Experimental constraints on hydrogen diffusion in garnet

Abstract

The incorporation mechanisms and diffusional loss of hydrogen in garnet have been experimentally investigated. A suite of gem-quality hydrous spessartine-and grossular-rich garnets were analysed by Fourier transform infrared spectroscopy (FTIR) and by ion microprobe (SHRIMP-SI) to determine the calibration coefficients for quantification of FTIR data. The excellent agreement between measured absorption and OH/O indicates that the same molar extinction coefficient can be used for spessartine and grossular. The coefficient of 14400 l mol − 1 cm − 2 proposed by Maldener et al. (Phys Chem Miner 30:337-344, 2003) seems the most appropriate for both minerals. A grossular with 6.4% andradite and 1.6% almandine containing 834 ppm H 2 O, and an almost pure spessartine with 282 ppm H 2 O, were selected for diffusion experiments. 1.5-mm cubes of garnets were heated between 12 h and 10 days at 1 atm under various temperature (750-1050 °C) and oxygen fugacity ( f O 2 ) conditions, (ΔQFM + 15.2 to − 3.0). Diffusion profiles were acquired from sections through the cubes using FTIR, with a deconvolution algorithm developed to assess peak-specific behaviour. Different families of peaks have been identified based on their diffusive behaviour, representing hydrogen incorporated in different H-bearing defects. A dominant, fast, strongly f O 2 -dependent oxidation-related diffusion mechanism is proposed) with a relatively low activation energy (158 ± 19 kJ mol − 1 ). This diffusion mechanism is likely restricted by availability of ferrous iron in grossular. At low oxygen fugacity, this diffusion mechanism is shut off and the diffusivity decreased by more than three orders of magnitude. A second, slower hydrogen diffusion mechanism has been observed in minor bands, where charge balance might be maintained by diffusion of cation vacancies, with much higher activation energy (≈ 200-270 kJ mol − 1 ). Spessartine shows clear differences in peak retentivity suggesting that up to four different H sites might exist. This opens exciting opportunities to use hydrogen diffusion in garnet as speedometer. However, it is essential to constrain the main diffusion mechanisms and the oxygen fugacity in the rocks investigated to obtain timescales for metamorphic or igneous processes.

Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps

“…In contrast, experiments on hydrogen diffusion in garnet of different composition, including Dora Maira specimens, imply different substitution mechanisms (e.g. Lu & Keppler, 1997; Padrón‐Navarta, Hermann, & O'Neill, 2014; Reynes, Jollands, Hermann, & Ireland, 2018). The latter is supported by the observation that the SW I and SW II bands in our samples are differently influenced by secondary water (see Section 6.1).…”

“…Apart from the fact that hydrogen diffusion can be as slow as that of other cations, depending on the OH substitution type and oxygen fugacity (e.g. Padrón‐Navarta et al., 2014; Reynes et al, 2018), pyropic garnet shows little difference in the H 2 O storage capacity in the pressure interval between 30 and 15 kbar (Lu & Keppler, 1997). As a result, diffusional water loss should not occur during decompression from UHP conditions to, at least, 15 kbar.…”

Micro- and nano-size hydrogrossular-like clusters in pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps

“…3 a IR single-crystal spectrum at room temperature (RT) and 80 K of synthetic pyrope (P-27) grown hydrothermally from a gel. The crystal thickness is 0.251 mm Lu and Keppler 1997;Reynes et al 2018). It was thought that these and other similar mechanisms "explain" the spectral differences between synthetic and natural garnets and among different natural crystals of the same species.…”

“…2 Si 3 O 12 , respectively, are, in general, more variable and complex (Rossman and Aines 1991;Maldener et al 2003;Phichaikamjornwut et al 2011;Dachs et al 2012;Reynes et al 2018) than those of Dora-Maira pyrope. OHstretching modes of a number of different energies are observable in the former.…”

“…Here, various defect OHsubstitutional mechanisms have been considered and proposed including, for example, {2H + ⇔ Ca 2+ } and [3H + ⇔ Al 3+ ] in the case of grossular (e.g., Basso et al 1984;Birkett and Trzcienski 1984;Basso and Cabella 1990), which has been most intensively studied. In addition, coupled substitutional mechanisms such as {Li + -H + ⇔ Mg 2+ }, [Mg/Fe 2+ -H + ⇔ Al/Fe 3+ ], (B 3+ -H + ) ⇔ (Si 4+ ) and (Al 3+ -H + ) ⇔ (Si 4+ ) have been proposed for pyrope and other garnets (e.g., Kühberger et al 1989;Khomenko et al 1994;Lu and Keppler 1997;Reynes et al 2018). The substitutions H + + Al 3+ = Si 4+ and H + + B 3+ = Si 4+ do occur in other mineral structures (Rossman 1986).…”