T107 Gemological Research in the 21st Century—Gem Minerals and Localities

Monday, 11 October 2021
1:30 PM - 5:30 PM
A107–A109

DEUTERIUM-DEPLETED COMPOSITIONS OF DIAMOND-HOSTED MINERAL INCLUSIONS FROM THE SUBCONTINENTAL LITHOSPHERIC MANTLE

Paper number: T107-16
Program: Technical Sessions
Time: 4:55 PM – 5:10 PM

Authors

Abstract

Most diamonds crystallize from carbon-rich metasomatic fluids at ~4 - 7 GPa and ~1150 °C in the Earth’s upper mantle at the base of cratons. During growth and lengthy residence in the mantle, diamonds may entrap anhydrous mantle minerals such as olivine, enstatite, and garnet. The trace water in these inclusions provides essential information about mantle characteristics including viscosity, cratonic strength and longevity, and solidus/liquidus temperatures. The source of water in mantle minerals likely varies spatially as a function of mantle dynamics and tectonics, and likely includes subducted crustal fluids and primordial nebular hydrogen ingassed during Earth’s formation. To evaluate hydrogen sources in the mantle, we used the NanoSIMS 50L at Arizona State University to analyze D/H ratios of olivine and enstatite inclusions in diamonds sourced from ~2.1 Ga old rocks of the Guiana Shield, in northern South America. The δD values (D/H normalized to VSMOW) of the anhydrous minerals range from -114 to -258 ‰, with a mean of -178±47 ‰, and FTIR derived lattice bound hydrogen contents of ~10 to 62 ppm H2O. Entrapment depths derived from elastic barometry and water contents do not correlate with δD values. Instead, the diamond-hosted inclusions display a diffuse cluster that extends to values that are more deuterium-depleted than any previous study of mantle materials from xenoliths or basaltic melts. These inclusions preserve relict compositions that are shielded by the diamond host from any subsequent alteration. Relatively depleted Mg (Fo ~ 90) compared to refractory mantle and moderately elevated Mn (~ 465 ppm) compositions of the inclusions indicate that the relic grains were part of the metasomatized mantle. The strongly negative δD values of diamond inclusions likely reflect the isotopic composition of metasomatic fluids percolating beneath cratonic roots and may be a product of isotope mixing between crustal and primordial nebular hydrogen.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-368232

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.