Explicating the behavior of Mn-bearing phases during shock melting and crystallization of the Abee EH-chondrite impact-melt breccia

Abstract

Literature data show that, among EH chondrites, the Abee impact-melt breccia exhibits unusual mineralogical characteristics. These include very low MnO in enstatite (<0.04 wt%), higher Mn in troilite (0.24 wt%) and oldhamite (0.36 wt%) than in EH4 Indarch and EH3 Kota-Kota (which are not impact-melt breccias), low Mn in keilite (3.6-4.3 wt%), high modal abundances of keilite (11.2 wt%) and silica (~7 wt%, but ranging up to 16 wt% in some regions), low modal abundances of total silicates (58.8 wt%) and troilite (5.8 wt%), and the presence of acicular grains of the amphibole, fluor-richterite. These features result from Abees complex history of shock melting and crystallization. Impact heating was responsible for the loss of MnO from enstatite and the concomitant sulfidation of Mn. Troilite and oldhamite grains that crystallized from the impact melt acquired relatively high Mn contents. Abundant keilite and silica also crystallized from the melt; these phases (along with metallic Fe) were produced at the expense of enstatite, niningerite and troilite. Melting of the latter two phases produced a S-rich liquid with higher Fe/Mg and Fe/Mn ratios than in the original niningerite, allowing the crystallization of keilite. Prior to impact melting, F was distributed throughout Abee, perhaps in part adsorbed onto grain surfaces; after impact melting, most of the F that was not volatilized was incorporated into crystallizing grains of fluor-richterite. Other EH-chondrite impact-melt breccias and impact-melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories.