Evaluating samples for trace element equilibration studies

The mineral chemistry and textural information gained from high resolution back scattered images has allowed us to confidently isolate appropriate assemblages which thoroughly equilibrated under eclogite facies conditions and can therefore be used for trace element partitioning studies of eclogite-facies processes in other areas. In general, the inner rim areas of zoisite and garnet, the core areas of amphibole, phengite (Chapter 2), paragonite and apatite as well as the large clinopyroxene grains are all part of the re-equilibrated eclogite stage (Trescolmen stage; Fig. 3).

However, the approach towards equilibrium proceeded to different degrees in the investigated samples. We use the variability of clinopyroxene chemistry within individual samples as one criterion for evaluating equilibrium.

Clinopyroxenes from samples Ad25, 50-14, 55-3 and 59-1 show the greatest scatter and are clearly heterogeneous (Fig. 19b). In the case of Ad25, the variation occurs on a ca. 20 µm scale (Fig. 20a) and cannot be resolved with the LAM set-up used in this study. For 50-14, 55-3 and 59-1, we were unable to correlate the chemical variation with textural observations and detailed element mapping is required. All other samples have restricted clinopyroxene compositions on an electron microprobe scale (<5µm, Fig. 19a) and are therefore well suited for trace element analysis with LAM (>40µm).

We obtained another criterion for equilibrium from the textural relationships between the different phases. From all samples with restricted clinopyroxene composition, only 50-2, 50-13, 52-1 and 77-5 are well foliated eclogites

(Table 1). In these samples, hydrous phases define the main foliation with clinopyroxene, implying that they dynamically recrystallized together. Deformation significantly enhances material transport (Messiga et al., 1995), whereas solid state diffusion must have been too slow during the short-lived high temperature history of the Trescolmen eclogites to produce equilibrium compositions among different phases (Watson, 1996). Eclogite facies phases in CHM30b, CHM30a and 55-4 are also in apparent major element equilibrium, but we cannot judge the likelihood of trace element equilibration in the absence of signs of dynamic recrystallization.

In summary, we can order our samples in terms of the probability of showing equilibrium trace element partitioning behaviour. The most ideal samples are 50-2, 50-13, 52-1 and 77-5, which were therefore also the most thoroughly analyzed. They are taken as reference samples in the following discussion and are marked separately in the following figures. CHM30a, CHM30b and 55-4 comprise the second group, which have homogeneous clinopyroxene compositions, but the lack of dynamic recrystallization textures suggests the need to apply caution.

The samples Ad25, 50-14, 55-3 and 59-1 belong to the third group in which major element inhomogeneities were observed in clinopyroxene and can therefore also be expected for trace elements.

For low-T undeformed eclogites from Liguria, Messiga et al. (1995) have shown that chemical equilibrium of the REE commonly occurs in microdomains. We have therefore limited our trace element analyses to only one sub-area per thin section. The smallest possible sub-area was about 1 cm², which was necessary in order to find enough large inclusion-free grains of each mineral for trace element analysis. This size is larger than the sub-areas analyzed by Messiga et al. (1995), but the higher temperatures (650^oC instead of 450^oC) in combination with dynamic recrystallization in some of the Trescolmen eclogites make larger equilibrium domains more likely.

Different chemical properties (diffusion rates, affinities to different phases, etc.) of many trace elements make it likely that the approach towards equilibrium is a function of both trace element and phase in question. Therefore it would be ideal if a homogeneity test as applied to major components in clinopyroxene could be performed for every trace element distribution in each analyzed phase. However, at this stage we can only give some estimates of the degree of homogeneity in our samples. Despite the large number of LAM analyses (>400 single spot analyses) accumulated for this study, many data points (i.e., for the concentration of one trace element in one phase in a particular sample) are based on only one or two analyses. Where three or more data points exist, we can use the standard deviation as a homogeneity test (Kretz et al., 1999). We use a threshold of 25% standard deviation as an upper limit for analytical scattering and a lower limit of 35% standard deviation as a strong indication for trace element heterogeneity on a 40-100 µm scale. Data points with an average concentration below 0.1 ppm were not evaluated for heterogeneity due to possible undetectable contamination and can therefore only be treated as maximum values (see analytical section). The results of this homogeneity test are shown in Tables 10 to 13, where homogeneous data points are marked in bold and inhomogeneous data points are marked by italics. Other data points are either below 0.1 ppm or based on less than 3 analyses. The threshold value of 25% is larger than the estimated precision of 10% for regular LAM analyses of standard glasses under constant conditions (Horn et al., 1997) and was chosen to compare data from different analytical set-ups and from different spot sizes. It should be noted that, in detail, we do not follow the procedure described by Kretz et al. (1999), in which threshold values were solely based on precision that was calculated from repeated analyses of standard glasses under constant conditions.

Table 10. Average trace element data for clinopyroxene by laser ablation microprobe in ppm. Numbers in bold are considered to represent homogeneous concentration of specified trace element in sample (≤25% std. deviation of ≥3 analyses); numbers in italics are averages of heterogeneous compositions (≥35% std. deviation of ≥3 analyses). 50-250-1352-1CHM30bCHM30aAd2577-550-1455-355-459-1 Li28251630-426440--- Be2.602.531.882.712.391.101.154.071.79-- B4.182.723.633.153.804.885.054.78--- Sr16882187183172828.2567571149 Y1.210.591.240.370.340.650.230.870.740.720.94 Zr2.442.002.542.192.352.134.051.851.922.742.10 Nb0.00460.0095<0.02<0.01<0.01<0.020.00700.00630.0150.0170.014 Ba0.0430.00360.0230.0310.00630.0760.0074<0.050.072<0.020.054 Ce0.820.280.460.0460.0550.0150.0170.10<0.030.160.20 Nd1.690.621.290.100.150.110.00350.31<0.30.440.84 Sm1.150.441.200.100.18<0.030.00840.34<0.30.360.67 Pb3.131.402.963.362.481.490.0401.302.371.665.04 Th<0.010.0100.00980.00070.00150.00200.00350.00210.0200.00070<0.01 U0.00680.00550.00380.00300.00120.00700.00060.00590.0440.0160.016

Table 11. Average trace element data for amphibole by laser ablation microprobe in ppm. For explanation of bold and italic numbers see Table 10. 50-250-1352-1CHM30bCHM30aAd2577-555-355-459-1 Li2.470.981.501.146.71.4018 Be5.833.183.482.221.261.051.572.22 B11.89.329.028.025.5810.311.64.30 Sr17199180313812.214.4467482 Y4.22.324.031.301.231.960.925.430.802.35 Zr6.035.516.102.962.693.457.632.821.773.29 Nb0.0480.110.0580.0260.031<0.020.030.0320.0170.015 Ba4.803.675.255.744.091.665.581.022.074.41 Ce0.230.140.180.00820.0370.0280.320.790.270.14 Nd1.140.260.78<0.040.10<0.030.0222.250.560.85 Sm1.770.511.600.0190.180.0170.00541.270.381.03 Pb6.945.639.021.012.440.660.132.283.497.38 Th0.0028<0.02<0.01<0.010.0006<0.010.00080.00260.00290.0025 U<0.010.00670.00670.00210.00940.00370.00150.00650.0013<0.01

Table 12. Average trace element data for phengites and selected trace element data for garnets by laser ablation microprobe in ppm. For explanation of bold numbers see Table 10.

PH E G R T

50-250-1352-1CHM30aAd2577-555-3CHM30b50-250-1352-177-5 Li1.112.200.654.20392.060.68<0.60.102.41 Be2.151.391.341.170.480.272.47<0.13 B22251814.53010.57.52.7 Sr597417409601285494130.0560.270.0500.25 Y0.0550.0490.0630.037<0.040.07612.21564276 Zr0.370.210.270.220.430.330.322.625.963.601.98 Nb0.190.510.210.320.0960.0550.62<0.01<0.090.0460.017<0.01 Ba2044131322802419117316660.0770.072<0.18 Ce<0.010.0780.0730.0020.00760.100.0470.0190.052<0.01 Nd<0.030.023<0.19<0.03<0.05<0.020.30<0.070.0300.0970.046 Sm<0.040.0110.20<0.01<0.060.00310.350.690.281.320.45 Pb38352129310.51240.351.500.151.050.012 Th<0.010.0029<0.010.0006<0.01<0.010.0036<0.03<0.08<0.02<0.04<0.01 U0.0300.00560.00470.0500.0021<0.010.014<0.03<0.050.0260.0011<0.19

Table 13. Average trace element data for paragonite, apatite, zoisite, clinozoisite and talc by laser ablation microprobe in ppm. For explanation of bold numbers see Table 10. PARAPAZOICZOTLC 50-250-1352-150-1450-250-1352-150-14CHM30bAd2577-577-555-4 Li252.47.1<0.15 Be9.07.09.7<1.0 B988314<5 Sr6428297128072584693828267643262333162833151711380.026 Y0.0570.0211066396885.8292121<0.01 Zr0.343.891.030.078<0.01 Nb0.0520.100.100.0160.0055 Ba231460.067<0.03 Ce0.0080.0111876518998405630430.0032 Nd<0.2<0.0136410635318825493045<0.03 Sm0.0720.0084190641831086161017<0.03 Pb741191361747454011135148157.60.082 Th<0.03<0.07<0.060.000300.370.180.170.120.780.710.351.100.0035 U<0.04<0.01<0.040.00412.251.090.470.75<0.290.690.120.22<0.01

Im Dokument Trace element mineral analysis in high pressure metamorphic rocks from Trescolmen, Central Alps (Seite 69-75)