4.7 Article

Potassium isotopic evidence for sedimentary input to the mantle source of Lesser Antilles lavas

Journal

GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 295, Issue -, Pages 98-111

Publisher

PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2020.12.013

Keywords

Potassium isotopes; Subduction zone magmatism; Crustal recycling; Lesser Antilles arc; Martinique Island

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This study used stable potassium isotopes to track the crustal signatures in lavas from Martinique Island in the Lesser Antilles arc. The results show that the variations in K isotopes in these lavas are related to chemical parameters and radiogenic isotope ratios, indicating modification of mantle sources by input of subducted sediments. Most lavas have <1% to 5% of subducted sediments added to their mantle sources, with some samples showing divergence likely caused by the addition of K-bearing fluids from slab dehydration.
Arc magmas derived from mantle melting often have trace element and isotopic signatures that indicate crustal contributions. The origin and extent of crustal contributions are critical constraints for quantifying crust-mantle recycling at subduction zones; however, it is difficult to distinguish between inputs from the downgoing oceanic slabs and that of the over-riding arc crust. Here we present a novel study using stable potassium (K) isotopes to fingerprint the long-debated crustal signatures in lavas from Martinique Island, Lesser Antilles arc. We find delta K-41 of Martinique lavas varies from -0.66 to 0.01 parts per thousand and correlates with chemical parameters and radiogenic isotope ratios. These correlations cannot be produced by assimilation of arc crust during magma ascent. Rather, they reflect mantle sources that have been modified by the input of subducted sediments. Most lavas display a strong negative correlation between delta K-41 and radiogenic isotopic compositions, from typical mantle values of -0.37 parts per thousand to much lower values of -0.66 parts per thousand , suggesting the addition of <1% to 5% subducted sediments to their mantle sources. Notable divergence in this correlation occurs for three samples with variably high delta K-41 (-0.28 to 0.01 parts per thousand), which is likely caused by the addition of isotopically heavy K-bearing fluids derived from slab dehydration. Our study indicates that a substantial fraction of large ion lithophile elements, including K in arc magmas and thus nascent crust, is inherited from subducted crust; K isotopes are useful tracers of this cycling process. (C) 2020 Elsevier Ltd. All rights reserved.

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