High-temperature hydrothermal alteration of tje Boehls Butte anorthosite: Origin of a bimodal plagioclase assemblage

Journal Article · Tue Jan 01 00:00:00 EST 2008 · Contributions to Mineral Petrol

OSTI ID:1055141

Mora, Claudia I ^[1]; Riciputi, Lee R ^[1]; Cole, David ^[2]; Walker, Karen ^[1]

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1. ORNL
2. Ohio State University

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 meteorichydrothermal 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 -6.2% V-SMOW, however, secondary ion mass spectrometry indicates spot-sized isotope values as low as -16%. Typical inter- and intra-plagioclase grain variability is 3 6%, and extreme heterogeneity of up to 20%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.

• Research Organization: Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: DE-AC05-00OR22725
• OSTI ID: 1055141
• Journal Information: Contributions to Mineral Petrol, Vol. 157, Issue 6
• Country of Publication: United States
• Language: English