Distribution of trace elements between garnet megacrysts and host volcanic liquids of kimberlitic to rhyolitic composition |
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| Authors: | Anthony J Irving Frederick A Frey |
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| Institution: | The Lunar and Planetary Institute 3303 NASA Road 1 Houston, TX 77058, USA;Department of Earth and Planetary Sciences Massachusetts Institute of Technology Cambridge, MA 02139, USA |
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| Abstract: | Abundances of rare earth elements, Hf, Sc, Co, Cr and Th in garnet megacrysts and their volcanic hosts or matrices are used to estimate garnet/liquid partition coefficients for these elements. Samples include pyropes from kimberlite and highly alkalic basalts, almandines from basalt andesite, dacite, rhyodacite and rhyolite and a spessartine-almandine from alaskite. The pyrope/host partition coefficients are fairly uniform and agree with experimental data within a factor of 2. The almandine/matrix data show more scatter (due in part to impurities in the garnet separates) but the partition coefficients tend to increase with increasing ratio of the matrix. The almandine/matrix partition coefficients are up to a factor of 10 higher than the pyrope/host partition coefficients. The spessartine-almandine is strongly enriched in heavy rare earths (~ 5000 times chondrites), Y, Sc and Co. The wide variation in garnet/liquid partition coefficients from kimberlites to rhyolites cannot be explained as an effect of temperature and we conclude that a major factor is the composition of the melt from which the garnet crystallized. |
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