Petrogenesis of HED clan meteorites: constraints from crystal size distribution

Abstract

Two diogenites (Johnstown and ALHA 77256) and two eucrites (Malotas (b) and Stannern) meteorites from the Howardite-Eucrite-Diogenite (HED) clan are investigated by petrography, mineral chemistry and using crystal size distribution (CSD) technique applied to pyroxene grains to demonstrate their crystallization history and post-magmatic processes. Among the dominant mineral phases, plagioclase is invariably anorthitic in all the samples. However, pyroxene has variable compositional ranges: En66-84Fs16-34Wo2-4 in Johnstown, En65-83Fs17-35Wo2-4 in ALHA 77256, En26-40Fs60-74Wo0-21 in Malotas (b), and En44-58Fs42-66Wo2-48 in Stannern, indicating diogenite pyroxenes are more Mg-rich and Ca-poor than the eucrite pyroxenes. The CSD result indicates that the diogenites show a near-linear slope with a major turning down of the curves at finer grains, which can be attributed to the onset of thermal annealing. Concave-up trends in the slopes of the diogenites are indicative of crystal accumulation at larger sizes leading to textural coarsening. The CSD plot of Malotas (b) eucrite suggests multiple mixing of magmas at a relatively deeper part of Vesta, whereas the lack of kink in the CSD pattern of Stannern eucrite indicates the crystal fractionation trend at smaller sizes above the diogenitic layer and its subsequent eruption on the surface of parent Vesta. This study suggests multiple stages of melting, crystallization, and subsequent sub-solidus recrystallization in the deep-seated diogenites, while the eucrites underwent a lesser amount of metamorphism at a shallower crustal level than the deep-seated diogenites.