T141 From Stardust to Planets: A Geological Tour of the Career of Harry Y. McSween Jr.

Monday, 26 September 2016
1:30 PM - 5:30 PM
Room 201

METAMORPHISM OF CK CARBONACEOUS CHONDRITES: SO, WHAT’S YOUR PETROLOGIC TYPE?

Paper number: T141-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Abstract

The CK chondrites are a group of highly-oxidized carbonaceous chondrites characterized by the presence of magnetite, Ni-bearing sulfides, and NiO-rich olivine. The CK chondrites are also the only carbonaceous chondrite group to exhibit the full range of thermal metamorphism (from petrologic type 3 to type 6). Though the majority of CK chondrites are equilibrated (types 4-6), thirty unequilibrated (type 3) CK chondrites have been discovered since 1990. Unequilibrated chondrites are particularly sensitive to the effects of metamorphism, and are thus divided into petrologic subtypes 3.0 – 3.9. Unequilibrated CK chondrites are thought to be metamorphosed to conditions associated with petrologic subtype 3.7 or higher. However, no criteria for establishing petrologic subtypes in the CK chondrites have been established. In this study, we examine the mineralogies and textures of seven unequilibrated and two equilibrated (type 4) CK chondrites to better characterize the extent of metamorphism experienced by the unequilibrated CK chondrites.

Though there are many geochemical indicators of metamorphism, homogeneity of chondrule olivine is one of the most useful. In unequilibrated chondrites, Fa content of olivine becomes increasing homogenous with petrologic type, eventually converging on equilibrated values (~30-32 mol% Fa in the CK chondrites). Though olivine homogeneity in most samples is consistent with petrologic types 3.8 or 3.9, the unequilibrated CK chondrites exhibit varying degrees of homogeneity. NWA 5343 is the least homogeneous, with matrix olivine ranging from Fa34.9-Fa37.9 and chondrule olivine from Fa0.3-Fa43.4. Percent mean deviation (PMD) of NWA 5343 is 17.1. When compared to the ordinary chondrites, this degree of heterogeneity and disequilibrium is more consistent with that of petrologic type 3.6 or 3.7. In contrast, the most homogeneous unequilibrated CK chondrite (DaG 431) has a PMD of 6.3. Abundances of CaO, Al2O3, MnO, CaO, and NiO in chondrule olivine also suggest that there are varying degrees of metamorphism within the CK chondrites.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-283567

© Copyright 2016 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.