Determination of trace element mineral/liquid partition coefficients in melilite and diopside by ion and electron microprobe techniques

Abstract

The use of the ion microprobe for quantitative analysis of Sr, Y, Zr, La, Sm, and Yb in melilite and pyroxene is evaluated. Three trace element-doped synthetic glasses of composition Ak_40, Ak_80, and Di_2AbAn were analyzed by ion microprobe (IMP) using ion yields determined from Corning glass standards. IMP-determined oxide concentrations in the Di_2AbAn glass agree well with electron microprobe (EMP) analyses (to within 6%), but IMP analyses of the melilite glasses deviate from EMP averages by up to 19%. The deviations are due to erroneous SiO_2 estimates caused by suppression of Si ion intensities by the enhanced concentrations of Ca and Al in the melilite glasses compared to the standards. Thus, in order to determine compositions of melilite, diopside, and glass from subliquidus experiments on each of the three starting compositions, we adopted a new set of ion yields such that IMP analyses of the three starting glasses reproduce the EMP average compositions. Further IMP and EMP comparisons of the subliquidus assemblages show that quantitative analyses of melilite, diopside, and glass can be obtained by IMP that are within 10% of the concentrations obtained by EMP, when ion yields determined from glass starting compositions are used. EMP-IMP comparison of crystal and glass analyses also suggests that a structural matrix effect may result in overestimation of SrO (10–12%) in melilite by IMP. Comparison of our data for Ak_12 and Ak_90 melilite compositions with literature results shows that melilite/liquid for REE^3+ determined by IMP decrease with increasing X_Ak (Ak_90: D_La = 0.038, D_Sm = 0.032, D_Yb = 0.0086; Ak: 0.67, 0.75, 0.25, respectively) while that for Sr (=Eu^2+) changes only slightly (0.99 to 0.78, respectively). Since X_Ak increases with decreasing temperature for all melilite with X_Ak<0.7, a progressively larger positive Eu anomaly is predicted for melilite as it crystallizes with falling temperature. Our diopside/liquid data are characterized by a large degree of scatter on most interelement correlation plots of apparent partition coefficients. The data cannot be understood in terms of simple models of boundary layer formation but require a complex surface partitioning explanation. Nevertheless, estimates of diopside/liquid are in excellent agreement with literature data.

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