Experimentally induced troilite melt pervasion in chondritic analog materials: A study for FeNi-FeS darkening in chondrites

Abstract

Iron sulfide and metal melt veins in chondritic materials are associated with advanced stages of dynamic shock. The shock-induced residual temperatures liquefy the sulfide component and enable melt distribution. However, the distribution mechanism is not yet fully understood. Capillary forces are proposed as agents of melt distribution; yet, no laboratory experiments were conducted to assess the role that capillary forces play in the redistribution of iron sulfide in post-shock conditions. To investigate this further, we conducted thermal experiments under reducing conditions (N₂(g)) using dunitic fragments, suitable chondritic analog materials that were doped with synthesized troilite (stoichiometric exact FeS). We observed extensive iron sulfide (troilite) migration that partially resembles that of ordinary chondrites, without the additional influence of shock pressure-induced fracturing. The iron sulfide melt infiltrated grain boundaries and pre-existing fractures that darkened the analog material pervasively. We also observed that the iron sulfide melt, which mobilized into grain boundaries, got systematically enriched in Ni from the surrounding host olivine. Consequently, FeNi metal fractionated from the melt in several places. Our results indicate that capillary forces majorly contribute to melt migration in the heated post-shock environment.