Effect of grossular on garnet-biotite, Fe-Mg exchange reactions: evidence from garnet with mixed growth and diffusion zoning

Article Abstract:

Geochemical studies of garnets, showing mixed growth and diffusion zoning, of the Wissahickon Group, southeastern Pennsylvania, reveal the effect of grossular content on garnet Fe-Mg exchange reactions. Variation in grossular content with nearly uniform Mg# and Mn concentrations suggests re-equilibration of garnet and matrix minerals with respect to iron, magnesium and manganese. Fe-Mg exchange reactions involving almandine-rich garnets and biotite disclose the minimal effect of calcium on such reactions. A calcium correction of garnets is futile for metapelitic geothermometry.

Research, Observations, Geochemistry

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Sub-critical fracture and associated retrogression of garnet during mylonitic deformation

Article Abstract:

Garnets of the Alpine Fault mylonites were notably fractured and a detailed study was conducted to explain fracture mechanisms andassociated retrogression. The Alpine Fault mylonites formed during progressive deformation that started at 5 Ma. The garnets grew during the Rangitata orogenyand were older than the Alpine Fault by more the 100 million years. Three fracture types were recognized: garnet fill, narrow retrogressive fill and wide, isolating islands of garnet, retrogressive fill. It was found that types two and three predominate over garnet filled fractures.

Environmental aspects, Faults (Geology), Mylonite

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Garnet + liquid equilibrium

Article Abstract:

New research were conducted to find the saturation conditions for garnet and silicate liquid. Starting conditions were natural basalt powders ranging from komatiite to nephelinite, which were partially melted at pressures between 25 and 100 kbar. Findings showed that equations of garnet-liquid equilibrium are simple and may be easily integrated into liquid line-of-descent models. Also, at high pressure, igneous garnets will not always contain Mj as the dominant mineral fraction.

Analysis, Measurement, Chemical equilibrium

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