MINERAL CHEMISTRY AND THE PETROGENESIS OF EUCRITES: I. NONCUMULATE EUCRITES

Abstract

Ion microprobe analyses of REE and other selected trace and minor elements, coupled with electron microprobe measurements of major elements, were made in pigeonite, plagioclase, and phosphates of eight noncumulate eucrites from the various chemical groups previously recognized.REE concentrations are homogeneous in individual pigeonite and plagioclase grains of equilibrated eucrites but, except for Ibitira which is highly equilibrated, show intergrain variations. REE zoning was observed in some pyroxene and plagioclase grains of Pasamonte and ALHA 76005 (two polymict eucrites with high abundance of unequilibrated clasts) and in some plagioclase grains of Sioux County and Chervony Kut (two equilibrated eucrites). Parent melt REE compositions were calculated for Pasamonte, Sioux County, and Chervony Kut, using the lowest REE abundances observed in the cores of igneously-zoned pyroxene and plagioclase grains and appropriate REE partition coefficients; they are remarkably similar to their respective whole rock abundances. The calculations also indicate that some plagioclase grains in Chervony Kut may be cumulus material. In ALHA76005, a positive Ce anomaly indicative of terrestrial REE redistribution was often observed in both pyroxene and plagioclase analyses. Coupled major and trace element analyses reveal that there are two types of zoned pyroxenes in Pasamonte: one was formed by igneous processes and the other owes its zoning to secondary alteration.The data generally support the model of Stolper (1977) for the origin of noncumulate eucrites, i.e., that the Stannern trend and main group eucrites were formed by different degrees of partial melting and the Nuevo Laredo trend by fractional crystallization. However, they also suggest chemical variability among eucrite magmas. The LREE/HREE ratios recorded in pyroxene and plagioclase indicate that noncumulate eucrites most likely derived from a source region with REEs in chondritic proportions.A search for 26Mg excesses from the decay of 26Al in Chervony Kut and Ibitira, the eucrites with the oldest reported ages, and in Pasamonte was negative.