Mineral Transformations in Low Grade Rocks

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Transmission electron microscope and other electron microbeam techniques are being emphasized in studies of diagenesis through biotite-grade metamorphism of fine-grained clastic sediments and igneous rocks. The focus is on charac- terization of clay minerals, for which the high resolution techniques permit direct structural, chemical and textural characterization. Comparison of characterized samples that cover the range of a given process permit mechanisms of transitions to be defined. This research emphasizes the parallel effects of changing temperature, water/rock ratio and stress in promoting reactions in metastable phases, for which transitions are in predictable Ostwald-step-rule-like changes toward the equilibrium state generally reached at greenschist facies conditions. Specific projects include: (1) Gulf Coast argillaceous sediments, with emphasis on illite/smectite relations (2) quanti- tative studies of the causes of variation in illite crystallinity (3) diagenesis through biotite-grade metamorphism, with emphasis on processes causing textural changes in southern New Zealand rocks, principally greywackes (4) low grade metamorphism with contact metamorphic overprint in Welsh pelites (5) alteration of marine basalts, emphasizing trioctahedral clay minerals in DSDP core 540B. Such studies lead to a better understanding of diagenesis in clay-rich rocks which will have important implications to oil and gas exploration.