Trace Element Evidence for the Formation of Antarctic Primitive Achondrites

Details Trace Element Evidence for the Formation of Antarctic Primitive Achondrites Trace Element Evidence for the Formation of Antarctic Primitive Achondrites

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.297t&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 297

Computer Science

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Scientific paper

Primitive achondrites, as represented by Winona, Acapulco and Lodran, have a grossly chondritic major chemical composition but indicate fractionated lithophile trace element features [1,2]. In order to investigate the petrogenesis of this type of unique meteorites, the fractionated REE patterns are of particular interest. As a part of a consortium study on the unique Antarctic meteorites [3], We have analysed five Antarctic primitive achondrites (Lordran-Acapulco type: Y-74063, ALH-78230, Y-74357, Y-8002; Winonite type: Y-75300) for lithophile trace elements (alkalis, alkaline earths and REE) by isotope dilution. The first group, Y-74063 and ALH-78230, has a slightly light-REE depleted pattern. The second group, Y-74357, shows the significantly light REE depleted fractionation with a relatively large negative Eu anomaly and lack of plagioclase component. The third group, Y- 75300 and Y-8002, indicates middle REE depleted, V-shaped pattern with a large positive Eu anomaly. This type of REE pattern has been reported for Winona 4189, Mt. Morris [1] and the unique clasts with dunitic mineralogy of Antarctic ordinary chondrites [4,5]. All these meteorites analysed in this work indicate an extreme depletion of rubidium but not necessarily of potassium. The mineralogy of these meteorites is, in general, related to the REE fractionations. Using the mineral/liquid partition coefficients of trace elements, the petrogenetic model calculations were performed. The results give indication on the origin of these meteorites. Y-74357 was formed through a small degree of partial melting (less than a few %) from a chondritic starting material. Y-74357 formed by a larger degree (~12%) of partial melting. The V-shaped REE pattern of third group may not be explained by a simple partial melting of chondritic material but by assuming the solid state equilibration within a reservoir with chondritic mineralogy. It is suggested that these meteorites might be derived from large chondrule-like materials depleted in phosphate and clinopyroxene. We are indebted to Dr. K. Yanai and H. Kojima for meteorite specimens studied in this work. References: [1] Prinz, M., Waggner, D.G. and Hamilton, P.J., Lunar Planet. Sci. XI, 902-904. [2] Palme, H., Schultz, L., Spettel. B., Weber, H., Wanke, H. and Christoph Michel-Levy, M. (1981) Geochim. Cosmochim. Acta 45, 727-752. [3] Nagahara H., Fukuoka T., Kaneoka I., Kimura M., Kojima H., Kushiro I. Tsuchiyama A. and Yanai K. (1990) Ast. 15th Symp. Antarct. Meteor. 92-94. [4] Nakamura N., Yanai, K. and Matsumoto, Y. (1984), Meteoritics 19, 278-279. [5] Warren, P.H. and Kallemeyn G.W. (1979) Proc. 19th Lunar Planet. Sci. Conf., 475-486.

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