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Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite
Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite E. BRUCE WATSON and C. J. CAPOHIANCO* Department of Geology.
Rensselaer
Polytechnic
Institute.
Troy. NY 12181. U.S.A.
Abstract-The solubllity of fluorapatite in I7 silica-rich melts in the system Na,O KzO AI,O, ~SIO, (with and without CaO or CaF,) was determined at I kbar water pressure and 750 900 C. Apatite saturation occurs at levels of dissolved P,05 ranging between 0.04 (kO.02) and 0.28 ( f 0.13)wt”,,. with only 4 values outside the 0.09-0.20 wt’,, range. The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that. under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P,05 (the mean of the experimental values) a source containing as little as 0.05% P,05 must be 35% melted before apatite is lost from the residue and no longer buffers the melt P,05 concentration at the saturation value. Higher abundances of P,O, in the source postpone the loss of residual apatite to still higher degrees of melting. and if the source PzOs content exceeds 0.14 wt?<::;, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds. The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. FractIonal crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.
INTRODUCTION OVER the past 15 years, major advances in our understanding of magma genesis have been achieved through quantitative modelling of trace element behavior during melting and crystallization processes. Further refinements in both modelling approaches and inferences are likely to develop in the near future, but progress is limited, particularly where felsic magmas are concerned, by lack of data in one fundamental area: that is. the stabilities of minor and accessory minerals that are effective concentrators of the tract elements commonly used in modelling. This paper represents part of a continuing program of experimental research on early-crystallizing accessory phases. It is preceded by studies of zircon solubility in felsic magmas (WATSON, 1979a), apatite saturation in basic and intermediate magmas at nearatmospheric pressure (WATSON, 1979b), and apatite stability in basic magmas at upper mantle temperatures and pressures (WATSON, 1980). Other important contributions on the topic of accessory phase crystallization are those of HELLMAN and GREEN (1979) on sphene in hydrous mafic compositions, KNUTSON and GREEN (1975) on an apatite-bearing megacryst assemblage, and the summary/review paper by GREEN (1981) on magmatic accessory minerals in general. The present paper centers specifically on the stability of apatite in felsic magmas at 1 kbar water pressure and temperatures of 75@9OO”C. The objective of
* Present address: Department of Geology. State University. Tempe. AZ 85281. U.S.A.
Arizona
the experiments was not simply to re-confirm what geochemists have already shown [i.e. that apatite can play a role in the trace element evolution of felsic magmas (e.g. ZIELINSKI and FREY. 1970; PRICE and TAYLOR, 1977; SIMMONSand HEDC~C.1978)], but to provide a basis for predicting the conditions of temperature and composition over which apatite will coexist with felsic melts. The experimental data are subsequently used to show. in a series of melting and crystallization models, how apatite can be expected to influence the behavior of phosphorus and the rare earth elements (REE) during felsic magma production and evolution. EXPERI.ZlENTS
We determined the stability of apatite in a variety of felslc melts by use of the same basic technique as employed by WATSON (1979b. 1980); that is. imposed saturation in apatite. In the present case. ground fluorapatite (55lO~m) comprised IO wt”,, of the starting materials. so equilibrium between apatite and melt was attained at run conditions by dissolution of apatite to the point of saturation. The usefulness of this method lies in the fact that it gtvcs. by simple analysis of the quenched glass for phosphorus, a measure of the amount of P,O, dissolved in a magma that is required for apatite crystallization at given P-T conditions. The bulk compositions of the startmg materials (minus apatite) are summarized in Table I. All are in the simple system Na,0~KZO~Al,03 Si02 + CaO or CaF2. and half are identical to compositions used by WATSON (lY7Ya) in a study of zircon crystallization in felsic melts. The composItions listed are all peralkaline [(Na + K).AI ranges from I.18 to 2.01 because it was impossible to obtain run
2349
2350
Si02
78.55
75.77
79.23
77.90
73.27
72.90
72.35
72.76
72 ,m
*lz",
il.53
lO.iL
79.00 il.50
8.46
10.40
14.36
14.29
14.18
14.2b
I:.!1
U*O
3.13
6.15
'r Yh
6.17
;.e!
6.40
6.3:
b.31
6.36
6_,’
v
4.19
r.ii
‘1 55
5.44
8. ii
j.96
5.93
5.89
5.92
:.:,.,
0.50
1.25
--
CaO
._
-
_-
CaP2
(Sa+K)/Al
1.18
1.40
1.66
------..---.
2.00
I .40
products (glasses) with uniform P content for compositions in which (Na + K)/AI was < 1. The mixes containing CaO or CaF, were included in the study in order to evaluate any possible effect of Ca and/or F (both of which appear in the fluorapatite solubility product) on apatitc crustallization behavior. With the exception of bulk compositions K and KPK. which were pre-fused to glasses, the starting materials for the apatite solution experiments were finely-ground oxide mixes prepared as described by W~rso~ (1979a). Each charge consisted of _ 15 mg of powder (including IO”,, apatite). which was sealed in Pt or PtYSAu, tubing with 15 wto,(,HzO. The experiments were run for I4 24 days in cold-seal pressure vessels at 1 kbar and 750.9OO’C. as summarized in Table 2. The products of runs made at temperatures above 800-C consisted of glass t apatitc $- vapor: other phases (quartz and feldspar) appearing in lower ternperature runs are indicated in Table 2.
run no.
Summary of run information and analyzed were made at 1 kbar water pressure.
bulk
c~rnp.~
PK PK IS PK PK K K K KPK KPK KPK PK PK LN AGl PPK AGl AG4 AG6 AG9
23 22 13 25 28 30 33
T,"C
900 900 800 800 900 800 750 900 800 750
334 334 334 336 336 334 336 330 334 336 330
750 750 900
330 330 334
800
336 334 552 552 576 576
900
900
850 850 850 850 added
apatite
P205
P205 in glass Macguarie
timc,h
1.1.8 1.18
Cl.70
i
1.18
1.in
._
As mentioned above, the composition +:ri~ablc 01 primary interest in these experiments is the amount of d~ssolved P,O, in apatite-saturated melts (see also WSYSC~U. 1979b, 1980). This value was determined by electron microprobe analysis of the quenched glasses. using two diA‘ercnt microprobe systems. The P,05 contents of glashc* in 0 of the 20 successful experiments were determined a\ the M.A.C. facility of Massachusetts Institute of ‘Icchnolog~. and all 20 were analyzed with the ETEC Autoprobe at Macquarie University (Australia). Similar operating conditions of 1.5kV and -0.025 PA sample currem were used at the two facilities, but the analyzing crystals wierc ditTe1ent (RAP at MIT., PET at Macquarie). In order LOavoid damage to the HzO- and alkali-rich glasses during analysis, a broad (- 15 pm) or rastered beam spot was utilized Because of the low concentrations of P,O, in the glasses, careful measurements of background count rates were
The obviously important task of confirming the attainment of equilibrium was approached in the following way: Since most experiments were apatite-solution runs, one or more runs was needed in which the equilibrium between apatite and melt was reached by apatite crystallization. To this end, a homogeneous, P,Os-bearing glass was prepared by melting a 97:3 mixture of composition PK and fluora-
2.
-
patite at 1400 C and I atm (loss of alkalies and lluol-in< during the 3-day fusion was prevented by sealmg the com ponents in a large Pt capsule). This glass was subsequenti) used as starting material for hydrothermal run numbers .: 6. and 16, which are reversals of 1, 8, and 24 rcspcctivcl! (see Table 2). The two approaches to equilibrium g~~c results that are in agreement within the errllrb ~‘1I’S~C~Imentation and analysis.
Rrrvrsu/.\
Table
1 .1x
--
-..
0.13 f 0.04 0.08 i 0.02 0.18
f
0.10
0.11 9.17 0.20 0.10 0.13 0.14 0.09 0.09 0.16 0.17 0.12 0.06 0.14 0.24 0.28 0.12 0.04
i 0.05 f 0.07 * 0.02 i 0.02 f. 0.03 * 0.01 i 0.01 i 0.03 f 0.05 f 0.03 f 0.05 zt 0.02 * 0.07 f 0.09 i 0.13 i. 0.08 F 0.02
contents
of glasses.
f 2 std error M.I.T.
0.09 f 0.01 * 0.05 0.14 f 0.04 0.20 * 0.04 0.17 h 0.01 0.10 i 0.01 0.13 l 0.02 0.10 * 0.03
All
rillls
additional
0.11
quartz
quartz quartz quartz 0.08 * 0.03
Eeldsp;l
a
excluding
b
phases
r
"reversal"' i.e., run in which apatite components were initially dissolved in the glass isee text). Run immediately below is the analagous apatite-solution experiment.
in addition
(see text)
to apatite,
melt,
and vapor
: j!hdses
‘3.51
P and REE in felsic magmas made on either side of the PK, peak. The background under the peak. obtained by interpolation between the offset values. was subtracted from the peak intensity, and the difl’crcncc was converted to P,Os concentration by comparison with an apatite standard and application of appropriate matrix correctlons. The P,O, concentrations reported for each run represent the mean of 8 15 analysis spot\. K?\irlr\ The analyzed PzOs contents of 20 melts (glasses) cocy1stin:! with apatlte arc listed in Table 7. The indlcatcd errors I & 2 standard errors) gibe a mcasurc of the variance among tht’ X I5 analysis points. and thus incorporate both X-ray counting error and sample heterogeneity. The uniformit) of I’ distribution in the glasses is apparently good for the h~ghl! pcralkalme compositions, but deteriorates progressivcly as (Na + K)‘AI decreases*. There is. however. no disdlsccrniblc dependence of apatlte solubility on melt alkalin1tl. 111marked contrast to the results obtained by WAISOU (lY7Ya) on flrcon snturatlon. Furthermore. despite the strong co{-relation between melt SiO, content and apatlte solubility that exists in basic and intermediate magmas (W.U?ON. 197Yb. 1980). there is no such interrelationship for the highly SIIICIC melts presently under consideration. Runs on bulk compositions containing 7X-7!, wt”,, SiOz yield a mean apatitr saturation value of 0.14 + 0.10 wt”,, d~~~olvcd P20,. which is indistinguishable from the 0. I3 - 0.03 wt”.. for runs with 72 73”.. SiO,. rThis result is not \urprl\ing 1;; \icw of the tendenc;‘of thk siOz vs P,O, apatlte saturatmn curves of WATSON (1979) to flatten out at
SiOL concentrations above -6Owt”,.] It seems reasonable to expect some positive dependence of apatite solubility upon temperature. especially among a group of runs in which the melt composition is constant o\cr changes in temperature. The temperature effect is apparently small. though. and perhaps obscured in most casc~ h> analytical uncertainty and sample heterogeneity. There 1s nevertheless some suggestion of a temperature dependence in the two series of runs for which the analyses arc most precise: 1.e. runs 19. I?. I4 and 17. 9. 5 (at 900. 800 and 750 C. respectively). The general lack of a strong tempclaturc effect is. again. consistent with the convergence of apntite saturation lsothcrms that occurs with increasing SiO, content of the melt (see WATSON. lY79b). 13ecausc of their comparatively low (Na + K),Al. the glasses with added CaO show generally poor homogeneity. Thus. although two of these experiments (25 and 28) yicldcd apparent high apatite solubilities relative to the Ca-free compositions. the tingurshahle from the more unccrtalnties are considered.
tanned
0 5”,, CaO.
numbers are in fact indistypical values when the large
Moreover.
run 22. which con-
resulted
in an apatite solubility loser than ,111other runs at 800 C. so it would be inappropriate to conclude that the presence of CaO increases the solubility of apatite in the melts examined. (as CaF,) to the melt The addition of 0.34 wt”,, fluorine has no discermble inlluence on apatite saturation behaviour (see Irun No 30). although it is possible that geologically unreallstlc F contents (e.g. 0.85 wt:‘,. as in run No. 33) do dccrcasc the solubility of apatitc. On the whole. apatlte solubihty IS relatively msensitive to temperature and composition effects. and no systematic dcpcndencles can be verified for the range of melts and condltlons studied. Thus. because all but 4 of the 17 measured apatite saturation levels fall within 0.06 of the mean of O.l4wt”,, dissolved P,Os, we will use this mean value for the illustrative melting and crystallization models discu\bed in the sections that follow.
*This effect can probably viscosity and:or increased (Na + K).AI.
be attributed P diffusivity
to decreased at higher
APATITE
IN THE SOURCE
REGIONS
OF FELSIC
MAGMAS
In this section. we use our results to show. first. how the apatite content of the source region and the P concentration in the liquid must evolve during cpisodes of partial crustal fusion to produce felsic melts. Constrained by this general model. we then consider the cffccts of residual apatitc on the REE characteristics of felsic magmas produced by partial melting of three reasonable source assemblages.
The first premise of all the models is that apatite saturation in fclsic magmas occurs at a P,O, concentration of 0.14 wt”,,. This is the mean of all the experimental determinations and is therefore a realistic value, but the reader should bear in mind the possible second-order effects of temperature and the composition variables discussed earlier. It IS probably reasonable to assume that any given crustal melting episode is more or less isothermal [See, for example. the discussion of anatexis by WIUKLEK (1967)]. so the P,O, saturation level. although not necessarily exactly 0.14 wt”,. should be close to this value and not subject to significant changes due to variation in temperature. [Interestingly, 0.14 wt’;, is a typical P,05 concentration for both I- and S-type granitoids containing more than 70% Si02 (e.g. WHITE et ~1.. 1977)]. It should also be noted at the outset that the term ‘felsic’ magma refers here to any composition with 7&80’/~~ SiOZ. Although WATSON (1979b. 1980) noted a strong dependence of apatite solubility upon at lower silica levels melt SiOz concentration (35560 wt”,;,,),there is, as noted previously, no discernible dependence at the high Si02 concentrations considered here. Thus. for the purpose of the illustrative models presented below, we feel that a constant P205 saturation value of 0.147<;,,is appropriate. This value should be relatively unaffected by changes in pressure (cf WATSON. 1980). An additional assumption implicit in the following models is that all phosphorus m the source region or residue is present as apatitc. The existence of other phosphate minerals (c.g. monazite) is certainly possible, but the ubiquitous occurrence of apatite in all rock types attests to the fact that it is by far the most common host for phosphorus. Phosphorus substitution in silicate minerals is considered negligible (cf. HENDERSON, 1968), although we recognize that garnet can accommodate fairly large amounts (up to 0.6 wt”,, P,O,) at upper mantle prcssures (THOMPSON, 1975). Because apatite is present in lower-crustal eclogites that contain as little as 0.04 wt”,, P,O, (GRIFFIN et ul.. 1979). however. we infer that very little P enters the garnet under these circumstances. The accuracy of our models is unavoidably affected by the texture and state of aggregation of the source and residue for two reasons. If, for example. the tex-
2352
E. BRKE
WATSON and C. J. CAPOBIANCO wt. % P2 05
wt .% P205 0.14 II
042
’
I26
084
i66
II
001
1 ’
003
g ’
005
sources containing
007
k
009
1 ”
Oil
1 h /
‘1
0.13
‘1
\
sourc8s conkrining greater than
I
0.14%
P, 0,
3
4
2
wt % apatite in
0
residue
Oi
02
03
L
wt.% apatW
Fig. 1. (a] Plot of P,O, (or apatite) in the residue vs ‘(,i melting for sources containing 0.18, 0.24 and 0.30 wt% P,O,. An apatite saturation value of 0.14”,, P20, in the melt is used. (b) Diagram showing evolution of P,Os in the melt and residue during melting of sources containing 0.05 and 0.10 WI”,, P20. (see text for further explanation).
ture of the residue is such that apatite crystals are from the melt by inclusion in major mineral phases (See, for example, WHITEand CWAPPELL. p. 17). then the system cannot behave as predicted by the experiments. Also . if residual apatite crystals become entrained in the departing melt fraction, the bulk characteristics of the resulting magma will not be those of liquid. These considerations are discussed whenever they are particularly important to our conclusions. A final note concerning our modeliing approach is that, when evaluating the effects of residual apatite on the REE contents of felsic melts. we choose to ignore other accessory phases such as sphene, zircon, allanite, and monazite. In view of the experimental results of HELLMANand GREEN(1979) and WATSON(1979a) on sphene and zircon, respectively, and the analytical/ petrographic study of MILLER and MITTL~EH~D~ (1981) on alfanite and monazite, this simpl~ying step undoubtedly detracts from the authenticity of the models. However, because our main aim is to evaluate separately the role of apatite. the simplification seems warranted. isolated
Given an apatite saturation value of O.l4wt% dissolved P,O, in felsic melts, two facts are immediately clear: (1) no felsic magma produced by crustal fusion can contain more than 0.14% P,O, (u&ss residual apatite crystals become entrained in the melt); and (2) no partial melt that leaves apatite in the residue can contain less than 0.14% P205. If the melt does contain <0.14”/, PZOs, any apatite present in the source rock was necessarily consumed during melting. The usefulness of these general observations can be improved by considering in detail how the P contents of residue and melt evolve as the degree of melting increases. Three general types of behavior are possible, depending on whether the b&k P,Os concen-
tration in the source is greater than, equal to, or less than the melt saturation value of 0.14 wt’:,,. If the source contains >0.14wt% P,05, the melts will be apatite-saturated (at 0.147; dissolved P,O,) f’or all degrees of melting, and the P,O, (apatite) content of the residue will increase as melting progresses. This case is illustrated in Figure La, where the abundance of apatite in the residue is plotted against “Cjmelting for sources containing 0.18, 0.24, and 0.30 wt”,, P,O,. The increase in proportion of residual apatitc is relatively small for degrees of melting below _ 50”,,. but increases rapidly for advanced degrees of melting. For sources containing ~0.14 wt% P,O,, there will be some initial melting intervat over which the melt remains apatite-saturated. but the progressive decrease in residual apatite over this interval results at some point in its complete consumption. The stage at which apatite is Iost from the residue depends, of course, upon how much P,Os (apatite) is ~~~it~aI~y present. As shown in Fig. lb, apatite is consumed at only 3.5”/0melting if the source P,O, abundance is 0.05 wt%, but an initial source concentration of O.lO”,, postpones complete loss of apatite until melting exceeds -7O”/,. In either case, once apatite is consumed, the P,05 content of the melt is no longer ‘buffered” at 0.147& and drops smoothly to the initial bulk source concentration when meltmg is complete. The simplest PzQS evolution is exhibited by systems in which the source contains exactly O.l4’,?,,, P,O,. In this case, the P,O, contents of melt and residue are both fixed at 0.14% for all degrees meIt~ng, The relevance of the preceding discussion to actual crustal melting episodes can be evaluated in the context of the following observations. First, it should be emphasized that the major-phase mineralogy of the source region is immaterial-any system that undergoes partial melting to produce felsic liquids is subject to the type of behavior illustrated in Fig. 1. because only the melt/apatite equilibria are important to
P and REE in felsic magmas
2353
P,O, evolution. It is also noteworthy that the range of source P,O, concentrations shown in Fig. 1 is within the range of values for potential felsic-melt source rocks. Australian post-Archean sediments (shales and graywackes), for example, have P,Os values ranging between 0.05 and 0.30wt% (NANCE and TAYLOR, 1976); rocks of basaltic composition average 0.25-0.32 wt% (TAYLOR, 1964; TURMAK and W~I~EPOHL, 1961): and the average andesite has 0.20-0.22 wt”/,, (CHAYES, 1969; JAKES and WHITE. 1971) which is close to the crustal average of 0.24 wt% (TAYLOR, 1964). The primary limitation of Fig. 1 is that it cannot account for residual apatite that becomes entrained in a rising melt fraction. Because entrainment would result in apparent PZO, concentrations in the melt higher than the saturation value. we can only hope that the process is restricted to relatively high degrees of melting. Anomalously low concentrations of P,05 in the melt (i.e. values below the saturation limit), on the other hand. would seem possible only if (1) apatite in the residue is included in major phases and thus isolated from the melt (e.g. WHITE and CHAPPELL, 1977) or (2) a P,O,-enriched vapor phase is exolved at some later stage in the evolution of the magma. Even subsequent fractionai crystallization of apatite cannot deplete the melt in phosphorus to a level below the saturation value, so if the complicating factors just noted are not operative, a felsic rock with content (e.g.
(Fig. I), its contribution to felsic melt and residue REE patterns can be quantitatively evaluated for any reasonable source composition and residue mineralogy. This exercise is carried out below for three general source types (graywacke, shale, and tholeiitic basalt) whose major mineral residual assemblages are effective to various degrees in retaining the REE during melting. An apatite saturation level of 0,14wt”,, dissolved P,O, is again assumed, although the actual value may be slightly higher or lower depending on the temperature of melting. ‘Graywacke’ source. As shown experimentally by WINKLER (1967) and confirmed in trace element models by ARTH and HANSON (19753, partial fusion of graywacke source material will produce felsic melts. The specific aim of Arth and Hanson was to duplicate measured trace element abundances of Precambrian quartz monzonites, which they did using a nonmodal. batch-equilibrium melting model that involved only the major residual phases of a highgrade gneiss assemblage of graywacke comp~)si~ion. In the present case, our purpose is to evaluate the influence of residual apatite on REF. behavior during batch melting of similar high-grade gneisses. The major element composition of these gneisses is specified only in the sense that %300/;: melting leaves a residue of 40’6 quartz. 307; plagioclase, 200;, hornblende, 5”,, biotite, 39; K-feldspar, 2”,,, garnet, and accessory apatite. Two different source P,O, contents (0.10 and 0.24 wtyb) are considered, which result in different amounts of residual apatite at a given stage of melting (see Fig. I). The crystal/liquid partition coefficients for Ce, Sm. Dy. and Yb. taken from the review of HANSON(19781, are summarized in Table 3. Aputite and the rure eurth elements The source REE characteristics resemble the graywacke of ARTY and HANSON (1975). Because of the selective incorporation of rare earth The chondrite-normalized REE pattern for the elements in apatite (NAGASAWA, 1970; NAGASAWA source graywacke is shown in Fig. 2 along with the and SCHNETZLIIR.1971), this mineral can play a sigcalculated patterns for liquids and residues of 5 and nificant role in determining the REE characteristics of some crust-derived magmas. The importance of apa300/(, batch melting. The residue characteristics are tite relative to the major residual phases is determined similar to those of the source, but slightly higher in the by two simple factors: namely, the concentration of middle and heavy REE due to the retention of these P,05 (and hence the amount of apatite) in the source, elements in amphibole and garnet. The effect of apatite and the abundance of major minerals in which the is noticeable only in the light REE. which are deREE are enriched relative to coexisting liquid. With pleted in the residue to a greater degree (and exhibit knowledge of the behavior of apatite during melting lower CeiSm) for the ‘low-P,O,’ source (see Fig. 2). Table
3.
Summary of mineral/melt partition coefficients used in REE models mineral/graniticmelt valuesa
mineralidacite values I
K-spar plag
hb
0.045 0.27 1.5 Sill 0.020 0.14 7.5 DY 0.006 0.065 13.0 Yb 0.006 0.05 8.0 Ce
OPX
CPX
gt
apat biot qtz
0.15 0.5 0.1 35 0.28 1.3 2.5 60 0.44 1.9 27.0 50 0.86 1.5 38.0 21
0.31 0 0.26 0 0.28 0 0.41 0
plagb hbbrc 0.20 0.09 0.05 0.04
0.9 4 6 5
cpxb
apacd
0.36 1.5 2.6 2.0
30 46 40 21
a from Hanson (1978), except in the case of quartz, which is assumed to incorporate no REE b typical values from Nagasawa and Schnetzler (1971) = from Arth and Barker (1976) d typical values from Nagasawa (1970)
t.
2354 loo‘,
,
,
,
,
,
,
,
,
HKI
(
WA~SOK and c‘. J.
(‘F
,
,
,
Liqutds produced by 5-30% Melting .im 0 57%
3
-
ap0we I” source
IO 24%PzOg) =o
24%
apat,te
_
I” swrce
source
Ce
I
Sm
I
I
I
I
Yb
DY
!
I
I
I
11
I
I
Residues of 540% Meltina
~APOHIAN(‘O
ary effects of apatite should nontheless be considered in matching model liquid REE abundances with those in igneous rocks. ‘S/z& ~SOWW.The intimate field associatton oI granutite-facies felsic rocks and apparent parttat melt products (e.g. PKIW and TAYLOK. 1977: Mc (‘.w~II~ and KABLE. 1978) justifies this second melting modei. which considers REE distribution hetwecn f&c melts and residues of charnockitic mineralogy. The as~mptions in this example are the same as for the preceding ‘graywacke’ source situation, and the crybtill Irqulti partition coefficients are also taken from t IS\~soh (1978). The differences here are that: (1) the source is assigned a REE pattern like that of the North American shale composite (NASC HGKIX 1’~
Residue 40% qtz 30% PIW 20%hb5 % bt 3 % ksp 2 % gt I
I
I
/
1000~
Rare
I
I
I
I
,
I
,
1
/
-r--r-
l---l
Liquids produced by 5 -30% Melting
/
8
I
I
Sm
Ce
I
earth
$1 DY
I
I
If
I Yb
elements
Fig. 2. REE characteristics of melts and residues produced by S_3Op,, melting of graywacke composition sources. The major phase residual assemblage shown pertains to all degrees of melting. The influence of apatite, which must melt as illustrated in Fig. 1, can be seen by comparing the REE patterns that result from melting of sources with 0.10 and 0.24 wt”,, PzOs (see text).
scurce
Ce
The melts are heavy REE-depleted, and, again, the influence of apatite is most apparent for the light REE: S’,, melting of the source containing 0.247,, P,OS yields a liquid with a Ce concentration of 55 x chondrites. whereas the same degree of melting of the source with 0.10’4 PzO, produces a liquid enriched in Ce to 69 x chondrites. On the whole. the influence of apatite on REE behavior during melting is relatively minor for this particular residual assemblage, in which amphibole and garnet are the ‘REE-dominating’ phases. The second* In view of the aluminous nature of the assumed shale source, it might be reasonable to include an Al-rich phase such as cordierite or AI,SiO, in the residue. These minerals would. however. have the same ‘diluting’ effect on the bulk REE content of the residue as does quartz, so replacement of some residual quartz by one of these aluminous phases has no etlect on the REE model. Garnet has intentionally been omitted from the residue so as to illustrate the case in which apatite plays the largest possible role in REE distribution.
‘00
1
I
Sm
I
1 /
I
YE
DY
I
,
, i-‘----
1
Residue -
sO”rCe
40%
plaq
30 % ksD
Residues of 5 - 30% Melting
Rare earth
elements
Fig. 3. Diagram analogous to Fig. 2 for S 30”,, melting LV shale composition sources. The effect of apatitc on the REE is more obvious in this case hccausc the rcGdup lack\ amphibole and/or garnet i~cc‘ text I.
P and
REE in felsic magmas
100
80
60
;z
5 5
40
s 20
60
70
80
%
Fig. 4.
Diagram
showing
90
IOC
REE in residue
that IS II? apatite
the fraction of each REE in the residue that resides in apatlte the degree of melting of the ‘shale’ sources (see Fig. 31.
contains no major mineral phase in which the REE are compatible. the accessory apatite exerts significant control over their abundances in both melt and residue. In the ‘high-P,O,’ case (0.24”,:,), the REE patterns of the residues of 5 and 30”/ melting are quite similar. and neither is displaced far below the source curve. The patterns of the liquids are light REE-enriched and distinctly concave up, the latter reature reflecting the preference of residual apatite for the middle REE. For the ‘low-P,O,’ source (O.lO”i,), the situation is quite different in that (1) the residue of 30”,, melting falls well below the source REE abundances, and (2) the liquids not only show considerably higher concentrations of all REE. but also lack the enrichment of Yb over Dy noted in liquids from the ‘high-P,O,’ source. The differences in REE behavior for the ‘high-” and ‘low-P,Os’ cases might be better illustrated in Fig. 4. which shows how the percentages of REE contained in residual apatite change as melting procoeds. The obvious feature of this diagram is that the proportion of REE in apatite inc~rrtrses with increased melting for the source containing 0.24 wt”;, PzO,. but tlrc~tr.sc~s as melting progresses in the ‘low-f’20,’ system. [It is appropriate to re-emphasize at this time that both source P,O, concentrations are rcaliatic values for the rock type being melted. so either- behavior could apply to natural melting situations. 1 .4 linal observation is that the effects of residual apatite on the REE must be taken into account before a proper estimate can be made of the degree of mcltmg that produced a given felsic rock. This is so because. except in the HREE. variations in the amount of residual apatite and changes in the degree of melting have similar effects on the REE patterns of liquids derived from a charnockitic source (see Fig. 3). ‘Q~r~t: tholeiitc,’ .SOII~YY. The last melting situation we nil1 examine is one in which felsic magmas are produced by partial fusion of the high-pressure assemblage appropriate to a quartz tholeiite compo-
nockite
as a function
01
sition (i.e. quartz eclogite). The plausibility of this particular melt-residue relationship has been demonstrated both by experiment (e.g. GREEN and RINGWMID, 1972) and by trace element studies (ARTH and HANSOU, 1975). As in the previous models. we consider two source P,O, levels (0.24 and 0.10 wtu,,). and we allow melting to proceed from 5 to 30”,,. Over this melting interval. the proportion of quartz in the residue is reduced from 15 to 57;, residual garnet is inis held creased from 30 to 40”/,,. and clinopyroxene constant at 55”,. The REE characteristics of the eclogite source resemble the early Precambrian tholeiites analyzed by ARTH and HANSON (1975. Fig. 5). The resulting REE patterns for melts and residues are summarized in Fig. 5. All patterns bear the expected imprint of residual garnet: the residues are enriched relative to the source in middle and heavy REE to an extent proportional to the degree of melting, and the liquids are severely HREE-depleted. Only in the LREE does the “<, melting have a significant effect on the melt characteristics. and it is here. too, that the influence of apatite is most noticeable. The ‘low-P,O,’ residue is considerably more LREEdepleted than its ‘high-P,O,’ counterpart. and the melts show concomitant changes in their degree of LREE enrichment. Overall. the contribution made by apatite to the REE patterns is certainly secondary to that of garnet. but nevertheless discernible in the degree of LREE enrichment or depletion. as well as in the Ce:Sm ratio. FRACTIONAL CRYSTALLIZATION OF APATITE Partial melting of crustal materials is not the only circumstance under which apatite can play a role in the geochemistry of granitic magmas. It is. in fact, inevitable that a felsic melt formed by fractional crystallization of an intermediate parent will have experienced saturation in apatite. because apatite solubility
F.
2356
BKL’CI WATSON
and C. J.
CAPOHIANCO
degree of andesite fractional crystallization to produce the rhyolite at. say, 80%. than a P,O, balance Liquids produced by allows direct computation of the amount of apatite 5 - 30 % Melting removed. An initial mass of 1OOg of andesite would contain 0.4g of P,O,, and, were apatite saturation not encountered, 80% fractional crystallization of major phases would leave a rhyolitic melt still COIN,taining 0.4g (or 2 wt%) of P,O,. Because saturation in apatite does occur, the rhyolite contains only 0.028 g of P,05, so the total PzO, removed as apatite amounts to 0.372 g (=0.89 g of apatite). Of the initial IOOg of andesite, then, 0.89 g (or 0.897:) was subp l Tholelite source tracted as apatite during differentiation. With this information, we can now proceed to evaluate the contribution of apatite to the REE pattern of the rhyolite. I “l”““c”‘Yb Sm Ce Dy We assume in this model complete equilibrium / I t I I I r 11 11 1 between crystals and melt with respect to Ce. Sm. Dy, Residues of and Yb. In order to average the effect of apatite and 5 -30% Melting the major phases over the differentiation series Residue :i:.:.:::: i$.$$ 0 57% aponte I” source andesite -+ rhyolite, we use REE partition coemcients 55 % cpx IO 24% P2051 30-400~ gt appropriate for minerals coexisting with dacitic melts .g 100 5_, 5% q+z z 0 24% opatite in source: ;; (0 10% P,qr (see Table 3). The REE pattern of the parent liquid is 2 that of TAYLOR(1969) for average circumpacific ande5 site, and is modified by removal of 550,; plagioclase, \ 20%; hornblende, and 5% augite. The resulting REE characteristics of two derivative rhyolites are shown in Fig. 6 in relation to the parent andesite. In one of these rhyolite curves, we have completely ignored the lol~“““;f”l,J Yb contribution of apatite, while in the other the necessSKI Cl? DY ary 0.S9y0 is represented. The obvious dill’erence Rare eorth elements between the two is that the ‘zero apatite’ curve shows Fig. 5. REE patterns resulting from 5 30”,, melting of considerably greater (and relatively uniform) enrichquartz eclogite sources initially containing 0.10 and ment in the REE. This is the same sort of upward 0.24 wt% P,O,. The range of residue modes is shown in the diagram; the low-quartz residue corresponds to 30”/:, displacement that would result from continued remelting. Note the effect of varying amounts of residual moval of the three major minerals. apatite on the light REE in mclrs and residues. Unlike the melting models discussed in the preceding section, the foregoing calculation of apatite fracin melts decreases so dramatically with failing temperature and increasing Si02 content (WATSON. 1979b, 1980; GREEN and WATSON, 1981). As noted previously, trace element geochemists have already recognized the likely involvement of apatite in the REE evolution of felsic magmas (e.g. SIMMONSand HEDGE, 1978), but no attempt has been made to independently evaluate the contribution of apatite relative to that of the major mineral phases. This calculation is actually possible, however, and requires only the following input: (1) the concentration of P,05 in the PMes removed : parent magma and in the felsic derivative; and (2) the 55% plop total fraction of crystals removed from the parental 20% hb liquid to produce the felsic magma under consider5% aug 0 or 0 89% opcwte ation. Suppose, for example, that the probable parental magma is broadly andesitic in composition and I I4 I I I I4 14 1 II 4 I contains 0.4 wt% dissolved PzO,*. while a related Yb ’ ce Sm DY rhyolite contains its saturation value of 0.14wt”,;, Rare earth elements P,05. If the major-element chemistry constrains the 100
,
,
,
,
,
,
I
/
I
I
I
I
,
I
&
1
i
(1979b. 1980) for apatite solubilities in magmas at various temperatures and press-
*See WATSON
intermediate ures.
Fig. 6. REE patterns of evolved rhyolitic melts produced by 80% fractional crystallization of the andesitic parent shown. The upper rhyolite curve results from removal of the major phases only, while the lower curve incorporates the required amount of apatite (see text for assumptions).
P and REE in felsic magmas
tionation effects is not validated principally by the experimental results presented in this paper. Low apatite solubihty in felsic melts is obvious for other reasons (e.g. apatite inclusions in the mafic minerals of granites; apatite phenocrysts in felsic volcanic rocks; severe depletion of phosphorus during differentiation at high silica levels), so there is nothing particularly novel about our conclusion that apatite can influence REE abundances in granites. The point we wish to emphasize is that apatite cannot, under most circumstances, be simply ignored in REE models of felsic magma formation. nor can it be called upon to crystallize in any amount that is convenient to fit a model to observed REE abundances. The systematics of apatite saturation have been established by experiment. so it is possible to at least qualitatively evaluate the role of apatitc in the REE chemistry of any differentiation series. SU.YlMARY The major conclusions of our experiments and subsequent models of P and REE behavior can be summarized as follows: 1. In the temperature range 75~900°C. felsic melts reach saturation in apatite at a level of -O.l4wt”,, dissolved P,O,. (This mean value represents a range in saturation levels that shows a positive correlation with temperature and perhaps CaO content, and a possible negative correlation with fluorine concentration). 2. The apatite (phosphorus) content of the source region and the extent of melting dictate whether apatite will be residual in crustal fusion episodes. Residual apatite is unavoidable if the source P,O, content exceeds the melt saturation value noted above. and increases in proportion as melting proceeds, Apatite may or may not be consumed in source regions whose Pz05 concentrations are less than that required to saturate a melt in apatite (depending on the degree of melting), but residual apatite necessarily decreases in abundance with increased melting. 3. The contribution of residual apatite to the REE patterns of crust-derived felsic rocks may be substantial if the source contains more than the saturation level of P,Os, and especially if garnet and/or amphibole is minor or lacking in the residue. 4. Saturation in apatite is inevitable during fractional crystallization of an intermediate parent to form a felsic magma. and may significantly alter the REE characteristics of the melt. Ac,hi~ol~/ed~c,nl~f~f,s -The authors
benefitted from continued discussion with J. F. BI:NDFR. R. H. FLOOD, T. H. GREF~, and C. F. MILLER. The critical comments of these individuals and of the olhcial reviewers greatly improved the initial manuscript. The electron microprobe analysis at Macquarie University was made possible through a Visiting Research Fellowship awarded to E.B.W. The experimental worh at R.P.I. was supported in part by the National Science Foundation Division of Earth Sciences. under N.S.F. Grant EAR7X-12980.
2357
REFERElVCES ARTH J. G. and HANSON G. N. (1975) Geochemistry and origin of the early Precambrian crust of northeastern Minnesota. Gwhim. Cosmochin~. Actu 39, 325-362. ARCH J. G. and BARKER F. (1976) Rare earth partitioning between hornblende and dacitic liquid and implications for the genesis of trondhjemitic tonalitic magmas. Gcolo,4, 534 536. CHAY~S F. (1969) Chemical composition of Cenozoic andcsite. In Proc. Andcsitc, Conf (ed. A. R. McBirney). I93 pp. State of Oregon Dept of Geology and Mineral Industries. GREEV T. H. (1981) Experimental evidence for the role of accessory phases in magma genesis. J. I/&.. Gc~othrrm. Rec. (in press). GR~F\ T. H. and RINC;U.OODA. E. (1972) Crystallization 01 garnet-bearing rhyodacitc under high-pressure hydrous conditions. J. G&. Sot,. Atr\t. 19, 203 21 2. GRF~~ T. H. and WATSO\ E. B. (1981) Experimental study of apatite saturation in natural hydrous magmas. with particular reference to erogenic magma series. Conr~ih. Mirwrcll. Pv~roi. (submitted). GRIFFIN W. L.. C,ZRSU.EI.~ D. A. and NIXON I’. H. (1979) Louver-crustal granulites and eclogites from Lesotho. South Africa. In The .l~unrlr .‘jun~p/~,. Inc /asion\ r,r Kin+ hcrlitea trrid OI/KY l.o/c,~rnic\ (eds F. R. Boyd and H. 0. A. Meyer). pp. 59 X6. Am. Geophys. Union. HAKSOY G. N. (1978) The application of trace elements to the petrogcnesis of igneous rocks of granitic composition. Enrtli Pl
2358
E. BRUC‘EWATSONand C. J. CAPOHIANCO
TAYLORS. R. (1964) Abundance of chemical elements in the continental crust: a new table. Geochim. Cosmochim. Acta 28, 1273-1285. TAYLOR S. R. (1969) Trace element chemistry of andesites and associated talc-alkaline rocks. In Proc. Andrsitc Conf (ed. A. R. McBirney), 193 pp. State of Oregon Dept of Geology and Mineral Industries. THOMPSON R. N. (1975) Is upper mantle phosphorus contined in sodic garnet? Earth P/met. Sci. I&r, 26. 417-424. TUREKIANK. K. and WEDEPOHLK. H. (1961) Distribution of the elements in some major units of the earth’s crust. Grol. Sot. Am. Bull. 72, 175 192. WATSON E. B. (1979a) Zircon saturation in felsic liquids: experimental results and applications to trace element geochemistry. Contrih. Mineral. Petrol. 70, 407-419.
WATSONE. B. (1979b) Apatite saturation in basic inter-mediate magmas. Geophys. Rrs. Letr. 6, 937 940. WA’~~ONE. B. (1980) Apatite and phosphorus in mantle source regions: an experimental study of apatitc/melt equilibria at pressures to 25 kbar. Etrrtlt Plmw~. St,;. 1,trrl 51, 322~335. Wlrrre A. J. R. and CHAPPELLB. W. (1977) lJltrametamorphism and granitoid genesis. Trcfonoph_wic~s 43, 7 12. Wtn-r~:A. J. R.. WILLIAMSI. S. and CHAPPELLB. W. 119771 Geology of the Berridale I : lOO,ooOsheet. Gcoloyit LIISUP rep!’of Nr,v South Walrs. Drpclrlm~~r of Mine\. Puhl. No 8625. 138 pp. WINKLEK H. (i. F. (1967) Prfrcycvwsi~ c!f M~‘tir,f~i,rp/~r~, Rocks. 2nd edn, 237 pp. Springer-Verlag. ZIELINSKIR. A. and FREY F. A. (1970) Gough Island. evaluation of a fractional crystallization model. (‘ottrrill Mincrol. Petrol. 29, 242.~254.
Rensselaer
Polytechnic
Institute.
Troy. NY 12181. U.S.A.
Abstract-The solubllity of fluorapatite in I7 silica-rich melts in the system Na,O KzO AI,O, ~SIO, (with and without CaO or CaF,) was determined at I kbar water pressure and 750 900 C. Apatite saturation occurs at levels of dissolved P,05 ranging between 0.04 (kO.02) and 0.28 ( f 0.13)wt”,,. with only 4 values outside the 0.09-0.20 wt’,, range. The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that. under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P,05 (the mean of the experimental values) a source containing as little as 0.05% P,05 must be 35% melted before apatite is lost from the residue and no longer buffers the melt P,05 concentration at the saturation value. Higher abundances of P,O, in the source postpone the loss of residual apatite to still higher degrees of melting. and if the source PzOs content exceeds 0.14 wt?<::;, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds. The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. FractIonal crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.
INTRODUCTION OVER the past 15 years, major advances in our understanding of magma genesis have been achieved through quantitative modelling of trace element behavior during melting and crystallization processes. Further refinements in both modelling approaches and inferences are likely to develop in the near future, but progress is limited, particularly where felsic magmas are concerned, by lack of data in one fundamental area: that is. the stabilities of minor and accessory minerals that are effective concentrators of the tract elements commonly used in modelling. This paper represents part of a continuing program of experimental research on early-crystallizing accessory phases. It is preceded by studies of zircon solubility in felsic magmas (WATSON, 1979a), apatite saturation in basic and intermediate magmas at nearatmospheric pressure (WATSON, 1979b), and apatite stability in basic magmas at upper mantle temperatures and pressures (WATSON, 1980). Other important contributions on the topic of accessory phase crystallization are those of HELLMAN and GREEN (1979) on sphene in hydrous mafic compositions, KNUTSON and GREEN (1975) on an apatite-bearing megacryst assemblage, and the summary/review paper by GREEN (1981) on magmatic accessory minerals in general. The present paper centers specifically on the stability of apatite in felsic magmas at 1 kbar water pressure and temperatures of 75@9OO”C. The objective of
* Present address: Department of Geology. State University. Tempe. AZ 85281. U.S.A.
Arizona
the experiments was not simply to re-confirm what geochemists have already shown [i.e. that apatite can play a role in the trace element evolution of felsic magmas (e.g. ZIELINSKI and FREY. 1970; PRICE and TAYLOR, 1977; SIMMONSand HEDC~C.1978)], but to provide a basis for predicting the conditions of temperature and composition over which apatite will coexist with felsic melts. The experimental data are subsequently used to show. in a series of melting and crystallization models, how apatite can be expected to influence the behavior of phosphorus and the rare earth elements (REE) during felsic magma production and evolution. EXPERI.ZlENTS
We determined the stability of apatite in a variety of felslc melts by use of the same basic technique as employed by WATSON (1979b. 1980); that is. imposed saturation in apatite. In the present case. ground fluorapatite (55lO~m) comprised IO wt”,, of the starting materials. so equilibrium between apatite and melt was attained at run conditions by dissolution of apatite to the point of saturation. The usefulness of this method lies in the fact that it gtvcs. by simple analysis of the quenched glass for phosphorus, a measure of the amount of P,O, dissolved in a magma that is required for apatite crystallization at given P-T conditions. The bulk compositions of the startmg materials (minus apatite) are summarized in Table I. All are in the simple system Na,0~KZO~Al,03 Si02 + CaO or CaF2. and half are identical to compositions used by WATSON (lY7Ya) in a study of zircon crystallization in felsic melts. The composItions listed are all peralkaline [(Na + K).AI ranges from I.18 to 2.01 because it was impossible to obtain run
2349
2350
Si02
78.55
75.77
79.23
77.90
73.27
72.90
72.35
72.76
72 ,m
*lz",
il.53
lO.iL
79.00 il.50
8.46
10.40
14.36
14.29
14.18
14.2b
I:.!1
U*O
3.13
6.15
'r Yh
6.17
;.e!
6.40
6.3:
b.31
6.36
6_,’
v
4.19
r.ii
‘1 55
5.44
8. ii
j.96
5.93
5.89
5.92
:.:,.,
0.50
1.25
--
CaO
._
-
_-
CaP2
(Sa+K)/Al
1.18
1.40
1.66
------..---.
2.00
I .40
products (glasses) with uniform P content for compositions in which (Na + K)/AI was < 1. The mixes containing CaO or CaF, were included in the study in order to evaluate any possible effect of Ca and/or F (both of which appear in the fluorapatite solubility product) on apatitc crustallization behavior. With the exception of bulk compositions K and KPK. which were pre-fused to glasses, the starting materials for the apatite solution experiments were finely-ground oxide mixes prepared as described by W~rso~ (1979a). Each charge consisted of _ 15 mg of powder (including IO”,, apatite). which was sealed in Pt or PtYSAu, tubing with 15 wto,(,HzO. The experiments were run for I4 24 days in cold-seal pressure vessels at 1 kbar and 750.9OO’C. as summarized in Table 2. The products of runs made at temperatures above 800-C consisted of glass t apatitc $- vapor: other phases (quartz and feldspar) appearing in lower ternperature runs are indicated in Table 2.
run no.
Summary of run information and analyzed were made at 1 kbar water pressure.
bulk
c~rnp.~
PK PK IS PK PK K K K KPK KPK KPK PK PK LN AGl PPK AGl AG4 AG6 AG9
23 22 13 25 28 30 33
T,"C
900 900 800 800 900 800 750 900 800 750
334 334 334 336 336 334 336 330 334 336 330
750 750 900
330 330 334
800
336 334 552 552 576 576
900
900
850 850 850 850 added
apatite
P205
P205 in glass Macguarie
timc,h
1.1.8 1.18
Cl.70
i
1.18
1.in
._
As mentioned above, the composition +:ri~ablc 01 primary interest in these experiments is the amount of d~ssolved P,O, in apatite-saturated melts (see also WSYSC~U. 1979b, 1980). This value was determined by electron microprobe analysis of the quenched glasses. using two diA‘ercnt microprobe systems. The P,05 contents of glashc* in 0 of the 20 successful experiments were determined a\ the M.A.C. facility of Massachusetts Institute of ‘Icchnolog~. and all 20 were analyzed with the ETEC Autoprobe at Macquarie University (Australia). Similar operating conditions of 1.5kV and -0.025 PA sample currem were used at the two facilities, but the analyzing crystals wierc ditTe1ent (RAP at MIT., PET at Macquarie). In order LOavoid damage to the HzO- and alkali-rich glasses during analysis, a broad (- 15 pm) or rastered beam spot was utilized Because of the low concentrations of P,O, in the glasses, careful measurements of background count rates were
The obviously important task of confirming the attainment of equilibrium was approached in the following way: Since most experiments were apatite-solution runs, one or more runs was needed in which the equilibrium between apatite and melt was reached by apatite crystallization. To this end, a homogeneous, P,Os-bearing glass was prepared by melting a 97:3 mixture of composition PK and fluora-
2.
-
patite at 1400 C and I atm (loss of alkalies and lluol-in< during the 3-day fusion was prevented by sealmg the com ponents in a large Pt capsule). This glass was subsequenti) used as starting material for hydrothermal run numbers .: 6. and 16, which are reversals of 1, 8, and 24 rcspcctivcl! (see Table 2). The two approaches to equilibrium g~~c results that are in agreement within the errllrb ~‘1I’S~C~Imentation and analysis.
Rrrvrsu/.\
Table
1 .1x
--
-..
0.13 f 0.04 0.08 i 0.02 0.18
f
0.10
0.11 9.17 0.20 0.10 0.13 0.14 0.09 0.09 0.16 0.17 0.12 0.06 0.14 0.24 0.28 0.12 0.04
i 0.05 f 0.07 * 0.02 i 0.02 f. 0.03 * 0.01 i 0.01 i 0.03 f 0.05 f 0.03 f 0.05 zt 0.02 * 0.07 f 0.09 i 0.13 i. 0.08 F 0.02
contents
of glasses.
f 2 std error M.I.T.
0.09 f 0.01 * 0.05 0.14 f 0.04 0.20 * 0.04 0.17 h 0.01 0.10 i 0.01 0.13 l 0.02 0.10 * 0.03
All
rillls
additional
0.11
quartz
quartz quartz quartz 0.08 * 0.03
Eeldsp;l
a
excluding
b
phases
r
"reversal"' i.e., run in which apatite components were initially dissolved in the glass isee text). Run immediately below is the analagous apatite-solution experiment.
in addition
(see text)
to apatite,
melt,
and vapor
: j!hdses
‘3.51
P and REE in felsic magmas made on either side of the PK, peak. The background under the peak. obtained by interpolation between the offset values. was subtracted from the peak intensity, and the difl’crcncc was converted to P,Os concentration by comparison with an apatite standard and application of appropriate matrix correctlons. The P,O, concentrations reported for each run represent the mean of 8 15 analysis spot\. K?\irlr\ The analyzed PzOs contents of 20 melts (glasses) cocy1stin:! with apatlte arc listed in Table 7. The indlcatcd errors I & 2 standard errors) gibe a mcasurc of the variance among tht’ X I5 analysis points. and thus incorporate both X-ray counting error and sample heterogeneity. The uniformit) of I’ distribution in the glasses is apparently good for the h~ghl! pcralkalme compositions, but deteriorates progressivcly as (Na + K)‘AI decreases*. There is. however. no disdlsccrniblc dependence of apatlte solubility on melt alkalin1tl. 111marked contrast to the results obtained by WAISOU (lY7Ya) on flrcon snturatlon. Furthermore. despite the strong co{-relation between melt SiO, content and apatlte solubility that exists in basic and intermediate magmas (W.U?ON. 197Yb. 1980). there is no such interrelationship for the highly SIIICIC melts presently under consideration. Runs on bulk compositions containing 7X-7!, wt”,, SiOz yield a mean apatitr saturation value of 0.14 + 0.10 wt”,, d~~~olvcd P20,. which is indistinguishable from the 0. I3 - 0.03 wt”.. for runs with 72 73”.. SiO,. rThis result is not \urprl\ing 1;; \icw of the tendenc;‘of thk siOz vs P,O, apatlte saturatmn curves of WATSON (1979) to flatten out at
SiOL concentrations above -6Owt”,.] It seems reasonable to expect some positive dependence of apatite solubility upon temperature. especially among a group of runs in which the melt composition is constant o\cr changes in temperature. The temperature effect is apparently small. though. and perhaps obscured in most casc~ h> analytical uncertainty and sample heterogeneity. There 1s nevertheless some suggestion of a temperature dependence in the two series of runs for which the analyses arc most precise: 1.e. runs 19. I?. I4 and 17. 9. 5 (at 900. 800 and 750 C. respectively). The general lack of a strong tempclaturc effect is. again. consistent with the convergence of apntite saturation lsothcrms that occurs with increasing SiO, content of the melt (see WATSON. lY79b). 13ecausc of their comparatively low (Na + K),Al. the glasses with added CaO show generally poor homogeneity. Thus. although two of these experiments (25 and 28) yicldcd apparent high apatite solubilities relative to the Ca-free compositions. the tingurshahle from the more unccrtalnties are considered.
tanned
0 5”,, CaO.
numbers are in fact indistypical values when the large
Moreover.
run 22. which con-
resulted
in an apatite solubility loser than ,111other runs at 800 C. so it would be inappropriate to conclude that the presence of CaO increases the solubility of apatite in the melts examined. (as CaF,) to the melt The addition of 0.34 wt”,, fluorine has no discermble inlluence on apatite saturation behaviour (see Irun No 30). although it is possible that geologically unreallstlc F contents (e.g. 0.85 wt:‘,. as in run No. 33) do dccrcasc the solubility of apatitc. On the whole. apatlte solubihty IS relatively msensitive to temperature and composition effects. and no systematic dcpcndencles can be verified for the range of melts and condltlons studied. Thus. because all but 4 of the 17 measured apatite saturation levels fall within 0.06 of the mean of O.l4wt”,, dissolved P,Os, we will use this mean value for the illustrative melting and crystallization models discu\bed in the sections that follow.
*This effect can probably viscosity and:or increased (Na + K).AI.
be attributed P diffusivity
to decreased at higher
APATITE
IN THE SOURCE
REGIONS
OF FELSIC
MAGMAS
In this section. we use our results to show. first. how the apatite content of the source region and the P concentration in the liquid must evolve during cpisodes of partial crustal fusion to produce felsic melts. Constrained by this general model. we then consider the cffccts of residual apatitc on the REE characteristics of felsic magmas produced by partial melting of three reasonable source assemblages.
The first premise of all the models is that apatite saturation in fclsic magmas occurs at a P,O, concentration of 0.14 wt”,,. This is the mean of all the experimental determinations and is therefore a realistic value, but the reader should bear in mind the possible second-order effects of temperature and the composition variables discussed earlier. It IS probably reasonable to assume that any given crustal melting episode is more or less isothermal [See, for example. the discussion of anatexis by WIUKLEK (1967)]. so the P,O, saturation level. although not necessarily exactly 0.14 wt”,. should be close to this value and not subject to significant changes due to variation in temperature. [Interestingly, 0.14 wt’;, is a typical P,05 concentration for both I- and S-type granitoids containing more than 70% Si02 (e.g. WHITE et ~1.. 1977)]. It should also be noted at the outset that the term ‘felsic’ magma refers here to any composition with 7&80’/~~ SiOZ. Although WATSON (1979b. 1980) noted a strong dependence of apatite solubility upon at lower silica levels melt SiOz concentration (35560 wt”,;,,),there is, as noted previously, no discernible dependence at the high Si02 concentrations considered here. Thus. for the purpose of the illustrative models presented below, we feel that a constant P205 saturation value of 0.147<;,,is appropriate. This value should be relatively unaffected by changes in pressure (cf WATSON. 1980). An additional assumption implicit in the following models is that all phosphorus m the source region or residue is present as apatitc. The existence of other phosphate minerals (c.g. monazite) is certainly possible, but the ubiquitous occurrence of apatite in all rock types attests to the fact that it is by far the most common host for phosphorus. Phosphorus substitution in silicate minerals is considered negligible (cf. HENDERSON, 1968), although we recognize that garnet can accommodate fairly large amounts (up to 0.6 wt”,, P,O,) at upper mantle prcssures (THOMPSON, 1975). Because apatite is present in lower-crustal eclogites that contain as little as 0.04 wt”,, P,O, (GRIFFIN et ul.. 1979). however. we infer that very little P enters the garnet under these circumstances. The accuracy of our models is unavoidably affected by the texture and state of aggregation of the source and residue for two reasons. If, for example. the tex-
2352
E. BRKE
WATSON and C. J. CAPOBIANCO wt. % P2 05
wt .% P205 0.14 II
042
’
I26
084
i66
II
001
1 ’
003
g ’
005
sources containing
007
k
009
1 ”
Oil
1 h /
‘1
0.13
‘1
\
sourc8s conkrining greater than
I
0.14%
P, 0,
3
4
2
wt % apatite in
0
residue
Oi
02
03
L
wt.% apatW
Fig. 1. (a] Plot of P,O, (or apatite) in the residue vs ‘(,i melting for sources containing 0.18, 0.24 and 0.30 wt% P,O,. An apatite saturation value of 0.14”,, P20, in the melt is used. (b) Diagram showing evolution of P,Os in the melt and residue during melting of sources containing 0.05 and 0.10 WI”,, P20. (see text for further explanation).
ture of the residue is such that apatite crystals are from the melt by inclusion in major mineral phases (See, for example, WHITEand CWAPPELL. p. 17). then the system cannot behave as predicted by the experiments. Also . if residual apatite crystals become entrained in the departing melt fraction, the bulk characteristics of the resulting magma will not be those of liquid. These considerations are discussed whenever they are particularly important to our conclusions. A final note concerning our modeliing approach is that, when evaluating the effects of residual apatite on the REE contents of felsic melts. we choose to ignore other accessory phases such as sphene, zircon, allanite, and monazite. In view of the experimental results of HELLMANand GREEN(1979) and WATSON(1979a) on sphene and zircon, respectively, and the analytical/ petrographic study of MILLER and MITTL~EH~D~ (1981) on alfanite and monazite, this simpl~ying step undoubtedly detracts from the authenticity of the models. However, because our main aim is to evaluate separately the role of apatite. the simplification seems warranted. isolated
Given an apatite saturation value of O.l4wt% dissolved P,O, in felsic melts, two facts are immediately clear: (1) no felsic magma produced by crustal fusion can contain more than 0.14% P,O, (u&ss residual apatite crystals become entrained in the melt); and (2) no partial melt that leaves apatite in the residue can contain less than 0.14% P205. If the melt does contain <0.14”/, PZOs, any apatite present in the source rock was necessarily consumed during melting. The usefulness of these general observations can be improved by considering in detail how the P contents of residue and melt evolve as the degree of melting increases. Three general types of behavior are possible, depending on whether the b&k P,Os concen-
tration in the source is greater than, equal to, or less than the melt saturation value of 0.14 wt’:,,. If the source contains >0.14wt% P,05, the melts will be apatite-saturated (at 0.147; dissolved P,O,) f’or all degrees of melting, and the P,O, (apatite) content of the residue will increase as melting progresses. This case is illustrated in Figure La, where the abundance of apatite in the residue is plotted against “Cjmelting for sources containing 0.18, 0.24, and 0.30 wt”,, P,O,. The increase in proportion of residual apatitc is relatively small for degrees of melting below _ 50”,,. but increases rapidly for advanced degrees of melting. For sources containing ~0.14 wt% P,O,, there will be some initial melting intervat over which the melt remains apatite-saturated. but the progressive decrease in residual apatite over this interval results at some point in its complete consumption. The stage at which apatite is Iost from the residue depends, of course, upon how much P,Os (apatite) is ~~~it~aI~y present. As shown in Fig. lb, apatite is consumed at only 3.5”/0melting if the source P,O, abundance is 0.05 wt%, but an initial source concentration of O.lO”,, postpones complete loss of apatite until melting exceeds -7O”/,. In either case, once apatite is consumed, the P,05 content of the melt is no longer ‘buffered” at 0.147& and drops smoothly to the initial bulk source concentration when meltmg is complete. The simplest PzQS evolution is exhibited by systems in which the source contains exactly O.l4’,?,,, P,O,. In this case, the P,O, contents of melt and residue are both fixed at 0.14% for all degrees meIt~ng, The relevance of the preceding discussion to actual crustal melting episodes can be evaluated in the context of the following observations. First, it should be emphasized that the major-phase mineralogy of the source region is immaterial-any system that undergoes partial melting to produce felsic liquids is subject to the type of behavior illustrated in Fig. 1. because only the melt/apatite equilibria are important to
P and REE in felsic magmas
2353
P,O, evolution. It is also noteworthy that the range of source P,O, concentrations shown in Fig. 1 is within the range of values for potential felsic-melt source rocks. Australian post-Archean sediments (shales and graywackes), for example, have P,Os values ranging between 0.05 and 0.30wt% (NANCE and TAYLOR, 1976); rocks of basaltic composition average 0.25-0.32 wt% (TAYLOR, 1964; TURMAK and W~I~EPOHL, 1961): and the average andesite has 0.20-0.22 wt”/,, (CHAYES, 1969; JAKES and WHITE. 1971) which is close to the crustal average of 0.24 wt% (TAYLOR, 1964). The primary limitation of Fig. 1 is that it cannot account for residual apatite that becomes entrained in a rising melt fraction. Because entrainment would result in apparent PZO, concentrations in the melt higher than the saturation value. we can only hope that the process is restricted to relatively high degrees of melting. Anomalously low concentrations of P,05 in the melt (i.e. values below the saturation limit), on the other hand. would seem possible only if (1) apatite in the residue is included in major phases and thus isolated from the melt (e.g. WHITE and CHAPPELL, 1977) or (2) a P,O,-enriched vapor phase is exolved at some later stage in the evolution of the magma. Even subsequent fractionai crystallization of apatite cannot deplete the melt in phosphorus to a level below the saturation value, so if the complicating factors just noted are not operative, a felsic rock with content (e.g.
(Fig. I), its contribution to felsic melt and residue REE patterns can be quantitatively evaluated for any reasonable source composition and residue mineralogy. This exercise is carried out below for three general source types (graywacke, shale, and tholeiitic basalt) whose major mineral residual assemblages are effective to various degrees in retaining the REE during melting. An apatite saturation level of 0,14wt”,, dissolved P,O, is again assumed, although the actual value may be slightly higher or lower depending on the temperature of melting. ‘Graywacke’ source. As shown experimentally by WINKLER (1967) and confirmed in trace element models by ARTH and HANSON (19753, partial fusion of graywacke source material will produce felsic melts. The specific aim of Arth and Hanson was to duplicate measured trace element abundances of Precambrian quartz monzonites, which they did using a nonmodal. batch-equilibrium melting model that involved only the major residual phases of a highgrade gneiss assemblage of graywacke comp~)si~ion. In the present case, our purpose is to evaluate the influence of residual apatite on REF. behavior during batch melting of similar high-grade gneisses. The major element composition of these gneisses is specified only in the sense that %300/;: melting leaves a residue of 40’6 quartz. 307; plagioclase, 200;, hornblende, 5”,, biotite, 39; K-feldspar, 2”,,, garnet, and accessory apatite. Two different source P,O, contents (0.10 and 0.24 wtyb) are considered, which result in different amounts of residual apatite at a given stage of melting (see Fig. I). The crystal/liquid partition coefficients for Ce, Sm. Dy. and Yb. taken from the review of HANSON(19781, are summarized in Table 3. Aputite and the rure eurth elements The source REE characteristics resemble the graywacke of ARTY and HANSON (1975). Because of the selective incorporation of rare earth The chondrite-normalized REE pattern for the elements in apatite (NAGASAWA, 1970; NAGASAWA source graywacke is shown in Fig. 2 along with the and SCHNETZLIIR.1971), this mineral can play a sigcalculated patterns for liquids and residues of 5 and nificant role in determining the REE characteristics of some crust-derived magmas. The importance of apa300/(, batch melting. The residue characteristics are tite relative to the major residual phases is determined similar to those of the source, but slightly higher in the by two simple factors: namely, the concentration of middle and heavy REE due to the retention of these P,05 (and hence the amount of apatite) in the source, elements in amphibole and garnet. The effect of apatite and the abundance of major minerals in which the is noticeable only in the light REE. which are deREE are enriched relative to coexisting liquid. With pleted in the residue to a greater degree (and exhibit knowledge of the behavior of apatite during melting lower CeiSm) for the ‘low-P,O,’ source (see Fig. 2). Table
3.
Summary of mineral/melt partition coefficients used in REE models mineral/graniticmelt valuesa
mineralidacite values I
K-spar plag
hb
0.045 0.27 1.5 Sill 0.020 0.14 7.5 DY 0.006 0.065 13.0 Yb 0.006 0.05 8.0 Ce
OPX
CPX
gt
apat biot qtz
0.15 0.5 0.1 35 0.28 1.3 2.5 60 0.44 1.9 27.0 50 0.86 1.5 38.0 21
0.31 0 0.26 0 0.28 0 0.41 0
plagb hbbrc 0.20 0.09 0.05 0.04
0.9 4 6 5
cpxb
apacd
0.36 1.5 2.6 2.0
30 46 40 21
a from Hanson (1978), except in the case of quartz, which is assumed to incorporate no REE b typical values from Nagasawa and Schnetzler (1971) = from Arth and Barker (1976) d typical values from Nagasawa (1970)
t.
2354 loo‘,
,
,
,
,
,
,
,
,
HKI
(
WA~SOK and c‘. J.
(‘F
,
,
,
Liqutds produced by 5-30% Melting .im 0 57%
3
-
ap0we I” source
IO 24%PzOg) =o
24%
apat,te
_
I” swrce
source
Ce
I
Sm
I
I
I
I
Yb
DY
!
I
I
I
11
I
I
Residues of 540% Meltina
~APOHIAN(‘O
ary effects of apatite should nontheless be considered in matching model liquid REE abundances with those in igneous rocks. ‘S/z& ~SOWW.The intimate field associatton oI granutite-facies felsic rocks and apparent parttat melt products (e.g. PKIW and TAYLOK. 1977: Mc (‘.w~II~ and KABLE. 1978) justifies this second melting modei. which considers REE distribution hetwecn f&c melts and residues of charnockitic mineralogy. The as~mptions in this example are the same as for the preceding ‘graywacke’ source situation, and the crybtill Irqulti partition coefficients are also taken from t IS\~soh (1978). The differences here are that: (1) the source is assigned a REE pattern like that of the North American shale composite (NASC HGKIX 1’~
Residue 40% qtz 30% PIW 20%hb5 % bt 3 % ksp 2 % gt I
I
I
/
1000~
Rare
I
I
I
I
,
I
,
1
/
-r--r-
l---l
Liquids produced by 5 -30% Melting
/
8
I
I
Sm
Ce
I
earth
$1 DY
I
I
If
I Yb
elements
Fig. 2. REE characteristics of melts and residues produced by S_3Op,, melting of graywacke composition sources. The major phase residual assemblage shown pertains to all degrees of melting. The influence of apatite, which must melt as illustrated in Fig. 1, can be seen by comparing the REE patterns that result from melting of sources with 0.10 and 0.24 wt”,, PzOs (see text).
scurce
Ce
The melts are heavy REE-depleted, and, again, the influence of apatite is most apparent for the light REE: S’,, melting of the source containing 0.247,, P,OS yields a liquid with a Ce concentration of 55 x chondrites. whereas the same degree of melting of the source with 0.10’4 PzO, produces a liquid enriched in Ce to 69 x chondrites. On the whole. the influence of apatite on REE behavior during melting is relatively minor for this particular residual assemblage, in which amphibole and garnet are the ‘REE-dominating’ phases. The second* In view of the aluminous nature of the assumed shale source, it might be reasonable to include an Al-rich phase such as cordierite or AI,SiO, in the residue. These minerals would. however. have the same ‘diluting’ effect on the bulk REE content of the residue as does quartz, so replacement of some residual quartz by one of these aluminous phases has no etlect on the REE model. Garnet has intentionally been omitted from the residue so as to illustrate the case in which apatite plays the largest possible role in REE distribution.
‘00
1
I
Sm
I
1 /
I
YE
DY
I
,
, i-‘----
1
Residue -
sO”rCe
40%
plaq
30 % ksD
Residues of 5 - 30% Melting
Rare earth
elements
Fig. 3. Diagram analogous to Fig. 2 for S 30”,, melting LV shale composition sources. The effect of apatitc on the REE is more obvious in this case hccausc the rcGdup lack\ amphibole and/or garnet i~cc‘ text I.
P and
REE in felsic magmas
100
80
60
;z
5 5
40
s 20
60
70
80
%
Fig. 4.
Diagram
showing
90
IOC
REE in residue
that IS II? apatite
the fraction of each REE in the residue that resides in apatlte the degree of melting of the ‘shale’ sources (see Fig. 31.
contains no major mineral phase in which the REE are compatible. the accessory apatite exerts significant control over their abundances in both melt and residue. In the ‘high-P,O,’ case (0.24”,:,), the REE patterns of the residues of 5 and 30”/ melting are quite similar. and neither is displaced far below the source curve. The patterns of the liquids are light REE-enriched and distinctly concave up, the latter reature reflecting the preference of residual apatite for the middle REE. For the ‘low-P,O,’ source (O.lO”i,), the situation is quite different in that (1) the residue of 30”,, melting falls well below the source REE abundances, and (2) the liquids not only show considerably higher concentrations of all REE. but also lack the enrichment of Yb over Dy noted in liquids from the ‘high-P,O,’ source. The differences in REE behavior for the ‘high-” and ‘low-P,Os’ cases might be better illustrated in Fig. 4. which shows how the percentages of REE contained in residual apatite change as melting procoeds. The obvious feature of this diagram is that the proportion of REE in apatite inc~rrtrses with increased melting for the source containing 0.24 wt”;, PzO,. but tlrc~tr.sc~s as melting progresses in the ‘low-f’20,’ system. [It is appropriate to re-emphasize at this time that both source P,O, concentrations are rcaliatic values for the rock type being melted. so either- behavior could apply to natural melting situations. 1 .4 linal observation is that the effects of residual apatite on the REE must be taken into account before a proper estimate can be made of the degree of mcltmg that produced a given felsic rock. This is so because. except in the HREE. variations in the amount of residual apatite and changes in the degree of melting have similar effects on the REE patterns of liquids derived from a charnockitic source (see Fig. 3). ‘Q~r~t: tholeiitc,’ .SOII~YY. The last melting situation we nil1 examine is one in which felsic magmas are produced by partial fusion of the high-pressure assemblage appropriate to a quartz tholeiite compo-
nockite
as a function
01
sition (i.e. quartz eclogite). The plausibility of this particular melt-residue relationship has been demonstrated both by experiment (e.g. GREEN and RINGWMID, 1972) and by trace element studies (ARTH and HANSOU, 1975). As in the previous models. we consider two source P,O, levels (0.24 and 0.10 wtu,,). and we allow melting to proceed from 5 to 30”,,. Over this melting interval. the proportion of quartz in the residue is reduced from 15 to 57;, residual garnet is inis held creased from 30 to 40”/,,. and clinopyroxene constant at 55”,. The REE characteristics of the eclogite source resemble the early Precambrian tholeiites analyzed by ARTH and HANSON (1975. Fig. 5). The resulting REE patterns for melts and residues are summarized in Fig. 5. All patterns bear the expected imprint of residual garnet: the residues are enriched relative to the source in middle and heavy REE to an extent proportional to the degree of melting, and the liquids are severely HREE-depleted. Only in the LREE does the “<, melting have a significant effect on the melt characteristics. and it is here. too, that the influence of apatite is most noticeable. The ‘low-P,O,’ residue is considerably more LREEdepleted than its ‘high-P,O,’ counterpart. and the melts show concomitant changes in their degree of LREE enrichment. Overall. the contribution made by apatite to the REE patterns is certainly secondary to that of garnet. but nevertheless discernible in the degree of LREE enrichment or depletion. as well as in the Ce:Sm ratio. FRACTIONAL CRYSTALLIZATION OF APATITE Partial melting of crustal materials is not the only circumstance under which apatite can play a role in the geochemistry of granitic magmas. It is. in fact, inevitable that a felsic melt formed by fractional crystallization of an intermediate parent will have experienced saturation in apatite. because apatite solubility
F.
2356
BKL’CI WATSON
and C. J.
CAPOHIANCO
degree of andesite fractional crystallization to produce the rhyolite at. say, 80%. than a P,O, balance Liquids produced by allows direct computation of the amount of apatite 5 - 30 % Melting removed. An initial mass of 1OOg of andesite would contain 0.4g of P,O,, and, were apatite saturation not encountered, 80% fractional crystallization of major phases would leave a rhyolitic melt still COIN,taining 0.4g (or 2 wt%) of P,O,. Because saturation in apatite does occur, the rhyolite contains only 0.028 g of P,05, so the total PzO, removed as apatite amounts to 0.372 g (=0.89 g of apatite). Of the initial IOOg of andesite, then, 0.89 g (or 0.897:) was subp l Tholelite source tracted as apatite during differentiation. With this information, we can now proceed to evaluate the contribution of apatite to the REE pattern of the rhyolite. I “l”““c”‘Yb Sm Ce Dy We assume in this model complete equilibrium / I t I I I r 11 11 1 between crystals and melt with respect to Ce. Sm. Dy, Residues of and Yb. In order to average the effect of apatite and 5 -30% Melting the major phases over the differentiation series Residue :i:.:.:::: i$.$$ 0 57% aponte I” source andesite -+ rhyolite, we use REE partition coemcients 55 % cpx IO 24% P2051 30-400~ gt appropriate for minerals coexisting with dacitic melts .g 100 5_, 5% q+z z 0 24% opatite in source: ;; (0 10% P,qr (see Table 3). The REE pattern of the parent liquid is 2 that of TAYLOR(1969) for average circumpacific ande5 site, and is modified by removal of 550,; plagioclase, \ 20%; hornblende, and 5% augite. The resulting REE characteristics of two derivative rhyolites are shown in Fig. 6 in relation to the parent andesite. In one of these rhyolite curves, we have completely ignored the lol~“““;f”l,J Yb contribution of apatite, while in the other the necessSKI Cl? DY ary 0.S9y0 is represented. The obvious dill’erence Rare eorth elements between the two is that the ‘zero apatite’ curve shows Fig. 5. REE patterns resulting from 5 30”,, melting of considerably greater (and relatively uniform) enrichquartz eclogite sources initially containing 0.10 and ment in the REE. This is the same sort of upward 0.24 wt% P,O,. The range of residue modes is shown in the diagram; the low-quartz residue corresponds to 30”/:, displacement that would result from continued remelting. Note the effect of varying amounts of residual moval of the three major minerals. apatite on the light REE in mclrs and residues. Unlike the melting models discussed in the preceding section, the foregoing calculation of apatite fracin melts decreases so dramatically with failing temperature and increasing Si02 content (WATSON. 1979b, 1980; GREEN and WATSON, 1981). As noted previously, trace element geochemists have already recognized the likely involvement of apatite in the REE evolution of felsic magmas (e.g. SIMMONSand HEDGE, 1978), but no attempt has been made to independently evaluate the contribution of apatite relative to that of the major mineral phases. This calculation is actually possible, however, and requires only the following input: (1) the concentration of P,05 in the PMes removed : parent magma and in the felsic derivative; and (2) the 55% plop total fraction of crystals removed from the parental 20% hb liquid to produce the felsic magma under consider5% aug 0 or 0 89% opcwte ation. Suppose, for example, that the probable parental magma is broadly andesitic in composition and I I4 I I I I4 14 1 II 4 I contains 0.4 wt% dissolved PzO,*. while a related Yb ’ ce Sm DY rhyolite contains its saturation value of 0.14wt”,;, Rare earth elements P,05. If the major-element chemistry constrains the 100
,
,
,
,
,
,
I
/
I
I
I
I
,
I
&
1
i
(1979b. 1980) for apatite solubilities in magmas at various temperatures and press-
*See WATSON
intermediate ures.
Fig. 6. REE patterns of evolved rhyolitic melts produced by 80% fractional crystallization of the andesitic parent shown. The upper rhyolite curve results from removal of the major phases only, while the lower curve incorporates the required amount of apatite (see text for assumptions).
P and REE in felsic magmas
tionation effects is not validated principally by the experimental results presented in this paper. Low apatite solubihty in felsic melts is obvious for other reasons (e.g. apatite inclusions in the mafic minerals of granites; apatite phenocrysts in felsic volcanic rocks; severe depletion of phosphorus during differentiation at high silica levels), so there is nothing particularly novel about our conclusion that apatite can influence REE abundances in granites. The point we wish to emphasize is that apatite cannot, under most circumstances, be simply ignored in REE models of felsic magma formation. nor can it be called upon to crystallize in any amount that is convenient to fit a model to observed REE abundances. The systematics of apatite saturation have been established by experiment. so it is possible to at least qualitatively evaluate the role of apatitc in the REE chemistry of any differentiation series. SU.YlMARY The major conclusions of our experiments and subsequent models of P and REE behavior can be summarized as follows: 1. In the temperature range 75~900°C. felsic melts reach saturation in apatite at a level of -O.l4wt”,, dissolved P,O,. (This mean value represents a range in saturation levels that shows a positive correlation with temperature and perhaps CaO content, and a possible negative correlation with fluorine concentration). 2. The apatite (phosphorus) content of the source region and the extent of melting dictate whether apatite will be residual in crustal fusion episodes. Residual apatite is unavoidable if the source P,O, content exceeds the melt saturation value noted above. and increases in proportion as melting proceeds, Apatite may or may not be consumed in source regions whose Pz05 concentrations are less than that required to saturate a melt in apatite (depending on the degree of melting), but residual apatite necessarily decreases in abundance with increased melting. 3. The contribution of residual apatite to the REE patterns of crust-derived felsic rocks may be substantial if the source contains more than the saturation level of P,Os, and especially if garnet and/or amphibole is minor or lacking in the residue. 4. Saturation in apatite is inevitable during fractional crystallization of an intermediate parent to form a felsic magma. and may significantly alter the REE characteristics of the melt. Ac,hi~ol~/ed~c,nl~f~f,s -The authors
benefitted from continued discussion with J. F. BI:NDFR. R. H. FLOOD, T. H. GREF~, and C. F. MILLER. The critical comments of these individuals and of the olhcial reviewers greatly improved the initial manuscript. The electron microprobe analysis at Macquarie University was made possible through a Visiting Research Fellowship awarded to E.B.W. The experimental worh at R.P.I. was supported in part by the National Science Foundation Division of Earth Sciences. under N.S.F. Grant EAR7X-12980.
2357
REFERElVCES ARTH J. G. and HANSON G. N. (1975) Geochemistry and origin of the early Precambrian crust of northeastern Minnesota. Gwhim. Cosmochin~. Actu 39, 325-362. ARCH J. G. and BARKER F. (1976) Rare earth partitioning between hornblende and dacitic liquid and implications for the genesis of trondhjemitic tonalitic magmas. Gcolo,4, 534 536. CHAY~S F. (1969) Chemical composition of Cenozoic andcsite. In Proc. Andcsitc, Conf (ed. A. R. McBirney). I93 pp. State of Oregon Dept of Geology and Mineral Industries. GREEV T. H. (1981) Experimental evidence for the role of accessory phases in magma genesis. J. I/&.. Gc~othrrm. Rec. (in press). GR~F\ T. H. and RINC;U.OODA. E. (1972) Crystallization 01 garnet-bearing rhyodacitc under high-pressure hydrous conditions. J. G&. Sot,. Atr\t. 19, 203 21 2. GRF~~ T. H. and WATSO\ E. B. (1981) Experimental study of apatite saturation in natural hydrous magmas. with particular reference to erogenic magma series. Conr~ih. Mirwrcll. Pv~roi. (submitted). GRIFFIN W. L.. C,ZRSU.EI.~ D. A. and NIXON I’. H. (1979) Louver-crustal granulites and eclogites from Lesotho. South Africa. In The .l~unrlr .‘jun~p/~,. Inc /asion\ r,r Kin+ hcrlitea trrid OI/KY l.o/c,~rnic\ (eds F. R. Boyd and H. 0. A. Meyer). pp. 59 X6. Am. Geophys. Union. HAKSOY G. N. (1978) The application of trace elements to the petrogcnesis of igneous rocks of granitic composition. Enrtli Pl
2358
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