Geochemical Zoning in Metamorphic Minerals

Abstract

Rock’s encode the sum of Earth processes that affected them during their “lifetimes,” and the purpose of most geological studies is to invert that information to refine our understanding of those processes. A metamorphic rock records not just a peak P – T condition, a single cooling rate, or a simple texture, but rather has undergone an evolving history of changes in P and T, mineral abundances, rim compositions, and textures, acting over its metamorphic lifespan, in response to heat flow, stress and strain, and inter- and intragranular movement of material. The greatest advances in understanding metamorphic rocks have been achieved through a recognition that metamorphism is a continuum, and by collection of data and development of models that directly address these continuum processes. Of the many approaches for investigating and interpreting metamorphic rocks, one of the most important is the characterization and quantitative modeling of geochemical zoning in metamorphic minerals. Geochemical zoning is particularly useful, because it is a quasicontinuous record of these metamorphic processes. There have been several previous reviews of chemical zoning in metamorphic minerals (e.g., Tracy, 1982; Loomis, 1983; Chakraborty and Ganguly, 1991; Spear, 1993). This review differs from them in three ways. (i) A catalogue was not compiled of all the minerals that exhibit zonation, and of the elements that are zoned. Although some minerals may be more obviously zoned than others, all minerals must be zoned in some element or isotope at some scale, and it is simply a matter of time before that zonation.