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Lower mantle diamonds from the Rio Soriso (Juina, Brazil) Hayman, Patrick

Abstract

The morphology, colour, fluorescence (FL), cathodoluminescence (CL), nitrogen content and aggregation state, internal morphology and mineral inclusion chemistry have been studied for sixty-nine alluvial diamonds recovered from the Rio Soriso, Juina area, Brazil. The majority of the Rio Soriso diamonds are colourless, but grey, yellow, brown and non-uniform colours are also observed. Diamonds fluoresce and cathodoluminesce a variety of shades and intensities of blue, turquoise and green. There is a correlation between FL and body colour, with most brown diamonds fluorescing turquoise or green. In general, diamonds with brighter CL have higher nitrogen concentrations. Diamond crystals are generally well resorbed, fragmented and plastically deformed, all of which contributed to the obscuration of the primary crystal habit. Most diamonds are classified as either tetrahexahedroids or dodecahedroids, but crystals of undetermined morphology are abundant. CL examination of polished diamond surfaces and plates indicate that some crystals developed through intermittent episodes of octahedral growth and resorption. Infrared spectroscopic studies show that the diamonds contain trace amounts of both nitrogen (0-541 ppm, averaging 72 ppm) and hydrogen. There is strong positive correlation between nitrogen and hydrogen concentrations. Nitrogen in most diamonds is fully aggregated as B centres (type IaB, 54%), but there is also a large proportion of nitrogen free stones (type Ha, 38%). A small population of diamonds contain nitrogen in the form of A centres (type IaA, 1.5% and IaAB, 7%), which is indicative o f residence in the upper mantle. Nitrogen contents typically decrease from crystal core to rim. Mineral inclusions recovered and analysed from 30 diamonds include: ferropericlase, MgSi-perovskite, CaSi-perovskite, 'olivine', tetragonal-almandine-pyrope-phase, pyrrhotite, magnetite, pyrope-almandine-grossular garnet and perovskite. Based on the mineralogy of diamond inclusions and diamond morphology, CL, FL, nitrogen content and aggregation state, the Rio Soriso suite was subdivided into six paragenetic groups which formed in the lower mantle, transition zone and upper mantle. These paragenetic groups are: 1) ultramafic lower mantle diamonds, 2) ultramafic diamonds sourced from the boundary between the upper and lower mantle, 3) mafic diamonds sourced from 580- 660 km, 4) mafic diamonds sourced from any sub-lithospheric depths, 5) eclogitic diamonds sourced from the upper mantle, and 6) peridotitic diamonds sourced from the upper mantle. The preferred explanation for the sampling o f such a large depth interval within the mantle (∼200 to >660 km) is that the diamonds were entrained in a plume that originated at the core-mantle boundary.

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