Hydrogen in mantle xenoliths from a forearc-backarc transect: heterogeneities at the km, cm and µm scale

Abstract:

In mantle xenoliths, the inter and intra-mineral distribution and speciation of hydrogen (H) may record valuable information about the surrounding mantle environment and interaction with the host magma. We had the unique opportunity to measure H concentrations and speciation in orthopyroxenes (opx) from mantle xenoliths from eight different localities along a forearc-backarc transect in Patagonia, using Fourier transform infrared spectroscopy (FTIR). In these samples, we distinguish three different types of spectra, based on the relative absorbance of the 3600-3590 cm^-1peak in comparison to the 3570 cm^-1peak: 1) spectra with a prominent band at 3600-3590 cm^-1, 2) spectra with a prominent band at 3570 cm^-1, and 3) The 3600-3590cm^-1and 3570 cm^-1 bands have the same height and are the prominent bands. All groups have absorbance bands between 3520 cm^-1and 3000 cm^-1, at 3520 cm^-1and 3420 cm^-1, and sometimes, at 3310 cm^-1and 3085 cm^-1. The results show variability between the different localities, which are separated by a few km to 200 km, within the same xenolith suites (from a single locality) and even within the same xenolith sample, and the same crystal. We observe considerable heterogeneity of spectra within samples from the backarc setting, and the similarities of some localities in the backarc setting with the forearc setting shows that it may not primarily be regional scale processes that affect the shape of the spectra. There are also local and in-situ influences. At the µm scale, we observed an apparent dehydration profile where the rim spectra exhibit the group 3 behavior while the core spectra exhibit the group 2 behavior. Whilst the total integrated absorbance shows a decrease from the core to the rim, a band at 3590 cm^-1shows an increase, which can be interpreted either as simultaneous, defect specific dehydration and hydration, or the result of redistribution of H between defects. Importantly, the variation of band positions and intensities in this single crystal is similar to the variation in the whole sample suite. To summarize, similar degrees of heterogeneity are observed between the forearc and backarc, between samples from the backarc, and even within individual grains. FTIR spectra of opx may be a useful tool for assessing the extent of subduction influence on mantle domains from across-arc mantle sections.