J. metamorphic Geol., 1999, 17, 187–193

Fluid evolution during metamorphism of the Otago Schist, NewZealand: (II) Influence of detrital apatite on fluid salinityM. P. SMITH* AND B. W. D. YARDLEYDepartment of Earth Sciences, University of Leeds, Leeds, LS2 9JT , UK

ABSTRACT Apatite occurs in the zeolite to greenschist facies metamorphic rocks of the Otago Schist, South Island,New Zealand, as both a groundmass constituent and as a hydrothermal phase hosted in metamorphicquartz veins. Groundmass apatite from low-grade rocks, ranging from the zeolite facies to the pumpellyite–actinolite zone, has chloride contents ranging from 0–1.4 wt%, and fluoride contents ranging from2.2–4.2 wt%, whilst groundmass apatite from the greenschist facies (chlorite to biotite zone) is virtuallypure fluorapatite. Vein apatite from all grades is also fluorapatite with little or no chloride. This differencein composition is interpreted as resulting from the preservation of the primary magmatic compositionsof detrital Cl-apatite grains, out of equilibrium with the metamorphic fluid, at low grades, whilst higher-grade groundmass apatite and neoformed apatite in quartz veins have compositions in equilibrium withan aqueous metamorphic fluid. The presence of detrital Cl-bearing apatite during the early stages ofmetamorphism may constitute a significant reservoir of Cl, given the low porosities of compacted sedi-ments undergoing prograde metamorphism. Calculations indicate that the release of Cl from detritalapatite in the Otago Schist, as a result of re-equilibration of apatite with the pore fluid, may have had asignificant effect on the salinity of the metamorphic fluid.

Key words: apatite; fluid salinity; metamorphic fluids; Otago Schist.

fugacities of HF° and HCl° in metamorphic fluids (e.g.INTRODUCTION

Yardley, 1985; Sisson, 1987; Nijland et al., 1993). Suchstudies have, however, concentrated on the derivationSedimentary pore fluids probably evolve continuously

from formation waters in deep basins into metamorphic of fluid compositions from apatite compositions atmedium to high grades of metamorphism, assumingfluids by progressive re-equilibration during burial,

heating and metamorphism (Hanor, 1994; Land, 1995; equilibrium. Here we report the results of a study ofthe change in the halogen content of apatite fromYardley, 1997). Interactions between the pore fluid

and its host rock which affect fluid salinity during this zeolite to greenschist facies metamorphic rocks fromthe Otago Schist, South Island, New Zealand, carriedprocess may have important consequences for later

metamorphic reactions such as dehydration and the out in order to examine possible relationships betweenthe halogen chemistry of apatite and the evolution ofconditions of phase separation in mixed H2O–CO2

fluids (Bowers & Helgeson, 1983; Yardley & Bottrell, fluid salinity during prograde metamorphism.1988; Johnson, 1991; Shmulovich et al., 1994). Theanion chemistry of the pore fluid also affects the cation GEOLOGYchemistry of fluids and hence the potential for mass

The samples analysed in this study are from the Otagotransfer via the formation of stable metal complexesSchist (sometimes also referred to in part as the Haast(e.g. Eugster, 1981). Understanding the anion chemistrySchist), South Island, New Zealand. The schists areof metamorphic fluids is therefore an important goalbelieved to have formed from the deformation andfor the understanding of a number of processes, asmetamorphism of accreted upper Palaeozoic andwell as for constraining the use of the halogens asMesozoic sediments during the Cretaceous Rangitatatracers of crustal fluid origins (e.g. Bohlke & Irwin,orogeny (Harper & Landis, 1967; Adams, 1979). The1992; Yardley et al., 1993).metasediments pass northwards into the very weaklyThe halogen content of hydrothermal apatite ismetamorphosed quartzo-feldspathic psammitic andcontrolled by the activities of chloride and fluoride inpelitic sediments of the Torlesse terrane (Bishop, 1972,the fluid from which it grew (Korzhinskii, 1981; Zhu1974), and to the south to the zeolite facies vol-& Sverjensky, 1991, 1992) and a number of studiescanogenic greywackes, pelites and minor volcanics ofhave used apatite compositions to determine thethe Caples terrane (Bishop et al., 1976). The boundarybetween these two terranes has been the focus of much*Current and correspondence address: Department of Mineralogy,recent research (Turnbull, 1979; MacKinnon, 1983;The Natural History Museum, London, SW7 5BD, UK (e-mail:

mars@nhm.ac.uk) Roser & Korsch, 1986, 1988; Frost & Coombs, 1989;

187© Blackwell Science Inc., 0263-4929/99/$14.00Journal of Metamorphic Geology, Volume 17, Number 2, 1999

188 M. P. SMITH & B. W. D. YARDLEY

Mortimer & Roser, 1992). The metamorphic grade groundmass of the wall rocks typically occurs asanhedral to subhedral grains, 30–50 mm in diameter,ranges from zeolite to prehnite–pumpellyite facies on

the margins of the schist belt through the chlorite zone commonly accompanied by phengite, chlorite, quartz,albite and minor titanite along with zeolites, pumpelly-of the greenschist facies to biotite zone towards the

centre of the schist belt (Landis & Coombs, 1967). ite and/or actinolite depending on the metamorphicgrade and protolith (Fig. 1b). In sample T1.2, morePeak temperatures of 350–370 °C have been indicated

for pumpellyite–actinolite facies metamorphism complex textural relationships are present in apatitegrains adjacent to the vein margin, where anhedral to(Yardley, 1982), and the peak temperature and pressure

for the greenschist facies metamorphism have been subhedral crystal cores, 20–40 mm in diameter, areovergrown by subhedral to euhedral apatite in zonesestimated as c. 390 °C at 4–5 kbar (Yardley, 1982;

Jamieson & Craw, 1987). Veins formed under greensch- 10–15 mm thick, containing included quartz grains(Fig. 1c,d).ist facies conditions during progade metamorphism

have been inferred to form at temperatures from 350–390 °C and 3–4 kbar on the basis of fluid inclusion

ANALYSIS AND RESULTShomogenization temperatures from W- and Au-mineralized veins, the above temperature estimates The fluoride and chloride contents of apatite were

analysed using a Cameca SX 50 electron microprobe,and arsenopyrite geothermometry (McKeag & Craw,1989; Craw & Norris, 1991; Ashley & Craw, 1995) and operating at an accelerating voltage of 15 kV, a beam

current of 10 nA and a 5 mm beam diameter in orderon the basis of fluid inclusion homogenization tempera-tures and chlorite geothermometry from unmineralized to reduce beam damage. The standards used were

KCl, which was polished before each calibration inquartz veins (Smith & Yardley, 1999). Post-peakmetamorphic W- and Au-mineralized veins, which are order to remove any effects of surface hydration, and

a sample of naturally occurring villiaumite (NaF).typically hosted in extensional structures formed duringexhumation of the schist, have been inferred to have Count times of 90 s on the peak and background were

used for both elements, giving detection limits offormed at a range of conditions during the retrogradeevolution of the schist, from 200–300 °C and 0.6–3 kbar approximately 0.05 wt% for F and 0.01 wt% for Cl.

Multiple analyses on homogenous individual crystals(Paterson, 1986; McKeag & Craw, 1989; Craw &Norris, 1991). were generally repeatable within ±0.1–0.2 wt% (2s)

for fluorine and ±0.05–0.1 wt% (2s) for chlorine,although it was typically difficult to collect a large

SAMPLESamount of data from a single crystal due to the smallgrain size of many apatite. The cation composition ofThe apatite-bearing samples come from metamorphic

rocks and quartz veins across the range of metamorphic apatite was not analysed for all grains, and idealstoichiometry was assumed for the purposes of massfacies and their characteristics are summarized in

Table 1. Sample localities are shown in Fig. 1 of Smith absorption corrections. There is little evidence forreciprocal effects between cation substitutions and the& Yardley (1999). At the lowest grades, the vein

samples showed no wall rock alteration. P–T estimates fluoride–chloride–hydroxyl substitution in apatite(Korzhinskii, 1981; Tacker & Stormer, 1989; Zhu &for vein formation based on fluid inclusion homogeniz-

ation temperatures and chlorite geothermometry com- Sverjensky, 1991, 1992). The results are shown inTable 2 and plotted in Fig. 2.pare well with the conditions for peak metamorphism

given above (Smith & Yardley, 1999). Vein apatite The most Cl-rich apatite occurs in the lowest gradesample (S2.1: zeolite/prehnite–pumpellyite facies), withtypically forms subhedral to euhedral prisms from

50 to 150 mm in length (Fig. 1a). Apatite in the significant Cl also present in apatite from both the

Table 1. Locations and descriptions of samples used in this study.

Sample Location (1550 000 sheet grid ref ) Terrane Host lithology (metamorphic gradea) Textural zoneb Relative vein agec

Rock samples

S2.1 H46 579 374 Caples Andesitic greywacke (Z/PP) TZ1

L5.2 H39 777 264 Torlesse Graphitic phyllite (PP/PA) TZ2A

T1.2 E42 412 474 Caples Andesitic greywacke cut by quartz vein (PA) TZ1/2A

L6.1 H39 524 294 Torlesse Graphitic phyllite (PA) TZ2B

W1.1 F42 Lake Wakatipu Caples Black phyllite (GS–Chl zone) TZ3A

Vein samples

HA1 G39 118 254 Torlesse Qtz-Chl vein in quartzo-feldspathic psammite (PA) TZ2B MP1

L1.2 G40 336 016 Torlesse Qtz-Chl vein in graphitic phyllite (PA) TZ3A MS2

L2.1 G40 438 995 Torlesse Qtz-Cal-Chl vein in graphitic phyllite (PA) TZ2B MS2

W3.1 F41 Nevis Bluff Caples Qtz-Chl vein in graphitic mica-chlorite schist (GS–Bt zone) TZ4 MP1

a Z, zeolite facies; PA, pumpellyite–actinolite facies; PP, prehnite–pumpellyite facies; GS, greenschist facies. b Textural zones after Bishop (1972) and Mortimer & Roser (1993): TZ1,

indurated, non-foliated medium-grained sandstone; TZ2A, slightly foliated meta-sandstone with widely spaced cleavage; TZ2B, penetratively and well-foliated semischist; TZ3A, strongly

foliated schist with segregation lamellae 1–10 mm long; TZ3B, as 3A but segregation lamellae >10 mm long and <2 mm thick; TZ4, as for 3A but segregation lamellae >2 mm thick.

c Relative ages: MP1, post-dates pervasive S1 fabric but probably pre-dates D2 folding; MS2, contemporaneous with D2 folding.

INFLUENCE OF DETRITAL APATITE ON FLUID SALINITY, OTAGO SCHIST, NEW ZEALAND 189

Fig. 1. Back-scattered electron images of apatite from rocks and veins in the Otago Schist. (a) Vein-hosted apatite (sample L1.2).(b) Detrital apatite in prehnite–pumpellyite facies schist (wall rock to sample L1.2). (c),(d) Detrital, Cl-rich apatite frompumpellyite–actinolite facies schist, with metamorphic, F-rich overgrowth (sample T1.2). Ap, apatite; Qtz, quartz; Ms, phengiticmuscovite; Chl, chlorite; Ttn, titanite; Pmp, pumpellyite; Ab, albite.

prehnite–pumpellyite and pumpellyite–actinolite facies. fluid, around a nucleus of detrital apatite. Theinterpretation that the metamorphic fluid was inChloride was not, however, detected in the single rock

sample analysed from the chlorite zone of the greensch- equilibrium with Cl-poor apatite, rather than theCl-rich apatite present in the low-grade rocks, isist facies (Fig. 2). In contrast, apatite from all the vein

occurrences is virtually pure fluorapatite. The high Cl supported by the low Cl-content of all metamorphicvein apatite analysed, irrespective of the host rockcontents of groundmass apatite from both terranes is

interpreted as reflecting a detrital origin for the grains. grade. The F-dominated, Cl-poor metamorphic apatitecomposition does not imply particularly F-rich orThis is consistent with the igneous provenance of the

sediments from both terranes (continental volcanics in Cl-poor fluids, but reflects the very different fraction-ation of F and Cl between minerals and fluid. Fig. 3the case of the Torlesse terrane and oceanic arc-related

in the case of the Caples terrane; MacKinnon, 1983; shows species predominance diagrams for apatite,constructed using the data of Zhu & Sverjensky (1991).Roser & Korsch, 1986; Frost & Coombs, 1989; Roser

et al., 1993), as Cl-rich apatite is relatively common in It can be seen that for an aqueous fluid at thetemperatures and pressures of interest, only very lowigneous rocks (e.g. Roegge et al., 1974; Candela, 1986).

Evidence for this interpretation is given by the apatite HF° activities relative to the activity of water arenecessary to produce fluorapatite, whereas aHCl° mustfrom sample T1.2 (Fig. 1c,d), where angular, inclusion-

free cores have relatively high Cl content be high relative to aHF° for the formation of asignificant mole fraction of chlorapatite. In metamor-(3.17–4.24 wt% F; 1.19–0.43 wt% Cl), and are

overgrown by subhedral to euhedral rims of Cl-poor phic fluids, the ratios aNaCl°/aHCl° and aKCl°/aHCl° arebuffered by equilibria between mica and feldspar andapatite (3.66–3.71 wt% F; Cl below detection levels).

This texture is interpreted as the result of growth of yield very low values of aHCl°, precluding significantsubstitution of Cl into OH-minerals such as apatitemetamorphic apatite, in equilibrium with the pore

190 M. P. SMITH & B. W. D. YARDLEY

Table 2. Electron microprobe analyses of fluoride and chloridefrom Otago Schist apatites.

n F (wt%) 2s Cl (wt%) 2s

Vein samples

HA1

2 3.65 0.11 <0.01 –

2 3.80 0.04 <0.01 –

2 3.78 0.13 <0.01 –

2 3.53 0.13 <0.01 –

2 4.07 0.15 <0.01 –

2 3.82 0.40 <0.01 –

2 3.45 0.11 <0.01 –

2 3.92 0.33 <0.01 –

3 3.15 1.49 <0.01 –

2 3.44 0.03 <0.01

L1.2 –

3 3.92 0.29 <0.01 –

4 4.21 0.25 <0.01 – Fig. 2. Results of electron microprobe analyses of fluoride and2 4.28 0.08 <0.01 – chloride in Otago Schist apatite.2 4.32 0.00 <0.01 –

2 3.96 0.32 <0.01 0.01

2 3.78 0.08 0.02 0.01

2 3.87 0.03 0.02 – DISCUSSION2 4.01 0.11 <0.01 –

2 4.19 0.26 <0.01 We interpret the halogen compositions of apatite fromL2.1 –

the Otago Schist as recording the replacement of3 4.36 0.13 <0.01 –

2 4.36 0.13 <0.01 – chloride-rich, detrital apatite with normal metamorphic2 4.19 0.08 <0.01 – fluorapatite. This may occur by diffusion, solution–2 3.87 0.14 <0.01 –

reprecipitation, or recrystallization of the primary2 4.09 0.13 <0.01 –

2 4.24 0.09 <0.01 apatite. Bulk dissolution of primary detrital apatite isW3.1 – unlikely except under low pH conditions (Ayers &2 4.28 0.10 <0.01 –

Watson, 1991), although the presence of apatite within2 3.70 0.01 <0.01 –

2 3.81 0.09 <0.01 – the quartz veins indicates that some phosphate was2 4.18 0.05 <0.01 –

carried in solution. The textural evidence from sample2 3.73 0.30 <0.01

T1.2 suggests that solution–reprecipitation, and poss-Rock samples

ibly recrystallization of the primary detrital apatite isW1.1

2 3.72 0.10 <0.01 – the dominant mechanism. This example clearly shows2 3.48 0.05 <0.01 –

the formation of metamorphic overgrowths on detrital2 3.47 0.01 <0.01 –

2 3.71 0.20 <0.01 – apatite cores. Diffusion of halogens in apatite cannotL6.1 be entirely ruled out at the higher grades sampled.2 3.48 0.10 0.19 0.00

The data of Brenan (1994) indicate that halogen3 3.25 0.42 0.03 0.00

2 3.32 0.09 0.23 0.03 diffusion in apatite would be significant at tempera-3 2.85 0.05 1.10 0.11 tures/pressures of 350–400 °C and 10 kbar, but the2 3.17 0.18 <0.01

lower pressures occurring during metamorphism of the2 3.38 0.26 0.02 0.03

2 2.49 0.11 1.32 0.34 Otago Schist would result in a lower the diffusion rate.L5.1

Yardley (1985) inferred that the Cl content of normal2 3.73 0.14 0.12 0.07

2 2.47 0.01 0.39 0.14 metamorphic apatite would be unlikely to buffer the2 2.60 0.19 0.28 0.01 fluid composition because of the high Cl content of2 3.06 0.28 0.28 0.05

the fluid phase relative to the solid. In view of the2 3.73 0.04 0.01 0.00

S2.1 above evidence for complete re-equilibration of detrital2 3.57 0.07 0.56 0.07 apatite during metamorphism, the low water5rock2 2.66 0.09 1.43 0.08

ratios likely during prograde metamorphism and the2 2.29 0.17 0.95 0.07

1 2.49 0.50 low fluid salinities reported from fluid inclusions in1 3.63 <0.01

both post-metamorphic and syn-metamorphic W- andT1.2

Core (1) 4.24 0.43 Au-mineralized and barren quartz veins in the OtagoRim (1) 3.66 0.01 Schist (from 0 to c. 3 wt% NaCl; Yardley, 1982;Core (1) 3.17 1.19

McKeag & Craw, 1989; Craw & Norris, 1991; SmithRim (1) 3.71 0.01

& Yardley, 1999), it is important to assess the possibleeffects of apatite Cl–F exchange on chloride in theOtago Schist fluid.

Most fluid inclusion data from quartz veins formed(Eugster, 1981). Zhu & Sverjensky (1991) calculatedthat at 750 °C and 2 kbar, for fluids from 0.5 to 5 molal during prograde metamorphism of the Torlesse terrane

of the Otago Schist comes from rocks of greenschistchloride and 500 ppm F, fluorapatite would alwaysdominate apatite solid solutions. facies or lower grade (Yardley, 1982; Hay & Craw,

INFLUENCE OF DETRITAL APATITE ON FLUID SALINITY, OTAGO SCHIST, NEW ZEALAND 191

Fig. 3. Activity diagrams showing predominance of end members for apatite at prehnite–pumpellyite and lower greenschist faciesconditions. Constructed using data from Zhu & Sverjensky (1991). Solid lines are constructed for equal mole fractions of each endmember composition in apatite. Dashed lines are constructed for XF-apatite=0.99.

1993; McKeag & Craw, 1989; Craw & Norris, 1991), mean P content of Otago Schist rocks from theTorlesse terrane (P2O5=0.15 ±0.06 wt% (n=42);and commonly indicate metamorphic fluids compar-

able in salinity to sea-water, or less saline, although Roser et al., 1993).2 An average initial apatite Cl content of 1.4 wt%.the degree of dilution is not large in view of the

extensive dehydration of minerals that may have 3 Complete loss of Cl from apatite to an homogenousfluid phase.occurred by the greenschist facies. Furthermore, Smith

& Yardley (1999) found inclusions with salinities 4 A water density of 1 g cm−3 and a rock density of2.6 g cm−3.ranging from 3.5 to 7.3 wt% NaCl eq. from a single

vein sample hosted in biotite zone rocks from the The results of the calculation are shown in Fig. 4,and indicate a significant increase in the molality ofNevis Bluff area. Higher salinity fluids have also been

found in quartz from internal boudinage structures chloride in the fluid phase if the porosity is <5%when detrital apatite recrystallizes. The porosities ofand veins from lower amphibolite facies schists in the

central Southern Alps (up to 10 wt% NaCl eq) by sediments in accretionary settings may be as low as5% once depths of 3–4 km have been reached (BrayHolm et al. (1989) and Craw & Koons (1989), but

these were associated with low salinity, CO2-rich & Karig, 1985), and may be much lower undermetamorphic conditions of the prehnite–pumpellyiteinclusions, and hence the primary fluid salinity may

have been overprinted by salinity changes caused by facies. The change in fluid composition due to apatitefluid immiscibility. In other accretionary terranes, fluidinclusion salinities in excess of those found in seawaterhave been reported from eclogites formed in subductioncomplexes in California and the Dominican Repubic(up to 5.3 wt% NaCl eq; Giaramite & Sorenson, 1994),but again the isolation of different processes affectingsalinity is difficult on the basis of currently availabledata. Overall therefore, fluid inclusion studies do revealthe persistence of relatively saline fluids during meta-morphism of ocean margin sediments, and this isdifficult to reconcile with the effects of breakdown ofhydrous minerals on marine pore waters without theavailability of an additional chloride reservoir.

An estimate of the contribution of Cl, released fromdetrital apatite, to the fluid salinity for a range ofporosities (and hence water5rock ratios) can be madeusing the following assumptions:

Fig. 4. The calculated change in chloride molality in an1 Apatite is the only P bearing phase present. This aqueous fluid due to the reaction of chloride-bearing apatiteallows a normative apatite content of the rock of (assuming an initial apatite Cl content of 1.4 wt%) to give pure

fluorapatite over a range in porosity. See text for discussion.3.46 g apatite per kg rock to be calculated from the

192 M. P. SMITH & B. W. D. YARDLEY

recrystallization may be important in determining not Coombs (University of Otago) for their advice andstimulating discussions. E. Condliffe helped with thejust the anion composition but also the metal speciation

and content of the metamorphic fluid. In the absence microprobe analyses. We would also like to thank P.M. Ashley for a constructive review, and D. Robinsonof any other mechanism, the process outlined above

would not change the total anion molality of the for editorial comments on an earlier version of thismanuscript.solution (i.e. the salinity, particularly as determined by

fluid inclusion microthermometry), due to the 151exchange of Cl for F. However, at the low water5rock

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