Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.538l&link_type=abstract
Meteoritics, vol. 30, no. 5, page 538
Other
Angrite, Coefficients, Partition, Crystallization, Kinetics, Fassaite, Meteorites, Angra Dos Reis, Lewis Cliff 86010, Rare Earth Elements
Scientific paper
As part of an ongoing effort to determine partition coefficients, D, for relevant planetary materials, we have determined Ds for REE and Sc, V, Rb, Sr, Y, Zr, Nb, Ba, Hf, Th, and U between fassaitic pyroxene and melt. We have focused on both equilibrium and kinetically-controlled partitioning and the application of the data to the petrogenesis of angrites. We used the synthetic ADOR starting material studied by [1] which was doped with trace elements at levels of 2 to 600 ppm The experiments were conducted in one-atmosphere, gas-mixing furnaces with the oxygen fugacity fixed at 1.2 log units above the iron-wustite buffer. Equilibrium experiments were equilibrated at 1225 degrees C for 24 hrs (ADE-103, 104) and the dynamic crystallization experiment was cooled directly from just above the melting temperature of 1265 to 1000 degrees C at 5oC/hr (ADE-106). Trace element abundances were measured with the PANURGE ion microprobe using the techniques and standards of [2]. The equilibrium Ds from ADE 103, 104 compare well with the data on REE determined by [3] considering the differences in the compositions of the angrite melts studied. Mckay et al. [3] point out that the Kds for fassaite do not differ significantly from diopside [4] and our data are even closer to the diopside values. Green [5] reviews recent experimentally-determined Ds for calcic pyroxene and our data compare well, although there are a few exceptions. The Ds for the heavy REE are slightly lower than those determined by Hart and Dunn [6] in a basaltic system. It is unexpected that this fassaitic pyroxene should compare so well with the normal calcic pyroxenes. In the cooling experiment, ADE-106, the REE and most of the other trace elements are enriched in the pyroxene as the Fe content increases, as in normal igneous zoning. The Ds at the Fe rich rims are systematically lower than the equilibrium values determined near the liquidus. This may be in part attributable to the crystallization of matrix phases that further enriched the residual melt in the incompatible elements, but we feel that the Fe-rich melt is also important. The normal and sector zoning in the fassaite have been documented by X-ray maps produced on a Cameca SX100 microprobe. The sector zoning is most apparent for Ti and Al and ion probe analyses in different sectors show that REE are enriched in Al-rich sectors. The normal zoning is evident in Mg and Fe and there is a normal zoning trend in the REE with the LREE elements showing a more pronounced increase with increasing Fe/Fe + Mg. The closed system fractionation trends, seen in these experiments and also in LEW 86010 are consistent with the igneous history suggested for that meteorite [3,7]. Kirschsteinite and a rhonite-like mineral have crystallized as matrix phases in ADE-106, along with spinel. The kirschsteinite has a complex intergrowth of Ca-rich and a Ca-poor zones, and both have strongly LREE-depleted patterns that resemble those in LEW 86010 [7]. The REE and trace elements are enriched in the Ca-rich zones. The rhonite-like mineral has a flat REE pattern almost indistinguishable from the melt. References: [1] Lofgren G. E. and Lanier A. B. (1992) EPSL, 111, 455-466. [2] Kennedy A. K. et al. (1993) EPSL, 115, 177-195. [3] McKay G. A. et al. (1994) GCA, 58, 2911-2919. [4] Grutzeck M. et al. (1974) GRL, 1, 273-275. [5] Green T. H. (1994) Chem. Geol., 117, 1-36. [6] Hart S. R. and Dunn T. (1993) Contrib. Mineral. Petrol., 113, 1-8. [7] Crozaz G. and McKay G. (1990) EPSL, 97, 369-381.
Fahey Albert J.
Lofgren Gary E.
Wasserburg Gerald J.
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