Basaltic reservoirs in the Earth’s mantle transition zone

Abstract

The formation and preservation of compositional heterogeneities inside the Earth affect mantle convection patterns globally and control the long-term evolution of geochemical reservoirs. However, the distribution, nature, and size of reservoirs in the Earth’s mantle are poorly constrained. Here, we invert measurements of travel times and amplitudes of seismic waves interacting with mineralogical phase transitions at 400–700-km depth to obtain global probabilistic maps of temperature and bulk composition. We find large basalt-rich pools (up to 60% basalt fraction) surrounding the Pacific Ocean, which we relate to the segregation of oceanic crust from slabs that have been subducted since the Mesozoic. Segregation of oceanic crust from initially cold and stiff slabs may be facilitated by the presence of a weak hydrated layer in the slab or by weakening upon mineralogical transition due to grain-size reduction.

Global Variability of Density Contrast Across the 660‐km Discontinuity

“…The modeling shows that the global average (5.3%) is consistent with a basalt fraction of 18%, which is in excellent agreement with that of pyrolite (Xu et al., 2008). For subduction zones, low density jumps of as much as 4.4 ± 0.3%, which correspond to basalt fractions of 34 ± 6%, favor a moderately basalt‐enriched MTZ, consistent with recent studies (Bissig et al., 2022; Tauzin et al., 2022). This basalt accumulation is associated with the density crossover between basalt and peridotite at 660 km depth, which facilitates the segregation of basalt from its harzburgitic residue and gravitational trapping in the MTZ (Anderson, 1979; Ringwood & Irifune, 1988).…”

Seismic evidence for global basalt accumulation in the mantle transition zone

“…( 33 ) and Tauzin et al. ( 43 ) first converted their differential travel time observations to depth using a 3D global seismic velocity model (S40RTS) and subsequently found that their estimates of MTZ thickness were consistent with the thicknesses predicted using the updated database, stx11 ( 19 ), for shallow mantle T pot estimates and a PMM composition.…”

“…Ritsema et al (51) used a self-consistent thermodynamic database stx08 (34) and found that differential travel times matched predictions from PMM at a T pot consistent with shallow mantle petrological estimates (52)(53)(54) and that PEA required an unrealistically high T pot . Waszek et al (33) and Tauzin et al (43) first converted their differential travel time observations to depth using a 3D global seismic velocity model (S40RTS) and subsequently found that their estimates of MTZ thickness were consistent with the thicknesses predicted using the updated database, stx11 (19), for shallow mantle T pot estimates and a PMM composition.…”

“…S20). Note that Waszek et al (33) and Tauzin et al (43) in essence did a similar correction by using a seismic model (with absolute wavespeeds determined by the seismic 1D reference profile) to convert their travel times to depth before comparing with the thermodynamic models. For stx08, we cannot simultaneously match MTZ differential travel times and the distance of the minimum in |R S660S | (Fig.…”

“…1A), give better global coverage than methods such as receiver functions, triplicated waveforms, ScS reverberations, or P′P′ precursors [e.g., (32)]. Previous studies estimated wavespeed and density (or impedance) contrasts across 410 and 660 using either stacked amplitudes or amplitude versus offset (AVO) analyses of SS and PP precursors (33,(37)(38)(39)(40)(41)(42)(43). Growth of the global dataset of seismic waveforms allows us to analyze an order of magnitude more data than previous studies.…”