Abstract
This study presents new secondary ion mass spectrometry (SIMS) reference materials (RMs) for measuring water contents in nominally anhydrous orthopyroxenes from upper mantle peridotites. The enstatitic reference orthopyroxenes from spinel peridotite xenoliths have Mg#s between 0.83 and 0.86, Al2O3 ranges between 4.02 and 5.56 wt%, and Cr2O3 ranges between 0.21 and 0.69 wt%. Based on Fourier-transform infrared spectroscopy (FTIR) characterizations, the water contents of the eleven reference orthopyroxenes vary from dry to 249 ± 6 µg/g H2O. Using these reference grains, a set of orthopyroxene samples obtained from variably altered abyssal spinel peridotites from the Atlantic and Arctic Ridges as well as from the Izu-Bonin-Mariana forearc region was analyzed by SIMS and FTIR regarding their incorporation of water. The major element composition of the sample orthopyroxenes is typical of spinel peridotites from the upper mantle, characterized by Mg#s between 0.90 and 0.92, Al2O3 between 1.66 and 5.34 wt%, and Cr2O3 between 0.62 and 0.96 wt%. Water contents as measured by SIMS range from 68 ± 7 to 261 ± 11 µg/g H2O and correlate well with Al2O3 contents (r = 0.80) and Cr#s (r. = -0.89). We also describe in detail an optimized strategy, employing both SIMS and FTIR, for quantifying structural water in highly altered samples such as abyssal peridotite. This approach first analyzes individual oriented grains by polarized FTIR, which provides an overview of alteration. Subsequently, the same grain along with others of the same sample is measured using SIMS, thereby gaining information about homogeneity at the hand sample scale, which is key for understanding the geological history of these rocks.
Highlights
Understanding the role of water, its amount and distribution in the Earth’s upper mantle and its influence on convective processes and geodynamics is a key task of igneous geochemistry
This study has demonstrated that when investigating pyroxenes from highly altered oceanic peridotite samples, secondary ion mass spectrometry (SIMS) offers significant advantages for quantifying intracrystalline H2O contents
The SIMS sampling volume is much smaller than that offered by Fourier-transform infrared spectroscopy (FTIR), allowing cracks and zones of alteration to be avoided
Summary
Understanding the role of water, its amount and distribution in the Earth’s upper mantle and its influence on convective processes and geodynamics is a key task of igneous geochemistry. Water decreases the mantle’s viscosity and mechanical strength via hydrolytic weakening of olivine (Demouchy et al 2012; Tielke et al 2017) and lowers peridotite melting temperatures Incorporated by various substitutions, such as protons attached to oxygen anions forming “hydroxyl defects” in the crystal structure (Bell and Rossman 1992; Libowitzky and Beran 2006), small amounts of water can be found in nominally anhydrous minerals (NAMs) such as pyroxenes and olivines derived from the Earth’s upper mantle. H loss has recently been observed in pyroxenes as well (Tian et al 2016; Denis et al 2018), orthopyroxene is generally considered to retain its upper mantle water concentration during exhumation (Gose et al 2011; Hao et al 2014; AzevedoVannson et al 2021) and can be used as a reliable tracer for water in the upper mantle
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