Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.522h&link_type=abstract
Meteoritics, vol. 30, no. 5, page 522
Other
2
Achondrites, Basaltic, Crystallization, Fractional, Elements, Rare Earth, Trace, Eucrites
Scientific paper
The origin of eucrites is still controversial. Since the early and extreme models that considered most of the eucrites to be the products of either fractional crystallization of more magnesian liquids [1] or low pressure partial melting of an olivine+pyroxene+plagioclase assemblage [2], it has been pointed out that each of these alternatives has its problems and that a number of processes, including partial melting, fractional crystallization, and magma mixing have to be invoked to interpret the genetic relationships between eucrites, diogenites, and howardites [e.g., 3 and refs. therein]. Most of the studies that have used minor and trace elements were made by analyzing whole rocks. The assumption was usually made that whole rock abundances (including of incompatible elements) represent the compositions of their parent melts. However, a substantial fraction of these elements are sited in minor and trace phases (REEs in phosphates, Ti in ilmenite...) that are heterogeneously distributed in eucrites. It is therefore legitimate to ask whether the differences in incompatible element concentrations that led to the classification of eucrites into a number of distinct groups are truly representative of their parent melt compositions or whether these differences simply reflect local variations of the abundances of these minor or trace minerals in the small samples typically analyzed. Here, we studied individual grains of pyroxene and plagioclase, the first two minerals to crystallize from a eucritic melt, in an attempt to derive independent information about the nature of the parent melts they grew in contact with. We report more than 400 analyses of pyroxene and plagioclase in ten eucrites representing all the chemical groups previously defined (Stannern and Ibitira from the Stannern trend, Nuevo Laredo and Lakangaon from the Nuevo Laredo trend, Chervony Kut and Sioux County from the Main group, the Moore County and Moama cumulate eucrites and the unequilibrated eucrites Pasamonte and ALHA 76005). Despite some complications (most eucrites have experienced reequilibration, plagioclase and pyroxene commonly are clouded by the inclusion of other minerals, extensive trace element remobilization due to weathering occurred in the Antarctic eucrite...), the data are generally consistent with the assumption that, for many eucrites, the whole rock REE concentrations are good approximations of their parent melt compositions. With a few exceptions, the data also support the group classification previously made and, indeed, confirm the accepted view of plagioclase and pyroxene cocrystallization. Expected igneous trends (i.e., increasing REE, Y, Ti and Zr concentrations with decreasing mg numbers) are clearly visible in Pasamonte pyroxene. In more equilibrated eucrites, Y, Ti, and Zr concentrations still typically are vestiges of the primary igneous characteristics. However, a number of our observations are in apparent contradiction with previous work. Incompatible element concentrations are higher in pyroxene of Ibitira than of Stannern (in contrast with the whole rock results). Whether the discrepancy is due to higher phosphate, ilmenite, and zircon contents in Stannern than in Ibitira, or whether it has another explanation is not yet clear. The high LREE/HREE ratios recorded in the silicates of the cumulate eucrites do not support the suggestion that non-cumulate and cumulate eucrites were derived from the same source region. In addition, the different LREE/HREE ratios in Moore County and Moama are not consistent with these two meteorites having formed by simple fractional crystallization from a common parent melt. References: [1] Mason B., Meteorites, 116-119, Wiley. [2] Stolper E. (1977) GCA, 41, 587-611. [3] Warren P. H. and Jerde E. A. (1987) GCA, 51, 713-725.
Crozaz Ghislaine
Hsu Weibiao
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