CHARACTER OF THE SUB-CRATONIC MANTLE OF THE GUIANA SHIELD INFERRED FROM DIAMOND-HOSTED INCLUSIONS

The composition of diamond-hosted inclusions provides insight into the character of the sub-cratonic lithosphere of the Guiana Shield. The Guiana Shield has long been an economic source of diamonds, however their scientific value as mantle xenocrysts has not yet been used to constrain the nature of the mantle beneath the Guiana Shield. Guyana’s Paloeproterozoic diamonds preserve an inclusion suite comprised of forsterite (Fo_89-92), enstatite (Mg^# - 0.92), and Cr-pyrope (Al/Cr - 1.8±0.1). Raman thermobarometry of entrapped olivine and pyrope inclusions preserve entrapment pressures of 4.4±0.9 GPa. Olivine and enstatite inclusions contain 19±11 ppm and 4±5 ppm H₂O, respectively. The inclusions indicate the diamonds were sourced from the shallow lithospheric mantle, which was composed of dry, moderately-depleted garnet-spinel harzburgite. When combined with inclusion thermobarometry, diamond residence temperatures project along paleo-geotherms between 37-49 mWm^-2. Low-to-moderate Fe³⁺ (8.7±4.2 wt.%) and low ƒO₂ (FMQ -2.19±1.08) in chromite inclusions indicates crystallization in strongly reducing conditions. Forsterite and chromite inclusions retain significant evidence for metasomatic alteration, including Mn-enrichment in forsterite and chromite rich in Cr and Zn. Diamond inclusions indicate that the sub-cratonic lithosphere of the Guiana Shield experienced a long, complex history that included episodes of partial melting and fluid-driven metasomatism, within highly reducing and dry conditions. The Guiana Shield has been relatively stable since Paleoproterozoic, meaning diamond inclusions may provide the best understanding of current conditions.