Isotopic disequilibrium during rapid crustal anatexis: implications for petrogenetic studies of magmatic processes

Abstract

The geochemical consequences of crustal anatexis are investigated over a range of mineral dissolution rates; 10 −10 cm/s (Mono Lake, CA), ∼10 −16 cm/s (Seram, Indonesia) and <10 −16 cm/s (regional migmatite terranes). Isotopic disequilibrium is established in all cases but surprisingly elemental partition coefficients appear close to equilibrium. This observation indicates that the melting and segregation rates are rapid enough to prevent full isotopic equilibration but that diffusion operates over a sub-100-μm length scale. A disequilibrium melting model is proposed in which diffusion maintains chemical equilibrium between the outer portions of minerals and melt but does not achieve full isotopic equilibrium between the entire protolith and melt. Preservation of isotopic disequilibrium in many metamorphic rocks has widespread implications. Dating metamorphic rocks using mineral–whole rock or mineral–mineral pairs may yield erroneous ages, as observed in the metasediments of Seram where ages range from −15 to 201 Ma, despite anatexis at ∼6 Ma. Consequently, some age estimates in the literature may be incorrect. Mono Lake is an example of how rapid melting will produce a sequence of chemically and isotopically distinct melts that are in isotopic disequilibrium with their crustal source. Hence, quantitative prediction of assimilation–fractional crystallisation (AFC) and anatectic processes requires full knowledge of the melting relations of the protolith. The chemical and isotopic disequilibrium associated with anatexis suggests that Nd model ages of granites and their protoliths will be incorrect when garnet and/or minor phases control a significant amount of the REE budget.