In situ analysis of Refractory Metal Nuggets in carbonaceous chondrites

Daly, L. et al. (2017) In situ analysis of Refractory Metal Nuggets in carbonaceous chondrites. Geochimica et Cosmochimica Acta, 216, pp. 61-81. (doi:10.1016/j.gca.2016.11.030)

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Abstract

Micrometre to sub-micrometre-scale alloys of platinum group elements (PGEs) known as Refractory Metal Nuggets (RMNs) have been observed in primitive meteorites. The Australian Synchrotron X-ray Fluorescence (XRF) beamline, in tandem with the Maia detector, allows rapid detection of PGEs in concentrations as low as 50–100 ppm at 2 μm resolution. Corroborating these analyses with traditional electron microscopy techniques, RMNs can be rapidly identified in situ within carbonaceous chondrites. These results dispute the assumption of most previous studies: that RMNs are unique to Ca–Al-rich inclusions (CAIs). We find that RMNs are, in fact, observed within all components of carbonaceous chondrites, such as the matrix, chondrules (consistent with observations from Schwander et al. (2015b) and Wang et al. (2007)), and sulphides; though the majority of RMNs are still found in CAIs. The chemistry of RMNs reveals a complex diversity of compositions, which nevertheless averages to CI chondrite abundance ratios. This implies that RMNs are the dominant, if not sole host phase for PGEs. One hundred and thirteen RMNs from this study are combined with reported compositions in the literature, and compared to condensation model compositions similar to Berg et al. (2009), RMNs derived experimentally by precipitation (Schwander et al., 2015a), host phase and host meteorite. Comparisons reveal only weak correlations between parent body processes (sulphidation) and nebular processes (condensation and precipitation) with RMN compositions. It appears that none of these processes acting in isolation or in tandem can explain the diversity observed in the RMN population. Our interpretation is that the Solar Nebula inherited an initially compositionally diverse population of RMNs from the Giant Molecular Cloud; that a variety of Solar System processes have acted on that population; but none have completely homogenised it. Most RMNs have experienced disk and asteroidal processing, but some may have retained a primordial composition. RMNs have been identified in pre-solar graphite grains (Croat et al., 2013). We anticipate that pre-solar RMNs will be present elsewhere in primitive meteorites.

Item Type:Articles
Additional Information:This work was funded by the Australian Research Council via their Australian Laureate Fellowship program.
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Daly, Dr Luke
Authors: Daly, L., Bland, P. A., Dyl, K. A., Forman, L. V., Evans, K. A., Trimby, P. W., Moody, S., Yang, L., Liu, H., Ringer, S. P., Ryan, C. G., and Saunders, M.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Geochimica et Cosmochimica Acta
Publisher:Elsevier
ISSN:0016-7037
ISSN (Online):1872-9533
Published Online:25 November 2016
Copyright Holders:Copyright © 2016 Elsevier Ltd.
First Published:First published in Geochimica et Cosmochimica Acta 216:61-81
Publisher Policy:Reproduced in accordance with the publisher copyright policy

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