Accessory-Mineral and Reaction-History Controls on Pelitic Mineral Trace-Element Partitioning: A Combined EMP and LA-ICP-MS Study.

(V22D-05)Joseph M. Pyle1, Frank S. Spear1, Roberta L. Rudnick2, William F. McDonough2, and

Ingo Horn21Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY, 12180, USA

2Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA

Abstract LA-ICP-MS analyses for approximately 20 elements in coexisting minerals have been determined for pelites from the garnet zone (GZ), staurolite zone (STZ), sillimanite zone (SZ), and migmatite zone (MZ) of central New England. Concentration in garnet of transition metals Sc (75 ppm GZ-830 ppm MZ), V (63 ppm GZ-742 ppm MZ), and Cr (74 ppm GZ-1100 ppm MZ) increases with grade, whereas garnet core concentration of HREE Gd (25 ppm GZ-4 ppm SZ), Dy (334 ppm GZ-23 ppm SZ), Er (541 ppm GZ-11 ppm SZ), and Yb (781 ppm GZ-9 ppm SZ) mimics Y behavior in garnet and decreases with grade.Garnet-biotite partition coef ficients have been determined for Sc (2 GZ-7 SZ), V (0.11 GZ-0.07 SZ), Ti (0.009 GZ-0.002 SZ), Cr (0.24 GZ-0.75 MZ), Co (GZ ~ STZ ~ SZ ~ 0.27), Zn (GZ ~ SZ ~ 0.12), and Ga (0.16 GZ, 0.14 SZ), and overlap values for garnet-biotite partition coefficients of Yang et al. (1999). Partition coefficients for Sc and Cr appear to have some temperature dependence, but overall T dependence is lower than that of net-transfer trace-element geothermometers, such as xenotime-YAG. For SZ and MZ biotite-muscovite partitioning, Kd Cr (1.56+0.62) > Kd V (1.29+0.2) > Kd Sc (0.46+0.04), which is in agreement with the findings of Dahl et. al (1993). The smaller interlayer site in muscovite favors substitution of cations smaller than K and this is reflected in biotite-muscovite partition coefficients for Ba (0.52+0.11) and Sr (0.025+0.0100).Y and HREE depletion in garnet with increasing grade reflects increasing fractionation of these elements (notably Y, Dy, and Gd) into accessory monazite. The strong observed T dependence has considerable potential for thermometry in monazite-bearing rocks. However, MZ garnet contains regions of high Gd (17 ppm), Dy (123 ppm), Er (175 ppm), and Yb (274 ppm) relative to SZ garnet. Neither monazite nor xenotime is found in MZ garnets; generation of melt may drastically lower P activity in solid phases and suppress crystallization of phosphates during the melting interval, with concomitant enrichment of HREE in garnet grown in this interval.Large variations in pelite mineral TE partition coef ficients may arise from selection of non-equilibrium pelite mineral pairs. One MZ sample contains three generations each, identified by Sc content of garnet and biotite; possible garnet-biotite Kd Sc values range between 0.7 and 40.3. Combination of non-equilibrium mineral pairs may produce spurious temperature dependencies where little or no T control of Kd exists, or, alternatively, mask temperature control of Kd.

Section I: Garnet composition, Garnet-Biotite element partitioning

Figures 1-5 display LA-ICP-MS and EMP data for garnet and biotite from pelitic samples as a function of metamorphic grade. This first group of figures is concerned with the absolute concentrations of Y, REE, and transition elements in garnet, and partitioning of transition elements between garnet and biotite. The interpretations derived from this group of figures are as follows;

1 ) Garnet yttrium concentration generally decreases with grade (garnet zone to silli-manite zone), but may increase in garnet grown in equilibrium with melt (Fig. 1). HREE (Dy, Er, Yb, Lu) distribution in garnet follows Y to a remarkable degree, whereas Gd distribution does not mimic Y as closely (Fig. 2). Transition metal concentration in garnet (Fig. 3) increases weakly with grade to sillimanite-zone samples, but garnet grown in equilibrium with melt shows a dramatic increase in the concentration of these elements (Fig. 3); this change is related to loss of muscovite and/or accessory phases during anatexis and resultant switch to garnet-melt partitioning.

2) Partitioning of transition elements between garnet and biotite at a variety of sample grades is shown in Fig. 4. Dgt-bt ([el]gt/[el]bt) increases slightly with grade for Sc and Cr, and decreases with grade for Ti; Dgt-bt for V, Zn, and Ga vary little with grade. However, the partition coefficients for Sc, Cr, and V increase significantly with "melt zone" garnet values. Selection of equilibrium garnet-biotite pairs for distribution coefficient calculation is difficult in samples with complex reaction histories. Migmatitic sample BF-14P contains several generations of biotite and garnet; three generations of biotite crossed against garnet give DScgt-bt values ranging from 2 to 40 (Fig. 5). Garnet grown during anatexis is most likely in equilibrium with high-Sc "core" biotite, biotite of this generation was likely consumed during anatexis, when high Sc/Cr/Y garnet grew. Matrix biotite grew either upon crystallization of melt, or during retrograde reactions that consumed garnet and muscovite and also produced sillimanite and/or limited staurolite.

Figure 6. Monazite BSE images. All scale bars 10 mm except in c (1 mm). See panel for details.

Figure 6 caption

a. Matrix monazite, BF-15A (garnet zone)Y 2O3 content: 1.32-1.56 wt% (5 spot analyses)1 1.5 wt% ThO2 core, 3.3 wt% ThO2 rim

b.Matrix monazite, BF-57B (staurolte zone))Y 2O3 content: 1.47-1.63 wt% (6 spot analyses)7.1 wt% ThO 2 core, 3.2 wt% ThO2 rim

c.Monazite inclusion in garnet, BF-78 (sillimanite zone)Y 2O3 content: 2.40-2.75 wt% (7 spot analyses)ThO 2 content: 3.6-3.9 wt% (dark phase is zircon)

d.Monazite inclusion in garnet, V7C (melt zone)spot 1: 1.50 wt% Y 2O3, 5.72 wt% ThO2spot 2: 2.85 wt% Y 2O3, 4.15 wt% ThO2

e.Matrix monazite, BF-78 (sillimanite zone)spot 1: 0.66 wt% Y 2O3, 4.14 wt% ThO2spot 2: 2.82 wt %Y 2O3, 3.43 wt% ThO2

f.Matrix monazite, BF-14P (melt zone)spot 1: 1.05 wt% Y 2O3, 4.55 wt% ThO2spot 2: 2.52 wt% Y 2O3, 4.08 wt% ThO2spot 3: 0.82 wt% Y 2O3, 4.59 wt% ThO2

Section II: Xenotime and Monazite Crystal Chemistry and its Relation to Metamorphic Grade

In addition to garnet, monazite and xenotime are the major sinks for Y and HREE in typical pelites, and as such play an important role in controlling distribution of these elements. Monazite is present in all but the most calcic samples examined and xenotime has been detected in 11 of the 20 monazite bearing samples. The second group of figures (Figs. 6-9) shows BSE images displaying the variety and complexity of monazite zoning, and summarizes crystal chemistry of the REE phosphates and the relation between composition and metamorphic grade. The findings in this section are as follows;

1) BSE images (Fig. 6) of monazite reveal the following: A) monazites in low grade samples (garnet and staurolite zone) generally contain more Th and less Y than higher grade samples. Complex zoning (as indicated by BSE images) is rare in these samples (Figs. 6a, 6b). B) In high grade samples (sillimanite and melt zone), monazite inclusions in garnet are generally more compositionally homogeneous than larger matrix monazites (Fig. 6c), though exceptions do exist (Fig. 6d). Most zoned high-grade matrix monazites contain a Th-rich, Y-poor core mantled by a region richer in Y and poorer in Th. This Y/Th variation may be repeated multiple times in the same grain (Figs. 6e, 6f), but high-grade monazites have been observed where this pattern is reversed, or zoning is too patchy and complex to interpret as concentric growth of Y-poor, Th-rich and Y-rich, Th-poor regions.

2) Chondrite-normalized monazite REE distribution is highly similar for different com-positional zones in the same grain and for monazites in different samples at all grades studied, with smoothly increasing depletion of heavier LREE and greater depletion of Y+HREE (Fig. 7).

3) Much of the compositional deviation from (REE)PO 4 monazite and (REE)PO4 xenotime is accomodated by the brabanite exchange vector, CaThREE-2 (Fig. 8), though the huttonite exchange vector ((Th,U)SiREE-1P-1) predominates in some monazites. For xenotime, the ZrSiREE-1P-1 (zircon) exchange vector predominates over the huttonite exchange vector. The total extent of either substitution is roughly four times greater in monazite than in xenotime; maximum huttonite component in analyzed monazite is 5.1 mole %, and maximum brabanite component is 7.9 mole %.

4) Xenotime yttrium content is approximately constant in all analyzed grains regardless of grade (Fig. 9B), and Gd and Dy also vary little with grade. However, monazite Y content correlates strongly with grade (Fig. 9A), with maximum yttrium content (3.5 wt% Y2O3) occurring in migmatitic sample V7C (peak P-T conditions of 8+1 kb, 820+30oC). Low-Y regions do exist in high-grade monazite; these are interpreted as either low-grade cores, or growth at higher-grade in a xenotime-absent assemblage. High Y monazites are interpreted as having grown in a xenotime-bearing assemblage.

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Figure 1. Garnet yttrium distribution maps. Numbers indicate concentration of Y (ppm) in arrowed region. grt-garnet zone, st-staurolite zone, sil-sillima-nite zone, melt-anatectic sample. Warmer colors indicate higher concentra-tion of Y. All scale bars = 100 mm.

Figure 2. Garnet Y and HREE concentration in four samples from four metamorphic grades. Analyses taken with LA-ICP-MS

Figure 3. Garnet transition element concentration in four samples from four metamorphic grades. Analyses taken with LA-ICP-MS. Gaps-concentration below detection limit of LA-ICP-MS.

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Figure 5. Garnet-biotite Sc partitioning for migmatitic sample BF-14P. Partitioning between gar-net and three distinct biotites (high-Sc garnet inclusion, low-Sc garnet inclusion, matrix biotite) are shown. Anatectic ("melt") high-Sc garnet is interpreted to have grown during consumption of high-Sc biotite. After Sc fractionation by garnet, low-Sc biotite grew on melt crystallization or later via reactions such as garnet+muscovite=sillimanite+biotite.

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Figure 7. Chondrite-normalized Y and REE distribution in pelitic monazites. BF-14P, melt zone, BF-78, sillimanite zone, BF-38B1 and BF-64, staurolite zone, 93-19A and BF-15, garnet zone.