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.

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H2O in the form of OH groups. H2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H2O, pyroxene from ~ 50 to ~ 310 µg/g H2O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H2O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H2O increase from ~ 450 to ~ 2500 µg/g H2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

The densification of the lower crust in collision and subduction zones plays a key role in shaping the Earth by modifying the buoyancy forces acting at convergent boundaries. It takes place through mineralogical reactions, which are kinetically favored by the presence of fluids. Earthquakes may generate faults serving as fluid pathways, but the influence of reactions on the generation of seismicity at depth is still poorly constrained. Here we present new petrological data and numerical models to show that in the presence of fluids, densification reactions can occur very fast, on the order of weeks, and consume fluids injected during an earthquake, which leads to porosity formation and fluid pressure drop by several hundreds of megapascals. This generates a mechanically highly unstable system subject to collapse and further seismic-wave emission during aftershocks. This mechanism creates new pathways for subsequently arriving fluids, and thus provides a route for self-sustained densification of the lower crust.

Garnet is a nominally anhydrous mineral that can incorporate several hundreds of ppm H2O in the 8 form of OH groups, where H + substitutes for cations in the garnet structure. In order to understand the 9 ManuscriptClick here to access/download;Manuscript;Manuscript_review2_Reynesetal Click here to view linked References Keywords: Quantitative compositional mapping -Garnet -FTIR mapping -Nominally anhydrous 31 minerals 32 33 Introduction 34Garnet is a common metamorphic mineral formed during prograde dehydration reactions such as 35 encountered during the subduction of oceanic crust and sediments. The garnet formula contains 36 neither H2O nor OH groups, nevertheless it can incorporate several hundreds of ppm H2O as OH 37 groups where H + incorporation is charge-balanced by cation substitutions. Because of the large P-T 38 stability field of garnet, the incorporated H2O can be transported inside the slab to the deep mantle and 39 might play an important role in the Earth's deep water cycle. It is essential to know where the OH 40 groups are located in the garnet structure, and which coupled substitutions exist for the incorporation 41 of H + . The dominant substitution found so far is the replacement of a Si 4+ cation by 4H + , known as the 42 hydrogarnet point defect (Cohen-Addad et al. 1967;Foreman 1968;Lager et al. 1987). Other point 43 defects have been proposed, involving H + substitutions for dodecahedral and octahedral cations 44 Andrut et al. 2002;Basso et al. 1984;Geiger et al. 1991). Coupled substitutions have been invoked 45 such as H-B and H-Li point defects (Lu and Keppler 1997) and even incomplete silicon vacancies, 46 like 3H + substituting for a Si 4+ and being compensated by Ti 4+ in octahedral site (Khomenko et al. 47 1994), or Fe 3+ or Fe 2+ together with one of two H + in the tetrahedral site in Ti-rich garnets (Kühberger 48 et al. 1989). Recent studies support that multiple point defects are present in single garnet grains 49 (Geiger and Rossman 2018;Reynes, et al. 2018). It has been shown that garnet composition does 50 influence the incorporation of OH groups in the garnet structure. Garnet is a complex solid solution 51 involving various endmembers. Spessartine (Mn-Al), almandine (Fe 2+ -Al), pyrope (Mg-Al) constitute 52 the widespread pyralspite subfamily, whereas grossular (Ca-Al), andradite (Ca-Fe 3+ ) and uvarovite 53 (Ca-Cr 3+ ) belong to the ugrandite subfamily. Garnet dominated by ugrandite endmembers can 54 incorporate up to several wt.% H2O (Rossman and Aines 1991), whereas pyralspite garnets usually 55 incorporate only a few hundreds of ppm (Aines and Rossman 1984a). Spessartine can incorporate 56 more water (up to 1000 ppm H2O, Arredondo et al. 2001) than pyrope and almandine (< 150 ppm 57 H2O). Because garnet has these various solid solutions and their affinity for H2O seems to differ, a 58 correlation between major elements and OH groups at a comparable scale is needed. Major and minor 59 element chemistry of garnet is usually measured by Electron Probe Micro-Analysis (E...