Magmatic sulfides in the porphyritic chondrules of EH enstatite chondrites

Abstract

The nature and distribution of sulfides within 17 porphyritic chondrules of the Sahara 97096 EH3 enstatite chondrite have been studied by backscattered electron microscopy and electron microprobe in order to investigate the role of gas-melt interactions in the chondrule sulfide formation. Our data suggest that most of the sulfides found in EH3 chondrite chondrules are magmatic minerals that formed after the dissolution of S from a volatile-rich gaseous environment into the molten chondrules. Troilite formation occurred via sulfur solubility 2 within Fe-poor chondrule melts followed by sulfide saturation, which causes an immiscible iron sulfide liquid to separate from the silicate melt. The FeS saturation started at the same time as or prior to the crystallization of low-Ca pyroxene during the high temperature chondrule forming event(s). Protracted gas-melt interactions under high partial pressures of S and SiO led to the formation of niningerite-silica associations via destabilization of the previously formed FeS and low-Ca pyroxene. We also propose that formation of the oldhamite occurred via the sulfide saturation of Fe-poor chondrule melts at moderate S concentration due to the high degree of polymerization and the high Na-content of the chondrule melts, which allowed the activity of CaO in the melt to be enhanced. Gas-melt interactions thus appear to be a key process that may control the mineralogy of chondrules in the different classes of chondrite.

Chondrule heritage and thermal histories from trace element and oxygen isotope analyses of chondrules and amoeboid olivine aggregates

“…() ascribed to an influx of Si (accompanied by S; Marrocchi and Libourel ; Piani et al. ) in the partially molten chondrules out of the ambient gas, which would have promoted the crystallization of enstatite near the margins, as commonly observed (Friend et al. ).…”

Chondrules in Enstatite Chondrites

“…the pyroxene/olivine ratio, sulfides) and the composition of the mesostasis (the main carrier of many incompatible elements). Compared to its ordinary chondrite counterparts, the latter is indeed depleted in Ca, Eu, Yb, Mn (Jacquet et al 2015), and enriched in S (0.2-0.5 wt%; Piani et al 2016), Cl (0-2 wt%; Piani et al 2016) and alkali like Na-more so in EH3 (5-13 wt% Na 2 O; than in EL3 (0-4 wt%; Schneider et al (2002) although Manzari (2010) finds up to 9 wt% in Elephant Moraine 920299). Oldhamite, whether inside or outside silicate chondrules, is typically enriched by 1-2 orders of magnitude in REE relative to CI, with frequent positive anomalies in Eu and Yb in EH3 and negative Eu anomalies in EL3 chondrites (Larimer & Ganapathy 1987;Crozaz & Lundberg 1995;Gannoun et al 2011;El Goresy et al 2017; Fig.…”

“…El Goresy et al 1988;Ikeda 1989b;Hsu 1998;Rubin 2010;Weisberg & Kimura 2012;Lehner et al 2013;Piani et al 2016). In EH3 porphyritic chondrules, Cr-Ti-bearing troilite, Sibearing Fe-Ni metal, oldhamite (CaS) and niningerite ([Mg,Fe,Mn]S) are the most abundant opaque phases, with average modal abundances of 2.1, 0.7, 0.5, 0.4 vol%, respectively, for Sahara 97096 (Piani et al 2016). Rare occurrences of minor caswellsilverite (NaCrS 2 ), two Cr-sulfides (minerals A and B of Ramdohr 1963), schreibersite ([Fe,Ni] 3 P) and perryite ([Fe,Ni] 8 [Si,P] 3 ) have also been reported El Goresy et al 1988;Piani et al 2016).…”

“…What would the origin of sulfides in EC chondrules then be? The absence of niningerite and oldhamite enclosed in low-Ca pyroxene of the Sahara 97096 porphyritic chondrules indicates that these sulfides are not preserved from the chondrule precursors, unlike (possibly) olivine or Fe-Ni metal grains (Piani et al 2016). At any rate, in order to avoid a rapid loss of sulfur from the molten chondrules (Yu et al 1996;Uesugi et al 2005), a high ambient partial pressure of sulfur is necessary (e.g.…”

“…Then, the sulfidation of silicates (olivine and pyroxene) by a S-rich gas at high temperatures (> 1000 °C) could result in the formation of Fe-, Ca-, or Mg-sulfides (Rubin 1983;Fleet & MacRae 1987;Lehner et al 2013). Piani et al (2016) noted that under the temperatures required for the sulfidation of solid silicates, with the chondrules being partially molten, the high partial pressure of sulfur should have interacted with the silicate melt. Thus sulfur would have dissolved in the silicate melt (hence the high S content of the mesostasis) and eventually saturated forming immiscible sulfide melts (e.g.…”