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

• Published in: Geology
• Main Authors: Giuliani, A., Kamenetsky, V. S., Phillips, D., Kendrick, M. A., Wyatt, B. A., Goemann, K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Geological Society of America 2012
• Subjects: Geology; 1907 Geology; Yakutia
• Online Access: https://espace.library.uq.edu.au/view/UQ:dbabf7c

Mantle xenoliths sampled by kimberlite and alkali basalt magmas show a range of metasomatic styles, but direct evidence for the nature of the metasomatising fl uids 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 fl uids 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 fi rst direct evidence for alkali-carbonate melts in the shallow lithospheric mantle (~110-115 km), above the diamond stability fi eld. 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 fi ndings extend the evidence for alkali-carbonate melts/fl uids permeating the Earth mantle outside the diamond stability fi eld 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 fl uid type documented here likely represent a major metasomatising agent in the Earth's lithospheric mantle.