Journal
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY
Volume 157, Issue 6, Pages 781-795Publisher
SPRINGER
DOI: 10.1007/s00410-008-0364-3
Keywords
Anorthosite; Oxygen isotopes; Hydrothermal alteration
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Funding
- NSF [EAR-8922638]
- US DOE [DE-ACO5-00OR22725]
- Oak Ridge National Laboratory [LAUR-08-2539]
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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.
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