Basalt Accumulation Fuels Warm Slab’s Subduction into Lower Mantle

doi:10.21203/rs.3.rs-3219497/v1

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Basalt Accumulation Fuels Warm Slab’s Subduction into Lower Mantle

https://doi.org/10.21203/rs.3.rs-3219497/v1

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How and why oceanic slabs subduct into the lower mantle is crucial in understanding the mantle dynamics and evolution, but remains unclear. Here we present seismic images of the mantle transition zone (MTZ) beneath the Sumatra subduction zone that reveal phase transformations of both the olivine and garnet systems, exhibiting a relatively warm slab in a ~250-km-wide passage from ~350 to ~750 km, then broadening to ~600 km wide at ~780 km depths. The amplitudes of converted waves at the interfaces of the MTZ base and the topmost of the lower mantle suggest up to ~75±25% of basalts in a basalt-harzburgite assemblage and thus intensive enrichment of oceanic crust near the upper-lower mantle boundary, and a gradual reduction of basalt content toward the depthsof ~780 km. Mineral physics modeling shows that such basaltic enrichment provides sufficient positive density contrasts and driving forces for downwelling of the slab assemblage into the lower mantle, and negative density contrasts and thereby buoyancy for the upward concentration of basalts in the uppermost lower mantle. The residual part of the slab assemblage subjected to the basalt reduction is rheologically more weak and conducive to itsflattening and broadening between 750 to 1200 km depths.

Earth and environmental sciences/Solid Earth sciences/Seismology

Earth and environmental sciences/Solid Earth sciences/Geodynamics

Subducting plates follow different paths in the mantle. While some are stagnant in the MTZ above 660 km depth¹^,², others sink straightly through the MTZ and stalling in the uppermost lower mantle³. The negative Clapeyron slopes of the post-spinel⁴ and akimotoite-bridgmanite⁵ phase changes can account for the stagnation of some subducting slabs at the base of the MTZ, but the mechanisms causing slabs to sink straightly into the lower mantle and stalling in the uppermost lower mantle remain unsettled. Geoscientists have attributed the straight slab sinking to factors such as hotter mantle⁶, downwelling geometry⁷, relatively large flexural rigidity of slabs with intermediate thermal parameters⁸, or younger and weaker slab incapable of inducing trench and plate retreat⁹, and have ascribed the slab stalling in the top of the lower mantle to the buoyancy of basaltic crust¹⁰^,¹¹, negative Clapeyron slope of post-garnet phase changes¹², and viscosity increase in the top of the lower mantle¹³^,¹⁴.

The subduction of oceanic slab brings both thermal and compositional heterogeneity into deep mantle. The top and bottom of the MTZ, or the 410- and 660-km seismic discontinuities, are generally linked to the olivine-to-wadsleyite and post-spinel phase changes of pyrolite mantle¹⁵. These phase changes have positive and negative Clapeyron slopes, and the subduction process may lead to shallowing and deepening of the top and bottom boundaries of the MTZ¹⁶, respectively. The phase change from wadsleyite to ringwoodite leads to the 520-km discontinuity observed within the MTZ. This pyrolite-based mantle discontinuity system becomes more complicated when the garnet components (subducted basaltic crust) are involved: The Clapeyron slopes of garnet-to-post-garnet phase changes¹⁷ lead to split 520-km discontinuities¹⁸observed in the middle of the MTZ¹⁹ and multiple interfaces observed near the base of the MTZ²⁰. The phase changes occur at different temperatures and pressures in slabs with different compositions. Seismic imaging of these phase change boundaries could help resolve which mechanisms dominate the mantle dynamics and subduction processes.

Seismic Imaging of the MTZ structure beneath the Sumatra subduction zone

The Sumatra subduction zone is an ideal region for seismic investigations of the subduction processes of oceanic slab through the MTZ, where a young ocean floor (~50 Ma)²¹ subducts steeply beneath the southern margin of the Eurasia plate and a slab has been imaged down to ~1200 km depth by various seismic tomography studies³^,²². This region has a relatively dense coverage of seismic stations available through the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (Fig. 1). The available data illustrated by the distribution of P-to-S-wave piercing points at 410 and 660 km (Fig. 1) is sufficiently dense to generate a high-resolution seismic profile across the subduction zone.

We used the P-wave receiver function (RF, P-to-S converted waves) technique, an effective tool for imaging the mantle discontinuities²⁰ and slab interfaces within the MTZ²⁵, to investigate the MTZ structure. We implemented a procedure similar to the common-conversion-point method but without plane-wave assumption; RFs were migrated from time to space in a 3-D volume beneath the seismic stations by dynamic ray tracing²⁶ from the sources to the seismic stations, and then corrected for the earth crust, ellipticity, station elevation, and the 3-D velocity inhomogeneity along the ray-paths using the TX2019slab model²⁷. The seismic images were finally obtained by projecting and stacking all the RFs onto the profile perpendicular to the strike of the subduction zone (Fig. 1).

The 410-, 520- and 660-km discontinuities are observed in the image constructed with all the 2752 RFs (Fig. 2a). We further improved the observation by applying an automatic screening procedure to the RFs, in which a RF is removed if it does not enhance the maximum stacked amplitude within a depth range of interest (Fig. 2b-g). This new procedure is aimed at minimizing RFs with large random noise or conversions from later P arrivals (e.g., pP, PcP) with improper ray parameters, improving the legibility of the structures in the image. To obtain more robust estimates of the RF amplitudes for the discontinuities, we then stacked the RFs for selected regions (Tables S1 and S2).

Intricate Transformations of the Olivine and Garnet Systems

There are two sets of discontinuities observed in our images: One involves the 410-, 520- and 660-km discontinuities, which can be traced continuously across our profile (Fig. 2b-d). The other discontinuities are not universally observed and can be observed mainly within the subduction zone, including the 350-, 560-, 730- and 780-km discontinuities (Fig. 2a, 2e-g).

North of the subducting slab, the 410-km discontinuity is at the normal depth²⁸^,²⁹^,³⁰, indicating that the mantle there has a normal, average temperature (Fig. 2b). The undulation of the 410-km discontinuity beneath the central section (~11 km uplift) suggests that the center of the subducting slab is colder than the average mantle by ~150 K, according to the Clapeyron slope for the transition from olivine to wadsleyite¹⁶ (see Extended Data Fig. S1). South of the slab, the 410-km discontinuity is at ~425 km depth, based on the stacked RF for the south section (Fig. 2b). The depressed 410-km discontinuity behind the subducting plate is not rare, which can also be observed behind the subducting Pacific plate³¹.

The 660-km discontinuity appears at a little shallower-than-normal depth north of the subduction zone and slightly depressed (a few kilometers) within the subduction zone (Fig. 2d). The depressed 410- and 660-km discontinuities south of the subduction zone suggests a hot mantle (~180 K) near the top of the MTZ and a cold mantle at the bottom of the MTZ. Measurements by the peak-to-peak waveforms of stacked RFs show that the Sumatra subduction zone has a thicker-than-normal MTZ of ~268 km, and its southern and northern sides have a relatively normal MTZ of ~254 and ~247 km, respectively (Table S1), which suggests similar MTZ thicknesses thereby average mantle temperatures on both sides of the subduction zone. Under-corrected low-velocity anomalies in the crust and shallow (<400 km) mantle in the southern section may contribute to the apparent depression of the 410- and 660-km discontinuities, but should have little influences on the observed MTZ thicknesses. Based on the parameters and the relationship between the MTZ thickness and temperature of ref.³², we estimated the mantle potential (adiabatic) temperatures(Tp)³³ of the MTZ: ~1420 K within the subduction zone and ~1580 K south and north of the zone (Fig. S2).

In the middle of the MTZ, the 520-km discontinuity shallows from 520 km to 490 km depths from south to north along our profile with a small undulation in the central section (Fig. 2c). It is near the normal depth south of the slab, while the local uplift (~13 km) of the 520-km discontinuity in the center of the subduction zone corresponds to ~150 K degrees lower temperature (Fig. S1). In contrast, the 560-km discontinuity can be observed only within a limited lateral extent under the subduction zone at depths of ~560 km (Fig. 2e). The 520- and 560-km discontinuities are generally associated with the transitions from wadsleyite to ringwoodite of the olivine component¹⁶ and from garnet to Ca-rich perovskite of the garnet component³⁴, respectively. They appear at similar depths for an average mantle temperature of 1500 K but move to different depths at lower temperatures¹⁸^,¹⁹. Our Perple X modeling³⁵ results show that a high concentration of basalt (see below) can also enlarge the separation of the discontinuities (Fig. S1). The observed separation (~54 km) of the two split discontinuities and the elevated 520-km discontinuity may suggest a potential adiabatic temperature Tp of 1420 K, if the mantle there has a basalt fraction f=0.4 (for reference, a pyrolitic mantle has f=0.2, oceanic crust f=1.0 and harzburgite f=0.0; Table S3). We note that the limited lateral extent of the 560-km discontinuity suggests that the significant garnet component may be confined within a 200- to 300-km wide passage of the subduction zone.

We also observed some weak conversions, distributed diffusely directly below the MTZ down to a depth of ~750km (Fig. 2a,f), suggesting wave-speed gradients near the top of the lower mantle. The maximum amplitudes of these conversions, delineated as the 730-km discontinuity, is clearly imaged, with even stronger conversions than the 660-km discontinuity, at depths of ~720 km within the subduction zone, but weaker than the 660-km discontinuity, at 680-700 km depths south and north of the subduction zone (Fig. 2f).

The 730-km discontinuity has been interpreted as the transition of non-olivine component, involving the transition from ilmenite (akimotoite) to Ca-rich perovskite (bridgmanite) in a relatively cold region¹⁷, or garnet to bridgmanite (gt-out) in less cold regions²⁰^,³². Recent high-pressure experiment using in situ X-ray diffraction multi-anvil techniques shows that the garnet to bridgmanite+corundum (br+cr) transition appears at similar depths¹². Our Perple X modeling shows that, at ~720 km depth, this discontinuity also corresponds to the appearance of the Ca-ferrite (cf) structure at the temperatures inferred from the MTZ discontinuities(Fig.S1). Both of the br+cr and br+cf transitions can account for the central portion of the discontinuity. South and north of the subduction zone, however, the discontinuity appears at depths much shallower than predicted by the small negative Clapeyron slope of the br+cr transition¹², and inconsistent either with the positive Clapeyron slope of the br+cf transition and the situation of relatively higher temperatures away from the center of the subduction zone. Therefore, other mechanisms, involving the segregation of oceanic crust entrained into³⁶ and trapped near the top of the lower mantle¹⁰^,¹¹, are required to account for all the observation of the 730-km discontinuity in the region.

At greater depths, one more discontinuity can be coherently imaged at depths of ~780 km near the top of the lower mantle, also with conversions stronger than the 660-km discontinuity (Table S1). It appears at depths similar to that predicted by the Perple X modeling for the gt-out transition at the temperature inferred for the subduction zone (Fig. S6d). The upward concaved shape is in good agreement with the large positive Clapeyron slope for the gt-out transition, and the undulation of the 780-km discontinuity (~27 km) suggests that the center of the subducting slab is colder than its southern periphery mantle by ~220 K, according to the Clapeyron slope in the Perple X modeling results (Fig. S1). The 780-km discontinuity has a lateral extent of ~600 km, much wider than the other aforementioned garnet discontinuities.

Basalt Accumulation toward the Upper-Lower Mantle Boundary and Buoyance for the Subduction and Basaltic Segregation Processes

Theoretically, an increase of basalt fraction may lead to a decrease of RF amplitudes observed from the 410-km and 660-km discontinuities and an increase of RF amplitudes from the 730-km³² and the 780-km discontinuities (Figs. 3, S3c-d and S4). Indeed, we observed RF amplitude decrease toward the center of the subduction zone for the 410-km and the 660-km discontinuities (Table S1), consistent with slab-ward increase of the basalt fractions. The observation of stronger conversion for the 780-km than for the 660-km discontinuity (Table S1) is consistent with a high-level concentration of basaltic component in the slab assemblage near the top of the lower mantle (Fig. 3d).

Nevertheless, stacking of RFs from undulating discontinuities over a relatively broad area may lead to an underestimation of the RF amplitudes and thereby an overestimation of the basalt fractions near the 410-km and 660-km discontinuities and an underestimation of the basalt fractions near the 780-km discontinuity. Therefore we used the data stacked in a relatively narrow region from the station PSPSI (Table S2), which is near the center of the subduction zone, to mitigate such an effect in the estimation of the basalt fractions of the slab assemblage (denoted by solid blue rectangles in Fig. 3; the estimations with data from broader regions are shown for comparison). Our estimation suggests that the basalt fractions are ~0.35 near the top of the MTZ (Fig. 3a), ~0.75 near the base of the MTZ (Fig. 3b), and ~0.25 near the depths of ~780 km (Fig. 3c). In contrast, the basalt fractions in the surrounding mantle are relatively lower, especially south of the subduction zone (Fig. 3a-b).

The post-garnet transition, which spreads over a large depth interval from 520 to 800 km³⁷ (or to ~780 km in the study region, Fig. S1), is rather diffuse. Our synthetic modeling shows that for a mechanically mixed basaltic slab assemblage (valid approximation under slow equilibration rate³⁹), such a phase transition involving the appearance of the Ca-ferrite structure cannot produce sufficient RF-amplitude observed for the 730-km discontinuity (Fig. S4e), unless a compositional segregation has also occurred at the discontinuity. Based on our observation, we infer that the 730-km discontinuity is due to a combination of a downward increase of Ca-ferrite and Ca-perovskite components and a downward reduction of basalt-fraction (on an order of ~0.1, Fig. S4f) in the slab assemblage.

If we ignore the downward reduction of the basalt fraction, we can follow ref.³² to make another estimation with the P780S/P660S ratio, and obtained an estimate of basalt fraction ~0.5 for the slab assemblage near the top of the lower mantle (Fig. 3d). In this case, the effects of the errors in the RF-amplitudes and the ray parameters on the estimation will be largely removed (Fig. S5). Because of the presence of downward reduction of basalt fraction, a decrease of basalt fraction near the 780-km discontinuity requires an increase of basalt fraction near the 660-km discontinuity to maintain the same P780S/P660S ratio (Fig. 3b-c). Thus, the above estimate (~0.5) can be considered as the lower and upper bound for the estimates of basalt fraction, respectively, for the 660-km and the 780-km discontinuities.

Figure 4a shows the density contrasts computed with the Perple X code³⁵ and the elastic parameter database of ref.³⁸ for a slab assemblage with Tp=1420 K and various basalt fractions with respect to a pyrolitic surrounding mantle with Tp=1580 K. We consider the slab assemblage to be either equilibrium assemblage (EA) or mechanical mixture (MM)³⁹ of basalt and harzburgite. An increase of basalt fraction (f) will lead to an increase of positive density contrast down to the bottom of the MTZ for either the EA or MM mantle. A slab assemblage with f≥0.3 has positive density contrasts throughout the MTZ, taking into account the effects of the phase change, thermal expansion and the depression of the 660-discontinunity. Thus, depressed 660-km discontinuity poses no hindrance to the sinking of the basalt-rich slab assemblage into the lower mantle. Our results also show that a basalt-enriched slab assemblage (f³0.4 for EA or f³0.3 for MM) will have negative density contrast thereby upward buoyancy and will tend to stall in the top of the lower mantle.

Basalt enrichment and a relatively warm thermal condition appear to be indispensable for the slab’s penetrating into the lower mantle. A hypothetical harzburgite slab depleted of basalt component under similar thermal condition will produce small to no positive density contrast in the MTZ and larger positive density contrast in the top of the lower mantle (red curves in Fig. 4a). This will prevent the slab from stalling either in the base of the MTZ or the top of the lower mantle. On the other hand, a slab under a relatively colder environment, the garnet-to-ilmenite transition above the 660-km discontinuity¹⁷^,²⁰ will lead to a significantly negative density contrast at the base of the MTZ, whether the slab assemblage is basalt-enriched or not (Fig. S6c). This will produce positive buoyancy for the slab stagnant at the base of the MTZ, but not for the stalling in the top of the lower mantle, consistent with the recent high-pressure experiment for the akimotoite-bridgmanite transition in a relatively cold region⁵. In a relatively warm region such as the Sumatra subduction zone, ilmenite, which is less dense than perovskite, is unstable and transforms into bridgmanite at depths shallower than the postspinel phase transition⁵^,¹⁷, leading to a loss of local buoyancy at the base of the MTZ (Fig. 4a) and facilitate a straight sinking of the slab through the MTZ. Such a thermal setting seems consistent with the fact that most of such subductions occur in and around the Pacific Large Low-Shear-Velocity Province²^,³.

Our results suggest that the basalt accumulation plays a crucial role in the mantle dynamics for the slab penetrating through the MTZ and the segregation and stalling of basalt component in the shallow lower mantle. In contrast, the dynamic effects of the depressed 660-km discontinuity become insignificant, because of the gentle Clapeyron slope of the postspinel transition in such a relatively warm region⁵. The dynamic effects of the negative Clapeyron slope of the br+cr transition¹² may also be largely canceled out by the positive Clapeyron slopes of the br+cf transition and the gt-out transition observed in this study (Fig. S1).

Compared to what have been previously reported for the global accumulation of basalt in the MTZ⁴⁰, we observed an extremely high concentration of basalt near the upper-lower mantle boundary, mainly within the subduction zone, and a significant removal of basalt component at depths ≥ 780 km. The observation of a ~250 wide vertical passage down to ~750 km depth suggest that the slab assemblage is still basalt-enriched and mechanically strong down to such a depth. The segregation of basalt from the slab and stalling in the periphery mantle may further facilitate the subduction of the remnant slab into deep mantle (Fig. S6d). At depths ≥ 780 km, a slab with less basalt component is rheologically more weak⁴¹, conducive to its flattening and broadening, as imaged by seismic tomography³^,²², between 750 to 1200 km depths(Fig. 4b). This weakness feature can be further strengthened by the resistance of viscosity increase¹³, likely derived from the spin transition of iron¹⁴, between 750 to 1200 km depths. Because the slab is no longer buoyant and there lacks any other suggested phase transition, the resistance of viscosity increase seems indispensable for the slab’s stagnation between 750 to 1200 km depths. But, the viscosity increase alone cannot account for the observations of the basaltic enrichment and segregation in the topmost lower mantle.

Data availability

All seismic data used in this study are available online through the IRIS data service (http://ds.iris.edu/ds/). The RF data are also available from the corresponding authors.

Code availability

The seismic ray tracing program used in the construction of the RF images is available at http://geodynamics.org/cig/software/packages. The thermodynamic code Perple X used for the mineral physics modeling is publically available at https://www.perplex.ethz.ch. Figures were created with the GMT software package(https://www.generic-mapping-tools.org). Specific data processing scripts are available from the corresponding authors.

Acknowledgments

This study is supported by the National Natural Science Foundation of China (42174109), National Key Research and Development Program of China (2016YFC0600201), and China Geological Survey (DD20221829 ) to D.S. and the U.S. Natural Science Foundation (1927133) to Y.S.

Author Contributions

D.S. and Y.S. performed seismological analysis and wrote the paper. D.S., J.S. and R.Z. processed the seismic data. D.S and H.X. conducted mineral physics modeling.

Competing interest statement: The authors declare no competing interests.

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