Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars’ deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 ± 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 ± 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m 2 .
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
Akimotoite, a MgSiO3 polymorph, present in the lower transition zone within ultramafic portions of subducting slabs and potentially also in the ambient mantle, will partition some amount of Al, raising the question of how this will affect its crystal structure and properties. In this study, a series of samples along the MgSiO3 akimotoite -Al2O3 corundum solid solution have been investigated by means of single-crystal X-ray diffraction in order to examine their crystal chemistry. Results show a strong non-linear behavior of the aand c-axes as a function of Al content, which arises from fundamentally different accommodation mechanisms in the akimotoite and corundum structures. Furthermore, two Al2O3-bearing akimotoite samples were investigated at high pressure in order to determine the different compression mechanisms associated with Al substitution. Al2O3-bearing akimotoite becomes more compressible at least up to a content of 20 mol% Al2O3, due likely to an increase in compressibility as the Al cation is incorporated into the SiO6 octahedron. This observation is in strong contrast to the stiffer corundum end-member having a KT = 250 GPa larger than that of the akimotoite end-member (KT = 205(1) GPa). These findings have implications for mineral physics models of elastic properties, which have in the past assumed linear mixing behavior between the MgSiO3 akimotoite and Al2O3 corundum end-members in order to calculate sound wave velocities for Al-bearing akimotoite at high pressure and temperature.
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