Squeezing water from a stone: H2O in nominally anhydrous minerals from
granulite xenoliths and deep, hydrous fractional crystallization
Abstract
Although ~10% of Earth’s water resides within
continents, HO distribution throughout the continental lithosphere and
partitioning of HO in nominally anhydrous minerals (NAMs) remain poorly
constrained. Models of continent formation and destruction depend on HO
content. We report HO contents in NAMs measured on petrographic thin
sections by secondary ion mass spectrometry (SIMS) of Proterozoic deep
crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and
garnet contain average HO contents of 560, 347, and 85 ppm,
respectively; reconstructed bulk rock HO ranges from ~75
to ~600 ppm. Inter-mineral HO ratios overlap
experimental mineral/melt D values, and are used to calculate HO of
melts last in equilibrium with the xenoliths. We propose that these
xenoliths represent cumulates fractionated from a primitive, hydrous (≥1
wt.% HO) melt at high (~1 GPa) pressures, similar to
conditions in modern subduction zones, and potentially associated with
widespread arc accretion that formed the core of North America in the
Precambrian.