Bland, PA

Cressey, G

Menzies, ON

Russell, SS

Meteoritics and Planetary Science

Spiral - Imperial College Digital Repository

QUANTIFYING THE MODAL MINERALOGY OF FINE-GRAINED CAIs IN ALLENDE.  O. N. Menzies1, P. A. Bland1, G. Cressey2 and S. S. Russell2.  1Department of Earth Science and Engineering, South Kensing-ton Campus, Imperial College, London SW7 2AZ, UK. 2Department of Mineralogy, The Natural History Museum, Lon-don, SW7 5BD, UK. E-mail: o.menzies@imperial.ac.uk.  Introduction: A plethora of isotopic and mineralogical research has been carried out on CAIs in Allende using a range of analyti-cal tools; however, the fine-grained inclusions whose mineralogy is more difficult to both identify and quantify have been less thoroughly studied. Most petrographic, and hence mineralogical, studies have been carried out on polished sections; but section work is limited due to the 2-dimensional nature of the sample and thus little work has been done to quantify the abundance of al-teration products in CAIs. Here we obtain modal mineralogical data for fine-grained CAIs using MicroSource XRD. This allows us to investigate samples as small as 0.018mm3 (as described for bulk carbonaceous chondrites by [1]). That MicroSource XRD is a grain-size-independent tool means that it is most beneficial in observing alteration products in fine-grained inclusions. Modal mineralogical data of bulk CAIs can be used to relate trace ele-ment and isotopic data to specific abundances of mineral phases, to identify fine-grained alteration minerals and to assess whether sequences of progressive alteration exist among CAIs.  Methodology: Twenty two CAIs were hand-picked from the meteorite Allende. Each inclusion was divided in two; one piece was made into a thick polished section and the other piece ground to a fine powder. BSE images and quantitative chemical analyses were carried out on the polished sections using a Jeol 5900LV SEM and Cameca SX50 electron microprobe. Quantitative modal mineralogical analyses were carried out on the powders using the MicroSource XRD with PSD system as outlined in [1].  Discussion: The CAIs were classified, according to the scheme of [2], by SEM on the polished sections; 13 were found to be either fine-grained, spinel-rich (fgsr) inclusions or amoe-boid olivine aggregates (AOAs). The modal mineralogy of each inclusion was quantified to within ~2wt% for each mineral phase. The bulk wt% of Ca and Al were calculated from the modal min-eralogical data and used to group inclusions that must have had similar primary mineralogies. Bulk wt% Fe, Na and Cl, calcu-lated in the same manner, were used to assess the amount of Fe-alkali-halogen metasomatism experienced.  Some of the findings of the XRD analysis that were not clear from the SEM study in-clude: 1) the so-called ‘fgsr’ inclusions contain only 7-23wt% Fe-rich spinel; 2) melilite remains in several fgsr inclusions suggest-ing that these inclusions were protected from intense alteration; 3) plagioclase, probably secondary in origin, is abundant in some fgsr inclusions; 4) a number of fgsr inclusions were altered pref-erentially by Fe-alkali-halogen metasomatism and others by al-teration of melilite to plagioclase; 5) the AOAs show evidence of having undergone more Fe metasomatism than Fe-alkali-halogen metasomatism although alteration proceeded to a similar maxi-mum degree as in the fgsr inclusions. Conclusions: This preliminary study provides the first modal mineralogy of CAIs by MicroSource XRD, allowing alteration sequences to be assessed. CAIs from less altered, primitive car-bonaceous chondrites are the next target of this work, to further investigate CAI formation and evolution. References: [1] Bland, P. A., et al. 2004. MAPS 39:3-16. [2] Grossman, L. 1975. GCA 39:433-454.  67th Annual Meteoritical Society Meeting (2004) 5076.pdf

Quantifying the modal mineralogy of fine-grained CAIs in Allende

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