Magmatic sulfides in the porphyritic chondrules of EH enstatite chondrites

摘要

The nature and distribution of sulfides within 17 porphyritic chondrules ofthe Sahara 97096 EH3 enstatite chondrite have been studied by backscatteredelectron microscopy and electron microprobe in order to investigate the role ofgas-melt interactions in the chondrule sulfide formation. Troilite (FeS) issystematically present and is the most abundant sulfide within the EH3chondrite chondrules. It is found either poikilitically enclosed in low-Capyroxenes or scattered within the glassy mesostasis. Oldhamite (CaS) andniningerite [(Mg,Fe,Mn)S] are present in about 60% of the chondrules studied.While oldhamite is preferentially present in the mesostasis, niningeriteassociated with silica is generally observed in contact with troilite andlow-Ca pyroxene. The chondrule mesostases contain high abundances of alkali andvolatile elements as well as silica. Our data suggest that most of the sulfidesfound in EH3 chondrite chondrules are magmatic minerals that formed after thedissolution of S from a volatile-rich gaseous environment into the moltenchondrules. Troilite formation occurred via sulfur solubility within Fe-poorchondrule melts followed by sulfide saturation, which causes an immiscible ironsulfide liquid to separate from the silicate melt. The FeS saturation startedat the same time as or prior to the crystallization of low-Ca pyroxene duringthe high temperature chondrule forming event(s). Protracted gas-meltinteractions under high partial pressures of S and SiO led to the formation ofniningerite-silica associations via destabilization of the previously formedFeS and low-Ca pyroxene. We also propose that formation of the oldhamiteoccurred via the sulfide saturation of Fe-poor chondrule melts at moderate Sconcentration due to the high degree of polymerization and the high Na-contentof the chondrule melts, which allowed the activity of CaO in the melt to beenhanced.