T225 Gemological Research in the Twenty-First Century—Gem Minerals and Localities

Wednesday, 12 October 2022
8:00 AM - 12:00 PM
503

COMPOSITION OF THE LITHOSPHERIC MANTLE AND VOLATILE RECYCLING BENEATH THE WEST AFRICAN CRATON RECORDED BY KOIDU DIAMONDS

Paper number: T225-15
Program: Technical Sessions
Time: 10:55 AM – 11:10 AM

Authors

Abstract

Diamonds that form in the lithospheric mantle are primarily associated with peridotite and eclogite host rocks. Peridotite is the dominant rock in the Earth’s upper mantle and is mainly composed of olivine, enstatite and diopside. Eclogite is comprised predominantly of garnet and omphacite, formed by subduction and high-pressure metamorphism of oceanic crust. During diamond formation minerals from mantle peridotite or eclogite may be encapsulated by diamonds. These minerals are shielded by diamonds from subsequent mantle processes, and their compositions broadly reflect conditions prevailing in the mantle at the time of diamond crystallization.

We present the first systematic study of 105 Koidu diamonds recovered from Sierra Leone, West African Craton (WAC), which includes 82 eclogitic diamonds, 18 peridotitic diamonds, and five diamonds with co-occurring peridotitic and eclogitic mineral inclusions. All mineral inclusions were analyzed for major element composition. Garnet and clinopyroxene inclusions were analyzed for trace element composition. The combined dataset indicates a multistage history for the diamond host rocks characterized by melt depletion and metasomatism beneath the WAC.

Diamond itself is a robust tracer of volatile recycling within the Earth as diamonds formed from carbon and nitrogen that are derived from oceanic crust have distinct isotope compositions compared to diamonds with an upper mantle source. In situ analysis of carbon and nitrogen isotope compositions in different growth zones of Koidu diamonds indicates that a subset (n = 24) grew episodically, formed from multiple sources of carbon and nitrogen during discrete growth episodes widely separated in time. These Koidu diamonds have cores precipitated from crustal carbon and nitrogen and rims precipitated from mantle-derived carbon and nitrogen. Formation of diamonds from different pulses of melts/fluids in two distinct episodes provides clues to the localized geodynamic processes and associated volatile recycling in the WAC.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-381567

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