Deciphering the nebular and asteroidal record of silicates and organic material in the matrix of the reduced CV3 chondrite Vigarano

Abstract

Abstract-We have conducted scanning electron microscope (SEM) and transmission electron microscope (TEM) studies of a variety of occurrences of matrix in the reduced CV3 chondrite breccia Vigarano. Matrix, which occurs as clastic interchondrule material and finer-grained rims, is dominated by morphologically variable olivines that host submicron, hercynitic spinel, and carbonaceous inclusions. Clastic matrix and fine-grained rims show significant differences in their olivine morphologies, abundance, and composition of olivine inclusions, and characteristics of the carbonaceous matter. We suggest that these differences are the result of different degrees of alteration of clastic matrix and rims and are not due to variability in their precursor materials. Textural and compositional characteristics of olivine in the matrix are consistent with formation by growth, possibly from an amorphous precursor material during asteroidal metamorphism, in the presence of limited quantities of aqueous fluids. Spinel inclusions in olivine may be nebular condensates that acted as seeds for nucleation of olivine or may have formed during metamorphism and were subsequently overgrown by olivine. Carbonaceous material occurs as nanometer-sized inclusions within olivine in both fine-grained rims and clastic matrix, but is most abundant as 100-200 nm grains, interstitial to matrix olivines. Most carbonaceous material is amorphous, but poorly graphitized carbon (PGC) also occurs as a minor component in both olivine inclusions and interstitial C. The widespread occurrence of fine-grained amorphous carbon grains in the interstitial regions between olivine grains may preserve the distribution and grain size of nebular organic material. No clear textural relationships exist between carbonaceous grains and the other mineralogical components of Vigarano matrix that could help constrain the origin of the organic grains (i.e., evidence for Fischer-Tropsch-type reactions). Finally, there are considerable differences between matrix olivines in Vigarano in comparison with those in oxidized CV3 chondrites. In particular, the mineralogy and morphology of the matrix olivines and the nature, composition, and distribution of inclusions in the olivine grains are distinct. Based on these differences, we conclude that matrix in the oxidized CV3 chondrites could not have formed by thermal processing of Vigarano-like material.

Nebular Versus Parent Body Processing

“…However, Krot et al (2004a) have proposed an alternative model for the formation of platy olivine that involves hydrothermal growth from solution, which does not necessarily produce 16 O-poor solids. This conclusion is supported by the observations of matrix in Vigarano ( Abreu and Brearley, 2011) in which platy matrix olivine compositions and the inclusion populations in olivine are consistent with growth during thermal metamorphism, probably in the presence of a fluid phase.…”

“…Rare calcite also occurs ( Abreu and Brearley, 2005) that appears to be extraterrestrial in origin. The ubiquitous presence of these alteration products interstitial to matrix olivines led Lee et al (1996) to conclude that alteration occurred in an asteroidal environment, a conclusion that is supported by the fact that the olivine grains appear to be the product of parent body thermal metamorphism ( Abreu and Brearley, 2011).…”

“…In the reduced CV chondrites, evidence for aqueous alteration appears to be minimal. Lee et al (1996) and Abreu and Brearley (2011) found evidence for incipient aqueous alteration in the matrix of Vigarano in the form of ferrihydrite and rare, very fine-grained saponite. Rare calcite also occurs ( Abreu and Brearley, 2005) that appears to be extraterrestrial in origin.…”

A XANES and Raman investigation of sulfur speciation and structural order in Murchison and Allende meteorites

“…In the case of Murchison, sulfides could be preaccretionary or/and resulting from parent body processes, as observed in the mineralogy of CM chondrites (Bullock et al. ; Abreu and Brearley ). But for CV chondrites, it is unclear (Bischoff ).…”

“…Sulfides can readily form from reactions between H 2 S and metal phases in the solar nebula at temperatures <687 K in a gas of solar composition and under thermodynamic equilibrium (e.g., Lauretta et al 1996Lauretta et al , 1997. In the case of Murchison, sulfides could be preaccretionary or/and resulting from parent body processes, as observed in the mineralogy of CM chondrites (Bullock et al 2010;Abreu and Brearley 2011). But for CV chondrites, it is unclear (Bischoff 1998).…”