Minerals mass redistribution

Finally, periodic non-destructive imaging by x-ray CT is used to view redistribution of mineral mass within the heated sample, as illustrated in Figure 2. X-ray CT records changes in density within the sample as a proxy for mineral removal or redistribution, with the scanner used in this study capable of resolving down to -1/1000 of the diameter of the sample. For sample diameters of 3-5 cm used in the following study, the minimum voxel size scales to the order of 30-50 pm. This resolution is at the limits of utility in defining mass redistribution - it is adequate to define mass redistribution in samples of Berea and limestone, but not for the relatively tighter fracture in novaculite. 

The stress-mediated redistribution of mineral mass by pressure solution may be represented by the three serial processes of mineral dissolution beneath the contacting asperity, transport along an interfacial water-film to the fracture void, and then redistribution by precipitation within the fracture void, or efflux from the sample. The asperities interpenetrate as mass is removed from contacting asperities, in a similar manner to the mechanisms that drive grain interpenetration and cementation in sandstones [Yasuhara et al., 2003], as illustrated in Figure 10. 

Changes in fracture transport characteristics that result from the removal or redistribution of mineral mass within a fracture may be constrained by calculating the evolution of permeability with the concurrent loss or redistribution of mineral mass. Mass loss is unambiguously recorded by the effluent flux, but redistribution may only be discerned by imaging. Independent measurements of fluid and mineral flux, concurrent with non-invasive imaging, are used to constrain the processes controlling mass redistribution within the fracture. 

During metamorphism and recrystallization, oxygen isotopes are redistributed among mineral phases, according to the mass-dependent equilibrium fractionations corresponding to the peak metamorphic temperature. The measured mineral-pair fractionations (usually for major minerals olivine, pyroxene, and feldspar) can then be used for metamorphic thermometry, yielding temperatures of 600 °C for an L4 chondrite, and 850 50 °C for several type-5 and type-6 chondrites (Clayton et al., 1991). Isotopic equilibration, even in type-6 chondrites, involves oxygen atom transport only over distances of a few millimeters (Olsen et al., 1981). 

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