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


Jed L. Mosenfelder1, Anette von der Handt 1, Anthony C. Withers2, Hélène Bureau3, Caroline
Raepsaet
4George R. Rossman5


1 Department of Earth and Environmental Sciences
University of Minnesota
 116 Church St. SE,
11 Minneapolis, Minnesota, 55455, U.S.A.
2 Bayerisches Geoinstitut
Universität Bayreuth,
 95440 Bayreuth, Germany.
3 Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC)
UMR 7590 Sorbonne Université, CNRS 
4 place Jussieu, 75252 Paris Cedex 05, France
4Service de Physique de l’état Condensé, SPEC, CEA, CNRS
Université Paris-Saclay
CEA Saclay, 91191 Gif sur Yvette, CEDEX, France.
5 Division of Geological and Planetary Sciences  
California Institute of Technology
Pasadena, CA  91125-2500, USA

ABSTRACT

           It is well known that some garnet compositions can incorporate hydrogen and/or fluorine at levels up to several weight percent. However, accurate measurement of these elements can be difficult at trace to minor levels of abundance so they are frequently ignored in routine chemical analysis. Furthermore, the mechanisms of H incorporation are still under debate and only one mechanism for F substitution is commonly considered. We employed infrared spectroscopy  (FTIR), elastic recoil detection analysis (ERDA), secondary ion mass spectrometry (SIMS), and electron probe microanalysis (EPMA) to measure H and F concentrations and constrain incorporation mechanisms in ten grossular garnets. We also present SIMS data for 11 spessartine garnets and two andradites. ERDA on three of the grossulars was used to obtain an infrared integral molar absorption coefficient (ei) for H2O of 13,470 L · mol-1·cm-2. H2O and F concentrations of the grossulars range from 0.017 to 0.133 wt% and 0.012 to 0.248 wt%, respectively. Correlations between 16OH and 19F in SIMS data and interpretation of FTIR spectra promt us to consider various coupled substitutions of H and F for Si, which can explain some high frequency IR absorption bands that have been previously attributed to "hydrogrossular clusters" (variably-sized clusters in which 4H substitutes for Si) or to inclusions of hydrous minerals. A strong correlation between 16OH and 19F in spessartines implies a similar role for H- F substitution in these garnets. Coupled H-F substitution is also probably relevant to some andradite-rich garnets, rare pyropes from the Dora Maira massif, and some synthetic garnets. Improvements in analytical methods for trace to minor H and F open up more possibilities for using these elements to calculate the activities of H2O and F-species in fluids that were in equilibrium with garnet-bearing phase assemblages, as well as constraining the recycling of these elements into the mantle via study of xenoliths.