SYSTEMATICS OF MAJOR ELEMENT PARTITIONING AMONG GRAPHITIC METAPELITES FROM WESTERN MAINE

An undergraduate thesis project supervised by Meg Thompson mapping Paleozoic rocks from the Boston Basin launched the senior author to a career of studying minerals from within a geological context, firmly rooted in fieldwork. In this paper, a progress report of a 20-year (and ongoing) project to complete chemical analyses of all elements present in all minerals in a suite of rock-forming minerals from the metapelites of western Maine will be presented. An extensive collection (>600 samples) of metapelites studied by Charles Guidotti and students covers the spectrum of metamorphic grades (chlorite through second sillimanite zone) under roughly isobaric 4 kbar conditions. The Mg-Fe contents of the minerals vary as a function of bulk composition and of coexisting oxide and sulfide assemblage. In order to reduce the compositional variables that may influence the minerals, samples are considered in terms of saturating phases (e.g. ilmenite for Ti) and oxidation conditions (e.g. graphite or magnetite). This approach makes it possible to utilize an isobaric Ti saturation surface for biotites from graphitic, peraluminous metapelites that contain ilmenite or rutile. This surface provides a basis for a Ti-in-biotite geothermometer and allows a way to evaluate Ti-substitution mechanisms in biotite.

Many detailed chemical analyses have been carried out on a subset of these samples with the addition of H, Fe²⁺, Fe³⁺, and/or Li determinations on biotite, muscovite, tourmaline, chlorite, staurolite, garnet and ilmenite. These data make it possible to use crystal-chemical insights into distributions of various cations among coexisting phases. In turn, the results inform our understanding of Mg/Fe²⁺ partitioning among mafic minerals as a function of temperature (i.e. influence on exchange thermometers), and demonstrate that there are distinct ranges of Fe³⁺ contents for Fe-bearing minerals in equilibrium with graphite, ilmenite, magnetite and/or hematite. This work also permits Fe³⁺ contents of silicates to be inferred from coexisting oxide phases in equilibrium.