THE ROLE OF GLOBAL FLUIDS IN THE GENESIS OF MANTLE HETEROGENEITIES AND ALKALINE MAGMATISM
L.N. Kogarko
Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
19 ul. Kosygina, Moscow, 117975, Russia
Keywords: Alkaline magmatism, carbonatites, mantle, metasomatism
Pages: 1213-1224
Subsection: COMPOSITION AND STRUCTURE OF CONTINENTAL LITHOSPHERE
Abstract
Alkaline magmatism originated at 2.5-2.7 Ga and continuously developed throughout the Earth's history. The appearance of alkaline rocks coincided in time with change in geodynamic regime: The plume tectonics was supplemented by plate tectonics. Global plate-tectonic events at the Archean-Proterozoic boundary caused the subduction of the strongly oxidized volatile-enriched oceanic crust and gave rise to a large-scale mantle metasomatism, which then led to the formation of volatile-enriched reservoirs, the sources of alkaline-carbonatite magmatism. Ion microprobe studies of the metasomatized mantle material revealed impurities of primary carbonate melts highly enriched in trace elements. Based on the results obtained, a new model of the two-stage genesis of Ca-rich carbonatites is proposed: (1) metasomatic wehrlitization and carbonatization of the mantle material, (2) partial melting of the wehrlitized mantle, resulting in either carbonate-rich melts or immiscible liquids (in the excess of alkalies) - silicate, carbonatite, and sulfide (at the high activity of sulfur).
Metasomatic fluids were supplied, most likely, with plumes ascending from the core-mantle boundary.
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