Global observations of reflectors in the mid-mantle with implications for mantle structure and dynamics

Abstract

Seismic tomography indicates that flow is commonly deflected in the mid-mantle. However, without a candidate mineral phase change, causative mechanisms remain controversial. Deflection of flow has been linked to radial changes in viscosity and/or composition, but a lack of global observations precludes comprehensive tests by seismically detectable features. Here we perform a systematic global-scale interrogation of mid-mantle seismic reflectors with lateral size 500–2000 km and depths 800–1300 km. Reflectors are detected globally with variable depth, lateral extent and seismic polarity and identify three distinct seismic domains in the mid-mantle. Near-absence of reflectors in seismically fast regions may relate to dominantly subvertical heterogeneous slab material or small impedance contrasts. Seismically slow thermochemical piles beneath the Pacific generate numerous reflections. Large reflectors at multiple depths within neutral regions possibly signify a compositional or textural transition, potentially linked to long-term slab stagnation. This variety of reflector properties indicates widespread compositional heterogeneity at mid-mantle depths.

Primordial Earth Mantle Heterogeneity Caused by the Moon-forming Giant Impact?

“…The upper layer of the Earth is compositionally similar to the disk, out of which the Moon evolves, whereas the lower layer preserves proto-Earth characteristics.As long as this predicted compositional stratification can at least partially be preserved over the subsequent billions of years of Earth mantle convection, the compositional similarity between the Moon and the accessible Earth's mantle is a natural outcome of realistic and high-1 arXiv:1904.02407v1 [astro-ph.EP] 4 Apr 2019probability Moon-forming impact scenarios 8 . The preservation of primordial heterogeneity in the modern Earth not only reconciles geochemical constraints 4, 5,9,10 but is also consistent with recent geophysical observations [11][12][13][14] . Furthermore, for significant preservation of a proto-Earth reservoir, the bulk composition of the Earth-Moon system may be systematically shifted towards chondritic values.As the only planet in our solar system, the Earth is orbited by a single and massive moon.The leading theory for the formation of the Earth-Moon system with its high angular momentum involves a giant impact followed by lunar aggregation from the impact debris disk 1 .…”

“…about radius R) is indeed consistent with restricted mixing. Sharp seismic-velocity contrasts at similar depths support this interpretation, and provide direct evidence for large-scale compositional mantle heterogeneity 13,14 . The preservation of primordial noble gases 10 , e.g.…”

“…Since we do know the isotopic composition of the Moon (XM) and the upper mantle (XU) as well as the masses and/or proto-Earth contributions in all reservoirs (from the SPH simulations; mi and/or fi), we are left with four unknown variables (XP, XT, XL, XJ) and four equations relating them. We use equations (11) and (12) to solve for XP and XT, which are then in turn used to calculate XL and XJ using (13) and (14). Remarks on the core entropy and EOS…”

Evidence of Subduction‐Related Thermal and Compositional Heterogeneity Below the United States From Transition Zone Receiver Functions

“…Sporadic and weak conversions are present between about 850‐km and 1,000‐km depth. Seismic layering in the lower mantle have been observed previously (e.g., Jenkins et al, ; Waszek et al, ), but there are no known mineral phase transitions that could account for these observations.…”

Dehydration Melting Below the Undersaturated Transition Zone

“…While the major components of the Earth's lower mantle are unlikely to store significant water, a group of hydrous magnesium silicates, particularly phases D and H, have been shown to be stable under the pressure and temperature conditions of the Earth's mantle (Nishi et al, 2014;Tsuchiya, 2013), particularly when considering stabilization with aluminum solid solutions (Pamato et al, 2015;Panero & Caracas, 2017), these minerals may be stable to the temperatures of a relatively cold geotherm, more likely to be found in relatively cold, downwelling areas. P-to-S conversions and SS and PP precursors have identified additional sharp changes in physical properties of the mantle below the base of the transition zone, at depths between 730 and 780 km depth distinct from other reflectors in the deep mantle (e.g., Jenkins et al, 2017;Waszek et al, 2018). While not a global signature, P-to-S conversions are observed at the base of the transition zone in downwelling regions beneath the southwestern North America (Schmandt et al, 2014) and Japan (Liu et al, 2016) indicate an abrupt velocity decrease 50-100 km beneath the 660 km discontinuity in which the P-to-S conversion changes polarity.…”