High-temperature hydrothermal alteration of the Boehls Butte anorthosite: origin of a bimodal plagioclase assemblage |
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| Authors: | Claudia I Mora Lee R Riciputi David R Cole Karen D Walker |
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| Institution: | (1) Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 84545, USA;(2) Chemistry Division, C-NR, Los Alamos National Laboratory, Los Alamos, NM 87545, USA;(3) Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6110, USA;(4) Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA |
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| Abstract: | 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 −6.2‰ 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. |
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| Keywords: | Anorthosite Oxygen isotopes Hydrothermal alteration |
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