From dry to drenched: variability of H2O contents in rutile from subducted rocks using quantitative in-situ FTIR spectroscopy and mapping

Rutile is potentially very important for the transport of water during subduction metamorphism after the breakdown of hydrous phases, as it is one of the most hydrous nominally anhydrous minerals (up to several 1000 μg/g H₂O) and commonly occurs in a variety of lithologies and P-T conditions across subduction zones.

We present results from quantitative in-situ Fourier Transform Infrared (FTIR) spectroscopy of rutile from different subduction settings (P-T, lithology, geothermal gradients) and high-resolution FTIR mapping to evaluate the variability of H₂O contents in rutile and retention of H⁺.

Observed H₂O contents are highly variable. Granulite facies rutile have low H₂O contents (<150 μg/g) with high-P granulites showing up to ~400 μg/g H₂O. The highest average H₂O contents were observed in low-T eclogite facies rutile (500–1700 μg/g). Amphibolite- and high-T – high-P eclogite facies rutile has intermediate H₂O contents (~200–400 μg/g). Rutile from UHP shows greatly varying H₂O contents (<10–700 μg/g).

FTIR maps of high-P granulite-, high-T – high-P eclogite facies and UHP rutile show evidence for diffusive H⁺ loss, while low-T eclogite- and amphibolite facies rutile are homogeneous or show growth zoning and thus retain their original H₂O contents. Therefore, the typically variable and lower H₂O contents at higher P-T conditions result from H⁺ loss at temperatures above ~650–700 °C.

Generally, H₂O contents are distinctive for specific subduction zone conditions, especially when coupled with Zr-in-rutile thermometry and trace-element geochemistry, e.g. high H₂O contents above 500 μg/g coupled with Zr contents below 200 μg/g indicate cold subduction geotherms.