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

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.