Extended chronologies of aqueous alteration in the CM2 carbonaceous chondrites: evidence from carbonates in Queen Alexandra Range 93005

Lee, M. , Lindgren, P., Sofe, M., Alexander, C. and Wang, J. (2012) Extended chronologies of aqueous alteration in the CM2 carbonaceous chondrites: evidence from carbonates in Queen Alexandra Range 93005. Geochimica et Cosmochimica Acta, 92, pp. 148-169. (doi: 10.1016/j.gca.2012.06.005)

Abstract

The Antarctic CM2 carbonaceous chondrite QUE 93005 contains four compositionally distinct carbonates, namely breunnerite, calcite, dolomite and a Ca-poor dolomite. These carbonates can form monomineralic grains, or may be intergrown as bimineralic grains consisting of dolomite plus breunnerite and dolomite plus calcite, or polymineralic grains containing an intergrowth of breunnerite, Ca-poor dolomite and calcite. Carbonates in all grain types have inclusions of Fe,Ni sulphides and/or Mg-Fe phyllosilicates. In the bimineralic grains dolomite crystallised first to be overgrown by breunnerite or partially replaced by calcite. Polymineralic grains are concentrically layered, with breunnerite crystallising first on pore margins to be later etched, then overgrown and partially replaced by Ca-poor dolomite that was itself partly dissolved prior to being overgrown by calcite. Calcite and dolomite have also cemented fractures that cross-cut the fine-grained rims to aqueously altered chondrules and were formed by expansion of the chondrules during their hydration. Overall, the sequence of mineralisation in QUE 93005 was: (1) dolomite, (2) breunnerite, (3) Ca-poor dolomite then (4) calcite. This secular change in carbonate composition and mineralogy reflects changing solution composition and probably also provenance. Mg-Fe phyllosilicates replaced dolomite, breunnerite and Ca-poor dolomite prior to calcite crystallisation and most or all of the sulphides formed after both the phyllosilicates and calcite. Following sulphide crystallisation the edges of carbonate grains were abraded, either by impact ‘gardening’ or as a consequence of fluidisation of the matrix during rapid loss of gas or vapour. Determination of the crystallisation age of dolomite via the MnCr system indicates that aqueous alteration of QUE 93005 began on or before 3.93 ± 0.23 Ma after the formation of the solar system. Overall, the water/rock ratio and <i>f</i>O₂ during alteration of QUE 93005 was similar to that of the CM1s and CR1s, but the lower degree of alteration of QUE 93005 overall suggests that alteration timescales were shorter, possibly due to loss of intergranular liquid water during fluidisation.

• Item Type: Articles
• Keywords: Meteorite, carbonates, nanoSIMS
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Lee, Professor Martin and Lindgren, Dr Paula
• Authors: Lee, M., Lindgren, P., Sofe, M., Alexander, C., and Wang, J.
• Subjects: Q Science > QB Astronomy; Q Science > QE Geology
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences > Geography
• Research Group: Earth Systems
• Journal Name: Geochimica et Cosmochimica Acta
• ISSN: 0016-7037
• Published Online: 22 June 2012
• Copyright Holders: Copyright © 2012 Elsevier
• First Published: First published in Geochimica et Cosmochimica Acta 92:148-169
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher

Project Code	Award No	Project Name	Principal Investigator	Funder's Name	Funder Ref	Lead Dept
49188	1	Spatial and temporal scales of aqueous alteration in icy planetesimals	Martin Lee	Science & Technologies Facilities Council (STFC)	ST/G001693/1	School of Geographical and Earth Sciences