MINERAL CHEMISTRY AND THE PETROGENESIS OF EUCRITES .1. NONCUMULATE EUCRITES

Ion microprobe analyses of REE and other selected trace and minor elem ents, coupled with electron microprobe measurements of major elements, were made in pigeonite, plagioclase, and phosphates of eight noncumul ate eucrites from the various chemical groups previously recognized. R EE concentrations are homogeneous in individual pigeonite and plagiocl ase grains of equilibrated eucrites but, except for Ibitira which is h ighly equilibrated, show intergrain variations. REE zoning was observe d in some pyroxene and plagioclase grains of Pasamonte and ALHA76005 ( two polymict eucrites with high abundance of unequilibrated clasts) an d in some plagioclase grains of Sioux County and Chervony Kut (two equ ilibrated eucrites). Parent melt REE compositions were calculated for Pasamonte, Sioux County, and Chervony Kut, using the lowest REE abunda nces observed in the cores of igneously zoned pyroxene and plagioclase grains and appropriate REE partition coefficients; they are remarkabl y similar to their respective whole rock abundances. The calculations also indicate that some plagioclase grains in Chervony Kut may be cumu lus material. In ALHA76005, a positive Ce anomaly indicative of terres trial REE redistribution was often observed in both pyroxene and plagi oclase 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 alterati on. The data generally support the model of Stolper (1977) for the ori gin of noncumulate eucrites, i.e., that the Stannern trend and main gr oup eucrites were formed by different degrees of partial melting and t he Nuevo Laredo trend by fractional crystallization. However, they als o suggest chemical variability among eucrite magmas. The LREE/HREE rat ios recorded in pyroxene and plagioclase indicate that noncumulate euc rites most likely derived from a source region with REEs in chondritic proportions. A search for Mg-26 excesses from the decay of Al-26 in C hervony Kut and Ibitira, the eucrites with the oldest reported ages, a nd in Pasamonte was negative.