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Nature of alkali-carbonate fluids in the sub-continental lithospheric mantle

journal contribution
posted on 2023-05-17, 13:36 authored by Giuliani, A, Vadim KamenetskyVadim Kamenetsky, Phillips, D, Kendrick, MA, Wyatt, BA, Karsten GoemannKarsten Goemann
Mantle xenoliths sampled by kimberlite and alkali basalt magmas show a range of metasomatic styles, but direct evidence for the nature of the metasomatising fluids is often elusive. It has been suggested that carbonate-rich melts produced by partial melting of carbonated peridotites and eclogites play an important role in modifying the composition of the lithospheric mantle. These mantle-derived carbonate melts are often inferred to be enriched in alkali elements; however, alkali-rich carbonate fluids have only been reported as micro-inclusions in diamonds and as unique melts involved in the formation of the Udachnaya-East kimberlite (Yakutia, Russia). In this paper we present the first direct evidence for alkali-carbonate melts in the shallow lithospheric mantle (∼110–115 km), above the diamond stability field. These alkali-carbonate melts are preserved in primary multiphase inclusions hosted by large metasomatic ilmenite grains contained in a polymict mantle xenolith from the Bultfontein kimberlite (Kimberley, South Africa). The inclusions host abundant carbonates (magnesite, dolomite, and K-Na-Ca carbonates), kalsilite, phlogopite, K-Na titanates, and phosphates, with lesser amounts of olivine, chlorides, and alkali sulfates. Textural and chemical observations indicate that the alkali-carbonate melt likely derived from primary or precursor kimberlite magmas. Our findings extend the evidence for alkali-carbonate melts/fluids permeating the Earth mantle outside the diamond stability field and provide new insights into the chemical features of previously hypothesized melts. As metasomatism by alkali-rich carbonate melts is often reported to affect mantle xenoliths, and predicted from experimental studies, the fluid type documented here likely represent a major metasomatising agent in the Earth’s lithospheric mantle.

History

Publication title

Geology

Volume

40

Issue

11

Pagination

967-970

ISSN

0091-7613

Department/School

School of Natural Sciences

Publisher

Geological Soc America

Place of publication

Boulder, USA

Rights statement

Copyright 2012 Geological Society of America

Repository Status

  • Restricted

Socio-economic Objectives

Expanding knowledge in the earth sciences

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