An ion microprobe study of the intra-crystalline behavior of REE and selected trace elements in pyroxene from mare basalts with different cooling and crystallization histories

Abstract

To study the effects of crystallization sequence and rate on trace element zoning characteristics of pyroxenes, we used combined electron microprobe-ion microprobe techniques on four nearly isochemical Apollo 12 and 15 pigeonite basalts with different cooling rates and crystallization histories. Major and minor element zoning characteristics are nearly identical to those reported in the literature. All the pyroxenes have similar chondrite-normalized REE patterns: negative Eu anomalies, positive slopes as defined by Yb Ce , and slopes of REE patterns from Ce to Sm much steeper than from Gd to Yb. The characteristics of the REE patterns of pigeonite and augite can be rationalized in terms of REE size and charge and M2 site characteristics. Numerous REE and other trace element (V, Sr, Zr, Na, Sc, Cr) zoning characteristics can be correlated with quadrilateral composition. The total REE abundances increase with increasing Ca in the M2 site and indicate that the distribution coefficients of REE between pyroxene and melt vary systematically with the Wo component. The total REE abundances also increase with decreasing Mg (Mg + Fe) and increasing Al prior to plagioclase crystallization. The Eu anomaly in pyroxene is not substantially affected by plagioclase crystallization because Eu 3+ is effectively partitioned into pyroxene relative to Eu 2+ (( Eu 2+ Eu 3+ ) = .03 in pigeonite) and the plagioclase effect is diluted by the low Eu distribution coefficient for the more abundant pyroxene. The Cr and V components behave similarly in that Cr and V decrease with increasing Ti Al in basalt sample 15058 and decrease with crystallization and at similar Ti Al . Na and Sc increase in abundance from pigeonite to augite with similar Ti Al in both 15058 and 15499. In 15058, Na decreases coherently with increasing Ti Al , whereas Sc shows a limited variability. Sr and Zr in 15058 are somewhat more enriched in the augite with low Ti Al relative to pigeonite. The augite does not become enriched in Zr or Sr until the pyroxene has high Ti Al and low Mg (Mg + Fe) . In 15499, Sr and Zr show slight enrichment in augite relative to pigeonite. These trace element zoning characteristics in pyroxene and the partitioning of trace elements between pyroxene and the melt are intimately related to the interplay among the efficiency of the crystallization process, the kinetics at the crystal-melt interface, the kinetics of plagioclase nucleation and the characteristics of the crystal chemical substitutions within both the pyroxene and the associated crystallizing phases ( i.e. plagioclase).