14. MAGNETIC-MINERALOGICAL STUDIES OF DSDP SITE 323 CORES1

V.I. Bagin, Yu. A. Bogdanov, and T.S. Gendler,P. P. Shirshov Institute of Oceanology, Academy of Sciences, USSR

INTRODUCTION

Using Mössbauer and rock magnetism methods,samples of cores from DSDP Site 323 were studied inour laboratory to determine some of their magneticmineralogical and magnetic physical properties as ameans of mineral identification and as a means of gain-ing some insight into the genesis of their iron forms.

Site 323 sediments were sampled from the oceanfloor to a hole depth of 701 meters where basalt was en-countered. For chronological purposes the descriptionproceeds from the basalt contact up. Our miner-alogical studies resolve three depth intervals: (1) typicaldeep-sea deposits from 701 to 638 meters; (2)terrigenous deposits with a large amount of mineralalteration from 638 to 506 meters; and (3) terrigenousdeposits with little mineral alteration from 506 to 0meters.

ANALYTICAL PROCEDURESWe studied three aspects of the samples from Site

323: Mössbauer effect, magnetic properties at roomtemperature, and thermomagnetic effects and proper-ties.

Mössbauer Technique

The Mössbauer methods (Wertheim, 1964) wereused mostly on the lower portion of the sedimentcolumn. The results are summarized in Table 1, andsome Mössbauer spectra are shown in Figure 1.Because of the very limited ferromagnetic constitutionof these samples, the ferromagnetic fraction was notsatisfactorily measured by Mössbauer spectra. All thesamples studied by this method have spectra with onesmall quadrupole doublet with a splitting value ofabout 0.054 mm/sec and isomer shift (with respect toCo57 [Pd]) of about +0.18 mm/sec. The doublet peaksare sharp and have components of equal intensity andequal line width. The observed characteristics of theseparameters indicate fine superparamagnetic particles ofgoethite FeO(OH) with an average size of about 180Å.

Spectra of the samples from levels higher in the sedi-ment column indicate that iron is mainly contained inthe clay minerals either only in the Fe3+ state (mont-morillonite) or in both the Fe2+ and Fe3+ states (illite,cronstedtite, chamosite).

Magnetic Properties at Room Temperature

The magnetic investigations at room temperatureconsisted of the following (see Table 2 for definition ofsymbols used): Determination of Irso as a function of

'Translated in part from Russian.

He; a.c. demagnetization of Irs0; determination of [Hòs]and [He's] and Hes; determination of the ratios Iso/Irsoand Irso/Irsomax (Table 1). The first parameter (Irso asa function of He) characterizes the relative amounts ofparamagnetic, superparamagnetic, single domain, andmultidomain grains of iron-containing minerals in thesamples (Bagin et al., 1969). The second parameter, Irs0characterizes the relative remanent saturation mag-netization of the various deposits. The results of roomtemperature magnetic determinations are given inTable 1.

Curves of Ir/Ir,,,^. as a function of He were twotypes: normal (Figure 2) and anomalous (Figure 3). Forthe curves of normal type (the normal type of deposit)the beginning of the saturation plateau (Hes) variesfrom 1000 to 3000 oe. For the anomalous type of curve(the anomalous deposit) the beginning of the saturationplateau (Hes) is greater than 7000 oe. For the normaldeposit the average value at the maximum slope (He's)is about 250 oe, and for the anomalous deposit themaximum slope (He's) is about 1.5 to 2 times greater.

The ratio Irso/Irsomax. varies greatly from sample tosample. The difference between the maximum and theminimum values of the ratio is as much as 1.5 to 2orders of magnitude. The curves (not shown) for Irs0 asa function of H are typical for deposits of normal type.One-half of Irso is usually demagnetized at 100 to 150oe. Only for the anomalous deposits is one-third of Irsonot demagnetized at 600 oe. The values of the ratioIso/Irso vary from 2.7 to 15.0. This means that there is awide range in grain size of the iron-containing mineralsin the deposits.

The results of magnetic investigations at roomtemperature show that the majority of samples containmagnetic minerals with a moderate value of magnetichardness. Only for the anomalous deposits weremagnetic phases found that require a great coercivefield. The large differences between the ratios ofIrso/Irso max. and Iso/Irso in Table 1 only indicate thatthe deposits contain iron in a number of differentforms.

Thermomagnetic StudiesFigure 4 shows curves of Is/Is0 and Irs/Irs0 as a func-

tion of temperature. The samples fall into five classes:Class 1. Magnetite and maghemite determine the

thermomagnetic curves of this class (Figure 4a, b). TheCurie point is about 575°C. Temperature changes causemineral alterations of maghemite. The Curie points andthe curves indicate maghemite.

Class 2. These thermomagnetic curves (Figure 4c, d)are typical for deep-sea deposits. The curves are in-dicative of the presence of nonstoichiometrical hema-tite and hydroxides of iron.

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V. I. BAGIN, Y. A. BOGDANOV, T. S. GENDLER

TABLE 1Summary of Results Obtained from the Mössbauer Technique and Magnetic Investigations at Room Temperature

Sample(Interval in cm)

1-1, 50-601-1, 140-1501-2, 34-431-4, 118-1273-1, 88-943-2, 14-20

3-2,61-70

4-2,7-2, 105-1107-3, 18-248-1, 120-121

10-1, 113-122

10-2, 75-85

10-3, 106-11411-1,22-3211-2, 137-14813-5, 106-11513-6, 145-15014-2, 1-814-2, 128-13515-1, 52-6015-2, 91-10015-3, 29-3615-4,51-6315-5, 89-9815-6, 19-2616-1, 57-6216-3,32-3716-4, 83-9418-2, 80-8618-3, 130-13918-4, 115-127

Hes(oe)

150050001500250025001000

>7500

150030003000

>7500

>7500

5000

350025001500150010003000200010001000100010001000700500

1000500700

10001000

Hc's(oe)

260320340400280230

550

100310320270

500

300

320350270290300320240240230230230260240240220220300250250

H(V2lrs0)oe

120_

120180

——

220

-_-

100

200

—

—_-—--

120——----

-_——

-

Iso/Irso

14.58.06.25.8

13.015.0

13.8

4.97.07.2

10.0

14.0

10.0

7.010.29.54.66.65.76.66.34.34.74.25.04.84.54.64.94.52.74.7

Irso/Irsomax

0.220.0540.1380.1450.155

—

0.055

0.6060.1830.1370.018

0.060

0.255

0.184_

0.3110.3940.7280.2600.9100.9101.0000.7280.8180.4580.3670.4000.8480.7340.4840.7340.421

OxidationState

_

__

Fe3+F e2+Fe3+Fe2+

--—

Fe 3 +

Fei2+

F e π

2 +

Fe3+Fe2+Fe3+Fe2+

_—---—

Fe3+——

Fe3+Fe3+

--

Fe3+Fe3+

—-—

Fe3+

IsomerShift

(mm/sec)

__——

0.04 ±0.040.98 ±0.040.15 ±0.061.08 ±0.02

-—-

0.08 ±0.030.98 ±0.030.78 ±0.030.10 ±0.061.25 ±0.060.15 ±0.061.07 ±0.02

——-—--

0.18 ±0.02_-

0.18 ±0.020.18+0.02

--

0.18 ±0.020.18 ±0.02

——-

0.18 ±0.02

QuadrupoleSplitting(mm/sec)

_

—__—

0.68 ±0.042.53 ±0.04

Vz 1.202.12 ±0.04

-—-

0.58 ±0.042.00 ±0.042.41 ±0.040.71 ±0.062.40 ±0.06Vz= 1.15

2.08 ±0.04——--—-

0.52 ±0.04—-

0.54 ±0.040.54 ±0.04

--

0.52 ±0.040.52 ±0.04

——-

0.48 ±0.04

RatioF e 3 + / F e 2 +

_

-——_

0.54

5.0

-—-

0.40

1.5

3.8

—-----————-----—--

-

Class 3. The thermomagnetic curves (Figure 4e, f) arecharacteristic of iron oxide forms, and they are typicalfor nonstoichiometrical hematite.

Classes 4 and 5. It is presumed that the curves ofFigures 4g, h, i are likely to be determined bytemperature transitions of cronstedtite. The 525 °Ctemperature corresponds to the beginning of thetemperature transition of cronstedtite as revealed bydifferential thermal analysis.

MAGNETIC-MINERALOGICAL DEPTHZONATION OF SITE 323

Interval 701 to 638 Meters; Typical Deep Sea Deposits

The magnetic-mineralogical data delineate threezones in this interval.

Zone A, 701 to 675 meters: Samples 16-1, 57-62 cm;16-3, 32-37 cm; 16-4, 83-94 cm; 18-2, 80-86 cm; 18-3,130-139 cm; 18-4, 115-124 cm. The thermomagneticand other magnetic characteristics are typical of deep-

sea deposits. The iron is mostly contained in the super-paramagnetic grains of αFeOOH. The ferromagneticfraction contains the nonstoichiometrical hematite.

Zone B, 675 to 660 meters: Samples 15-3, 29-36 cm;15-4, 54-63 cm; 15-5, 89-98 cm; 15-6, 19-26 cm. Thethermomagnetic data are typical for highly alteredterrigenous deposits. The iron is contained in non-stoichiometrical hematite. The hydroxide iron formsare less common here than in Zone A.

Zone C, 660 to 638 meters: Samples 13-6, 145-150cm; 14-2, 1-8 cm, 128-135 cm; 15-1, 52-60 cm; 15-2, 91-100 cm. Zone C is similar to Zone A.

Interval 638 to 506 Meters; Terrigenous DepositsWith Highly Altered Minerals

We used Samples 10-1, 113-122 cm; 10-2, 75-85 cm;10-3, 106-114 cm; 11-2, 137-148 cm; 13-5, 106-115 cm.Except for Sample 10-1, 113-122 cm, the magneticproperties of the deposits are similar to those of Zone Bat 675 to 660 meters below the sea floor. Cronstedtite is

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MAGNETIC-MINERALOGICAL STUDIES OF CORES

18-4, 115-124 cm

TABLE 2Definition of Symbols Used in this Report

-2 -1 0 +1

Velocity (mm/sec)

+2

Figure 1. Some Mossbauer spectra of Site 323 sedimentsamples.

likely present in Sample 10-1, 113-122 cm. Samples 10-1, 113-122 cm and 10-2, 75-82 cm correspond to the in-termediate zone (638 to 506 m) in the terrigenousdeposits. Hematite is found in the samples from 638 to506 meters.

Interval 506 to 0 Meters; Terrigenous Deposits With Lit-tle Alteration of Minerals

Based upon magnetic-mineralogical characteristics,this interval can be divided into many zones. Forsimplicity we divided the interval into three groups ofmagnetic-mineralogical characteristics regardless of thesample's depth in the interval 506 to 0 meters.

Group 1: Samples 1-1, 50-60 cm; 1-3, 78-88 cm; 3-1,88-94 cm; 4-2. The ferromagnetic fraction consists oflow-temperature oxidation magnetite (magnetite andmaghemite). The material of these samples was likelyderived from pre-existing rocks, with littlemineralogical alteration during transportation.

Group 2: Sample 3-2, 61-70 cm. In contrast to thesamples of Group 1, magnetite and maghemite werenot found. Only nonstoichiometrical hematite wasfound here, which may indicate a highly oxidizingregime at the time of the material's formation.

Irs

Irs0

Irsomax ~

Is

Is0

Ir

IrmaxHe

Hds

Hes

He's

oeHa.c. -d.c.Isi

Is2—

H(%Irs0) -

remaining Irs0 at a particular temperature as thesample is heated or cooledinitial saturation remanencemaximum value of Irso for given collection ofsamplesmagnetization in a strong field at a particular temp-eratureinitial saturation magnetizationnormal remanencemaximum normal remanence for each sampleartificial magnetic field to which a sample is sub-jectedreverse d.c. magnetic field required for completedemagnetization of Irso

minimum amount of magnetic field required toreach the beginning of the saturation plateau (Fig-ure 2)The magnetic field at a point of maximum rate ofchange of Ir/Irmax on the Ir/Irmax versus Hecurve of Figure 2oerstedsa.c. fieldalternating currentdirect currentIso

saturation magnetization at 20°C after first heatingapproximately equal tomedian destructive a.c. field

Group 3: Samples 1-1, 140-150 cm; 1-2, 37-43 cm; 1-4, 118-127 cm; 7-2, 105-110 cm; 7-3, 18-24 cm; 8-1, 120-121 cm. According to the literature (Betekhtin, 1950),cronstedtite formation occurs soon after deposition.The low values of remanent saturation magnetizationand high values of the ratio Iso/Irso require that the ironis mainly contained in paramagnetic silicate minerals.

REFERENCESBagin, V.I., Brodskaya, S.Yu., Petrova, G.N., and

Pechersky, D.M., 1969. Study on the ferromagnetic frac-tions in basalts: Izv. Acad. Nauk. USSR, ser. pkizikazemli, no. 11, p. 66-76.

Betekhtin, A.G., 1950. Mineralogy: Moscow (Gosgeolizdat).Wertheimer, G.K., 1964. Mössbauer effect, principles and

applications: New York and London (Academic Press).

£ 0.5 -

8000

Figure 2. A plot typical of normal (more common) samplesof Ir/Irrmx. as a function of the magnetic field. Notethe positions of Hes and He's.

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V. I. BAGIN, Y. A. BOGDANOV, T. S. GENDLER

1000 2000 3000 4000 5000 6000 7000 8000

Figure 3. A plot typical of anomalous samples of Ir/Irmax

as a function of the magnetic field. Note the positionsof Hes and He's.

Figure 4a. A plot of the saturation magnetization in a 5000oe field as a function of progressive and regressive tem-perature for class 1 samples. Note the difference betweenthe heating curve and the cooling curve due to mineralalteration during heating.

Figure 4b. Remanence magnetization of class 1 samples asa function of temperature for progressive heating andthen for progressive cooling.

Figure 4c. A plot of the saturation magnetization in a 5000-oe field as a function of progressive and regressive tem-perature for class 2 samples. Note the differences be-tween the heating curve and the cooling curve due tomineral alteration during heating.

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MAGNETIC-MINERALOGICAL STUDIES OF CORES

Figure 4d. Remanence magnetization of class 2 samples asa function of temperature for progressive heating thenfor progressive cooling.

600

Figure 4e. A plot of the saturation magnetization in a 5000-oe field as a function of progressive and regressive tem-perature for class 3 samples. Note the differences be-tween the heating curve and the cooling curve due tomineral alteration during heating.

" 0.5 •

0 100 200 300 400 500 600 700

Figure 4f. Remanence magnetization of class 3 samples asa function of temperature for progressive heating andthen for progressive cooling.

Figure 4g. A plot of the saturation magnetization in a 5000-oe field as a function of progressive and regressive tem-perature for class 4 samples. Note the differences be-tween the heating curve and the cooling curve due tomineral alteration during heating.

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V. I. BAGIN, Y. A. BOGDANOV, T. S. GENDLER

Figure 4h. Remanence magnetization of class 4 samples asa function of temperature for progressive heating andthen for progressive cooling.

600

Figure 4i. A plot of the saturation magnetization in a 5000-oe field as a function of progressive and regressive tem-perature for class 5 samples. Note the differences be-tween the heating curve and the cooling curve due tomineral alteration during heating.

326