Theory and Applicability of a Recrystallized Grain Size Paleopiezometer

Abstract

An approximate theoretical relation is derived which relates stress during steady state creep to both subgrain size and dynamically recrystallized grain size. The relation results from equating the dislocation strain energy in the grain boundary to that in the enclosed volume. Available data on metals and silicates are in excellent agreement with the theory. For paleopiezometry, the recrystallized grain size must be preserved by quenching, by cooling under stress, or by inhibition of grain growth by intimate mixture of two or more phases. In general, stress may be underestimated using rocks in which grain size has been reduced by dynamic recrystallization, especially if the grain size is very small. Stress may be overestimated using coarse grained rocks in which the grain size has increased toward the steady state value. Quantitative limits remain to be established. The theoretical relation can in principle be applied to any metal or mineral if only the effective isotropic elastic moduli and the Burgers vector are known. When used as a paleopiezometer, the technique indicates that high stresses on the order of 100 MPa are not infrequently associated with mantle diapirism and with large scale thrust faulting. Consideration of the Mt. Albert ultramafic body suggests that texturally inferred stresses from peridotite massifs and from ultramafic xenoliths in alkali olivine basalts might reflect either horizontal variations in stress across a rising diapir or else a vertical variation in stress as defined by the pyroxene geobarometer (Mercier et al. 1977). In either case the stresses are probably characteristic of local diapirism. Stresses characteristic of global upper mantle flow might be inferred from xenoliths originating from above kimberlite-producing diapirs.