High P-T Elastic Properties of OH-Bearing Majoritic Garnet

John Daniel Lazarz, Northwestern University, Earth and Planetary Science, Evanston, IL, United States, Sylvia-Monique Thomas, University of Nevada Las Vegas, Las Vegas, NV, United States, Sergey N Tkachev, University of Chicago, Center for Advanced Radiation Sources, Chicago, IL, United States, Joshua P Townsend, Sandia National Laboratories, Albuquerque, United States, Craig R. Bina, Northwestern University, Department of Earth and Planetary Sciences, Evanston, United States and Steven D Jacobsen, University of Colorado Boulder, Department of Earth Science, Boulder, United States
Abstract:
The mantle transition zone (TZ) is believed to be primarily composed of three constituents: wadsleyite, ringwoodite, and majorite garnet (Ringwood, 1975). Laboratory sound velocity measurements for wadsleyite and ringwoodite alone are too high to match TZ seismological models (Li et al., Science, 1998; Sinogeikin et al., JGR, 1998), while majorite yields significantly lower sound velocities (Sinogeikin et al., GRL, 2002; Gwanmesia et al., PEPI, 2009). Taken together, a compositional model such as pyrolite yields a good fit to seismology within uncertainties, with the major discrepancies being that pyrolite yields slightly larger velocity jumps and shallower velocity gradients than seismology (Li and Liebermann, Science, 2007; Irifune et al., Nature, 2008). Hydration of ringwoodite in the transition zone is expected to reduce seismic velocities. If the lower part of the TZ is hydrated, as some recent studies suggest (Pearson et al., Nature, 2014; Schmandt et al., Science, 2014), the proportions of ringwoodite and majoritic garnet in the TZ should be re-evaluated.

Velocity gradients in the TZ are likely related to the gradual eclogite-garnetite transition. Over the TZ pressure range (~13-24 GPa), the dissolution of pyroxene into garnet gradually increases, resulting in a complex depth-varying garnet-majorite solid solution, ranging from M4Si4O12 majorite (Mj) to M3Al2Si3O12 garnet (Gt), where M is Mg, Fe, Ca0.5Mg0.5, etc. (Akaogi and Akimoto, PEPI, 1977; Bina and Wood, GRL, 1984; Gasparik, CMP, 1989). Several studies have considered the compositional dependence of majoritic garnet elastic moduli (Liu et al., PEPI, 2000; Sinogeikin et al., EPSL, 2002; Sinogeikin and Bass, GRL, 2002; Murakami et al., EPSL, 2008), but few have considered both composition and hydration state under the high-pressure and high-temperature conditions of the TZ. Here we combine in situ X-ray and Brillouin measurements to determine the elastic constants of various majoritic garnet compositions under pressure and temperature conditions of the TZ, up to 25 GPa and 1000°C. The elastic moduli dependence on composition and hydration of majorite garnet are of first-order importance when seeking to constrain mineralogical models of the mantle.

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