Hydration State of the Transition Zone and Lowermost Mantle

Project: Research project

Project Details

Description

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
StatusFinished
Effective start/end date1/1/1512/31/18

Funding

  • National Science Foundation (EAR-1452344-002)

Fingerprint

Explore the research topics touched on by this project. These labels are generated based on the underlying awards/grants. Together they form a unique fingerprint.