SILICA-RICH OBJECTS IN THE ACFER 182 CH CHONDRITE: A NEW VIEW"

MARIA EUGENIA VARELA

Meteoritics and Planetary Science

Wiley

Lugar: New Jersey; Año: 2020

1945-5100

Five silica-rich objects (SRO) from Acfer 182 were studied. They have6 cryptocrystalline textures characterized by micro-emulsion and amoeboid7 patterns that point towards the co-existence of pyroxene- and silica-normative8 liquids that were quenched. Both objects have variable contents of refractory9 lithophile elements. Their positive Yb vs. La correlation around primordial values10 suggests a cosmochemical process (e.g., a gas/liquid condensation) as11 responsible for SRO formation. The bulk trace element abundances of12 amoeboid-and emulsion-type SRO, as well as, their fractionation do not support13 an origin through high temperature processes. Conversely, their formation might14 have taken place while cooling of the nebular gas in two different chondrule-15 forming regions characterized by having different evolution paths. Cooling of16 these dust-enriched regions might lead to the condensation of pyroxene-rich17 liquids first, follow by formation of Mg-rich and SiO2-rich liquids, provided18 irradiation and annealing were active in these regions. Irradiation could be the19 process involved both in the formation of cristobalite (with annealing ~1200 K)20 and in triggering a spinoidal decomposition causing un-mixing of the enstatite21 liquid into two coexisting phases, such as Mg-rich and SiO2-rich liquids, the22 precursors of the SRO in Acfer 182. Formation of emulsion-and amoeboid-type23 objects may be the result of exposing those chondrule-forming regions to24 different degrees of radiation.