Seismic velocities of CaSiO3 perovskite can explain LLSVPs in Earth’s lower mantle

Abstract

Seismology records the presence of various heterogeneities throughout the lower mantle e.g. 1,2 , however, the origins of these signals, whether thermal or chemical, remain uncertain and therefore much of the information they hold about the nature of the deep Earth is obscured. Accurate interpretation of observed velocities requires knowledge of the seismic properties of all of Earth's possible mineral components. Calcium silicate perovskite (hereafter "calcium perovskite") is believed to be the third most abundant mineral throughout the lower mantle. Here we measure the crystal structure, compressional and shear wave velocity of calcium perovskite samples , and provide direct constraints for calcium perovskite's adiabatic bulk and shear moduli. We observe that titanium incorporation into calcium perovskite stabilises the tetragonal structure to higher temperatures, and that the shear modulus of calcium perovskite is significantly lower than is predicted by computations 3-5 or thermodynamic datasets 6. When combined with literature data and extrapolated our results suggest subducted oceanic crust will be visible as low velocity anomalies throughout the lower mantle. In particular we show that large low-velocity provinces (LLVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal-cubic phase transition in Ti-bearing calcium perovskite. The lower mantle is vast, extending from the seismic discontinuity observed at ~ 660 km depth to the core-mantle boundary (CMB) at ~ 2890 km. Tomographic images demonstrate that despite a smooth variation of vp, vs and ρ in 1D velocity models the lower mantle is heterogeneous and regularly refertilised by subducting slabs 7,8. Sluggish diffusive re-equilibration and incomplete mechanical mixing e.g. 9 means that large-scale patterns of mantle convection may be directly observed via tomographic velocity anomalies and/or the distribution of seismic scatterers. Identifying the causes of heterogeneities requires accurate mineralogical models of Earth's mantle to facilitate comparisons between geophysical observations and predicted seismic velocities. However, a major uncertainty in many models e.g. 10,11 has been the influence of calcium silicate perovskite (capv, here corresponding to Ca[SixTi[1-x]]O3) on velocity, despite the widespread expectation that it is the lower mantle's third most abundant phase comprising 5-10 and 24-29 vol.% of peridotitic 12 and basaltic 13 assemblages respectively. Uncertainties stem from a sparsity of reliable measurements of capv's physical properties, which are technically challenging because CaSiO3 is unrecoverable 14 , undergoing spontaneous amorphisation at room temperature during decompression. The widely used thermodynamic model of Stixrude et al. 6 predicts that capv is significantly faster than PREM 15 , and therefore slow velocity anomalies are difficult to explain using recycled crustal material. Whilst

Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

“…The discrepancy between our results and Thomson et al (2019) 54 resulted from the usage of different elastic and velocity data for Ca-perovskite. The calculated data used in this study are from previous ab initio molecular dynamic simulations 36 , while the Ca-perovskite data adopted in Thomson et al (2019) 54 were extrapolated from low pressure to the deep mantle conditions. Since velocities measured for Ca-perovskite 54,55 are considerably lower than computational predictions at the conditions of the uppermost lower mantle, the extrapolated data would be expectedly lower than theoretical calculations 36 under deep mantle conditions.…”

“…A recent work conducted by Thomson et al (2019) 54 suggested that subducted oceanic crust would be visible as low-seismicvelocity anomalies throughout the lower mantle when data are extrapolated to the lower-mantle conditions. The discrepancy between our results and Thomson et al (2019) 54 resulted from the usage of different elastic and velocity data for Ca-perovskite.…”

“…A recent work conducted by Thomson et al (2019) 54 suggested that subducted oceanic crust would be visible as low-seismicvelocity anomalies throughout the lower mantle when data are extrapolated to the lower-mantle conditions. The discrepancy between our results and Thomson et al (2019) 54 resulted from the usage of different elastic and velocity data for Ca-perovskite. The calculated data used in this study are from previous ab initio molecular dynamic simulations 36 , while the Ca-perovskite data adopted in Thomson et al (2019) 54 were extrapolated from low pressure to the deep mantle conditions.…”

“…The calculated data used in this study are from previous ab initio molecular dynamic simulations 36 , while the Ca-perovskite data adopted in Thomson et al (2019) 54 were extrapolated from low pressure to the deep mantle conditions. Since velocities measured for Ca-perovskite 54,55 are considerably lower than computational predictions at the conditions of the uppermost lower mantle, the extrapolated data would be expectedly lower than theoretical calculations 36 under deep mantle conditions. It is still unknown what results in the discrepancies in sound velocities of Caperovskite between theoretical and experimental studies, and future research is needed to solve this problem.…”

Effects of Heat‐Producing Elements on the Stability of Deep Mantle Thermochemical Piles

“…Exploring potential MORB compositions would require testing various specific MORB compositions and Si/Fe enrichment ratios. Recently, it has been suggested that Ca‐perovskite in MORB can play a major role in explaining LLSVP velocities (Thomson et al, ). This is an interesting scenario for HPE‐enriched piles, as aluminous Ca‐perovskite is also shown to easily incorporate HPEs (Gautron et al, ; Perry et al, ).…”