Velbel, M. A., J. T. McGuire, and A. S. Madden. 2007. Scanning electron microscopy of garnet from southern Michigan soils: Etching rates and inheritance of pre-glacial and pre-pedogenic grain-surface textures. Developments in Sedimentology 58:413-432.

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Garnet surface textures are known to be modified by processes in several portions of the sedimentary cycle: (1) weathering (first-cycle), (2) erosion, transportation, and deposition by moving water or ice, and (3) sandstone diagenesis. Previous studies have not assessed, however, the relative importance of inherited and newly formed grain-surface features of sand-size garnet grains. Garnets from soils developed by weathering of glacial (till and outwash) deposits at agricultural and forested sites in Michigan’s Lower Peninsula were examined by scanning electron microscope to determine possible associations of grain-surface textures with parent material provenance and soil processes. All Michigan soil localities examined contain garnets that exhibit surface features apparently inherited from their pre-existing sedimentary source rocks, and/or features formed during glacial transport. Widespread preservation of inherited pre-pedogenic surface textures suggests that garnet in Michigan soils has undergone little weathering since deglaciation. Some garnet grains from both forested and agricultural soils, however, have well-formed 1–10 μm etch pits on their surfaces that may have formed either in garnet source-area regoliths or by weathering in the present soil.

At published garnet dissolution rates experimentally determined under weathering conditions, formation of 1–10 μm etch pits would take decades to centuries. Given that mineral weathering rates in nature are almost invariably one to three orders of magnitude slower than in the laboratory, the small size of the observed Michigan soil–garnet etch pits is broadly consistent with the age of the Michigan soils. This suggests that the “response time” of garnet surface textures to environmental perturbations is long relative to the time since the last deglaciation; the time required to develop large, extensive, and abundant weathering textures on garnet is longer than the age of Michigan soils. Even millennia under present weathering conditions are not sufficient to destroy pre-pedogenic and pre-glacial (e.g., source-rock) garnet surface textures and replace them with characteristic garnet weathering textures.

DOI: 10.1016/S0070-4571(07)58015-5

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Undergraduate Research

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