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- Title
Composition of the Sub-Cratonic Mantle of the Guiana Shield Inferred From Diamond-Hosted Inclusions.
- Authors
Bassoo, R.; Befus, K. S.
- Abstract
The composition of diamond-hosted inclusions provides insight into the character of the sub-cratonic lithosphere of the Guiana Shield. Guyana's Paleoproterozoic diamonds preserve an inclusion suite comprised of forsterite (Fo ∼89.3–91.8), enstatite, chromite, and Cr-pyrope. Raman thermobarometry of entrapped olivine and pyrope inclusions indicate entrapment pressures of ∼5.3–7.0 GPa. Unpolarized Fourier transform infrared spectroscopic measurements of forsterite and enstatite inclusions produce low absorbances from OH. Using established calibrations, those absorbances indicate forsterite and enstatite contain median values of 26 ± 14 and 14 ± 18 ppm H2O, respectively, and suggest a high effective viscosity of 1023.7 ± 2.1 Pa∙s for the lithospheric mantle. When combined with inclusion thermobarometry, diamond residence temperatures suggest paleo-geotherms ranging from 35 to 40 mW m−2. Low-to-moderate Fe2O3 content (1.9 ± 0.8 wt.%) and low oxygen fugacity (log ƒO2 (ΔFMQ) −1.6 ± 1.1) determined from chromite inclusions indicate crystallization in reducing conditions. Forsterite and chromite inclusions retain evidence for metasomatic alteration, including Mn-enrichment in forsterite and chromite rich in Zn. These characteristics indicate that the sub-cratonic lithosphere of the Guiana Shield experienced episodes of partial melting and fluid-driven metasomatism of dry, strongly viscous, and moderately depleted garnet-spinel harzburgite. The Guiana Shield has been relatively stable since the Paleoproterozoic, meaning diamond inclusions may also provide the best means for understanding current conditions in the region's lithospheric mantle.
- Subjects
GUIANA Highlands; SOUTH America; EARTH'S mantle; INTERNAL structure of the Earth; PLATEAUS; FOURIER transform infrared spectroscopy
- Publication
Geochemistry, Geophysics, Geosystems: G3, 2021, Vol 22, Issue 6, p1
- ISSN
1525-2027
- Publication type
Article
- DOI
10.1029/2021GC009841