Crystallography of refractory metal nuggets in carbonaceous chondrites: a transmission Kikuchi diffraction approach

Daly, L. et al. (2017) Crystallography of refractory metal nuggets in carbonaceous chondrites: a transmission Kikuchi diffraction approach. Geochimica et Cosmochimica Acta, 216, pp. 42-60. (doi:10.1016/j.gca.2017.03.037)

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Abstract

Transmission Kikuchi diffraction (TKD) is a relatively new technique that is currently being developed for geological sample analysis. This technique utilises the transmission capabilities of a scanning electron microscope (SEM) to rapidly and accurately map the crystallographic and geochemical features of an electron transparent sample. TKD uses a similar methodology to traditional electron backscatter diffraction (EBSD), but is capable of achieving a much higher spatial resolution (5–10 nm) (Trimby, 2012; Trimby et al., 2014). Here we apply TKD to refractory metal nuggets (RMNs) which are micrometre to sub-micrometre metal alloys composed of highly siderophile elements (HSEs) found in primitive carbonaceous chondrite meteorites. TKD allows us to analyse RMNs in situ, enabling the characterisation of nanometre-scale variations in chemistry and crystallography, whilst preserving their spatial and crystallographic context. This provides a complete representation of each RMN, permitting detailed interpretation of their formation history. We present TKD analysis of five transmission electron microscopy (TEM) lamellae containing RMNs coupled with EBSD and TEM analyses. These analyses revealed textures and relationships not previously observed in RMNs. These textures indicate some RMNs experienced annealing, forming twins. Some RMNs also acted as nucleation centres, and formed immiscible metal-silicate fluids. In fact, each RMN analysed in this study had different crystallographic textures. These RMNs also had heterogeneous compositions, even between RMNs contained within the same inclusion, host phase and even separated by only a few nanometres. Some RMNs are also affected by secondary processes at low temperature causing exsolution of molybdenite. However, most RMNs had crystallographic textures indicating that the RMN formed prior to their host inclusion. TKD analyses reveal most RMNs have been affected by processing in the protoplanetary disk. Despite this alteration, RMNs still preserve primary crystallographic textures and heterogeneous chemical signatures. This heterogeneity in crystallographic relationships, which mostly suggest that RMNs pre-date their host, is consistent with the idea that there is not a dominant RMN forming process. Each RMN has experienced a complex history, supporting the suggestion of Daly et al. (this issue), that RMNs may preserve a diverse pre-solar chemical signature inherited from the Giant Molecular Cloud.

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., Saxey, D. W., Reddy, S. M., Fougerouse, D., Rickard, W. D.A., Trimby, P. W., Moody, S., Yang, L., Liu, H., Ringer, S. P., Saunders, M., and Piazolo, S.
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:05 April 2017
Copyright Holders:Copyright © 2017 Elsevier Ltd.
First Published:First publishe in Geochimica et Cosmochimica Acta 216:42-60
Publisher Policy:Reproduced in accordance with the publisher copyright policy

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