Hydration State of the Transition Zone and Lowermost Mantle

PROJECT SUMMARY Overview: Page A Evidence is building in support of a long-working theory that the mantle transition zone (TZ, 410-660 km) contains a significant - if not the largest - geochemical reservoir of H2O in the Earth. This proposal builds on two recent discoveries (1) a ringwoodite inclusion in diamond containing near-saturated (1.5 wt%) amounts of H2O, indicating local hydration of the TZ and (2) regional-scale evidence for a hydrous TZ from signatures of dehydration melting below 660 km beneath much of North America. The latter, interdisciplinary study integrated the PI’s NSF early CAREER research in mineral physics with new data from the US-Array through collaboration with NSF-Earthscope scientists. It remains to be determined how heterogeneously and how globally the TZ may be hydrated. In this proposal, Brillouin spectroscopic experiments will be conducted at high pressure-temperature (P-T) conditions on a new suite of synthetic OH-bearing majoritic garnets, as well as further compressibility and Brillouin measurements on wadsleyite and ringwoodite to better constrain the effects of hydration on P-T derivatives of the elastic moduli. These critical gaps in the thermoelastic database for hydrous mantle materials will result in a forward model of TZ velocity as a function of water content, Fe-content, and temperature. Whereas the TZ water storage capacity appears much higher than the lower mantle, what remains completely unknown is how H will behave at the base of the silicate mantle. In exploratory areas of the proposed work, preliminary results using both experiments and computational methods suggest that there is a stable OH-post-bridgmanite phase. Proposed calculations on OH-post-bridgmanite thermodynamic properties and lattice-preferred orientation will be used to provide mineral physics constraints on core-mantle boundary seismic structure. Intellectual Merit : Hydration of the mantle transition zone has implications for understanding deep mantle melting and geochemical filtering at the discontinuities, constraints on the bulk composition of the Earth, and the origin of Earth’s water. Deep-mantle hydration may indeed be a necessary ingredient for planetary plate tectonics and the long-term stability of large liquid oceans. This proposal involves multiple lines of research on the Earth?s water cycle from atomic to geophysical scales. Experiments utilize the latest technology at large-scale user facilities such as in-situ high P-T Brillouin scattering, as well as a unique method co-developed by the PI called GHz-ultrasonic interferometry. In addition, computational methods will be used to evaluate the properties of a hydrated base-layer of the mantle. Mineral physics data collected over the past twenty or so years on hydrated mantle silicates, as well as some new critical gaps to be measured, will be used to generate a publically-available thermoelastic database from which forward modelling of transition zone composition and hydration state may be conducted using existing and forthcoming seismic data but especially high-resolution data from the US-Array beneath the east coast of North America where a large low-velocity zone may exist within and above the transition zone. Broader Impacts : The proposed research in mineral physics spans topics in solid-Earth geophysics, mineralogy, geochemistry, petrology, high-pressure materials science, and condensed matter physics. Graduate and undergraduates involved in this study will therefore be exposed to a broad range of science and tra