A mapping approach for the investigation of Ti–OH relationships in metamorphic garnet

Abstract

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...

Coupled hydrogen and fluorine incorporation in garnet: New constraints from FTIR, ERDA, SIMS, and EPMA

“…Fine-scale zoning of H in garnets such as those from Asbestos or the garnets studied by Jamtveit and Hervig (1994) or Reynes et al (2020) is probably best studied using microanalytical techniques such as SIMS or FTIR conducted with a focal plane array (FPA) detector, which is becoming more widely available (the FPA detector has a higher spatial resolution than normal MCT detectors such as those we used; e.g., Reynes et al 2020). We have also demonstrated that F can be measured reliably with EPMA at levels down to ~50 g/g, and it should be possible to achieve higher analytical throughput at lower cost with EPMA compared to SIMS.…”

“…The role of Ti in H incorporation in grossular and andradite was recently studied by Reynes et al (2020). They proposed coupled substitution of Ti on the Y site and 2H for Si, as follows: samples in this study, centered at 3546, 3599, and 3657 cm -1 in GRR1386.…”

“…They recognized that verification of this substitution relies on accurate determination of Fe 2+ /Fe 3+ as well as H and F content, beyond the scope of their study. Various mechanisms involving coupled substitution of Ti and H have also been considered, primarily for pyrope (Bell and Rossman 1992;Khomenko et al 1994;Geiger et al 2000) but also for grossular (Reynes et al 2020). Partial substitution of H for the Ycation in a hydrogrossular was inferred by Kalinichenko et al (1987), based on NMR, and at least two studies have inferred substitutions other than (4H) Si × in H-rich garnets based on crystal chemical arguments (Birkett and Trcienski 1984;Basso and Cabella 1990).…”

“…The imperfect correlation of F to the integrated absorbance of the high-frequency bands (Fig. 8a) could be related to the additional presence of F in sites with no OH (e.g., Equation 4), vagaries of the fitting exercise, or perhaps other mechanisms completely unrelated to F -such as the cluster model of Geiger and Rossman (2020a,b) or coupled substitution of H and Ti (Reynes et al 2020), which we discuss below. Therefore, more work including ab initio calculations is needed to verify our hypothesis that clumped OH-F defects are present in grossular.…”

Low-temperature infrared spectrum and atomic-scale structure of hydrous defects in diopside

“…For a symmetry lower than orthorhombic, the orientation of the principal axes of the dielectric tensor of the pristine or defective crystal is not fully constrained by the crystal symmetry. The principal axes of the index and absorption tensors may thus differ, which complicates the straightforward computation of the infrared absorption spectrum (e.g., Sambridge et al, 2008). To circumvent this difficulty, the anisotropy of the electronic contribution to the dielectric tensor was neglected as this anisotropy marginally affects the value of the absorption coefficient.…”