Twin Sisters dunite: Petrology and mineral chemistry

Petrologic study of the Twin Sisters dunite, an alpine-type ultramafic body about 90 km², has led to the establishment of three broad stages in the recrystallization of the body. The first stage is represented by “primary,” now highly strained, large olivine crystals, which were subsequently partly recrystallized and/or mylonitized in the upper mantle. The development of chromite pods and lenses occurred in this first stage. The second stage, separated from the first by deformation, is manifest by the segregation, probably also in the upper mantle, of at least two generations of olivine-bearing orthopyroxenite and clinopyroxenite veins. The third stage is represented by the generation of metamorphic minerals exemplified by the following reactions: enstatite + diopside + H₂O = forsterite + tremolite; enstatite + H₂O = talc + forsterite; chromite + forsterite + enstatite + H₂O = ferritchromite + chlorite.

Electron microprobe study of coexisting minerals indicates that olivine, orthopyroxenes, and clinopyroxenes are remarkably homogeneous. Minor but systematic variations in mineral compositions do exist. Olivine in chromite pods and lenses is more forsteritic and contains nearly twice as much NiO than olivine in the main dunite and pyroxenite veins. Chromite compositions show two distinct but parallel trends — one for chromite present in the main dunite and in veins and the other for chromite in pods and lenses. As in olivine, X_Mg in chromite pods and lenses is higher than that in chromite from the main dunite. Unlike the trends for the Kilauean chromites, total Fe increases with increasing Cr₂O₃ content, whereas Al₂O₃ and MgO decrease. Calcic amphiboles are heterogeneous, and important end members in different samples are pargasite, edenite, and tremolite. Anthophyllite coexists with calciferous amphiboles. Magnesian cummingtonite was also found.

A consistent apparent equilibration temperature of 1100 to 1200 °C, derived from the olivine-chromite geothermometer, is 138 to 333 °C higher than that derived from the Ca content of orthopyroxene and clinopyroxene pairs. Temperature of unmixing in pyroxenes is about 115 °C lower than that indicated by their bulk composition.