Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds |
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| Authors: | Wuyi Wang Shigeho Sueno Eiichi Takahashi Hisayoshi Yurimoto Tibor Gasparik |
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| Institution: | (1) Center for High Pressure Research, State University of New York, Stony Brook, NY 11794-2100, USA e-mail: wang@sbmp04.ess.sunysb.edu, US;(2) Institute of Geoscience, The University of Tsukuba, Tennotai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan, JP;(3) Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo 152-8551, Japan, JP |
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| Abstract: | Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined
using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast
to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements
(REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the
lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations
were observed with their major element compositions. Analyses of randomly selected two to three points within every grain
of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity,
and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the
trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian
diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets
become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in
high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism
by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by
carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature
are the main reasons for the large variations of REE patterns and other trace element concentrations.
Received: 27 April 1999 / Accepted: 1 March 2000 |
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