Carbonatite-metasomatism signatures hidden in silicate-metasomatized mantle xenoliths from NE China
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Date
2018-03
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Geological Journal
Abstract
Lithium isotopes measured in situ in minerals from typical silicate‐metasomatized mantle xenoliths from NE China show that olivine has higher Li abundances of 1.2–4.0 ppm and higher δ7Li values of −0.12–15.46‰ than those in orthopyroxene (Li = 0.56–3.0 ppm; δ7Li = −11.84–7.31‰) and clinopyroxene (Li = 0.21–2.1 ppm; δ7Li = −9.81–14.75‰). The Li distributions between these coexisting minerals show a prominent signature of carbonatite metasomatism. The correlation of δ7Li with Li abundance in orthopyroxene is in accordance with petrological expectation of preferential reaction of orthopyroxene with carbonatite melt. The carbonatite metasomatic agent inferred from the Li isotope systematics is compatible with petrological and geochemical features such as the presence of apatite, appearance of wehrlite, and presence of high‐Mg# minerals and low Ti/Eu in clinopyroxene. The complicated Li isotope composition of olivine implies that carbonatite metasomatism occurred after silicate metasomatism. These features indicate that the predominant silicate metasomatism is extensively recorded by petrology, mineralogy, and geochemistry, whereas Li isotope systematics is more sensitive to subordinate carbonatite metasomatism. The metasomatic melts are inferred to be relatively enriched in 6Li and thus suggests that they were probably derived from a highly dehydrated slab.
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Keywords
Li isotopes, Lithospheric mantle, Mantle metasomatism, Mantle xenoliths