High-temperature hydrothermal alteration of the Boehls Butte anorthosite: origin of a bimodal plagioclase assemblage

The Boehls Butte anorthosite consists predominantly of an unusual bimodal assemblage of andesine and bytownite-anorthite. Oxygen isotope compositions of the anorthosite were profoundly altered by high temperature, retrograde interaction with meteoric-hydrothermal fluids that varied in composition from isotopically evolved to nearly pristine meteoric water. Oxygen isotope ratios of bulk plagioclase separates are in the range +7.0 to -6.2aEuro degrees V-SMOW, however, secondary ion mass spectrometry indicates spot-sized isotope values as low as -16aEuro degrees. Typical inter- and intra-plagioclase grain variability is 3-6aEuro degrees, and extreme heterogeneity of up to 20aEuro degrees is noted in a few samples. High-temperature hydrothermal alteration of intermediate plagioclase is proposed to explain the origin of bytownite-anorthite in the anorthosite and creation of its unusual bimodal plagioclase assemblage. The anorthite-forming reaction created retrograde reaction-enhanced permeability which, together with rapid decompression, extension, and unroofing of the anorthosite complex, helped to accommodated influx of significant volumes of meteoric-hydrothermal fluids into the anorthosite.