Abundance and partitioning of OH in a high-pressure magmatic system: megacrysts from the Monastery kimberlite, South Africa

David R. Bell1*, George R. Rossman1, Rory O. Moore2

1 Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, California 91125-2500, U.S.A.

2 Mineral Services Canada, 1300-409 Granville Street
Vancouver, BC V6C 1T2, Canada

* Present address: Department of Chemistry and Biochemistry
Arizona State University, Tempe, AZ 85287, U.S.A.

The Journal of Petrology


Concentrations of OH, major and trace elements were determined in a suite of mantle-derived megacrysts that represent the crystallisation products of a kimberlite-like magma at ~ 5 GPa and ~ 1400 – 1100ºC.  OH concentrations, determined by single-crystal FTIR spectroscopy, display the following ranges (ppmw H2O): olivine 54-262, orthopyroxene 215-263, garnet 15-74, clinopyroxene 195-620, and zircon 28-34.  High OH concentrations in olivine imply mantle conditions of origin, with limited H loss during ascent.  OH is consistently correlated with megacryst composition, exhibiting trends with Mg# that are similar to those of other minor and trace elements and indicating a record of high-pressure magmatic evolution.  H substitution is not coupled to minor elements in olivine, but may be in ortho- and clinopyroxene.  The OH – Mg# trends of garnet and clinopyroxene show inflections related to co-precipitation of ilmenite, suggesting minor element (Ti) influence on OH partitioning.  During differentiation, relative OH enrichment in clinopyroxene and olivine is consistent with proportional dependence on water activity, while that in garnet suggests a higher power law dependence and/or influence of temperature.  Inter-mineral distribution coefficients (D's) for OH between cpx, opx, olivine and zircon are thus constant, whereas partitioning between these minerals and garnet shows a factor 4-10 variation, correlated regularly with composition (and T).  Calculation of solid-melt partition coefficients for H at 5 GPa over the range of magmatic evolution (1380 – 1250ºC ) yields: ol 0.0053-0.0046, opx 0.0093-0.0059, cpx 0.016-0.013, gt 0.0014-0.0003, bulk (garnet lherzolite-melt) 0.0063-0.0051. These are consistent with experimental studies and similar to values inferred from MORB geochemistry, confirming the moderate incompatibility of H in mantle melting.

Correction: In table 2, page 1542, the Integrated molar absorption coefficient (l/mol per cm2) is incorrect. The value, 4740, was calculated for the wrong thickness. The correct value is about 36,500.