Apatite dissolution into peraluminous haplogranitic melts: An experimental study of solubilities and mechanisms

Geochimica et Cosmochimica Acta,Vol. 58.No. 19. 04. 4127-4145. 1994 Copyright 0 1994 kevier science Ltd Printed in the USA. All rights reserved 00 I6-7037/94 %.OO + .OO

-

Pergamon

0016-7037(94)00197-9

Apatite dissolution into pe~luminous ~plog~itic melts: An experimen~l study of solubilities and mechanisms MICHAEL B. WOLF and DAVID LONDON School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, USA

(ReceivedAugust 16, 1993; accepted in revis~~rm

march 3 1, 1994)

Abstract-Apatite ( Ap) dissolution and solubility in the peraluminous haplogranite system were investigated in three sets of experiments at 750°C and 200 MPa (PHIL), as tinctions of Ap grain size, abundance (melt: Ap ratio), and melt ASI [alumina saturation index: mol. A1203/(Na20 + K20 + CaO)]. Between an AS1 of l-I.1 (m~aluminous to mildly ~raluminous), the solubility of Ap, as measured by PZ05 in the melt, is low (0.1 wt% PZ05), which agrees with previous work. In equilibrium runs with more strongly peraluminous compositions, however, Ap solubility increases linearly with ASI, to PZ05 - 0.63 wt% in melt at AS1 = 1.3, and can be defined by a simple equation: P20s = -3.4 -t 3.1 X AS1 (R = 0.833), which agrees within error with previous calculations reported in the literature. In experiments where Ap grains are sparce and widely separated (i.e., simulating Ap distribution in an Ap-bearing peraluminous protolith undergoing anatexis) the dissolution of Ap involves transient disequilibrium that promotes higher solubility of PZOs (up to nearly six times the ~uilib~um values) adjacent to Ap at a given bulk ASI of melt, and this increases the Al:Si of melt in the Ap dissolution aureole. The approach to equilibrium between Ap and melt is hastened by increasing the Ap:melt ratio within the local melt pool (e.g., the capsule) or by decreasing Ap grain size (for a similar melt:Ap ratio). Diffusivity calculations, using inverse error function methods for the normal diffusion of P and Ca, yield DP - lo-” cm’/s and Dca - lo-” cm*/s, confirming prior results that diffusion of P away from Ap surfaces is the rate-limiting factor in Ap dissolution. These results, combined with previous work at higher temperatures but on similar hydrous melts, yield an activation energy (E) for P diflZusion of 16.5 kcal /mol and a frequency factor (Do) of 1.4 X 10e3 cm*/& Phosphorus diffusion through the melt is accompanied by uphill diffusion of Al, resulting in an increase of up to 2 wt% Al203 in melt near the Ap. Within the Ap dissolution aureole, melt contains one additional Al cation for every P cation (i.e., AAI:AP = 1:I ) which is suggestive of an associated AlPO.+ complex. The exchange reaction in melt is represented by AlPSi_, , in which 2 TO., clusters are created in the place of 1; this exchange should be accompanied by discemable changes in melt properties (e.g., density, molar volume, viscosity). The results have applications to ~uilib~um and local di~uilib~um effects during anatexis of Apbearing metapelites, and to the saturation limits of Ap during crystallization of peraluminous magmas. The dissolution of flux-bearing minerals such as Ap (e.g., those containing P, B, or F) may influence the stability and solubility of other trace element-bearing minerals during anatexis of metapelites. Texture (e.g., grain size and distribution, and spatial relationship to other minerals) plays an important role in reaction kinetics and, at least temporarily, in mineral solubilities. The disequilibrium behavior of accessory minerals has critical consequences for REE m~elling of granites. INTRODUCTION

creases with the normal fractionation trend toward increasing Si contents of melt. The apparent explanation of this latter effect was that in melts, both Si and P exist as network-forming (T) oxyanions that compete for charge-balancing cations. In a later expe~ment~ study, KOGARKO et al. ( 1988) related the effect of Si content to the more generalized NBO/T ratio (NBO: nonbridging oxygen; MYSEN et al., 198 I), a measure of the degree of polymerization in melt. Noting high P contents of whole rocks and alkali feldspar from peraluminous granites, pegmatites, and rhyolites (commonly with ~70 wt% SiOZ in the whole rock), RCHAVANT et al. ( 1987) LONDON f 1992), and LONDON et al. (1990) proposed that the solubility of Ap was higher in peraluminous compositions than in those less aluminous. LONDON et al. ( 1990) related the higher solubility of P in such siliceous melts and related alkali feldspar to the elevated ASI, aluminum ~tumtion index ( mol. Al203/ [ Nat0 + &O + CaO ] ), which is supported by spectroscopic evidence for AI-P complexing (MYSEN et al., 1981; GAN and HESS, 1992). Sub-

APATITE (Ap) PLAYS AN important role in determining the abundance and fractionation of REEs, Th, and U among comagmatic suites of igneous rocks. This is especially true for felsic systems, wherein REEs, Th, and U reside primarily in accessory phases such as Ap, rather than in the major rock-forming minerals. It is for these reasons that the solubility of Ap in silicate melts, especially felsic compositions, has been the subject of considerable interest and effort. Pioneering experimental studies by WATSON( 1979a) and coworkers (WATSON and CAPOBIANCO,198 1; GREEN and WATSON, 1982; HARRISONand WATSON, 1984) established that the solubility of Ap in metaluminous to peralkaline magmas is a function of melt composition and temperature. For hydrous, siliceous compositions similar to metaluminous granites, the ~lubility of Ap was found to be small, measured as -0.1 wt% P205 (WATSON and CAPOBIANCO,1.981) . Apatite solubility decreases with falling temperature, and de-

4127

4128

M. B. Wolf and D. London

sequent experiments by MONTEL et al. ( 1989) and PJCHAVANT et al. ( 1992) confirmed that the solubility of Ap is

considerably higher in peraluminous melts (AS1 > 1) than in compositionally similar metaluminous ones. To elaborate on the reconnaissance study of PICHAVANTet al. ( 1992)) we have set out to further investigate the solubility of apatite as a function of ASI, and to better understand how, mechanistically, the Al content of melt affects the dissolution of apatite (WOLF and LONWN, 1993a). Our three main sets ofexperiments show that, in addition to the effect of ASI, the grain size and amount of Ap in a local melting region (e.g., melt: Ap ratio within a capsule, which is roughly equivalent to the proportion of starting haplo~nitic powder to apatite, or Pdr: Ap weight ratio) greatly affect the apparent solubility of Ap (by up to a factor of six). Thus, differences in experimental methods that may simulate variations in natural rocks may affect the rate of approach to equilibrium which, in turn, can lead to large differences in the apparent solubilities of accessory minerals. Reconnaissance experiments containing fluorite in P-rich haplogranitic melts reveal much lower Ap solubilities in melt regardless of ASI, demonstrating that Ap solubility is a complex function of the activity product of all the Ap-forming components in meit, especially of Ca in addition to P (HARRISON and WATSON, 1984; E&n. 3a), and is limited by the av~lability of the least abundant component, or essential structurat constituent (SUN and HANSON, 1975 ) . EXPERIMENTAL

METHODS

Natural Ap from Durango, Mexico, with a composition close to Ca,( PQ9)sF, was mixed with five different P-and Ca-free haplogranitic powders made from nearly eutectic proportions of finely ground natural quartz (Brazil), albite (Brazil), and orthoclase (Switzerland), and varying amounts of activated amorphous Al203 (obtained by decomposing gibbsite at 4OOY). Initial compositions project to the feldspar side but close to the 200 MPa H@-saturated Qz-Ab-Or minimum ( Ab~~0r26.&ze.s) (TUTTLE and BOWEN, 1958 ), with measured ASI ranging from 1.05 to 1.20(Table 1). The differences between the calculated and measured com~sitions ofthe initial haplogranite powders in Table 1 are a result of saturation of melt by an aluminous

phase, apparently corundum; the corundum forms clots that are sufficiently distinct and coarse-grained that they could be easily avoided in all of the glass analyses. Mineral powders were used instead of gels or glasses to more closely simulate natural conditions and possibly, even the structure of natural melts. The melts produced from these mineral powders am homogeneous and thus, do not complicate interpretation of the data. Apatite grain size and distribution were varied in three different sets of experiments to study Ap dissolution kinetics: ( 1) a few coarse grains of Ap (semi-isolated) in a relatively large volume of baplogranite powder (with length scales between Ap grains generally longer than P diffusion length) (Fig. 1a); (2) very fine-grained Ap (mostly < 15 jrrn dia.) mixed with haplo~nite powder (Fig. 1b) ; and ( 3 ) an aggregate of coarsegrained Ap ( -fin-supine) in a relatively small volume of haplogranite powder (with length scales between Ap grains generally shorter than P diffusion length) (Fig. lc) . The coarse-grained experiments were conducted using 0.1-0.4 mm3 blocky Ap fragments: semi-isolated Ap runs contained -2- 10 mg Ap ( - 10 grains/capsule) and -30-45 mg powder, and Ap aggregate runs contained -20 mg Ap and - 10 mg powder. As shown below, the powder to Ap weight ratio (Pdr:Ap or melt:Ap, for a given Ap grain size) is an important parameter in the dissolution process. In a few experiments, clear colorless fluorite (Fl) from Iron Mountain, New Mexico was dissolved into haplogranitic melt (norm wt% Ab460r27Qz27,ASI = 1.05-i .20) containing 3 wt% PzOs to promote Ap crystallization. Two sets of Ap crystallization experiments were done: one with single, coarse-grained FI to study the kinetic effects of F1 dissolution (Ca and F diffusion) and Ap growth; the other with very finely ground Fl to approach ~uilib~um. In ah runs, the Ap (or Fl), rock powder, and Hz0 in excess needed for saturation of melt were sealed in 20 X 3 mm Au capsules (with powder mix confined to 5 X 3 mm portion of capsule). In the Ap dissolution experiments, all five powder compositions were used in the series of two-, four-, and eight-week, semi-isolated Ap runs and in the four-week Ap aggregate and very fine Ap runs. Capsules were run subhorizontally in water-pressurized cold-seal vessels at 200 MPa. Pressure was measured with a factory-calibrated Heise bourdon tube gauge, with fluctuations of ~5 MPa over the course of experiments. A temperature of 750°C was controlled by external and measured by internal Chrome]-Alumel thermocouples, with an estimated total error + 10°C. Oxygen fugacity was not controlled, but the intrinsic oxygen fugacity of these vessels is between the NNO and FMQ buffers (e.g., HUEBNER, 1971). Runs were quenched isobarically in air ( 5I O”C/ s) Capsules were reweighed to test for leaks. Chemical analyses were obtained by wavelength~is~rsjve spectrometry ( WDS) on a Cameca SX-50 electron microprobe (EMPA)

Table 1. Initial compositions of haplogranite powders and aptite Pow&r

SiO2

A1203

Na20

calculated compositions (anbydrous basis) HG 76.28 14.10 5.69 GASI. 1 75.62 14.84 5.63 GASI. 74.97 15.59 5.59 GASI. 74.32 16.31 5.54 GASI. 73.69 17.02 5.50 measured ~m~sitio~ HG 76.41 GAS1.l 75.81 GASi. 75.20 GAS13 75.07 GASIA 74.85 Durango

Apatite

(reno~~i~ 14.08 14.60 is.36 15.56 15.80 P2G5

40.78

KY&’

Total

3.92 3.89 3.86 3.82 3.79

100.00 100.00 loO.CQ 10.00 100.00

on anhydrous basis, 5.42 4.09 5.45 4.14 5.38 4.97 5.27 4.10 5.33 4.03 F

3.53

ASI?

AS$

1.04 1.10 1.17 1.23 1.29

--1: --

totals) e;Ti;for 89.39 91.49 91.90 91.80 I-&@

1.43

1: ---

1.05 1.09 I.16 1.19 1.20

TOtal

99.76

Note: All initial compositions have Ab:Or:@= 4526528.5. ASIF = calculated initial ASI: (added Alfi+calc.Alfi from Ab+Or)i(calc.Na~+K~ from Ab+Or). ASIF = averaged measured AS1 from Ap-absent runs (750°C. 200 MPa, ~-saturated. 4-weeks). Differences between ASC and AS$ possibly due to undissolved or recrystallized Al microlites (and are within error of each other: la - 0.05). Ap analysis from wet chemistry. R&@ = rare earth oxides (mostly Ce and La, WOLF and LONWN unpub. data).

4129

Sohtbility of apatite in peraluminous granitic melt

for Ca (on FET) and Al (on TAP). Beam parameters for glass (quenched melt) analyses were: for isolated Ap grain nun+- 15kV, 2 nA, 10 gm spot size, with peak counting times of 180 s (Si, P, F), 120 s (Al, Ca), 60-40 s (Na, K), and background counting times half that of the peaks; for Ap aggregate runs-15 kV, 2 nA, 10 &rn spot size, with peak counting times of 60 s (Si ), 120 s (Al, Ca) ,40 s ( Na, K), and i 5 kV, 20 nA, 7 Itm spot size, with peak counting times of 60 s (P, F); for very fine Ap runs-15 kV, 2 nA, I5 Mm spot size, with peak counting times of 60 s (Si), 120 s (Al), 40 s (Na, K), and 15 kV, 20 nA, 12 pm spot size, with peak counting times of 60 s (P, F, Ca). The low beam current (2 nA) minimized voiatiiization the mobile elements (especially Na); tests using 1, 5, and 10 s repetitive analyses showed no Na loss for total analysis times up to 2 min, so correction factors have not been applied to the NazO values. The PAP correction procedure was used (POUCHOU and PtCHOIR, 1985). The detection limit for P was 0.08 wt% P20s (with a 2 nA beam) and 0.04 wt% P,O, (with a 20 nA beam) at 30 above mean background. In the coarse-grained experiments, Apglass interfaces were selected such that the beam fluorescence volume did not impinge upon Ap beneath the glass surface. Lines of analyses were started 7 pm away from Ap to avoid fluorescence overlap. The

lines were ail perpendicular to the surface strike of the Ap-glass interfaces. Digital WDS Ka X-ray images of P, Ca, F, Si, and Al were made to study the diffusion of these elements in the melt near Ap, to assess melt homogeneity, and to ensure that Ap grains immediately below the sample surface did not contribute to the analyses of glass. In the very fine-grained Ap runs, the attempts to measure only dissolved Ap components and to avoid small undissolved Ap grains were confirmed by the hom~eneity of the glass analyses and comparisons with the data from the coarse-grained Ap aggregate runs. RESULTS

Apatite Dissolution as Semi-Isolated Grains The results from experiments with low Ap:meIt proportions, in which Ap grains are essentially isolated from each

other by melt, are discussed first. In these experiments, the melt does not achieve saturation in Ap components over the duration of the experiments; hence, diffusion profiles extend from the margins of Ap grains to background values of the bulk melt. Apatite DissoIutio~ as a Function of ASI

FIG. 1. Backscattered electron photomicrographs of representative runs from the three sets of experiments ( Ap: apatite; Gi: glass): (a) semi-isolated, coarse Ap grains, (b) very fine-grained Ap, and (c) coarse Ap aggregate.

using crystalline standards with TAP, PET, or PCl diffmction crystals Ruoroapatite for P (on PET), topaz for F (on TAP or PCI), aibite for Si and Na (on TAP), orthoclase for K (on PET), and bytownite

Table 2, part A lists the averaged glass analyses from points closest to Ap grains for the five bulk compositions at two-, four-, and eight-week durations. The average from the one fifteen-week run (HG-50) also is listed. Two additional averaged analyses are listed from four-week runs (HG-38 and HG-39 ) , to which a further excess of A&O3 had been added in an attempt to increase the local ASI around Ap grains; that this increase did not occur attests to an upper limit to the solubihty of the aluminous phase under these conditions. Table 2, part B Iists individual analyses from points along one profile from one run (HG-20); the P205 data shown in this table as well as similar data from other runs are plotted in Fig. 2. Diffusion profiles of P20s are shown in Fig. 2a-d for the four different run durations. Differences among curves within each of the figures illustrate the strong influence of AS1 on Ap dissolution, as measured by P205 (wt%) in the glass. A comparison of similar symbols between figures, however, reveals that the same compositions used in runs of different durations ended up with different values of AS1 (also compare the ASI, [ASI of nearest point to Ap] and ASIpti [average AS1 of profile] between similar sta~ingcom~sitions in Table

4130

M. B.

Table 2A. Glass dyses

336 69.27 1.17 12.83 0.23

sio9 A$3

:g

3.70 0.00 4.% 0.09

E

0.05 0.01 0.11

LaI ASI,,, ASI.vs

91.01 0.12 1.05 1.06

Rtmx’ Powder

FIG-16

Pdr Ap

2::

K

(h)

0.75 0.12 0.01 0.00

sdm:3

70.44 790 0.23 13.13

0.14

4.01 0.20 s I&al ASI,

ASIpt Run# Powder

Pdr:Ap

0.02

1.22

1.19 0.01

1.21 0.00

HG-17 sdm:3 FIG-18 sdm:3 IX&l9 sdm:3 I&‘J$Q sdm:3 f HG_39 GASI.1 GASI. GASI. SPAG GASI. 13.0 12.3 12.1 9.6 10.4 694 68.57 789 0.44 64.78 790 0.62 64.87 789 0.39 61.45 790 0.13 60.89 17.20 13.86 0.14 14.85 c.@ 16.19 0.11 16.34 0.06 4.14 0.20 4.22 0.05 4.13 0.13 4.29 0.16 4.76

89.53 0.08 0.02 0.67 1.30 0.00 1.28 0.04

92.14 0.99 O.&j O.c@ 1.40 0.02 1.39 o.oc

89.61 0.03 0.06 0.37 1.36 0.M 1.33 o.@t

0.02

1.17 0.M HG-21

sdm:3

g

1.23

O.&t

1.24 0.04 HG-22 GASI. 1

sdm:3

3.0

0.32 0.22

3.67 0.15 0.06 O.C@ o.@3 0.02

HG-23 GASI.

sdtx3

HG-24

3.0

70.31 1442 0.15 13.60 0.0.5 5.02 0.04

sdm:3

GAS13 2.9

68.69 1440 0.34 14.51 0.21 4.69 0.03

67.89 1440 0.04 15.11 0.08 4.47 0.08

3.68 0.32 0.05 0.80 0.01

0.49 3.71 1.01 0.02 0.03 0.01

92.51 0.09 0.23 0.04 1.08 0.02

93.19 0.05 u.@2 0.22

92.78 0.09 O.@ 0.33

92.74 0.07 O.@ 0.13

I.08 0.00

1.07 0.01

61.72 16.42

1.06

0.00

1.18 0.0~ 1.18 0.00

analyaea from apatfte dbolution

1.23 0.01 1.22 o.@zl

3.66 3.63 0.51 0.00 90.65 1.35

__

sdm:3

FIG-50

67.18 1442 0.07 15.46 0.19 4.42 0.07

3.62 0.56 1.40 0.07 0.01 o.@t 92.73 0.09 0.28 0.w

1.27 0.01 1.25 0.01

64.14

64.90

65.62

666.04

66.73

67.08

67.23

3.62 4.39

15.56 3.62 4.42

15.10 4.30 3.84

14.91 4.36 3.75

14.81 4.41 3.67

14.49 4.40 3.64

14.57 4.45 3.66

14.57 3.74 4.50

3.23 2.86 0.55 0.45 90.14 0.00 89.74 0.22

2.30 0.39 89.97 0.05

1.71 0.32 89.77 0.00

1.49 0.30 89.98 0.05

0.92 0.27 90.02 0.19

0.69 0.43 0.3 1 0.24 89.93 O.OG 90.06 0.13

0.15 0.30 90.29 0.08

0.08 0.22 90.54 0.21

1.32

I.32

1.28

1.27

1.23

1.23

1.35

sdm:3

0.84 0.20 0.03 0.09 0.@3 0.01 0.W 0.m 0.02

sdm:8

10.5 2518 65.23 16.18 4.40 3.56 2.19 0.45 0.00 92.01 1.36 1.32

0.09 0.03 0.05 0.04 0.02 0.01 0.00 0.07 0.01 0.01

aureole -

15.89 3.76 4.39

1.32

0.07 0.02 0.07 0.03 O.tr3 0.m 0.25 0.01

GASIA

63.20

3.70 4.53

62.09 16.10

m-25

GASI. 3.0

3.99 0.16 0.07 0.02 0.01 0.03

SiO? AI&3

17.12 4.98 3.58 3.65 0.59 0.00 91.30 1.30 __

0.15 0.02

91.52 0.14 0.78 0.M

1.19

694 61.37 0.21

0.19 0.0-l ,000 0.07 0.10 0.09 0.11

91.77 0.06 o.@$ 0.17

G&a

ASI,

1.11 0.00

1.19

336 65.97 15.85 4.82 3.57 1.29 0.64 0.11 91.57

0.23 0.13 0.00 o.@3 0.09 0.05 o.@ 0.33

3.75 3.20 0.51 0.M 0.01 0.o-I

Table 23.

P& #IO Lal

0.14 0.00 0.04 0.06 0.03 0.05 0.48 0.01

336 65.80 15.09 4.74 3.57 1.14 0.54 0.02 90.39

0.21 O.@ 0.00 o.Ca o.Cn 0.01 0.02 0.X 0.01 0.01

3.69 2.73 0.45 0.06 0.19 0.01

13.02 4.75

:g

13.68 4.97 3.65 0.36 0.15 0.10 91.30 1.10

336 66.02 14.33 5.00 3.72 0.85 0.32 0.04 90.28 1.12 1.14

nms -

3.59 0.34 1.66 0.01 0.10

70.78 1442 0.45

ASIpr

336 68.69 0.38

~~~~

3.50 0.39 0.92 0.05 o.05 003

&!I; 0

&al ASl,

from d&solution aureoles In se&-isoktcd

0.19 0.13 3.81 o.rJ3 0.00 0.06

AI203 N%@

g

Wolf and D. London

1.26

1.25

Note: AmtIysesof hydratedglasses(75ooC.2&l MI%)fromEMP,see text foranalyticalconditions. Pointsware&apatite= average of nearest points from different profiles in each run (eaclt -7 w from Ap). Pdr:Ap = weight ratio of powder to apatite in each run. sdm:# = standard deviation of the mean of # of nearest points. ASI@ = measured Alumina Saturation Index for nearest points: fA~(Na@+K@tCaO)~ mol. basis. ASIpr = the average measured AS1 from multiple averaged profiles (unweighted).

2, part A). In addition

to variations in ASI, two other parameters affect the amount of PzO=,added to the melt-the run duration and the powder:apatite (Pdr:Ap) weight ratio. For a given starting composition, the concentration of P builds up near the dissolving Ap between two and four weeks, is reduced between four and eight weeks. but rises again from

eight to fifteen weeks (e.g., note the rise [Fig. 2a-b] and fall [Fig. 2b-c] and rise [Fig. 2c-d ] of the curves with solid circles). The lengths of the profiles (diffusion aureoles) increase with time, because P is continually diffusing away from the Ap (for the curves with solid circles, the profiles lengthen from 100 pm after two weeks to 300 ym after fifteen weeks ) .

4131

Solubility of apatite in peraluminous granitic melt 2.0-L.

.

’

’ . ’ ’ . . .

’ . ’ .

L.5 j ASI(ar) = 1.23

: Ic

P,Os

1.0 (WI.%) 0.5 0.0

0

50

100

150

200

w5 (wt.%)

0

50

IOU

150

203

Distance from Apatite-Glass

250 Interface

300

350

(pm)

FIG.

2. P,Os concentration profiles (hydrous basis) in glass, away from dissolving apatite, for four run durations (750°C): (a) two weeks, (b) four weeks, (c) eight weeks, and (d) fifteen weeks. Each symbol represents one of the five starting compositions, but all five compositions are not shown in each figure for clarity (see Table 1 for initial measured AS1 of each composition): HG, solid triangles; GASI. 1, open diamonds; GAS1.2,open circles; GASl.3, solid squares; GASI.4, solid circles. The ASI of the point nearest to the Ap [ ASlo,] is shown for each profile [mol. A1203/( Na20+KzO+Ca0)]. Typical powder to apatite weight ratios (Pdr:Ap) for each set of durations are also listed.

results in an apparent but transient high value that falls with continued run duration. The PzOs levels (wt%) in the analyses of glass closest to isolated Ap crystal fragments have previously been taken to be the closest approximation of Ap solubility for a given condition (HARRISON and WATSON, 1984), on the well founded assumption that the melt closest to Ap was most nearly saturated in Ap components. This approximation, however, does not hold for these runs in which a high degree of chemical interaction occurs between dissolving Ap components and melt (compare with results from Ap aggregate and very finely ground Ap runs, below). To the extent that the dissolution of accessory-phase components induces transient compositional changes among the major melt components within the dissolution aureole, the apparent solubilities of accessory minerals as determined from singlecrystal experiments may be much higher than the true equilibrium values (i.e., for the bulk composition of melt). Such is the case for these experiments with isolated Ap grains. The P data from all points closest to Ap from all of the runs are compiled in Fig. 3. Because apatite solubility is a function of the silica content of the melt (HARRISON and WATSON, 1984)) only runs with similar SiOz should be compared. Variations in the Si02 content of starting bulk compositions are small, and the starting bulk compositions themselves (without Ap) are homogeneous (Table 1). In these semi-isolated experiments, however, the SiOz content of resultant glass adjacent to Ap grains is more variable, is lower than in the starting bulk composition, and is determined largely by the solubility of Ap. These effects, which are discussed below under “Concentration Profiles and Diffusivities,” mean that the SiOz content and AS1 of melt cannot be varied independently in these experiments, and we suspect that the much of the scatter among points in Fig. 3 results from variations in SiOz. Most of the runs contain 65-7 1 wt% SiOz in resultant glass (compare to Table 1, starting compositions), except for three four-week runs; data from these lower silica runs (61 wt% SiOr; small solid circles; HG-20,

4.0:

’

’

’

’

’

’

3

’

’

!

3.5 3.0 2.5 PzOs 2.0 (wt.%) 1.5 1.0 0.5 0.0

As discussed below, the abundance and distribution of Ap plays a very important role in controlling Ap dissolution. Thus, only the runs with similar Pdr:Ap ratios in Fig. 2 should be compared, i.e., the P205 contents in eight-week runs (Fig. 2c) should not be compared with those in the two-, four-, and fifteen-week runs (Fig. 2a,b,d). For a given AS1 of bulk melt at large Pdr:Ap ratio ( isolated Ap grains), the P205 content of melt at the Ap-melt interfaces

1.0

1.1 MI:

1.2

1.3

I A1~0~(Na20+K@+Cs0)

1.4

1.5

mol. ]

FIG. 3. P205 vs. ASI in glass from points nearest apatite from semiisolated Ap grain runs (four different durations). Line is fit through data from runs with 65-7 I wt% SiO2. Smaller solid circles, displaced above the line, are from runs with fmal glass compositions with 61 wt% Siq. In the peraluminous haplogranite system, apatite solubility increases linearly with ASI. Elevated P,O, contents, especially noticeable at higher ASI, are due to transient high Ap solubility.

4132

M. B. Wolf and D. London

38, 39) have been separated from the other four-week data and plot at distinct& higher values of P,Os for a given ASI (OS-I.5 wt% P205 higher). This increase in Ap solubility with decreasing SiOz qualitatively conesponds with the results of HARRKON and WATSON (1984) in the metaluminous granite system. For the runs containing 65-7 1 wt% Si02, the transient Ap solubility increases linearly with increasing AS1 for all run durations and rises to quite high levels (3 wt% P205 at AS1 = 1.4). The two- and eight-week data coincide (same slope and upper limits, and similar SiOz contents; TabIe 2A). The four-week data may form a line of slightly steeper slope (constrained only by the low-P205 run ). The fifteenweek data fall along the lines defined by both the two-/eightweek and four-week runs. The data from the runs with 657 1 wt% SiO, define a line represented by the equation, P205 = -7.1 + 6.75 X ASI (R = 0.9128). It must be noted that, in these semi-isolated Ap runs, there is no change in apparent solubility adjacent to Ap grains as a function of the distance to other Ap grains, i.e., PZOs contents are just as high within narrow (20-50 pm widths) melt-filled spaces between Ap grains as they are along the free faces of those same Ap grains (with the next closest Ap grain hundreds of pm away, and well beyond the Ap dissolution aureole).

Distance from 0 68!‘*““““.“‘~!

40

Apatite (pm) 80 120

,1 sio2/...H-

160

j

FIG. 4. Concentration profiles(hydrous basis) of SiO*,A1203,PZOs, CaO, Na,O, and KzO from one EMP traverse (see Table 2B, HG20 profile). Arrows show direction of diffusion due to apatite dissolution. P,Os and CaO dissolveinto melt from apatite and diffuse away. A120, in melt diffuses uphi into apatite dissolution aureole. Pan of Si02 profileis due to dilution effects.There are no gradients in Na20 or K20.

Concentration Profiles and Diffusivities In Fig. 4, the concentrations of SiOZ, Al203, CaO, and PZ05 in glass vs. distance away from Ap are shown in one traverse from a strongly peraluminous four-week run. Not surprisingly, the dissolution of Ap has produced a large concentration gradient in PZ05, and to a lesser extent, in CaO (also see HARRISON and WATSON, 1984). The arrows show the direction of diffusion of P and Ca in the melt, down their concentration gradients (away from Ap). In addition, Ap dissolution has produced large concentration gradients in Al and Si. The changes in Al and Si do not result simply from dilution of melt by Ap components (Ca, P, and F). The A&O3 content of melt near the Ap is 2 wt% higher than the initial 14.6 wt% (recorded in this profile at > 120 pm); dilution by Ap components would lower Al in melt. Thus, AI has diffused up its concentration gradient, exhibiting uphill diffusion. The gradient in A1203 is probably the result of increasing local Al solubility with Ap dissolution (Al complexing with P, see below). Although the presence of alumina microlites within the strongly peraluminous runs (Al microlite-over~turated) adds a layer of complexity to the dissolution and diffusion processes, it is clear from the weakly peraluminous runs (to which microlites were added but completely dissolved: AS1 5 GASI.2, Tables 1, 2) that Al diffusion is not just a very localized (a few pm) but a longrange process fat least hundreds of pm up to a few mm). The concentration gradients of SiO, can be attributed partly, but not completely, to dilution of silica by the addition of A1203, CaO, and P205. As shown by Fig. 4, the dissolution of Ap in a strongly peraluminous granitic melt locally, but dramatically, changes the melt composition, not only by the addition of Ap components, but in other elements as well. The changes are less pronounced as the ASI of the melt becomes less ~mluminous and are hardly noticeable at an ASI = 1, because the magnitude of the effect is a function of the apparent solubility of Ap, which does increase with ASI. WDS X-ray intensity maps of P, Ca, Al, and Si (Fig. 5ad) also reveal the compositional gradients in a peraluminous melt that developed around Ap after eight weeks at 750°C and 200 MPa. Differences in the diffusion lengths along different crystal faces for a given element probably are not due to differential crystallographically controlled dissolution rates but to differences in the intersection angle between the sample surface and the Ap-glass interface (longer diffusion profiles due to angles < 90” ). These images are a check on interfacesurface o~en~tions, which are important because the shortest diffusion length must be used to obtain the true diffusivity. Figure 5a-d shows the increased concentrations of P, Ca, and Al, and the decreased concentration of Si near Ap, as illustrated in the profiles of Fig. 4. No detectable F concentration gradient was found in the glasses because F concentrations are close to the detection limit and F diffusion is so rapid that no substantial gradient is preserved (M. B. Wolf and D. London, unpubl. data). Figure 5e-h consists of X-ray maps of a reconnaissance fluorite dissolution run in which Ap crystallized from P-rich melt (discussed below under “Results from Fluorite Dissolution f Apatite Crystallization Runs”). Euhedral feldspar crystallized in most Ap dissoIution runs ( Fig. 5b-d), Most feldspar crystals possess Ab cores that probably are relicts of the starting material, and all have calcic

Solubility of apatite in peraluminous granitic melt

FIG. 5. False color digital WDS Ka X-my images of (a) P, (b) Ca, (c) Al, and (d) Si, showing compositional gradients that developed around dissolving apatite.( Ap) after eight weeksat 7WC, 200 MPa (PM&. Reds and yellows reveal higher concentrations of each clement, relative to the greens and blues within each frame. Note diffmnces in lengths of dissolution aureoles along various grain faces,due to ditferences in the obliqueness of the angle between the polished sur&ce and the gmin face. The shortest distances am taken as the true diffusion lengths. (e-h) highlight the crystallization of Ap f&m a P-rich melt as Ca and F (not shown) are added by the dissolution of iluoritc (Fl) (75WC, 200 MPa (PM&, one week).

4134

M. B. Wolf and D. London

T&s&

F~~~~~~~~t~~~

Rm

BG-25 HG-25 He-L?4 core core core

BG-25 HG-25 Ho-25 IF%25 Ho-24 rim rim rim nm lim

SiO2

68.12

68.38

68.45

62.66

62.%

63.66

63.49

64.51

A1203

19.70

19.77

19.78

23.29

23.14

22.58

22.78

22.43

0.17

0.03

0.08

2.99

3.09

2.87

3.45

2.79

11.38

11.67

11.28

931

9.18

9.23

9.32

938

K@

0.66

0.16

0.64

0.94

0.86

0.93

0.88

0.92

Pg5

0.06

0.05

0.08

0.80

0.60

0.26

0.02

0.03

lax32

99.93

99.83

99.55

99.94

loo.05

%.I9

cao NazO

Total

100.10 loo.07

NCSIU Ab

95.60

99.05

83.85

82.49

81.24

7734

80.61

An

0.46

0.00

0.00

10.23

12.12

13.04

17.35

13.86

or

3.94

0.95

3.81

5.92

539

5.72

5.31

5.53

Note: feidtipar me rdict my&Is (corm)hmed by new growth;EMP dyses. Feldsparampieea 4% of thecharge.

nearest Ap (after HARRNON and WATSON, 1983). An example of the calculations is given in Table 4 and plotted in Pig 6. Similar calculations were made from all of the profiles, and these diffusion coefficients are tabulated in Table 5 and plotted in Pig 7. E&ctive binary diffision with constant EBDC cannot adequately describe the uphill diffisive behavior of Al (e.g., ZHANGet at.,1989),and a rn~ti~rn~ne~t diffusion model would be more appropriate, given the interrelationships between P and Al (e.g., L&SAGA, 1979). In view of the complications associated with multicomponent diffusion calculations in complex systems ( ZHANG et al., 1989), the results for Al diffusion are not presented here. The inverse error function method was chosen. to facilitate comparison with previous calculations of P and Ca di&sion by WATSON and HARRISON (1984). The estimated total error is less than a factor of 3. Figure 7 plots the diffusion coefficients (D) vs. the averaged AS1 from the points along each profile (AS&,,,+,). Plotting D vs.ASI,m (instead of initial bulk ASI) is justified because both parameters are a function of the final melt composition along the length of each profile. Over the range of ASI,,rn from 1.12 to 1.42, thediffusion coefficient forP(&) is lo-” cm’/s, and DC,,ranges from 2 X lo-i0 to 3 X IO-"cm’/s.

rims (Table 3),withCa derived from the dissolution of Ap. In addition to feldspar, the strongly peraluminous experiments contain fine-grained microlites of an aluminous phase (possibly corundum, Pig. 5~) which are the recrystallized relics of the added alumina (gibbsite). The presence Ofmicdites outside of the Ap di~iution aureoles indicates that the melt is oversaturated with respect to this ~umino~ phase. They generally are not present within the aureoles, apparently because of their increased solubility in these P-rich zones (see below). Effective binary diffusion coefficients (EBDC) have been calculated for Ca and P for each measured compositional profile from two-, four-, and eight-week runs, which involved plotting the inverse error t%nction (erf -’ ) of the concentration ratio vs. a distance/time parameter and measuring the slope of the resulting line. Diffusion coefficients (D,in cm’/ s) were obtained by squaring the slope of the lines:

where x = distance from Ap (cm), t = time (s), C,,* = concentration along profile, and Co = concentration in glass

Table 4.

X(an)

(x103) 0.00

Exam& of caicuhth~ of diffusioncoefficientsfor PsOg and Ca0 (from EC-28)

x

(_,

1_Ferf“i-$

c

5 0.00

3.23

o.OoOo

O.NlO

Cl

(

.)

x

0.55

l-$erf-ll-g 0

(

o.OMo

O.ooO 0.157

1.58

4.92x10-7

2.86

0.1131

0.101

0.45

0.1760

3.17

9.84x10-7

2.30

0.2886

0.262

039

0.2849

0.258

4.75

1.48x106

1.71.

0.4699

0.444

0.32

0.4174

0.389 0.434

6.34

1.97x10-6

1.49

OB94

0.522

0.30

0.4610

7.92

2.46x 10-6

0.92

0.7142

0.755

0.27

0.5064

0.484

9.51

2.95x10-6

0.69

0.7861

0.879

0.3 1

0.4374

0.409

11.10

3.44x10-6

0.43

0.8670

1.062

0.24

oss72

0.543

t (8)= 2592ooo; c, = 3.23 Slope of theline containingthesepoints= Diffusionooeffcient (cm2/s); (slope)2=

,)

c, = 0.55 3. I~~~ZXIO-6 1.0x10-”

6.091~10-~ 3.7x10-‘1

4135

Solubility of apahte in peraluminous granitic melt

4*o1ww I

wm

P

J

2.0 106

pi=zGq cocff.= (slopcy

Diff.

droxyl groups), i.e., that two cations, one Al plus one P, replace one Si cation in the melt. Results from Very Fine-Grained Apatite Runs Because the P205 contents at the Ap-melt interfaces decreased substantially in semi-isolated Ap runs from one to two months, another set of experiments was done to obtain data as near to equilibrium as possible (for the bulk composition of the melt). Very fine-grained Ap was mixed into the haplogranite powders, essentially eliminating the time lag associated with dissolution and diffusion of Ap components from coarse-grained Ap (Fig. 1b). Table 6 .lists the averaged glass compositions from the six runs (a six-week run was done with the GASI. powder, in addition to the standard four-week run; no significant difference was found between these two runs). The very low errors (standard deviation of the mean) associated with these analyses attest to the homogeneity of the glasses, i.e., no compositional gradients were found. Through careful examination by BSE imaging, we are confident that the values represent true glass compositions and not small additions of very fine-grained, undissolved Ap. The P205 content of the glasses are plotted vs. AS1 in Fig. 10, similar to the plot of the semi-isolated Ap grain runs in Fig. 3. Figure 10 clearly shows that Pz05 is a linear function of ASI from 1.1 to 1.3, similar to the results

Y=I.196xI~1=tg~X104

0.0 18

0.5

0.0

1.0

E&(1-

4.0 106-,

’

’

’

’

*

ca

0.0

1.5

$) *

’

’ p_

0

0.2

0.4 Ed-‘(l-

0.6

0.8

$) D

FIG. 6. Examples of diffusion coefficient calculations and fits to the data of (a) P and (b) Ca, based on inverse error function method.

Table 5. Diffusion coeffiiients

from semi-isolated

apatite

grain dissolution experiments (75OT, 200 MPa)

Run# Powder Measured

D (cm2/s) 905

GASl.# .. 1.1

ASl(,fi .1.11. .

(hours)

13

1.2

1.14

336

2.1 X lo-”

1.0x 1vJ

Changes in Phosphorus, Aluminum, and Silicon in the Melt

13

1.2

1.16

336

8.3 x lo-‘2

2.6x 1U’O

13

1.2

1.13

336

8.7 x lo-‘*

1.1x 10’0

14

1.3

1.18

336

1.3 x 10-l’

8.5 x 10”

Figure 8a-b plots the change in P vs. the change in Al in the melt on a cation basis (per 8 oxygens) from the EMP analyses. Data from the points nearest Ap are shown in Fig. 8a (as in Fig. 3). Data from the entire length of one particular profile are shown in Fig. 8b as an example of what all the profiles look like. Because there is no P in the initial melt, AP represents the total number of cations of P added to the melt from the dissolving Ap. AA1 is the change in the number of Al cations from that of the P-absent glass: EMP analyses of Al in glass from the Ap dissolution aureole minus the averaged Al content of the measured starting composition from runs without Ap (ten to twenty analyses each). The data from both Fig. 8a and b fit 1: 1 lines very well, indicating that excess Al (up to 15% more Al than initially in the melt) and P coexist throughout the Ap dissolution aureole in a 1: 1 relationship. Figure 9a-b plots groups of EMP data similar to Fig. 8ab, except as the change in Al plus P vs. the change in Si in the melt on a cation basis (up to an 8% decrease in total Si). The data for the compiled points nearest Ap (Fig. 9a) and along the profile (Fig. 9b) fit 1:- 1 lines extremely well, indicating that an AlPSi..., exchange reaction occurs throughout the Ap dissolution aureole (possibly, charge balanced by hy-

14

1.3

1.21

336

6.5 x lo-‘”

4.2 x 10”

15

1.4

1.21

336

4.3 x lo-‘*

2.5 x 10”

15

1.4

1.21

336

8.6 x lo-‘*

1.2x 1wo

17

1.1

1.22

789

6.9 x lo-”

1.3x 1cP

18

1.2

1.26

790

8.0 x lo-‘*

4.5 x 10”

19

1.3

1.36

789

7.7 x lo-‘*

4.9 x 10”

19

1.3

1.37

789

1.3 x lo-”

2.2 x lcr”

20

1.4

1.42

790

9.3 x lo-”

5.4 x 10”

20

1.4

1.35

790

1.8 x lo-”

5.4 X 10”

20

1.4

1.28

790

1.0 x lo-”

3.7 X 10”

23

1.2

1.18

1440

6.3 x lo-‘*

7.5 x 10”

23

1.2

1.18

1440

4.8 x lo-‘*

1.5 x 10”

24

1.3

1.22

1440

1.0 x lo-”

24

13

1.23

1440

5.3 x 10-12

25

1.4

1.25

1442

3.4 x lo-”

25

1.4

1.24

1442

9.0 x 10-12

7.6 x 10”

25

1.4

1.26

1442

1.5 x lo-”

7.5 x 10”

50

1.4

1.32

2.518

1.4 x 10-l’

1.4 x 10’0

The diffusivities of P and Ca do not change with ASI, and, for a given ASI, do not change as a function of time.

HG.n ‘L

Time

Note:

336

1.3 x lo-”

cao

1.4 x 10’0

1.3x lo-‘0 5.6 x 10” 1.9x lo-‘0

ASl(,n = averaged AS1from all points along profile [md. &QW@ + W + W)l

4136

M. B. Wolf and D. London

of Fig. 3. However, the P205 contents for a given ASI are much lower than the very high values shown in Fig. 3, with a maximum near 0.6 wt% P,05 instead of 2 wt% Pz05, at an ASI of 1.3. Extrapolation of the trend to ASI = I .4 yields 1.1 wt% PzOs instead of 3.3 wt% P205. In this experimental configuration, the distance between adjacent Ap grains is small, and dissolution aureoles overlap before any long-range modification of melt composition takes place. The melt (glass) compositions remain uniform (i.e., bulk composition plus Ap components). Because diffusion distances between aureoles are short, there should be no appreciable decoupling of Ca from P, and ideally, the P/Ca cation ratio in melt should approach that of Ap (P/Ca cation ratio of Ap is 0.6). Figure 11 shows, though, that the P/Ca cation ratio is close to one (0.9 for the four-week data [R = 0.9561). The difference is accounted for by the amount of Ca and P incorporated intogrowing feldspar (Table 7; feldspar comprised 6 ~01% of these runs, determined by BSE mapping). Qualitatively, more feldspar grew in the more peraluminous runs (Table 6); with higher ASI, more Ap dissolved, adding more Ca to the melt and inducing more ternary feldspar growth. The partition coefficient, D$$y”, for P between the feldspar and melt is 0.63-0.74 at an ASI of 1.26-1.27 (from Tables 6 and 7), which agrees well with calculated partition coefficients from some natural Bolivian peraluminous volcanics (LONDON, 1992) and previous experiments in peraluminous granitic systems (LONDON et al., 1989.

0.15AP O.lO0.05 0.00

0.00

0.05

0.10 0.15 A Al

I 1 1 P & Al complexing along! profile HG-20 -(catloll -* basis)

0.25 0.20 1

0.20

0.25

1

R = 0.9888

t

0.1.5-

AP O.lO-

Point nearest apatite (P205=3.= Wt%)

1993).

0.05Point farthest from apatite

0.000.00

I

0.05

I

0.10

I

0.15

I

0.20

0.25

A Al FIG. 8. The change in P ( AP) vs. the change in AI (AAl) in the melt (cation basis) from (a) points nearest apatites from all traverses and (b) points along the entire length of one traverse. The very good fit to the I:1 line indicates that throughout the apatite dissolution aureole, each P is associated with one Al. possibly as an AlPOd complex.

Results from Coarse-Grained 109

10~10

4

D (cm%) 10 ”

0

i

*sl,,, FIG. 7. Diffusivities of (a) P and (b) Ca as a function of AS1 and time (Dc. > Dp), calculated with the inverse error function method. Diffusivities are constant as a function of ASI and time.

Apatite Aggregate Runs

Because of the substantially different values of P205 vs. AS1 found between the semi-isolated coarse-grained Ap runs and the very fine-grained Ap runs, another set of experiments was devised which combined the coarseness of the former runs with the ubiquitous Ap distribution of the latter runs (Fig. I c) The averaged glass compositions from the coarsegrained apatite aggregate runs are listed in Table 8. These analyses were taken from the glass matrix between grains of Ap, but with no regard for distance from Ap (except that x > 10 pm from Ap), because no compositional gradients were found in these glasses. Figure I2 plots PZ05 vs. ASI for glasses from these runs. Comparison of Fig. 12 with Fig. 10 reveals that these two sets of data are essentially identical, thus confirming the accurate analyses of both and the validity of the results. Figure 13 plots P vs. Ca (cation basis) for all analyzed points from these Ap aggregate runs ( lo-26 pts/ run). Comparison of the data in this figure (slope = 0.9, R = 0.932)

Solubility of apatite in peraluminous granitic melt

I (Al + P = Si) exchange in melt near apatites (cationbasis)

0.7

.

4137

* a . ’ * * ’ ’ * * . ’ ’ * ’ 0 A HG-41

O.fj- 0 . A

HO-42 HO-43

R = 0.9941

. Dulation: all 03 _ 4 weeks except

M = -1.00

. HG-33for 6 w 200 MPa

0.2 -

-0.3

-0.2

A Si

I

0.3 bJ

-0. I

0.0

I

FIG. 10. P205 vs. ASI in glass from very fine-grainedAp runs. These glasses have no compositional gradients and thus have essentially reached equilibrium. Note the linear relationship but much different slope and maximum, relative to the transient high PZOJ values in the coarse, semi-isolated Ap grain runs (Fig. 3). Compare with Fig. 12.

(Al + P = Si) exchange along profile HG20 with those in Fig. 11 again shows the similarities in the two datasets.

(cation basis) R = 0.9998 M = -0.96

Results

from Fluorite Dissolution/Apatite

Crystallization

Runs

AtA

Point farthest from apatite (P205=0.08 wt% at 150 pm) *

0.0

-0.3

I

I

-0.2

-0.1

A Si

0.0

FIG. 9. The change in Al + P vs. the change in Si in the melt (cation basis) from (a) points nearest apatites from all traverses and (b) points along the entire length of one traverse. The very good fit to the I:- I line indicates that the exchange reaction AlPSi_, occurs throughout the apatite dissolution aureole and that two TO4 clusters are created in the place of one.

Table 6. Average glass analyses from very finely-ground

RUlt# Powder Pdr Ap he (h) Fspgrowth

HG41 HG 16.0 720 minor

sdm:9

HG42 GASI. 1 16.0 720 minor

sdm:9

HG43

Figures 5e-h are WDS X-ray maps of a reconnaissance Ap crystallization run (dissolving, coarse-grained fluorite (Fl) provides Ca and F to P-rich (3 wt% P205) granitic melt; 75O”C, 200 MPa (PH20), one week). Apatite solubility is very low (x0.1 wt% P,05 in melt within the Ap crystallization zone, regardless of ASI of initial melt; WOLF and LONDON, 1993a). Figure 14 depicts the approximate concentration of PZOs, CaO, and F in melt as a function of distance from the R crystal (M. B. Wolf and D. London, unpubl. data), with the Y-axis (distance) oriented to correspond with Fig. 5e-h. Fluorine diffuses rapidly throughout the capsule (-0.4 wt% F several mm from Fl). The advance of the Ap crystallization front away from Fl (at a rate of lo-’ cm/s) is controlled by the diffusion of Ca in excess of that which combines with

apatite runs

sdm:13

HG-44

sdm:12

HG-32

sdm:13

HG-33

sdm:13

GASIA GASI. GAS13 GASIA 16.2 15.8 14.5 14.5 719 719 738 1050 abundant abundant abundant abundant Si@ 70.69 0.28 70.16 0.13 69.83 0.11 69.05 0.14 67.61 0.14 66.91 0.24 Al203 13.18 0.05 13.81 0.05 14.03 0.02 14.26 0.03 14.83 o.O4 14.90 0.05 NryLo 4.50 0.05 4.51 0.01 4.28 0.04 4.20 0.03 4.19 0.05 4.15 O.cJ3 K20 3.59 0.04 3.79 0.02 3.75 o.aJ 3.71 O.Q3 3.63 0.04 3.57 0.03 405 0.06 0.01 0.13 0.01 0.25 0.01 0.40 0.00 0.57 0.M 0.54 0.01 Cao 0.10 0.01 0.18 0.02 0.29 0.01 0.39 0.02 0.54 0.01 0.59 0.01 F 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.03 0.10 0.03 Total 92.12 0.30 92.57 0.11 92.43 0.13 92.01 0.18 91.47 0.19 90.76 0.2-I ASI 1.15 0.01 1.17 0.01 1.21 0.01 1.23 0.01 1.26 0.01 1.27 0.01 Note: sdm# = standard deviation of the mean of # analyses; Pdr:Ap = weight ratio of powder to apatite in runs; Feldspargrowth:minor - ~1%; abundant - still -zS%.

4138

M. B. Wolf and D. London Tabk 7. Average feldspar compositioas from two peralumiaous, very fhely-ground apatite runs HG-32 sdm:7

RUll Powder time @) SiO2

GASIA 1050

Ak@3 Cao

61.74 0.26 23.52 0.07 4.26 0.07

Na+ K20 P2Q F Total

8.59 0.73 0.36 0.03 99.23

62.09 23.62 4.22 8.74 0.72 0.40 0.04 99.82

0.08 0.01 0.01 0.01 0.25

75.9 19.6 4.5

Ab An or Note:

HG-33 sdm:9

GASI. 738

0.06 0.04 0.04 o.a3 0.01 0.M 0.01

Oh2

Ca

0.07

FIG. 11.Phosphorus vs. calcium (cation basis) in glass from very fine-grained Ap runs. Glasses contain an excess of P, relative to Ca: P stoichiometry in Ap ( 1.67), due to preferential incorporation of Ca into growing plagioclase. Compare with Fig. 13.

76.6 19.0 4.4

sdm:X = standard deviation

of the mean of # analyses

the parameter ASI, Al saturation index. The ASI is normally defined as mol. A1203/( NazO + KzO + CaO), or more generally as Al/( ZM’ + 2ZM2+), where M+ and M2+ are any charge-balancing cations (e.g., alkalies or alkaline earths) that permit Al to be incorporated as a network-forming component of melt. We have utilized these simple definitions of AS1 in this paper, though we recognize that at high concentrations of components such as phosphorus (or B or F), melt speciation reactions may occur that change the structural role of P and the true meaning of ASI as above-namely, the extent to which Al becomes a network-forming TO4 component. If P forms an associated 1: 1 complex or melt species with Al only, then a fraction of total Al is removed from a network-forming role with Si, and the representation of AS1 becomes (Al - P)/( ZM+ + 2ZM2+). If P tends to associate more with alkalies (e.g., the 1:1 component NaPO,: see LONDON et al., 1993), then they are no longer available for charge balance on Al and hence, ASI = Al/(BM+ + 2ZM2+ - P). An additional question is what AS1 values to use when ASI varies through the system, as it does in the dissolution experiments with large, semi-isolated Ap grains? It is ob-

almost all of the P (and some F) to form Ap from the initially

P-rich melt. In this case, the abundance of Ca is the essential structural constituent that limits the saturation of Ap (ESC; see SUN and HANSON, 1975). Another set of Fl dissolution/ Ap crystallization runs was done using very finely ground Fl mixed into the P-rich granite powders (with initial AS1 from 1.05 to 1.20 and SiOZ between 66-70 wt%), run at 750°C and 200 MPa (PH20) for four weeks). In a plot of PzOs in melt vs. ASI, Fig. 15 reveals the very low solubility of Ap in these runs, regardless of ASI. These melts contain -3 wt% F and 0.04-O. 16 wt% CaO and are Fl-saturated. Lines fit to the data from the Ap dissolution runs (Figs. 3, 10, 12) are shown for comparison. Figure 15 clearly shows that ASI alone does not control Ap solubility, as pointed out by BEA et al. ( 1992). DISCUSSION Apatite Solubility in Peraluminous

Melts

Before discussing the relationships of apatite solubility to melt composition, it is necessary to consider the meaning of

Table 8. Average glass analyses from coarw apatite aggregate runs Rllll Pow&r Pdr:Ap time 01) Fsp growth SiO2 Ak% N;yo K20 P2% CaO F Total ASI

HG45

adm: 22x5

HG 0.5 790 none 70.74 12.97 4.78 3.53 0.05 0.16 0.05 92.22 1.08

0.10 0.a 0.02 0.07 0.00 0.01 0.00 0.16 0.01

HG-46 sdm: HG47 %:K6 sdm: HG49 Sdm: HG48 23:K8 sdm: 24:K5 26:K6 GASI. GASI. 1 GASI. GASI. 0.5 0.5 0.5 0.5 790 790 790 790 minor none none minor 69.48 13.49 4.66 3.58 0.13 0.27 0.06 91.27 1.12

0.07 o.o.3 0.03 0.04 0.00 0.01 0.00 0.11 0.01

68.31 14.22 4.49 3.61 0.28 0.43 0.07 91.27 1.18

0.13 0.04 0.04 0.04 0.01 0.01 0.00 0.44 0.01

67.52 14.37 4.44 3.61 0.43 0.50 0.05 91.44 1.18

0.13 0.05 0.03 O-03 0.01 0.01 0.00 0.11 0.01

67.65 14.71 4.23 3.67 0.61 0.62 0.07 91.29 1.22

0.09 0.05 0.03 0.06 0.01 0.01 0.00 0.30 0.02

HG-51 $n: GASI. 0.5 2518 abundant 67.43 14.98 4.73 4.02 0.33 0.34 0.08 91.92 1.17

0.014 0.05 0.03 0.04 0.02 0.01 0.00 0.16 0.01

Note: sdm:#:K# = standard deviation of the mean of # analyses for all oxides except K$) & ASI (which follow the K) ~1%; abundant still ~5%.

PdrAp = weight ratio of powder to apatite in the runs; Feldspar growth: minor

4139

Solubility of apatite in peraluminous granitic melt

0.6 05 04

.

p205

03

(ti%) 0.2 0.1 0.0

FIG. 12. PrOr vs. ASI in glass from coarse-grained Ap aggregate runs. These glasses have no ~m~sition~ gradients and thus have essentially reached equilibrium. These runs give the same results as the very fine-grained Ap runs (Fig. 10).

viously meaningless to correlate the bulk AS1 of the system with the solubility of Ap, as AS1 is greater in the region of apatite di~lution than in the starting bulk com~sition. As utilized above, the ASI refers either to the spot nearest Ap margins (Fig. 3) or the average over the diffusion profile (Table 2A), both of which are greater than the bulk ASI of the system. Our work does confirm the low solubility of Ap (0.1 wt% P20s, Fig. 3) in me~luminous melts with AS1 x 1 (cf. WATSON and CAPOBIANCO, 1981; HARRISON and WATSON, 1984). Figures 10 and 12 show PzOs c 0.1 wt% in melts with ASI * 1.1. In addition, we have confirmed the positive correlation of Ap solubility with AS1 in silicic melts, as suggested by PKHAVANT et al. ( 1987) and LONDON et al. ( 1990) and demonstrated expe~men~ly by MONTEL et al. ( 1989) and PICHAVANTet al. ( 1992). The Ap dissolution data from the semi-isolated Ap grain runs show transient high Ap solubilities-much higher than equilibrium values (Figs. 2, 3). The apparent complication of the rise, fall, and rise in P,05 between Fig. 2a-d can be resolved if the weight ratio of rock powder to Ap is taken into account. Note that the

Fl

1

WE%

2

3 _J

FIG. 14. Schematic diagram showing approximate concentrations of P205, CaO, and F in melt (wt% along X-axis) as a function of distance (Y-axis) away from dissolving fluorite (Fl) grain (data derived from run shown in Fig. 5e-h). The melt is initially P-rich (-3 wt% P205 in Apfree region}. As PI dissotves, Ca and F are added to the melt, inducing Ap c~s~lli~tion. After one week, Ap ~~~i~tion front (Ap Xtl Front; arrow) has advanced 650 grn into melt. Calcium is the essential structural constituent of Ap at the front.

two-, four-, and fifteen-week runs all have a Pdr:Ap ratio of about 10, but that the eight-week runs have a PdrzAp ratio of 3 (Table 2A). The eight-week runs have Pdr:Ap ratios that fall between the high values of the other semi-isolated Ap grain runs and the low values of the Ap aggregate runs (0.5; Table 8 ) . The relatively low Pro5 contents in the eight-week runs (Fig. 2c) are a consequence of this low Pdr:Ap ratio (relative to the four- and fifteen-week runs, Fig. 2b,d; as noted

0.03

0.02

0.b

Ca RG. 13. Phosphorus vs. calcium (cation basis) in glass from coarsegrained Ap aggregate runs. Ghrsses contain an excess of P, relative to CazP ~oichiome~ in Ap ( 1.67),due to preferential in~~mtion of Ca into growing plagioclase. These runs give the same results as the very fine-grained Ap runs (Fig. 11).

FIG. 15. P205 vs. AS1 in glass from fluorite dissolution/Ap crystallization runs (solid diamonds; very tine-grained F1, 75O”C, 200 MPa (Pn&, four weeks). Compare with data from Ap dissoiution runs from Fig. 3 and Figs. 10 and 12 (lines). Apatite sotubility does not depend solely on melt ASI but is a function of the activity product of the constituents comprising Ap.

4140

M. B. Wolf and D. London

above, the low P205 contents in the two-week runs, Fig. Za, result from limited AQ dissolution due to the short run duration). This comparison reveals how sensitive the rate of AQ dissolution is to the abundance of Ap in the local melting region (i.e., the capsule in these experiments or an isolated, peraluminous melt pool in a melting rock). The rate of AQ dissolution, however, is not affected by the proximity of other Ap grains if the melt:Ap ratio is large (with abundant Al available for complexing with the P of the Ap). Large concentration gradients still are present in the fifteenweek run, so even much longer durations are needed to approach equilib~um with the semi-isolated (or single) crystal di~lution method, at 750°C and 200 MPa. Through time, we would anticipate the slope of the line in a plot of P205 vs. AS1 to decrease from that in Fig. 3 to those in Figs. IO and 12. The trend in Fig. 2 of decreasing P205 content with time at the Ap-melt interface probably continues with increasing time until equilibrium is reached. Eventually, those values would correspond with the lower P,O, contents found in the Ap aggregate and very finely ground Ap glasses (Figs. 10, 12). These latter sets of runs were devised to determine equilibrium Ap solubilities, and we believe we have achieved this goal, as detailed above. A line has been fitted through the combined data ( 106 points) from Figs. 10 and 12 to derive a simpte relationship between P20, and AS1 for peraluminous melts at 750°C with Si02 - 69 wt% (67-71 wt%): P205 = -3.4 4 3.1 X ASI (R = 0.833). PICHAVANT et al. (1992) have derived an Arrhenius-type equation for this relationship which takes variations in temperature and melt SiO2 content into account. Comparison of our data (Figs. IO, 12, and the above equation) with a line projected along 750°C in Fig. 2 ( Si02 = 72 wt%) Of PKHAVANT et al. ( 1992) reveals that the two lines have similar slopes, although our data are displaced to higher values of ASI (by 0. 1), i.e., for a given ASI, we find PzOs contents -0.45 wt% lower than those calculated by PICHAVANT et al. (1992), though well within the errors in their formulation. This relatively small difference cannot be attributed to the slight differences in SiOz, because an increase in SiOz decreases the amount of P205 in the melt in equilibrium with apatite. The match is remarkably good, considering all of the differences between the experiments. The Assaciatiou of Aluminum and Phosphorus The afhnity of P for Al has been inferred from spectroscopic studies of gIasses (e.g., MYSEN et al., 1981; CAN and HESS, 1992), from the coupledAlPSi& substitution in feldspar (e.g., SIMPSON, 1977; LONDON, 1992; LONIXN et al., 1990), and from effects of ASI on AQ solubility (e.g., MONTEL et al. 1989; P~CE~AVANT et al., 1987, 1992; cf. BEA et al., 1992). LONDON et al. ( 1993) also noted that the association of Al and P was apparently strong enough to remove a substantial fraction of Al from fully charge-balanced coordination in melt (with AS1 = I ), and by the AlPSi substitution in alkali feldspar, to create a normative ( Na,K)&Ou component of melt. The I : 1 correlation between the number of P cations and excess Al cations in the apatite dissolution aureoles (Fig. 8 ) provides yet another measure of the affinity of P for Al, and

of the stoichiomet~ of the normative component formedAlPOd or a hydroxylated species such as AlOP(0H)6 (cf. RYERSON and HESS, 1980; MYSEN et al., 1981). Of course, we cannot confirm that AlPOd or similar species actually exist as coordinated complexes at melt temperatures. There is no change in the absolute or relative concentrations of Na and K in the Ap diffusion aureole from their abundances in the starting homogeneous melt. This fact leads us to suggest that for these peraluminous melt compositions, P interacts predominantly with Al, not alkalies (i.e., it is AI, not alkalies, that provides coordination with P to promote dissolution of Ap). This is basically true of the changes in melt composition that accompany the A1PK2 exchange in feldspars (LONDON et al., 1993); there is little fractionation of P between Or and Ab, and changes in melt composition reflect a decreasing Si content more than changing Na:K of melt (melt fractionation follows a path along an Orzs isopleth ) . The Exchange Reaction AlPSi_, The data in Fig. 9a and b indicate that an AlPSi-, exchange reaction occurs throughout the Ap dissolution aureole, i.e., that two cations, one Al plus one P, replace one Si cation in the melt. This su~titution is distinct from the charge-balanced AlPSi_ su~titution in alkali feIdspars (SIMPSON, 1977; LONDON, 1992; LONDON et al., 1990, 1993). There are no real problems, steric or chemical, in maintaining local charge balance in the HzO-saturated melt, as there would be in the ordered structure of crystalline phases. The AlPSi_, exchange in melt, however, should be accompanied by discernable changes in melt properties (e.g., density, molar volume, viscosity ). Changes in ASI and PzOs Contents as a Function of Time The changes in ASI between two-, four-, eight-, and fifteenweek runs (Figs. 2, 3) must be addressed because of their relevance to the discussion of diffusivity. Figure 3 shows that in runs with the most peraluminous starting composition (GASI.4), the final melt ASI at Ap-glass interfaces increases from I .25 (two weeks; solid triangles) to 1.43 (four weeks: solid circles), decreases to 1.28 (eight weeks; open squares), and increases again to 1.4 (fifteen weeks; open diamonds). The high AS1 near Ap margins is the result of increased aluminate solubility by the addition of P to the melt from Ap and subsequent aluminophosphate complexing. PICI-IAVANT et al. ( 1992) also reported that the solubility of andalusite was increased by the availability of P from dissolving Ap. It is clear that ~raluminosity also greatly affects the solubility of AQ, and the effects of P on AI and AI on P are interrelated. The slope of the line through the data in Fig. 3 is well constrained by all of the data between 65-71 wt% SiOz. The variation of both P205 and ASI through time, up and down this line, may be a result of the differential rates of aluminate and apatite dissolution and Al and P diffusion, in addition to complications resulting from the complexing (and subsequent diffusion) of aluminophosphate species. These data may indicate that the dissolution rate of AQ is not constant but is a function ofthe availability ofA in the system (which depends on the melting reaction within the rock).

Solubility of apatite in peraluminous granitic melt

(cm%) _11_

0

WL4w

0

WL8w

l

WL15w

13ooT

I

-12 6

SiKYC

1100-C

I

I

I

7

I

7soT

I 9

I 10

lo&K FIG. 16. Arrhenius plot of P diffusion coefficients either calculated from gradients in P20s from semi-isolated Ap grain runs ( W, 75O”C,

two to fifteen weeks, this study); from HARRISONand WAIXON ( 1984), HW84; or from P~CHAVANT et al. ( 1992), PMR92. All runs from hydrous (26 wt% H20) granitic compositions. A fit to the data yields an activation energy of 16.5 kcal/mol and a frequency factor of 1.4 X 10e3 cm’/s.

Diffusivities Figttre 7a shows that Dp is -lo-” cm2/s and is independent of melt ASI. The diffusion of P is slower than that of Ca and appears to be the rate-limiting factor in Ap dissolution in peraluminous melts, as in metaluminous compositions (HARRISON and WATSON, 1984). Figure 16 is an Arrheniustype plot of diffusivity data from Fig. 7a in addition to those from HARRISON and WATSON ( 1984) and PICHAVANTet al. ( 1992 ) . The cluster of P diffusivities determined in our study falls along the projection of the higher temperature, HzOrich Dp line of HARRISONand WATSON ( 1984; Fig. 2)) which suggests that we have recorded true diffusivities. The line fit to all of the data in Fig. 16 represents a refinement to that calculated by HARRISONand WATSON ( 1984) and yields an activation energy (E) for P diffusion of 16.5 kcal/mol and a frequency factor (Do) of 1.4 X 10m3 cm2/s.

4141

Calcium diffusion ranges from 2 X lo-” to 3 X lo-” cm*/s and may depend slightly on melt AS1 (Fig. 7b). Calcium diffusion is 3- 10 times faster than and thus, decoupled from P diffusion (cf. HARRISON and WATSON, 1984), resulting in shallower concentration profiles (Fig. 4 ) and nonintegrated Ca/P ratios in the melt that are less than the Ap stoichiometry of 1.67 (cation basis). Other reactions, such as the consumption of Ca as an An component of plagioclase (Fig. Sb, Table 3; also see L~NWN et al., 1989; BEA et al., 1992) or saturation in fluorite would further decrease the Ca/P ratio in melt, even at equilibrium. The entire Ca diffusion profiles fit error function calculations well, in contrast with the results of HARRISON and WATSON ( 1984)) in which excess Ca was measured near Ap. We used a larger EMPA beam diameter that may have resulted in a lower integrated Ca concentration near Ap; however, HARRISON and WATSON’S ( 1984) deviations from a good fit to erf -’ come from O-50 pm away from Ap, and we have two to six points in this zone in all of our profiles (e.g., Fig. 4, Table 2B). Thus, our very good fits to erf -I are not due to the lack of data in this zone. As discussed above, we have not calculated diffusion coefficients for Al, due to its uphill behavior, in addition to the complexities associated with the persistence of the alumina microlites in some of the melt compositions. Qualitatively, we can show that Al diffusion is a long-range process, not confined solely to pm-length diffusion adjacent to dissolving alumina microlites. Two of the three experimental methods used in this study are depicted in Fig. 17a,b: (a) shows the semi-isolated Ap grain setup and (b) shows the Ap aggregate setup (also see Fig. la, c). In the semi-isolated Ap grain runs, some Ap grain faces are only a few to tens of pm apart. This local geometry is similar to that within the Ap aggregate runs, with Ap grain faces generally separated by narrow regions of melt. In the semi-isolated Ap grain runs, one might expect to see a variation in major element composition around grains depicted in Fig. 17a due to Ap dissolution, from strongly changed compositions along the free faces

a

PK. 17. Schematic diagrams of (a) the semi-isolated Ap grain runs, (b) the Ap aggregate runs, and (c) possible mineral relationships within a melting rock. Shading within melt qualitatively depicts relative P205 content (darker = P-rich). Larger Pdr:Ap ratio in (a) provides an abundant source of Al to complex with P and enhance Ap dissolution. Aluminum must diffuse relatively long distances toward the Ap (arrows). Smaller Pdr:Ap ratio in (b) inhibits longrange Al diffusion and transient high Ap solubilities. Compare Figs. 3 and 12, and Tables 2 and 8. (c) During anatexis, relatively large pools of peraluminous melt may enhance Ap solubility, which may lead to dissolution of nearby accessory minerals (Zr-zircon; Mnz-monazite).

4142

M. B. Wolf and D. London

(more dissolution) to less changed compositions between adjacent grains (less dissolution)-compositions akin to those found in the Ap aggregate runs (Fig. 17b). This expectation is not fulfilled; the melt composition at the Ap-melt interface is constant for a given run, regardless of the distance to other Ap grains. Aluminum must diffise readily from the relatively large reservoir of peraluminous melt surrounding the Ap grains, not only to the free faces of the grains, but also along the narrow regions of intergranular melt, in many cases for distances of hundreds of pm (Fig. 17a). A similar effect, though in the opposite direction, occurs for the components derived from the Ap. Because the Ap aggregate runs have much lower Pdr:Ap ratios, they do not contain the abundance of dissolved excess Al needed to drive or sustain the high rate of Ap dissolution, and concentration gradients remain small (Fig. 17b). The melt compositions homogenize much faster due to the much smaller pools of melt, relative to the semi-isolated Ap grain runs. Figure 17c schematically illustrates a small region of a partially melted, Ap-bearing rock. In this case, zircon and monazite crystals are located close by, the melt is peraluminous, and the melt fraction has increased more rapidly than the diffusion of P through the melt; thus, Ap solubility is not constrained, temporarily, to remain at the lower equilibrium values (Figs. 10, 12, 17b). Transient, enhanced Ap solubility, with its concomitant changes in local melt composition, may affect the stability of nearby accessory minerals (Fig. 17~). Structural and Chemical Changes in Melt around Apatite

The dissolution of apatite promotes substantial changes in the composition of adjacent melt if that melt is peraluminous and silicic, as would be derived from melting of the assemblage Qtz-Ab-Mu (e.g., ICENHOWER and LONDON, 1993 ). The cumulative effects of these changes on the bulk properties of melt are difficult to predict without knowledge of the interactions, if any, among the AlPOd and Siz04 components of melt. An AlPOd species of melt is created which, so far as we know, is not incorporated into the Si04 framework of the melt. One hypothesis would be that the decrease in silica content of melt alone would be accompanied by a drop in the degree of melt polymerization. Though phosphorus is normally viewed as a network-forming component ( MYSEN et al., 198 I), the existing spectroscopic data do not show evidence for B-O-P coordination in glasses (MYSEN et al., 198 1; GAN and HESS, 1992) and hence, imply that phosphate species of melt exist as discrete subunits, not connected to the silicate TO4 framework. Thus, even though the compositional data along Ap dissolution profiles imply that two network-forming cations (Al + P) substitute for one (Si), there may yet be a net reduction in the overall polymerization of melt. The addition of P, a network-forming component (?), to fully polymerized (NBO/T = 0) anhydrous haplogranitic melts produces a drop in melt viscosity ( DINGWELL et al., 1993). We suspect that here, too, P interacts with Al in such a way as to reduce the extent of melt polymerization, and hence these components together should not be construed as network-formers, equivalent to SiOd (cf. RYERSON and HESS, 1980).

PETROLOGICAL APPLICATIONS The potentially important ramifications of these experimental results are that with the availability of excess Al in melt, as for example in the production of peraluminous magmas by anatexis of metapelites (e.g., ICENHOWER and LONDON, 1993)) the enhanced solubility of Ap induces a transient, disequilibrium change in melt composition that promotes much higher P205 concentrations in melt than at equilibrium. Because the dehydration-melting of metapelite generally begins with the breakdown of muscovite and biotite (e.g., LE BRETON and THOMPSON, 1988; VIELZEUFand HOLLOWAY, 1988; PATINO DOUSE and JOHNSTON, 199 1 ), and the reaction of these minerals leads to peraluminous compositions, most melts formed during the initial stages of metapelite anatexis will be more aluminous (with potentially higher P205 contents) than subsequent, larger volumes of melt. The elevated ASI of initial melts may not necessarily be easily recognized in migmatite terranes, because peritectic reaction during cooling of leucosomes may result in back-reaction, mica growth and modification of melt composition (ELLIS and OBATA, 1992). Biotite growth as leucosome-rimming selvages during cooling (e.g., LE BRETON and THOMPSON, 1988) will lead to lower calculated melt ASI than was actually present in the melt, if these Al-rich regions crystallized from the leucosomes but are not recognized as such and thus, not included in calculations of melt composition. Higher PZOs concentrations due to increased peraluminosity would increase the degree of melting in the vicinity of Ap (LONDON et al., 1993). The compositional changes in melt, which we interpret to reflect a decrease in the extent of polymerization, would tend to enhance the dissolution of other nearby accessory phases of high field-strength elements, such as zircon, monazite, and oxides, which are commonly associated with apatite (e.g., WATSON, 1979b; HARRISONand WATSON, 1983; ELLISON and HESS, 1986, 1988; MONTEL, 1986; RAPP and WATSON, 1986; DICKINSONand HESS, 1985; KEPPLER, 1993; WOLF and LONDON, 1993b; M. B. Wolf and D. London, unpubl. data). The dissolution of flux-bearing minerals (e.g., those containing P, B, or F) may influence the stability and solubility of trace element-bearing minerals during both equilibrium and disequilibrium anatexis of metapelites ( Fig. I 7~). Equilibrium generally has been thought to be an a priori consequence of the high temperatures and long durations achieved during anatexis. However, manifestations of disequilibrium melting have been found in natural rocks, and recognition of this condition is becoming more common (e.g., DOUGAN, 1981; MEHNERT and BUSCH, 1982; WEBER and BARBEY, 1986; NASLUND, 1986; KACZOR et al., 1988; COPEI-AND et al,, 1988; BARBEY et al., 1989; SAWYER, 1991; MAAL.@E, 1992; HARRIS et al., 1992; SROGI et al., 1993; WATT and HARLEY, 1993). The common occurrence of inherited zircon and monazite in granitic rocks (e.g., WILLIAMSet al., 1983 ) indicates that trace element and isotopic equilibrium is rarely achieved completely during anatexis (WATSON and HARRISON, 1984). The differences in solubilities and reaction kinetics of different accessory minerals may lead to the decoupling of isotopic systematics (HOGAN and SINHA, 199 1). The solubility of trace element-bearing accessory minerals

Solubility of apatite in peraluminous granitic melt

has a profound influence on geochronological and geochemical modelling ( VON BLANCKENBURG, 1992). HARRIS and INGER( 1992) have pointed out that the ~~uitib~um behavior of accessory minerals has critical consequences for REE modelling of granites. The dynamic disequilibrium melting model of IWAMORI ( 1993) suggests that “discquilihrium processes may largely control chemical variations in igneous rocks.” Texture (e.g., grain size and distribution, and spatial relationship to other minerals) plays an irn~~nt role in reaction kinetics and, at least temporarily, in mineral solubilities; the location within a rock of the solubility-enhancing flux-bearing minerals may govern the location of early melts (leucosomes) (e.g., JOHANNES, 1988) and the solubility of trace element-bearing minerals and thus, the addition or withholding of trace elements to or from the melt (e.g., WEBERet al., 1985; BACON,1989; WATSONet al., 1989; SUZUKI et al., 1990; REID, 1990). Melts locally may contain higher concentrations of normally refractory or insoluble mineral components, and very small melt domains may exist that are com~~tion~ly quite distinct from the bulk melt. For example, the degree to which apatite might dissolve (and add REEs to the melt) depends on the quantity and size of apatite grains residing within a particular multigrain junction, where melting begins (compare P contents of melts between the three main sets of experiments). Apatite crystals generally are dispersed throughout natural rocks (isolated from one another), even in Ap-rich rocks. As the semi-isolated Ap grain experiments show, apatite dissolution may strongly alter the local melt composition, at least temporarily, which may affect the solubilities of other associated accessory minerals. The extent to which such small domains of melt might either segregate (e.g., WICKHAM, 1987; SAWYER, 199 1; WOLF and WYLLIE, 199 1) or equilibrate by mixing (with larger pools of melt) will govern when and how trace elements are fractionated from Apbearing source regions. In addition to temperature and silica content (HARRISON and WATSON, 1984), melt polymerization in general ( KOGARKO et al., 1988), AS1 ( MONTEL et al., 1989; PICHAVANT et al., 1987, 1992; LONDON, 1992; LONDON et al., 1990) and location within grains or along grain boundaries ( WATSON et al., 1989; BACON, 1989), we can now include rock texture, specifically the distribution and size of Ap grains (in relationship to the size of local melt pools), as another real consideration in understanding the dissolution and saturation of apatite in melt. Moreover, WOLF and LONDON ( 1993a; M. B. Wolf and D. London, unpubl. data) note that activities of Ca and F are also factors (Figs. 5e-h, 14, 15 ), so that Ap solubility must be viewed as an activity product of individual species (HARRISON and WATSON, 1984; BEA et al., 1992), and that Ap-forming components may represent only a fraction of these species in melt. In the light of the significant differences in apatite solubility between the peraluminous and metaluminous felsic systems (e.g., WATSON and CAPOBIANCO198 1; GREEN and WATSON, 1982; PICHAVANTet al., 1992), modifications must be made to models of anatexis of Ap-bearing rocks, with concomitant changes to REE models involving Ap. In contrast with the results of WATSON and CAPOBIANCO( 198 1) in the metaluminous system, peraluminous felsic melts can be P-rich, so

4143

high P20J contents of the resultant rocks do not necessarily imply residual Ap crystal entrainment in these magmas (see also BEA et al., 1992). In fact, because of the relatively high solubility of Ap in peraluminous melts, anatexis can consume all of the Ap in the source rock. However, disequilibrium melting processes, controlled by local melt compositions and grain geometries (i.e., distribution, size), may strongly influence the ultimate P concentration in the magma and the amount of Ap left in the source rock. For example, the transient, high solubility of isolated Ap grains will depend on the degree of melting (the melt fraction at any given temperature), because higher melt:Ap ratios drive the enhanced Ap dissolution. The solubility of Ap could vary during anatexis as both melt fraction and composition change. If the rate of major-phase melting can outpace or equal Ap dissolution (and P diffusion), then the transient high ~lubiliti~ determined from the disequilibrium, semi-isolated Ap experiments (Fig. 3) may be more relevant to anatectic processes than those determined from the equilibrium, Ap aggregate, and very finegrained Ap experiments (Figs. 10, 12), at least in terms of the initial dissolution behavior of Ap. The data from these latter runs represent Ap ~lubility limits that can be applied to the crystallization of Ap from peraluminous haplogranitic melts; this could include Ap recrystallization from local, P-oversaturated melt resulting from enhanced Ap dissolution during the initial stages of anatexis. Modifications also must be made to fractional crystallization models of P-bearing magmas. A complication to simple models, discounted by WKCKIN and CAPOBIANCO( 198 1) but demonstrated by LONDON ( 1992, 1990, 1993), is that major-phase mineralogy can have a large effect on the P content of melts, because feldspar can accommodate significant amounts of P$%, through the AlP& substitution (Tables 3,7; see also BEA et al., 1992). Feldspar will become enriched in Al and P through this substitution (ASI > 1 ), so the ASI of a melt can decrease during feldspar crystallization, lowering Ap solubility and inducing Ap crystallization (in addition to the effects of decreasing temperature and increasing Si02). The AIPSii2 substitution in feldspar may be partially offset by this competing process of Ap c~s~lli~~on. However, the Ap crystals themselves may become included within the growing feldspar (GREEN and WATSON, 1982; HARRISON and WATSON, 1984). The incorporation of P (either structurally or as Ap inclusions) along with Al into feldspar will decrease P in the melt. This combination of the lowered Ap ~lubility and lowered P con~ntmtion may effectively remove much of the P from the melt. The AIP& substitution in feldspar may hinder late-stage P enrichment in peraluminous felsic melts, although P in residual melt still will increase with fractionation. Ackn~w~~dgm~nts-akin research was supported by the NSF EPSCoR program, grant EHR-9 108977 1,as part of colla~rative research between the University of Oklahoma and the University of Tulsa on fluid-rock interactions. George Morgan VI’s invaluable assistance with EMP data collection is greatly appreciated. Insightful discussions with Jonathan Icenhower, Tom Dewers, and George Morgan VI were very helpful. The manuscript was greatly improved by comments by Chris Tacker and thorough reviews by Michel Pichavant and Rick Ryerson. Editorial handling: P. C. Hess

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M. B. Wolf and D. London REFERENCE

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