x ~ 0.6) titanomagnetite. With a low Curie temperature,

it is feasible that the remanence has been altered due to viscous processes since

Tertiary times. However, there is no significant difference in remanence direction

between flows with low Curie temperatures and those dominated by higher Curie

temperature, Ti poor titanomagnetite. It is therefore not thought likely that the late

acquisition of remanence is likely to have occurred in these Barrington Tops

lavas. However, the small variation between flow directions found at Barrington

Tops could indicate that there was some form of wide spread remagnetisation

event at some point after extrusion. There is no other evidence for this.

Another suggestion for the anomalous pole, advocated by Idnurm (1985a),

is that there is incomplete cancellation of palaeosecular (PSV) variation.

Barrington, Liverpool and Nandewar volcanoes all show successive flows with

similar remanence directions. This directional grouping could be caused by rapid

extrusion of the lavas. If there is episodic extrusion of lava then to eliminate

secular variation SV, unit weights should be assigned to the group means as

opposed to individual flows. At Barrington Tops it was noted that there were

ferruginised layers between some flows, indicating there was a significant length

of time between some flows and not others. However, all the flow means are very

similar with no clear grouping of directions. If the flow means are grouped in

213

some way whilst this will not change the pole position significantly it will

increase the α95 making the pole less anomalous. The total time for the extrusion

of the lava pile at Barrington Tops is not definitively known. It has been

suggested that the spread of K-Ar ages (Section 8.3) could be erroneous (N.

Thomas, pers. comm.) and the lava pile was extruded rapidly. However, the

Wellman et al. (1969) study which included a greater number of flows than the

present study found both normal and reversed directions indicating a longer time

interval than the present study, yet the deduced palaeopoles are the same.

An alternative explanation for the anomalous pole is that significant non

dipole components of the field are present. Global data covering the Cenozoic (0-

65 Ma) indicate that the GAD hypothesis is valid over this time period (Kent &

Smethurst, 1998). However, it has been suggested (e.g. Wilson, 1970; Hailwood,

1977; Livermore et al., 1984; Chauvin et al., 1996) that there was a significant

non dipole component of the geomagnetic field in the Tertiary. Schneider & Kent

(1990) (and previously Coupland & Van der Voo (1980)) examined selected

Tertiary palaeomagnetic data to assess the non dipole component, and concluded

that the non dipolar field was essential quadropolar. The evaluated non dipole

field is not sufficient however, to explain the Barrington pole (Idnurm, 1985a)

(nor for example, does it explain the Tertiary pole for Central Asia obtained by

Chauvin et al. (1996)).

If the dipole field is weak then the non dipole field will have a greater

influence. McFadden et al. (1991) compiled global PSV data through time and

evaluated the relative contributions of primary (dipole) and secondary

(quadropolar) dynamo families. The angular dispersion of PSV for 45-80 Ma

varies between 11° and 24° depending on latitude. The total angular dispersion

(S) from the present study is 9°, so that the between site dispersion is 8° when the

within site dispersion is removed (as described in McFadden et al., 1991). It can

be seen that the 8° dispersion is less than that from the global compilation of

PSV. However, the contribution from the secondary family (which is latitude

independent) is evaluated as 9.7 ± 1.5° which fits more closely to the 8°

dispersion found at Barrington Tops. Therefore the directional grouping could be

a result of extrusion occurring at a time when the amplitude of secular variation

214

(SV) was very small. The strength of the dipole field can be determined from

palaeointensity analysis.

8.8 Conventional Thellier Experiments The Coe (1967a) version of the Thellier technique with pTRM checks

(Section 2.3) has been carried out on sister cores of all the samples used in the

rock magnetic analyses (i.e. two per site). There was no sample pre selection

despite the evidence for low temperature oxidation in some flows. Heating steps

were carried out from 100°C in 50° steps up to 500°C and then 25° steps to

600°C. The laboratory field used was 50 µT.

8.8.1. Results Representative Arai plots with pTRM checks are shown in Fig. 8.18 and

Fig. 8.19. It can be seen that non- ideal behaviour is exhibited by all samples.

Three criteria had to be met for a palaeointensity estimate to be made. Firstly,

there must be a straight-line segment on the Arai plot in the temperature region

that defines the characteristic remanence (taken from the directiona l study).

Secondly, in the selected interval, the direction of magnetisation during the

Thellier experiment should be in the characteristic remanence direction and not

swing towards the direction of the magnetising field. Thirdly, the pTRM checks

should indicate the previous pTRM determination. Only 4 palaeointensity

estimates could be made out of the 42 Thellier experiments that were performed.

The Arai plots are shown in Fig. 8.19 and the results with associated Coe

statistics are in Table 8.9.

Table 8.9 Palaeointensity estimates

Sample N T interval f g σb/b q Ha µT uncertainty05-03 3 100-300 0.32 0.41 0.047 2.79 15.00 5.3414-06 4 250-450 0.20 0.54 0.311 0.35 6.18 17.6020-01 4 200-350 0.49 0.65 0.083 3.84 7.15 1.8621-05 3 200-300 0.12 0.49 0.299 0.20 12.38 63.40

215

01-02

0

0.2

0.4

0.6

0.8

1

1.2

0 0.05 0.1 0.15 0.2 0.25

trm gained

nrm

rem

aini

ng

02-04

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8

trm gained

nrm

rem

aini

ng

03-02

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

trm gained

nrm

rem

aini

ng

04-03

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3

trm gained

nrm

rem

aini

ng06-01

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14 16

trm gained

nrm

rem

aini

ng

07-03

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

trm gained

nrm

rem

aini

ng

08-05

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3

trm gained

nrm

rem

aini

ng05-04

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

trm gained

nrm

rem

aini

ng

Figure 8.18 Representative Arai plots (one per site), with pTRM checks (lines).

216

09-03

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

trm gained

nrm

rem

aini

ng

10-02

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6

trm gained

nrm

rem

aini

ng

12-06

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

trm gained

nrm

rem

aini

ng

13-07

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7

trm gained

nrm

rem

aini

ng14-01

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7trm gained

nrm

rem

aini

ng

15-06

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2trm gained

nrm

rem

aini

ng

16-06

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5trm gained

nrm

rem

aini

ng

17-03

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5trm gained

nrm

rem

aini

ng

Figure 8.18 Contd. Representative Arai plots (one per site), with pTRM checks (lines).

217

18-01

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3trm gained

nrm

rem

aini

ng

19-04

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8trm gained

nrm

rem

aini

ng

20-05

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10trm gained

nrm

rem

aini

ng

22-04

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2trm gained

nrm

rem

aini

ng

Figure 8.18 Contd. Representative Arai plots (one per site), with pTRM checks (lines).

218

05-03

y = -0.300x + 1.043R

2 = 0.998

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6trm gained

nrm

rem

aini

ng

21-05

y = -0.236x + 0.868R

2 = 0.913

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2trm gained

nrm

rem

aini

ng

20-01

y = -0.142x + 1.874R2 = 0.986

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10

trm gained

nrm

rem

aini

ng

14-06

y = -0.112x + 0.484R2 = 0.816

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4

trm gained

nrm

rem

aini

ng

Figure 8.19 Samples which produced palaeointensity estimates. Triangles are the accepted points and the dashed line the line of best fit.

219

8.8.2. Discussion

Only 4 out of the 42 determinations gave palaeointensity estimates. The

samples were from flows that did not show evidence of low temperature

oxidation. Two of the palaeointensity estimates use only 3 points and the other

two estimates, 4 points. The estimates are not of high quality as shown by the q

factor and the high uncertainty. However, the dipole appears weak at Barrington

Tops in the Tertiary as the results indicate that the field intensity was probably

around 10 µT, which corresponds to a VDM of 1.3 x 1022 Am2.

The low success (10 %) of these Thellier experiments is consistent with

the rock magnetic results that indicated the high susceptibility to thermal

alteration of the majority of the samples. The non- linear Arai plots and the

extreme failure of pTRM checks are indicators of alteration. The creation of new

magnetic material is also indicated by the formation of CRM in the direction of

the magnetising field used in the Thellier experiment, seen as a swing in the

direction of magnetisation from the characteristic remanence direction towards

the magnetising field direction. The creation of higher blocking temperature

material indicates that the correction methods of McClelland & Briden (1996)

and Valet et al. (1996) would not be suitable.

The success of the Thellier experiments could, perhaps, be improved with

a better choice of temperature steps. A greater number of low temperature heating

steps would be advisable for samples that exhibited the characteristic remanence

direction at low temperatures. Over the temperature interval that produced the

palaeointensity estimates (Table 8.9) better estimates could be made if there were

a greater number of data points. It would also be better to carry out the Thellier

experiments in a laboratory field that is nearer to the estimated palaeointensity.

220

8.9. Microwave Experiments

It is important for the development of the microwave technique that it can

be extended for use with older rocks, in particular ones that contain multi

directional components. Microwave demagnetisation and a preliminary intensity

investigation using microwave demagnetisation / remagnetisation has been

carried out with the Tertiary Barrington Tops basalt.

8.9.1. Microwave Demagnetisation

One standard core per site was selected, and two oriented mini cores from

each standard core underwent microwave demagnetisation. The mini cores were

oriented as described in Section 4.2.1. Selected, representative OVP and intensity

plots of microwave and thermal demagnetisation are shown in Fig. 8.20 and the

evaluated directions listed in Table 8.10. Similar behaviour is shown by thermal

and microwave demagnetisation (e.g Fig 8.20a and f) however, for most samples

the transition between different components of magnetisation is generally more

gradual with microwave demagnetisation (as was also found for some Hawaiian

samples (Section 7.6)). For some samples (Table 8.10 and Fig. 18.20c, d) this

smearing of components means that it is not possible to isolate the primary

component of magnetisation. Only one sample, 04-03 (Fig. 8.20e) exhibited

markedly different behaviour, with the thermal and two microwave experiments

all showing different behaviour.

Table 8.10 Directional results from thermal and microwave demagnetisation.

Thermal Microwave Microwave MeanSample T range D I MAD P range D I MAD P range D I MAD D I α95 k01-08 250-480 126.9 10.3 3.4 8-75 127.2 4.2 1.6 17-50 102.4 -16.9 1.2 119.1 -0.8 31.4 16.502-06 200-300 276.7 74.0 4.0 42-60 230.3 54.6 4.6 29-55 235.3 51.0 5.6 240.6 60.9 25.0 25.503-01 250-540 203.2 67.4 2.5 25-75 249.9 54.6 5.5 17-118 216.3 64.5 2.2 226.6 63.6 20.3 37.904-03 350-450 160.0 38.5 5.905-05 300-575 160.9 74.5 3.006-06 300-560 172.5 68.0 6.3 0-110 147.3 74.3 2.5 25-112 195.6 76.2 2.7 171.0 73.7 11.9 108.307-06 350-550 160.5 72.4 2.7 42-55 158.9 52.9 3.408-07 450-570 164.6 67.6 0.8 55-105 178.4 74.0 1.3 17-138 180.6 72.3 1.2 173.6 71.4 6.7 340.309-02 150-550 212.3 60.2 4.7 42-95 215.7 52.0 3.0 55-110 229.3 50.9 3.5 219.6 54.6 11.2 122.910-0512-08 350-570 180.1 56.0 3.2 37-125 180.9 55.0 1.9 17-110 180.3 56.5 2.8 180.4 55.8 1.2 10280.213-07 300-540 177.1 47.5 2.914-06 250-540 192.6 63.1 2.5 42-105 181.1 79.4 15.215-06 200-500 191.4 48.2 1.5 25-88 177.3 53.5 3.1 25-95 156.0 49.5 1.9 175.0 51.3 18.1 47.516-04 150-480 184.1 68.7 1.1 25-46 213.2 76.6 2.5 17-46 211.6 58.4 4.9 203.1 68.4 16.7 55.317-03 150-500 195.5 72.8 1.5 8-46 158.9 81.1 1.5 8-50 112.8 63.0 1.9 146.9 75.7 24.4 26.718-02 200-400 210.1 64.8 2.0 21-58 196.9 71.9 3.6 17-65 133.2 84.9 5.4 198.2 75.0 19.3 41.819-01 150-540 203.1 59.7 3.0 55-112 261.9 81.9 7.620-05 200-450 137.1 71.0 4.021-04 200-550 212.3 54.8 5.1 125-150 205.9 57.0 11.122-04 200-500 228.8 -4.9 2.2 8-62 224.0 5.8 1.7 8-50 248.5 5.1 4.6 233.7 2.0 22.0 32.4

221

W E

S DOWN

N UP

Hor izonta l

V e r t i c a l

S A M P L E E B T 1 0 1 - 0 8 A

FIELDS20 100 150 200 250 300 350 400 450 480

500 520 540

W E

S DOWN

N U P

H o r i z o n t a l

V e r t i c a l

S A M P L E E B T 1 B T 0 1 0 8 D

FIELDS0 8 12 17 21 25 29 33 37 42

46 50 58 66 75

W E

S DOWN

N U P

H o r i z o n t a l

V e r t i c a l

S A M P L E E B T 1 B T 0 1 0 8 B

FIELDS0 8 17 25 29 33 42 50 55

0 20 40 60

Power Watts

0 20 40 60 80

Power Watts

0 100 200 300 400 500 600

Temperature °C

222

N S

W D O W N

E U

Horizontal

Vert ical

SAMPLE EBT107-06A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

5 5 0 5 7 0

N S

W DOWN

E U

Horizontal

Vert ical

SAMPLE EBT1BT0706D

F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 5 6 0 6 8

7 5 8 8 1 0 0 1 1 2

N S

W DOWN

E U

Horizontal

Vert ical

SAMPLE EBT1BT0706B

F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 5 6 2 7 0

8 0 9 2 1 0 5 1 1 8 1 3 0 1 4 2

0 100 200 300 400 500 600

Temperature °C

0 20 40 60 80 100 120

Power Watts

0 50 100 150

Power Watts

N S

W D O W N

E U

Horizontal

Vert ical

SAMPLE EBT114-06A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 4 8 0

5 0 0 5 2 0 5 4 0

N S

W D O W N

E U

Horizontal

Vert ical

SAMPLE EBT1BT1406A

N S

W D O W N

E U

Horizontal

Ver t ica l

SAMPLE EBT1BT1406B

0 100 200 300 400 500 600

Temperature °C

0 50 100 150

Power Watts

0 50 100 150

Power Watts

Figure 8.20c and d.

07-06

14-06

Thermal

Thermal

Microwave

Microwave Microwave

Microwave

Vertical Horizontal

c)

d)

25W

25W

33W

33W

350°C

350°C

150°C

150°C

33W 42W

42W

223

F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 8 6 8 7 8

8 8 1 0 0 1 1 2 1 2 5 1 3 8

F I E L D S0 8 1 7 2 5 3 3 4 2 5 0 5 8 6 8 8 0

9 2 1 0 5 1 1 8 1 3 0 1 4 2

W E

S D O W N

N UP

Horizontal

Ver t ica l

SAMPLE EBT104-03A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

W E

S D O W N

N UP

Hor izonta l

V e r t i c a l

S A M P L E E B T 1 B T 0 4 0 3 D

FIELDS0 8 1 7 2 5 3 3 3 7 4 2 4 6 5 0 5 4

5 8 6 2 7 0 8 0 9 0 1 0 0

W E

S D O W N

N UP

Hor izonta l

Ver t ica l

SAMPLE EBT1BT0403B

FIELDS0 8 1 7 2 5 3 3 4 2 5 0 5 5 6 2 7 0

8 0

0 20 40 60 80

Power Watts0 20 40 60 80 100

Power Watts

0 100 200 300 400 500

Temperature °C

N S

W D O W N

E U

Horizonta l

Ver t ica l

SAMPLE EBT112-08A

F I E L D S2 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

5 5 0 5 7 0

N S

W D O W N

E U

Horizontal

Ver t ica l

SAMPLE EBT1BT1208A

N S

W D O W N

E U

Horizontal

Ver t ica l

SAMPLE EBT1BT1208B

0 100 200 300 400 500 600

Temperature °C

0 50 100 150

Power Watts

0 50 100 150

Power Watts

Figure 8.20 Representative OVP and intensity plots using thermal and microwave demagnetisation; a) 01-08, b) 18-02, c) 07-06, d) 14-06 e) 04-03 and f) 12-08.

Vertical Horizontal

e)

04-03

12-08

Thermal

Thermal

Microwave Microwave

Microwave Microwave

f)

37W

37W

37W

37W

350°C

350°C

62W

62W

33W

33W

70W

70W

224

Where primary directions were obtainable from both microwave demagnetisation

determinations the mean direction of the three determinations (two microwave

and one thermal) has been evaluated and listed in Table 8.10. The α95 values have

a large range, from 1.2° to 31°. The error is a combination of errors associated

with sample orientation and the uncertainty in the microwave determined

directions. For some samples, only three power steps isolate what is interpreted as

the primary direction.

Table 8.11 Comparison of thermal and microwave directions with no field data included.

Sample Thermal Microwave Difference Microwave Difference D I D I DT-DM IT-IM D I DT-DM IT-IM

01-08 106.6 -9.1 101.3 -12.4 5.3 3.3 101.6 -15.0 5.0 5.902-06 200.0 48.5 219.3 26.5 -19.3 22.0 224.6 27.0 -24.6 21.503-01 178.2 6.0 202.8 0.6 -24.6 5.4 183.6 3.3 -5.4 2.706-06 157.1 16.0 164.6 23.2 -7.5 -7.2 167.0 11.3 -9.9 4.707-06 210.3 61.4 249.5 58.6 -39.2 2.808-07 157.5 -6.0 164.5 -9.5 -7.0 3.5 163.1 -10.6 -5.6 4.609-02 247.0 60.0 215.7 52.0 31.3 8.0 274.0 60.7 -27.0 -0.712-08 128.4 48.8 128.1 47.8 0.3 1.0 128.4 49.3 0.0 -0.514-06 146.6 42.4 168.9 41.0 -22.3 1.415-06 153.5 77.1 192.6 77.3 -39.1 -0.2 228.7 66.9 -75.2 10.216-04 147.8 62.0 158.8 52.3 -11.0 9.7 132.1 49.4 15.7 12.617-03 168.4 61.9 184.7 54.3 -16.3 7.6 220.6 51.1 -52.2 10.818-02 144.7 51.8 158.3 54.3 -13.6 -2.5 182.6 47.8 -37.9 4.019-01 238.2 57.8 273.7 83.2 -35.5 -25.421-04 143.6 26.7 147.0 30.0 -3.4 -3.322-04 25.7 0.5 21.3 11.4 4.4 -10.9 46.0 9.3 -20.3 -8.8

To investigate the sample orientation error, field data has been removed

from the thermal and microwave directions so that the declination and inclination

values as measured by the SQUID and spinner magnetometers can be compared

(Table 8.11). The difference between thermal and microwave declination values

starts at –75°, then –39° up to +31° and the difference between inclination values

range from -25° to +22°. The range of differences in declination is greater than

for the inclination values. This is as expected from sample orientation error, as

the dominant error is in the horizontal plane (see Section 7.6.2). The differences

can be compared to those obtained in the study of Hawaiian lava (Section 7.6.2),

excluding the anomalous –75° declination difference. The differences are listed in

Table 8.12. The range of declination differences is less for the Barrington Tops

study indicating that using the wafering saw, to orient the samples to the

microwave system, is more accurate than the pen line used in the Hawaii study.

The mean declination difference is still significantly negative indicating that the

225

stick and mark on the wall used to orient the samples to the microwave system is

not correct. The range of inclination differences for the Barrington Tops samples

is greater than the Hawaiian study despite extreme care being taken in the drilling

of samples. It is therefore likely that this increased range (and non zero mean) is

indicative of error in the isolation of the primary component and not core

orientation.

Table 8.12 Comparison between the differences in thermal and microwave directions with no field data for the Hawaiian and Barrington Tops studies.

DT-DM IT-IM

N min max mean min max meanHawaii 26 -51 42 -16 -14 18 -0.6Barrinton Tops 28 -39 31 -13 -25 22 3

To summarise, for the Barrington Tops samples that underwent

microwave demagnetisation the presence of secondary components can cause

difficulties in the isolation of the primary component of magnetisation. The

difficulty in removing the secondary components indicates that the microwave

unblocking spectra differ from the thermal unblocking spectra. The orientation of

the samples needs to be improved, in particular the orientation of the sample to

the microwave system, before directions determined with the microwave system

are sufficiently accurate.

8.9.2. Microwave Palaeointensity Study Due to time constraints it was only possible to carry out a preliminary

microwave palaeointensity study. Samples from 8 sites were investigated. Six

samples (two from each rock magnetic group) were chosen that exhibited a stable

characteristic remanence in the expected direction during microwave

demagnetisation experiments. In addition to these samples, a sample (01-08) from

site 01 interpreted as containing an IRM from a lightning strike was investigated.

A sample from core 20-01 also underwent microwave palaeointensity analysis as

this core gave the best estimate of the palaeofield using the Thellier technique.

The ceramic method was used initially so that the directional stability

could be monitored. If the ceramic method produced a successful result the

perpendicular field method was then performed to check for consistency. No

226

alteration checks were carried out. The laboratory field was 20 µT for all

experiments. Unoriented samples were used throughout.

The sample that yielded the best estimate of the palaeofield using the

Thellier method (20-01) was not part of the microwave demagnetisation study so

its behaviour to microwave exposure was not previously known. Unfortunately,

as can be seen in Fig. 8.21 during microwave demagnetisation a stable

characteristic direction of magnetisation was not obtained. It was, therefore, not

possible to obtain a palaeointensity estimate using the microwave technique for

this sample.

N S

W DOWN

E U

Horizontal

Vertical

SAMPLE EBT1BT2001I

FIELDS0 8 17 21 25 29 33 37 42 46

52

Figure 8.21 OVP for sample 20-01 showing that no stable direction is obtained from microwave demagnetisation.

Site 01 is interpreted as having been affected by lightning strike resulting

in a strong stable remanence with no viscous component. A sample (01-08)

underwent palaeointensity analysis using the ceramic method. The results are

shown in Fig. 8.22 along with the Thellier result for comparison. The direction of

the NRM during the experiment, plotted on the OVP, illustrates the stability of

remanence. As the remanence is an IRM, as opposed to a TRM, ideal behaviour

would not be expected for palaeointensity analysis and this was indeed the case. It

52W

52W

8W

8W

227

is interesting to note that during the microwave experiment as the power is

increased the NRM reduces as expected, but the amount of pTRM gained also

reduces. This continues until the last three power steps when the pTRM gained

increases as expected. This behaviour is not seen in the Thellier analysis.

W E

S DOWN

N UP

Horizontal

Ver t i ca l

SAMPLE EBT1BT0108 I

F I E L D S0 8 1 7 2 1 2 5 2 9 3 3 3 7 4 2 4 6

01-08 Ceramic Method

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 200 400 600 800

TMRM gained

NR

M r

emai

nin

g

01-08 Thellier Method

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5

trm gained

nrm

rem

aini

ng

Figure 8.22 Comparison of microwave ceramic method and Thellier palaeointensity results for sample 01-08 containing an IRM from lightning strike. The OVP is from NRM

directions during the microwave experiment (unoriented sample).

The two rock magnetic type A samples (16-04 and 18-02) exhibited

anomalous behaviour during the microwave palaeointensity experiment using the

ceramic method. The results are shown in Fig 8.23 with the Thellier results for

comparison and the OVP of the NRM directions during the microwave

experiment. For sample 16-04, during the microwave experiment the NRM

intensity increased for two increasing power steps. This can be seen in both the

Arai and OVP plots in Fig. 8.23a. It is possible that this is due to the incomplete

removal of the TMRM formed in the previous power step. Without the experiment

being repeated with a sister sample it is not possible to say more about this result.

Sample 18-02 produces a good straight- line on the Arai plot however it has a

228

positive slope as opposed to the expected negative slope. For each increasing

power step the NRM decreases as expected, but the amount of TMRM gained for

increasing power decreases. This is similar to the behaviour seen by the sample

01-08 affected by lightning strike and would lead to the conclusion that this

sample could contain an IRM as opposed to a TRM.

16-04 Ceramic Method

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.5 1 1.5 2 2.5 3

TMRM gained

NR

M r

emai

nin

g

16-04 Thellier Method

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6

trm gained

nrm

rem

ain

ing

N S

W DOWN

E U

Horizontal

Vert ical

SAMPLE EBT1BT1604I

F I E L D S0 8 1 2 1 7 2 1 2 5 2 7 3 2 3 5 3 9

4 3

Figure 8.23a

a)

8W

8W

0W

0W

229

18-02 Ceramic Method

y = 0.2831x + 0.04R2 = 0.902

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4

TMRM gained

NR

M r

emai

nin

g

18-02 Thellier Method

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5

trm gained

nrm

rem

aini

ng

W E

S DOWN

N UP

Horizontal

Vert ical

SAMPLE EBT1BT1802I

F I E L D S0 8 1 7 2 1 2 5 2 7 3 1 3 5 3 9 4 5

5 2 5 8 6 5

Fig. 8.23 Comparison of microwave ceramic method and Thellier palaeointensity results for type A rock magnetic samples a) 16-04 and b) 18-02. OVPs are of NRM directions during

the microwave experiment (unoriented samples).

The results from the two samples from rock magnetic group B are shown

in Fig. 8.24. Sample 09-02 failed to give a palaeointensity estimate however an

estimate could be made from sample 03-01.

Sample 09-02 exhibited behaviour similar to 01-08 which contains an

IRM (Fig. 8.24a). For the first four power steps the TMRM gained decreased with

increasing power, but for the final three power steps TMRM gained increased.

However, from the OVP it can be seen that the smoothest trajectory, converging

to the origin is obtained for the final four power steps. The previous three power

steps (used in the palaeointensity experiment) do not extend the trajectory and

thus could be due to a secondary component of magnetisation. This would

explain the anomalous Arai plot, but even if the first three points on the Arai plot

are discarded the remaining points do not produce a good straight line. It is

therefore not possible to obtain a palaeointensity estimate from this sample.

27 W

65 W

b)

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230

Sample 03-01 behaved well during microwave palaeointensity analysis

using the ceramic method and also using the perpendicular field method (Fig

8.24b). The palaeointensity estimates and associated quality factors are listed in

Table 8.13. The perpendicular method gave a higher estimate, 18.5 ± 2.4 µT, than

the ceramic method estimate, 10.7 ± 1.2 µT. The discrepancy between the two

estimates could be due to a number of factors including sample anisotropy. No

palaeointensity estimate could be made from the Thellier experiment result;

indeed there is no straight line segment on the Arai plot.

N S

W DOWN

E U

Horizontal

Ver t i ca l

SAMPLE EBT1BT0902I

F I E L D S0 4 0 4 2 4 5 5 0 5 8 6 5 7 2 8 2 9 0

09-02 Ceramic Method

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

TMRM gained

NR

M r

emai

nin

g

09-02 Thellier Method

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5

trm gained

nrm

rem

ain

ing

Figure 8.24a

45 W

65 W

72 W

82 W

90 W

a)

65W

65W

231

03-01 Ceramic Method

y = -0.536x + 1.427R2 = 0.998

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5

TMRM gained

NR

M r

emai

nin

g

03-01 Perpendicular field method

y = -0.922x + 2083R

2 = 0.989

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5

TMRM gained

NR

M r

emai

nin

g

03-01 Thellier method

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5

trm gained

nrm

rem

ain

ing

W E

S DOWN

N UP

Horizontal

Vert ical

SAMPLE EBT1BT0301I

F I E L D S0 8 2 5 3 3 4 5

5 5 6 2 7 0 8 0

8 5 9 2 9 8 1 0 2 1 0 8

Fig. 8.24 Comparison of microwave and Thellier palaeointensity results for type B rock magnetic samples a) 09-02 and b) 03-01. OVPs are of NRM directions during the microwave

ceramic method experiment (unoriented samples).

The two samples with group C rock magnetic characteristics exhibited

similar behaviour to microwave palaeointensity analysis. Both samples produced

palaeointensity estimates as shown in Fig. 8.25 and Table 8.13. The

palaeointensity estimates evaluated using the ceramic, and the perpendicular field

methods produced consistent results. It is interesting to note that the Arai plot

produced using the Thellier method exhib its a straight- line segment with a

gradient that would indicate a palaeointensity similar to that derived from the

microwave analysis for both samples. The segment is composed of only 3 points

for sample 08-07 and 4 points for sample 06-06. The pTRM checks over the

temperature intervals in question fail completely. Without alteration checks being

carried out for the microwave analysis it is not possible to say conclusively that

b)

Fe = 10.7 ± 1.2 µT

Fe = 18.5 ± 2.4 µT

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232

no alteration has occurred during experimentation. However the consistency

between the two microwave results using different methods, and the quality of the

estimates lead me to believe that they do give a good indication of the

palaeointensity of the samples.

Table 8.13 Palaeointensity estimates using microwave palaeointensity analysis

Sample Method N P interval f g σb/b q Ha µT Uncertainty03-01 Ceramic 9 55-108 0.58 0.69 0.018 22.0 10.74 1.18

perp field 7 60-92 0.76 0.78 0.047 12.6 18.54 2.3708-07 Ceramic 8 62-108 0.76 0.79 0.033 18.5 14.33 1.63

perp field 6 70-92 0.71 0.77 0.029 18.6 14.02 1.5906-06 Ceramic 5 50-98 0.34 0.39 0.043 3.1 17.02 5.74

perp field 11 50-115 0.73 0.75 0.018 29.7 17.58 1.86

The success of the microwave palaeointensity analysis in this preliminary

study is related to the rock magnetic properties of the samples, with the samples

exhibiting the greatest high temperature deuteric oxidation (type C) yielding the

greatest success. No palaeointensity estimate was obtained from the two type A,

low temperature Curie point samples. One of the type B samples produced an

estimate whilst the other failed. The palaeointensity estimates obtained using the

microwave technique are similar to the ones obtained using the Thellier technique

providing further indication of a low dipole moment in the early Tertiary.

It is desirable to extend the microwave palaeointensity study to include

many more samples and to implement alteration tests. If this is carried out then a

much better idea could be obtained of the strength of the magnetic field recorded

by the Barrington Tops lava.

233

08-07 Perpendicular Field method

y = -0.700x + 1.206R2 = 0.997

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2

TMRM gainedN

RM

lost

08-07 Ceramic Method

y = -0.714x + 1.077R2 = 0.994

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1 1.2 1.4

TMTM gained

NR

M r

emai

nin

g

W E

S DOWN

N UP

Horizontal

Vert ical

SAMPLE EBT1BT0807I

F I E L D S0 1 7 3 3 5 0 5 5 6 2 7 0 7 5 8 0 8 8

9 5 1 0 0 1 0 8

08-07 Thellier method

y = -0.260x + 0.903

R2 = 0.993

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4

trm gained

nrm

rem

ain

ing

06-06 Ceramic Method

y = -0.848x + 1.067

R2 = 0.994

0

0.2

0.4

0.6

0.8

1

1.2

0 0.1 0.2 0.3 0.4 0.5 0.6

TMRM gained

NR

M r

emai

nin

g

Perpendicular field method

y = -0.878x + 0.998

R2 = 0.997

0

0.2

0.4

0.6

0.8

1

1.2

0 0.2 0.4 0.6 0.8 1

TMRM gained

NR

M r

emai

nin

g

N S

W DOWN

E U

Horizontal

Vert ical

SAMPLE EBT1BT0606I

F I E L D S0 8 1 7 3 3 4 8 5 8 7 0 8 0 8 2 9 0

9 2 9 8 1 0 2 1 1 0

06-06 Thellier method

y = -0.322x + 0.932R2 = 1.000

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4

trm gained

nrm

rem

aini

ng

Fig. 8.25 Comparison of microwave and Thellier palaeointensity results for type C rock magnetic samples a) 08-07 and b) 06-06. OVPs are of NRM directions during the microwave

ceramic method experiment (unoriented samples).

Fe = 14.3 ± 1.6 µT

Fe = 14.0 ± 1.6 µT 0.26 x 50 =13

Fe = 17.0 ± 5.7 µT

Fe = 17.6 ± 1.9 µT 0.32 x 50 = 16

a)

b)

234

8.10. Summary The study area, Barrington Tops, was chosen to extend Australian and

early Tertiary palaeomagnetic data. The samples proved to be very interesting

rock magnetically, but not as suitable for conventional palaeointensity analysis as

had been hoped.

The majority of the flows studied are dominated by what appears to be a

primary high Ti titanomagnetite that converts to a nearer magnetite phase on

heating. Some flows show evidence of having undergone varying degrees of low

temperature oxidation. A few flows contain a single Ti poor titanomagnetite

thought to result from high temperature deuteric oxidation. The mineral magnetic

study could be extended to include reflected light microscopy.

The directional study using thermal demagnetisation confirmed the results

of the previous palaeomagnetic study by Wellman et al. (1969). Little variation is

seen between flow means, giving a section mean declination of 190°, inclination

of 64° and α95 of 6°. The corresponding palaeopole has been questioned as

anomalous when compared to data from sediment sequences and the Indian

APWP. Additional palaeomagnetic studies from different locations are required

to gain a better understanding of the geomagnetic field in Australia in the early

Tertiary.

The rock magnetic analyses indicate that the majority of samples are

highly susceptible to thermal alteration, thus rendering them problematic for

conventional Thellier palaeointensity analysis. The success was extremely low (as

predicted) however, a crude estimate from 4 samples suggested that the intensity

of the field was weak. More success was had with microwave intensity analysis

using samples in which the primary component of magnetisation could be

isolated with microwave demagnetisation, but only a preliminary study has been

carried out. Hence, no firm conclusions regarding the field intensity can be made

until the microwave study is extended. Importantly though, the dipole field

appears weak. This may explain the anomalous palaeopole as the non dipole field

will have greater influence when the dipole field is weak. This is in agreement

with previous studies (e.g. Wilson, 1970; Hailwood, 1977; Livermore et al.,

1984; Chauvin et al., 1996) that suggested the presence of a significant non

dipole component of the geomagnetic field in the Tertiary.