Hydrogen incorporation in orthopyroxene from Southern Patagonian mantle xenoliths: the role of cooling upon emplacement

Peridotites contain Nominally Anhydrous Minerals (NAMs), notably, olivine, orthopyroxene and clinopyroxene, which can contain from a few up to hundreds of ppm of water in their structure as hydroxyl groups (OH-). Mantle xenoliths are one of the primary sources to estimate the water budget of the Earth’s upper mantle. Here we present a comprehensive study of orthopyroxene from mantle-derived xenoliths across a ∼300 km long transect in the back-arc region of Southern Patagonia. Spinel harzburgites, lherzolites and olivine websterites were collected, these record equilibration temperatures ranging from 850 to 1100 °C, all within the spinel peridotite stability field. Orthopyroxene was studied using polarized and unpolarized Fourier transform infrared (FTIR) spectroscopy. Major and trace element (including hydrogen) chemistry of olivine, clinopyroxene and orthopyroxene indicate that the hydrogen content is neither correlated to primary composition of mantle minerals nor to a metasomatic overprint recorded by incompatible trace elements in pyroxenes. Variability in the water contents in olivine, clinopyroxene and orthopyroxene and H-loss profiles in olivine and orthopyroxene are provide evidence of modification of the original mantle water contents, re-equilibration with the host-magma and H-loss upon ascent and/or eruption. Orthopyroxene spectra are subdivided based on the relative ratio of the peak height of the 3590 cm-1 and 3570 cm-1 peaks (PH3590/PH3570): group 1 has PH3590/PH3570 > 1 and group 2 has PH3590/PH3570 < 1. The 3570 cm-1 and 3590 cm-1 peaks are present in the majority of orthopyroxene FTIR spectra. Fast cooling rates during eruption are associated with PH3590/PH3570 > 1 and slow cooling rates are associated to PH3590/PH3570 < 1. The peak shift from 3590 cm-1 to 3570 cm-1 observed in single crystals is dependent on temperature and cooling rates, inter-site reactions and is related to H-loss. H-profiles from mantle orthopyroxene suggest that they experienced eruption – cooling timescales of just a few minutes to hours showing the potential of using frozen H-profiles in orthopyroxene to extract timescales and infer cooling mechanism at the surface.