Water contents of nominally anhydrous orthopyroxenes from oceanic peridotites determined by SIMS and FTIR

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.

NanoSIMS analysis of water content in bridgmanite at the micron scale: An experimental approach to probe water in Earth’s deep mantle

“…This indirectly confirms the validity of our analytical methods and the calibrated water contents of pyroxene crystals from the harzburgite samples. Moreover, although there have been ongoing efforts to characterize new Opx and Cpx water reference materials (e.g., Wenzel et al, 2021;Zhang et al, 2023), much more work is still needed considering the large chemical composition range of pyroxene (Figure 1C). Future work on water abundance reference materials may include potential candidates such as recently characterized natural Cpx reference materials for in situ strontium isotopic analysis by laser ablation multicollector inductively coupled plasma mass spectrometry (Zhao et al, 2020) and natural Cpx reference materials for in situ trace element abundances by laser ablation inductively coupled plasma mass spectrometry .…”