Chemical compositions of Fe-rich relict olivines from cosmic spherules, understanding their links with ordinary and carbonaceous chondrites

Abstract

Fe-rich relict olivine grains are found in a small percentage of cosmic spherules, which are studied here to determine the nature of their precursors. We examined 128 Fe-rich relict olivine grains with Fa >10 mol% from 53 cosmic spherules of different types collected from Antarctica (Antarctica micrometeorites [AMM]) and deep-sea sediments (DSS) of the Indian Ocean. Fe-rich olivines identified in cosmic spherules are close analogs of type II chondrule olivines formed in the early solar system. The olivine analysis shows well-defined trends in molar Fe/Mn versus Fe/Mg with an affinity for ordinary and carbonaceous chondrites. The minor oxides in olivine are in ranges such as MnO ~0.1-0.8 wt%, Cr2O3 ~0-0.7 wt%, CaO ~0-0.6 wt%, and Al2O3 ~0-0.2 wt%, respectively. The chemical composition suggests that the precursors for these Fe-rich olivine-bearing cosmic spherules consist of ordinary chondrites (~21%-23%, AMM-DSS), carbonaceous chondrites (~17%-36%, AMM-DSS), and a large fraction overlapping both carbonaceous and ordinary chondrites (~41%-62% AMM-DSS). The elemental ratios Fe/Si/CI and Mg/Si/CI for the Fe-rich relict olivines ranging between the values 0.5-1.0 and 1.1-1.7 are compatible with IDPs, Comet 81P/Wild 2 as well as the Asteroid Itokawa and Ryugu, which are indistinguishable from carbonaceous and ordinary chondrites. In addition, pyroxene and olivine assemblages in their Fa versus Fs mol% show strong similarities to EOC chondrites. Our results on Fe-rich relict olivines show that these grains in cosmic spherules are less common than Mg-rich olivines, which show a narrow range of chemical compositions identical to those from ordinary chondrites and carbonaceous chondrites, indicating a supplementary contribution of an ordinary chondritic component to the micrometeorite source of dust.