Constraints on the origin of mantle-derived low Ca garnets

Article Abstract:

Multiple melt extraction of mantle residues formed low calcium (Ca) garnets with chromium(III) oxide (Cr2O3) greater than 4%, while high-pressure ultramafic melt interval in the Archean mantle formed chromium-poor low Ca garnets. Cr-rich low Ca garnets in spinel-containing diamonds were formed from the crystallization of very depleted Cr-containing protoliths in shallow Precambrian mantle levels. In addition, oxygen isotope analysis could be used to determine whether the protolith was dry or serpentinized before it underwent subduction and metamorphism.

Analysis, Mineralogical research, Rocks, Igneous, Xenoliths, Komatiite

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Ferric iron in mantle-derived pyroxenes and a new oxybarometer for the mantle

Article Abstract:

Mantle-derived spinel lherzolite xenoliths, with coexisting clino- and orthopyroxes, were analyzed for ferric iron content in order to determine the oxidation state of the mantle. The results of the study indicated greater ferric iron content in clinopyroxenes than in orthopyroxenes of the spinel lherzolite. Reactions involving olivine-, orthopyroxene- and clinopyroxene- saturated spinel lherzolites had oxygen fugacities which conformed with the results of earlier studies involving the olivine-orthopyroxene-spinel oxybarometer.

Research, Usage, Oxygen, Pyroxene

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The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa

Article Abstract:

An analysis describing hydroxyl distribution in garnets is presented. Garnets from subcontinental mantle were examined for OH content using infrared (IR) spectroscopy. The examination revealed IR spectra in the 3400-3700 cm(super-1) region with constant absorption patterns, which were bound within the stone crystal and contaminated by low intensity OH absorptions. The samples were taken from rock types in the form of xenoliths in kimberlites of south Africa.

Natural history, Earth, Southern Africa, Mantle (Geology), Africa, Southern

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