Mica Mountain Muscovite: A New Silicate Hydrogen Isotope Standard Reference Material

Abstract:

A new standard reference material consisting of finely ground muscovite flakes has been developed and utilized at Washington State University to calibrate hydrogen isotope ratio (D/H) measurements to the VSMOW scale. This standard was prepared from a single crystal 'book' of a muscovite-bearing pegmatite near Deary, ID. The value we obtained for this muscovite standard (MMM) is: δD VSMOW = -79.1 ± 2.0‰ relative to NBS-30 biotite at -65.7‰ compared to a VSMOW value of 0.00‰. This mean value was determined for the muscovite and has been used as our working standard. There have been many recent geological applications to continuous flow isotope ratio mass spectroscopy. When hydrogen isotope ratios are of interest, a suitable standard for hydrogen in silicate systems often is not available. With supplies of the older NBS-30 biotite standard exhausted, much D/H data measured on silicate minerals have been linked to the VSMOW scale via non-silicate reference materials which may not behave similarly to minerals under study. Some recent studies have shown the NBS-30 standard to have poor intra-laboratory agreement with that material's measured and accepted isotopic values (Qi et al., 2014). Many laboratories which would measure D/H in silicate minerals would benefit from using a silicate-based standard for hydrogen. With further characterization, this muscovite may also be useful as a standard for silicate oxygen ratios as well as for some major element cations. This muscovite standard gives consistent values and it is easy to work with and does not leave much combustion residue. Also, because muscovite contains little iron, metal-hydride formation and associated fractionation factors is greatly reduced during the sample combustion. A new silicate-hydrogen standard is needed by the community, and this work represents an example of what a replacement standard material could look like.