期刊
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY
卷 158, 期 2, 页码 197-222出版社
SPRINGER
DOI: 10.1007/s00410-009-0378-5
关键词
Lithium; Li isotopes; Sub-arc mantle; Mantle xenoliths; Metasomatism; Kamchatka
资金
- NSF [EAR 0609689]
- Sonderforschungsbereich 574 at Kiel University [159]
Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are derived from the sub-arc mantle and record element transfer from the slab to the arc. Olivine and orthopyroxene in the harzburgites have Li isotopic compositions (delta Li-7 = +2.8 to +5.6) comparable to estimates of the upper mantle (delta Li-7 similar to +4 +/- A 2). The pyroxenite veins, which represent modal metasomatism and may therefore provide information about the metasomatic agent, have mantle-normalized trace element characteristics that suggest overprinting of their mantle source by an aqueous, slab-derived fluid. These include relative enrichments of Pb over Ce, U over Th and Sr over Nd. Li is enriched relative to the HREE, and ortho- and clinopyroxene from the veins are in Li elemental and isotopic equilibrium with each other and the surrounding harzburgite. Vein samples (delta Li-7 = +3.0 to +5.0) do not record a significant slab-derived delta Li-7 signature. These observations can be reconciled if slab Li diffusively re-equilibrates in the mantle wedge. Modeling demonstrates that Li equilibration of small (1-2 cm width) veins or melt conduits is achieved at mantle wedge temperatures within 10(1)-10(5) years. We conclude that strongly fractionated Li isotopic signatures cannot be sustained for long periods in the sub-arc mantle, at least at shallow (< 70 km) depths.
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