Restoration of the elemental and stable-isotopic compositions of diffusionally altered minerals in slowly cooled rocks

Abstract

In polymineralic plutonic igneous and metamorphic rocks, slowly cooled crystals seldom retain their initial chemical compositions. This paper introduces a new, simple and widely applicable material-balance method that recovers the former compositions of minerals – regenerating in many rock types their chemical memory of the environment in which they formed – without a priori knowledge of temperature or pressure or diffusion kinetics. Restored stable-isotopic, trace-element, and/or major-element compositions provide a basis for interpretations of petrogenetic processes and conditions, including recovery of peak temperature and pressure, depth, and average diffusion distance during re-equilibration. Case studies illustrate applications of the mineral-restoration technique to regional crustal dynamics, ore metallogeny, and igneous fluid dynamics and petrogenesis. The first illustrative case addresses the controversial origin of decimetre-thick, modally graded rock layers in the Skaergaard intrusion. Layer-wide mineral-chemistry gradients that previously were ascribed a primary origin are here shown to be due to sub-solidus diffusive re-equilibration amongst minerals that initially were chemically uniform. This finding redefines the constraints to be satisfied by fluid-dynamic models of chemical differentiation processes in the magma chamber, and eliminates the basis of prior interpretations of the modally graded layers as products of in situ crystallization on the magma chamber's floor. In another case, lower crustal olivine-chromite cumulates underwent a long two-stage history of mineral-composition readjustment spanning >500° C. The technique introduced here removes the effects of the second-stage solid-state diffusion, recovering mineral compositions that represent the igneous solidus temperature at the termination of the metasomatic stage. The third example removes effects of retrograde diffusive ion-exchange from garnet, hornblende, and clinopyroxene in order to restore the rock's chemical memory of its pressure and depth of crystallization. The depth corresponds to a measure of extreme Cenozoic uplift and erosion (∼58 km) along the Main Mantle Thrust, which juxtaposes the underthrusting Indian Plate and the over-riding Kohistan island-arc terrain in the Pakistani Himalayas.