DEFINING THE CRYSTALLOGRAPHIC FINGERPRINT OF EXTRATERRESTRIAL TREASURES. L. V. Forman1, P. A. Bland1, N. E. Timms1, L. Daly1, G. K. Benedix1 & P. W. Trimby2. 1Dept. Applied Geology,

Curtin University, GPO Box U1987, Perth, WA 6845, Australia. 2Australian Centre for Microscopy and Microanal-ysis, The University of Sydney, NSW 2006, Australia.

Introduction: In describing fine grained extraterrestrial materials it is common practice to employ a geochemi-

cal shorthand, e.g. quoting the average composition of a particular mineral (frequently olivine in matrices), to con-strain chemical variance and reveal some aspect of the petrogenesis of the sample. Typically, this geochemical fin-gerprint in olivines is expressed as a percentage of one of the olivine end members, fayalite (Fa) and forsterite (Fo), which indicate the proportions of Fe and Mg within each grain (e.g. [1]).

Determining the crystallographic characteristics of the same matrix grains would enable any relationship be-tween grain shape, size and crystal axis coordination to be identified. Whilst grain morphologies of matrix grains in chondrites are often quoted (e.g. [2, 3]), an accumulation of crystallographic data over a large area would allow for a standardised crystallographic fingerprint to be developed for the entire matrix of the meteorite. An understanding of the connection between crystallographic preferred orientation (CPO) and shape preferred orientation (SPO), specific to a sample of interest, allows for quick fabric interpretation and an indication of the growth and formational history of the sample. Furthermore, as the information required can be obtained using a relatively inexpensive but sensitive imaging technique, such quantification could be easily reproduced for a range of samples at a variety of scales.

Methods: In this study, we used electron backscatter diffraction (EBSD) mapping to identify phases and quanti-fy their grain shapes, sizes, and crystallographic orientations in the CV3 meteorite Allende over a large area (4 mm2, ~55800 data points). The analysed data are presented as maps, which can display various crystallographic parame-ters; grain size, grain shape, and primary crystal axis orientations for example. For each grain, the orientations of the major crystallographic axes are quantified, and then displayed collectively on lower-hemisphere equal-area plots to characterize the ‘shape’ and strength of CPOs. Each grain was assigned an ellipse oriented to reflect the physical longest axis of that grain and colour coded to reflect the ellipse orientation (Fig. 1A &B). Contouring of the crystal-lographic data (Fig. 1C) allowed for a comparison between SPO and CPO.

Preliminary results: The {001} poles of many of the olivine matrix grains clustered to produce a weak fabric (Fig. 1C). When compared with the SPO maps (Fig. 1A & B), we found grain shape to be closely related to the {001} pole fabric therefore implying the two parameters may be co-dependent during grain formation. Further investigation in this study will fully determine the signature Allende matrix grain crystallography, complementary to the geochemical signature (Fa45-56 [1]). This approach will enable us to examine shape and shape orientation changes around large and prominent meteoritic features, such as chondrules, and compare such results to CPO data for the same grains. We could also investigate how grain shape might impact fabric formation. By determining the average crystal shape, size and crystallographic axis coordination, we can define the crystallographic fingerprint of a chon-dritic sample mapped with EBSD. We suggest that identification of key crystallographic features; SPO, CPO, and their relationship should routinely form part of a comprehensive description of meteorite samples. We believe this to be important and complimentary to the geochemical data that is commonly determined for meteoritic matrix grains.

References: [1] Peck J. A. 1984. 15th Lunar and Planetary Science Conference pp. 635-636. [2] Scott E. R. D. and Krot A. N. 2003. In Meteorites, comets, and planets. Elsevier Press. pp. 143–200. [3] Brearley A. J. and Jones R. H. 1998. In Planetary materials. Mineralogical Society of America. pp. 3-1–3-307.

Figure 1 EBSD data of case-study application: a) large area map of ellipse orientation, b) expanded map of ellipse orientation, c) contoured CPO data.

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