Petrography, stable isotope compositions, microRaman spectroscopy, and presolar components of Roberts Massif 04133: A reduced CV3 carbonaceous chondrite.

Jemma Davidson,Conel M O'D Alexander,Harold C Connolly,Ian A Franchi,Richard C Greenwood,Henner Busemann,Devin L Schrader,Kenneth J Domanik,Dante S Lauretta,Alexander Verchovsky

doi:10.1111/maps.12377

Abstract

Here, we report the mineralogy, petrography, C-N-O-stable isotope compositions, degree of disorder of organic matter, and abundances of presolar components of the chondrite Roberts Massif (RBT) 04133 using a coordinated, multitechnique approach. The results of this study are inconsistent with its initial classification as a Renazzo-like carbonaceous chondrite, and strongly support RBT 04133 being a brecciated, reduced petrologic type >3.3 Vigarano-like carbonaceous (CV) chondrite. RBT 04133 shows no evidence for aqueous alteration. However, it is mildly thermally altered (up to approximately 440 °C); which is apparent in its whole-rock C and N isotopic compositions, the degree of disorder of C in insoluble organic matter, low presolar grain abundances, minor element compositions of Fe,Ni metal, chromite compositions and morphologies, and the presence of unequilibrated silicates. Sulfides within type I chondrules from RBT 04133 appear to be pre-accretionary (i.e., did not form via aqueous alteration), providing further evidence that some sulfide minerals formed prior to accretion of the CV chondrite parent body. The thin section studied contains two reduced CV3 lithologies, one of which appears to be more thermally metamorphosed, indicating that RBT 04133, like several other CV chondrites, is a breccia and thus experienced impact processing. Linear foliation of chondrules was not observed implying that RBT 04133 did not experience high velocity impacts that could lead to extensive thermal metamorphism. Presolar silicates are still present in RBT 04133, although presolar SiC grain abundances are very low, indicating that the progressive destruction or modification of presolar SiC grains begins before presolar silicate grains are completely unidentifiable.

Highlights

Roberts Massif (RBT) 04133 was found as a single stone weighing 459.4 g during the 2004 Antarctic Search for Meteorites (ANSMET) field season at Roberts Massif, Antarctica
We report a multitechnique study of the petrography, stable isotope compositions, Raman spectral characteristics, and presolar components of RBT 04133
Chondrites can be classified on the basis of their petrography and mineralogy (e.g., Weisberg et al 2006), bulk element abundances (e.g., Kallemeyn et al 1989), stable O-isotope compositions (e.g., Clayton and Mayeda 1999), and the C and N isotopic compositions of their insoluble organic matter (IOM; Alexander et al 2007), there is often some overlap between groups

Summary

Introduction

Roberts Massif (RBT) 04133 was found as a single stone weighing 459.4 g during the 2004 ANSMET field season at Roberts Massif, Antarctica. This Antarctic find was initially classified as a petrologic type 2 Renazzo-like carbonaceous (CR) chondrite with a B/C weathering grade (Weisberg et al 2008). We report a multitechnique study of the petrography, stable isotope compositions, Raman spectral characteristics, and presolar components of RBT 04133. Chondrites can be classified on the basis of their petrography and mineralogy (e.g., Weisberg et al 2006), bulk element abundances (e.g., Kallemeyn et al 1989), stable O-isotope compositions (e.g., Clayton and Mayeda 1999), and the C and N isotopic compositions of their insoluble organic matter (IOM; Alexander et al 2007), there is often some overlap between groups. The petrologic subtypes of type 3 (i.e., least altered) samples can be determined by various means, including: Cr2O3 content of FeO-rich olivine (for types 3.00 to 3.2; Grossman and Brearley 2005), Ni and Co content of metal (e.g., Kimura et al 2008), and the Raman spectral parameters of IOM (e.g., Quirico et al 2003; Bonal et al 2006; Busemann et al 2007)

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Conclusion