Ion microprobe investigation of exsolved pyroxenes in cumulate encrites: Determination of selected trace-element partition coefficients

Abstract

We have examined the partitioning of selected minor and trace elements in exsolved pyroxenes from cumulate eucrites by secondary ion mass spectrometry (SIMS). Solid-solid partition coefficients (D's) for Al, Cr, Ti, seven REEs, Sr, Y, and Zr have been determined between low Ca- and high-Ca pyroxenes from Moore County, Binda, Serra de Magé, and Moama. Equilibrium temperatures of the co-existing pyroxenes were calculated to constrain the relevant temperatures for the partition coefficients. Trace-element abundance patterns are similar for pyroxenes from the four cumulate eucrites and thus indicate similar distribution behaviors of the trace-elements during exsolution. There is a significant HREE enrichment over LREEs in the low-Ca pyroxenes and abundances of the REEs in augite are greater than those found in the low-Ca pyroxene hosts. This REE behavior reflects the pyroxene crystal structures. The calculated solid-solid partition coefficients are found to vary systematically as expected from pyroxene crystal chemical arguments. Calculated subsolidus equilibration temperatures are within the expected range produced by pyroxene solvus temperature-composition relations, and consistent with petrographic observations of the cumulates. The trace-element patterns and abundances indicate that three of the four cumulates examined (Binda, Moama, Serra de Magé) were formed from the same melt or similar melts crystallizing under similar conditions in their parent body. Moore County has much higher trace-element concentrations and this suggests that it either formed from a more evolved melt than the other three cumulates and crystallized later in the fractionation sequence, or alternatively, it was derived from a different melt system.