Pyroxene Chemistry in Polymict Eucrite Northwest Africa 6475: Contrasts with Juvinas, Stannern and Igdi, and Evaluation of Models for Eucrite Magmatic Evolution

Abstract

The eucrite meteorite suite is the mafic component of a silicate profile that formed from a magma ocean on a minor body, potentially the asteroid Vesta. There are four chemical divisions of eucrites: cumulate (CE), main group trend (MGT), Nuevo-Laredo trend (NLT), and Stannern trend (ST). This study examined pyroxenes in previously classified monomict eucrites Juvinas (MGT), Igdi (NLT), and Stannern (ST). The newly classified meteorite NWA 6475, a polymict eucrite with large igneous clasts, was also examined. All four meteorites are reequilibrated in major elements, so true liquid compositions cannot be calculated. Pyroxene major and minor element (Si, Ca, Fe, Mg, Cr, Ti, Al, Mn, Ni, Na) concentrations were determined by electron microprobe. Pyroxene trace element (Sc, V, Sr, Y, Zr, Nb, REE, Hf, Ta) concentrations were determined by laser-ablation sourced ICP-MS. MGT, NLT, and ST compositional trends are reproduced in pyroxene compositions. The modal NWA 6475 pyroxenes have major element chemistry similar to the MGT, but uncommon CE and NLT pyroxenes were observed. In trace element chemistry, NWA 6475 pyroxenes fall along the ST, but closer to Juvinas than Stannern. Oxygen isotopes in NWA 6475 are consistent with the main HED suite. The presence of three major element groups in a single meteorite strongly suggests that all can originate on a single body. Contrasting this, the model relation between MGT and NLT magmas (pyroxene fractionation) is not consistent with observed pyroxene compositions in Juvinas and Igdi. NWA 6475 does not fall along a consistent silicate or oxide fractionation curve from Juvinas. CE pyroxenes remain an enigma and were observed not as individual clasts, but as inclusions in an MGT-dominated clast. Trace element mixing curves that connect MGT and ST magma compositions are not physically realistic. Assimilation and fractional crystallization curves fail to connect MGT and ST compositions. All of this suggests that while some may be, not all eucrites are cogenetic. Minor and trace element pyroxene compositions should be treated, along with oxygen isotopes, as a method for separating eucrites into cogenetic suites.