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

• Published in: Mineralogy and Petrology
• Main Authors: Wenzel, Kirsten T., Wiedenbeck, Michael, Gose, Jürgen, Rocholl, Alexander, Schmädicke, Esther
• Other Authors: Deutsche Forschungsgemeinschaft, Bavarian Equal Opportunities Sponsorship - Realization Equal Opportunities for Women in Research and Teaching, Friedrich-Alexander-Universität Erlangen-Nürnberg
• Format: Article in Journal/Newspaper
• Language: English
• Published: Springer Science and Business Media LLC 2021
• Subjects: Geochemistry and Petrology; Geophysics; Arctic
• Online Access: http://dx.doi.org/10.1007/s00710-021-00757-9; https://link.springer.com/content/pdf/10.1007/s00710-021-00757-9.pdf; https://link.springer.com/article/10.1007/s00710-021-00757-9/fulltext.html

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, Al 2 O 3 ranges between 4.02 and 5.56 wt%, and Cr 2 O 3 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 H 2 O. 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, Al 2 O 3 between 1.66 and 5.34 wt%, and Cr 2 O 3 between 0.62 and 0.96 wt%. Water contents as measured by SIMS range from 68 ± 7 to 261 ± 11 µg/g H 2 O and correlate well with Al 2 O 3 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.