CHLORINE IN THE EARTH’S MANTLE AS AN INDICATOR OF THE GLOBAL RECYCLING OF OCEANIC CRUST
E.V. Asafov1, A.V. Sobolev1,2, V.G. Batanova1,2, M.V. Portnyagin1,3
1Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991, Russia
2Universite Grenoble Alpes, Institut des Sciences de la Terre, CNRS, F-38041, Grenoble, France
3GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148, Kiel, Germany
Keywords: Melt inclusions, komatiites, mantle petrology, transition zone, water in mantle, geochemistry of volatiles, plume magmatism, Archean mantle, plate tectonics
Abstract
Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.
DOI: 10.15372/RGG2020161
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