Porous, S‐bearing silica in metal‐sulfide nodules and in the interchondrule clastic matrix in two EH3 chondrites

Abstract

Two new occurrences of porous, S-bearing, amorphous silica are described within metal-sulfide nodules (MSN) and as interchondrule patches in EH3 chondrites SAH 97072 and ALH 84170. This porous amorphous material, which was first reported from sulfidebearing chondrules, consists of sinewy SiO 2 -rich areas containing S with minor Na or Ca as well as Fe, Mg, and Al. Some pores contain minerals including pyrite, pyrrhotite, and anhydrite. Most pores appear vacant or contain unidentified material that is unstable under analytical conditions. Niningerite, olivine, enstatite, albite, and kumdykolite occur enclosed within porous silica patches. Porous silica is commonly interfingered with cristobalite suggesting its amorphous structure resulted from high-temperature quenching. We interpret the S-bearing porous silica to be a product of silicate sulfidation, and the Na, Ca, Fe, Mg, and Al detectable within this material are chemical residues of sulfidized silicates and metal. The occurrence of porous silica in the cores of MSN, which are considered to be preaccretionary objects, suggests the sulfidizing conditions occurred prior to final parent-body solidification. Ubiquitous S-bearing porous silica among sulfide-bearing chondrules, MSN, and in the interchondrule clastic matrix, suggests that similar sulfidizing conditions affected all the constituents of these EH3 chondrites.

Earth’s water may have been inherited from material similar to enstatite chondrite meteorites

“…ppm H 2 O) or the aubrite pyroxene fraction, respectively. The remaining ~65% to 20% of H measured in the bulk rock may be contributed by other unknown carriers such as sulfur-rich carbon-bearing porous amorphous silica (22), due to an underestimation of the H content in EC pyroxenes, and/or could derive from some pervasive terrestrial contamination, although a limited amount of contamination is expected otherwise it would have erased the thermal metamorphism effect visible on the δD vs. 1/H diagram (Fig. S5).…”

“…Atmospheric escape to space, often invoked for atmospheric species including water vapor, could potentially achieve such enrichments, but light noble gases do not support this hypothesis. Ne-Ar isotopic variations in the mantleatmosphere system do not follow elemental/isotopic variations that would sign atmospheric escape; they are best reproduced by mixing between a solar-like component trapped in the solid Earth and a carbonaceous chondrite-like component with a low 20 Ne/ 22 Ne ratio (5). A contribution of carbonaceous chondrite-like materials, especially CIs, to the surface inventory would also be consistent with the observed H and N isotopic variations (Fig.…”

“…The modal abundance of low-calcium pyroxene in ECs is ~50 vol % (15), so enstatite pyroxenes could account for ~15 or ~58% of the bulk H content of ECs, considering the highest H content of OC pyroxenes (1300 wt ppm H 2 O) or the aubrite pyroxene fraction, respectively. The remaining ~65 to 20% of H measured in the bulk rock may be contributed by other unknown carriers such as sulfur-rich carbonbearing porous amorphous silica (22), owing to an underestimation of the H content in EC pyroxenes, and/or could be derived from some pervasive terrestrial contamination, although a limited amount of contamination is expected (otherwise, it would have erased the thermal metamorphism effect visible on the diagram of dD versus 1/H; fig. S5).…”

Global Modeling of Nebulae With Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles

“…3.2.2, the models for α t = 10 −4 actually have no refractory iron over a wide region from ∼ 0.4AU -2AU, as it has all been incorporated in troilite leaving a large reservoir of H 2 S gas. It may be that this reservoir of H 2 S gas goes on to affect mineralogy in other ways that are beyond the scope of this paper (Lehner et al 2013(Lehner et al , 2017, see section 4.3). Alternately, some of the iron could be sequestered in silicates.…”

Global Modeling of Nebulae with Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles

“…As we explained in Section 3.2.2, the models for α t = 10 −4 actually have no refractory iron over a wide region from ∼0.4 to 2 au, as it has all been incorporated in troilite leaving a large reservoir of H 2 S gas. It may be that this reservoir of H 2 S gas goes on to affect mineralogy in other ways that are beyond the scope of this paper (Lehner et al 2013(Lehner et al , 2017; see Section 4.3). Alternately, some of the iron could be sequestered in silicates.…”