Tracing crustal recycling using the acoustic signature of calcium perovskite
- Date: Friday 19 October 2018, 15:00 – 16:00
- Location: Earth and Environment - Seminar rooms 1 and 2, 8.119
- Type: Seminars, Earth and Environment, Institute of Geophysics and Tectonics
- Cost: Free
Institute of Geophysics and Tectonics seminar. Speaker Andy Thompson, University College London.
The travel times of seismic waves through Earth’s interior provide one of the few direct constraints on the physical properties of mantle rocks. If carefully compared with experimental and computational measurements of the elastic properties of minerals this data may be interpreted in terms of mantle mineralogy, chemistry and temperature; information that is not otherwise directly attainable. Calcium silicate perovskite (capv) is believed to be the third most abundant mineral in Earth’s deep mantle, constituting ~ 10 % of pyrolite or up to ~ 30 % of subducted basalt at lower mantle conditions. However, predictions of vp and vs from computational studies vary by ~ 25% and disagree with experimental measurements made at room temperature. There have been no published experimental measurements of capv’s acoustic velocities at mantle conditions (high P and T), and it is apparent that the elastic properties of capv are currently uncertain and are insufficient for the reliable interpretation of seismic data.
We have performed synchrotron multi-anvil experiments to simultaneously measure the crystallography and acoustic velocity of Ca[SixTi1-x]O3 samples at conditions extending to ~ 15 GPa and 1700 K. We have observed that acoustic waves travel through polycrystalline calcium perovskite samples significantly slower than is predicted in computational studies, and that calcium perovskite undergoes structural distortions that cause large changes in acoustic properties. When extrapolated via thermodynamically consistent equations of state, our results suggest subducted oceanic crust will be visible as a negative velocity anomaly throughout the lower mantle. The large low-velocity provinces (LLVPs) likely result from moderate enrichment of recycled crust, and mid-mantle discontinuities can be well explained by a tetragonal-cubic phase transition in calcium perovskite.