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Squeezing Water From a Stone: H 2 O In Nominally Anhydrous Minerals From Granulite Xenoliths and Deep, Hydrous Fractional Crystallization
Author(s) -
Chin Emily J.,
Curran Sean T.,
Farmer G. Lang
Publication year - 2020
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2020jb020416
Subject(s) - geology , geochemistry , granulite , xenolith , petrography , proterozoic , biotite , fractional crystallization (geology) , continental crust , crust , mantle (geology) , tectonics , quartz , geomorphology , paleontology , facies , structural basin
Abstract Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile‐rich state and calc‐alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution—in the deep crust—remains poorly constrained. Here, we report H 2 O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H 2 O contents ranging from 75–760, 233–410, and 42–139 ppm, respectively. Reconstructed bulk rock H 2 O contents range from ~60 to ~650 ppm. Intermineral H 2 O ratios overlap experimental mineral/melt D values and are used to calculate H 2 O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H 2 O) 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.