Cold Slab Research Articles

Widespread D″ ${\mathbf{D}}^{\mathbf{{\prime\prime}}}$ Anisotropy Beneath North America and the Northeastern Pacific and Implications for Upper Mantle Anisotropy Measurements

AbstractObservations of seismic waves that have passed through the Earth's lowermost mantle provide insight into deep mantle structure and dynamics, often on relatively small spatial scales. Here we use SKS, S2KS, S3KS, and PKS signals recorded across a large region including the United States, Mexico, and Central America to study the deepest mantle beneath large swaths of North America and the northeastern Pacific Ocean. These phases are enhanced via beamforming and then used to investigate polarization‐ and propagation direction‐dependent shear wave speeds (seismic anisotropy). A differential splitting approach enables us to robustly identify contributions from anisotropy. Our results show strong seismic anisotropy in approximately half of our study region, indicating that anisotropy may be more prevalent than commonly thought. In some regions, the anisotropy may be induced by flow driven by sinking cold slabs, and in other, more compact regions, by upwelling flow. Measured splitting due to lowermost mantle anisotropy is sufficiently strong to be non‐negligible in interpretations of SKS splitting due to upper mantle anisotropy in certain regions, which may prompt future re‐evaluations of upper mantle anisotropy beneath North and Central America.

Deep‐Focus Earthquake Mechanisms at the Subducting Nazca Plate (Peru‐Brazil Border): Cold Slab Behavior in a Warm Plate

AbstractWe calculate focal mechanisms and centroid depths for deep‐focus earthquakes (DFEs) along the Peru‐Brazil border. We obtained a total of 28 focal solutions for events with magnitudes between 4.2 and 7.5 Mw and occurring between 2014 and 2022. Focal mechanisms indicate predominance of normal faulting, demonstrating a rather uniform down‐dip compression (DDC) regime within the plate. The orientations of the nodal planes suggest that earthquakes tend to occur along faults parallel to the local slab strike, although other fault types are documented. Stress orientations derived from the focal mechanisms agree with patterns expected if faulting were initiated by transformational faulting on a metastable olivine wedge (MOW) under DDC. Centroid depths range between 557 and 659 km, defining a narrow seismic zone within the lower portion of the subducting plate and an aseismic upper portion. We suggest that DFEs nucleate through transformational faulting within a narrow MOW preserved at a colder slab segment right above the lower mantle and juxtaposed to a shallower, warmer segment at around 500 km depth. This thermal complexity was possibly produced through flat subduction initiated by the subduction of the Nazca Ridge. We speculate that subduction of other aseismic ridges is possibly controlling the thermal state of the Nazca slab as a whole and, consequently, the depth distribution of DFEs along the South America subduction front.

The fate of ultramafic-rich mélanges in cold to hot subduction zones: Implications for diapirism (or not) and chemical geodynamics

Buoyant ultramafic-rich (serpentine- or chlorite-rich) mélange diapirs in sediment-starved subduction zones can transport slab material to arc sources. While the buoyancy of chlorite-rich mélanges was previously investigated, serpentine-rich mélanges were never explored. Thus, the overall contribution of ultramafic-rich mélanges to buoyancy, the conditions for diapir formation, and their fate in subduction zones are not well constrained. Here, we investigate the partial melting behavior and the associated density transformations of a serpentine-rich matrix (5–10 wt.% H2O) with minor sediments (9:1 ratio) at fore-arc (∼65 km) to sub-arc (∼95 km) depths (2–3 GPa and 800–1250 °C) and compare to that of chlorite-rich mélanges from the literature. Our results show that the solidus of serpentine-rich matrices is between 1050 and 1100 °C and requires either diapiric rise of the mélange into the hotter mantle wedge or interactions with a hotter asthenosphere through slab tears to partially melt and produce basaltic melts, whether in hot or cold slab channels. Chlorite-rich mélanges may account for the sources of some arc lavas, but partial melting of serpentine-rich mélanges produce melts depleted in CaO, TiO2, alkalis, and are highly enriched in MgO compared to basaltic arc lavas. Both serpentine-rich and chlorite-rich matrices dehydrate to form denser peridotite and lose buoyancy at ∼800 °C and ≥1000 °C, respectively. Even if diapirism initiates in such mélanges near the slab-mantle interface, they would likely lose buoyancy upon ascent into the hotter mantle wedge resulting in stalled or failed diapirs. Diapir growth (τa) is controlled by the interplay of density, thickness and viscosity of the mélange, as well as the timescale of slab subduction (τs) and thermal structure of the subduction zone. We observe that the onset of diapirs in cold subduction zones requires mélanges that may sometimes be thicker than that observed by field and geophysical studies, while hot subduction zones overall require thinner mélanges. Thus, ultramafic-rich mélange diapirs may occur but only under specific conditions and when the diapiric ascent timescale is faster than the thermal equilibration barrier of ∼800–1000 °C (especially at the core of the mélange). Dehydration or partial melting of ultramafic-rich mélanges can affect the large ion lithophile element (LILE), volatiles, and high-field strength element (HFSE) budgets in the mantle wedge. Partial melting (caused by a diapiric rise or slab tear) does not fractionate LILEs from HFSEs at T ≥ 1100 °C and if the mélange has a lower LILE/HFSE to begin with, that signature is transferred to arc sources. Dehydration releases aqueous fluids rich in fluid-mobile elements (LILE and volatiles) relative to HFSE. Thus, the characteristic high LILE/HFSE signature of aqueous fluids is transferred to arc magma sources. Given high LILE/HFSE ratio is a ubiquitous arc magma signature, but slab tears are not, and diapirism in ultramafic-rich mélanges is highly conditional, this study corroborates that aqueous fluids released from sediment-starved mélanges are the predominant mass transfer agents rather than diapirs.

Inefficient nitrogen transport to the lower mantle by sediment subduction

The fate of sedimentary nitrogen during subduction is essential for understanding the origin of nitrogen in the deep Earth. Here we study the behavior of nitrogen in slab sediments during the phengite to K-hollandite transition at 10–12 GPa and 800–1100 °C. Phengite stability is extended by 1–3 GPa in the nitrogen (NH4+)-bearing system. The phengite-fluid partition coefficient of nitrogen is 0.031 at 10 GPa, and K-hollandite-fluid partition coefficients of nitrogen range from 0.008 to 0.064, showing a positive dependence on pressure but a negative dependence on temperature. The nitrogen partitioning data suggest that K-hollandite can only preserve ~43% and ~26% of the nitrogen from phengite during the phengite to K-hollandite transition along the cold and warm slab geotherms, respectively. Combined with the slab sedimentary nitrogen influx, we find that a maximum of ~1.5 × 108 kg/y of nitrogen, representing ~20% of the initial sedimentary nitrogen influx, could be transported by K-hollandite to the lower mantle. We conclude that slab sediments may have contributed less than 15% of the lower mantle nitrogen, most of which is probably of primordial origin.

Present day mantle structure from global mantle convection models since the Cretaceous

SUMMARY Using forward mantle convection models starting at 140 Ma, and assimilating plate reconstructions as surface velocity boundary condition, we predict present-day mantle structure and compare them with tomography models, using geoid as an additional constraint. We explore a wide model parameter space, such as different values of Clapeyron slope and density change across 660 km, density and viscosity of the thermochemical piles at the core–mantle boundary (CMB), internal heat generation rate, and model initiation age. We also investigate the effects of different strengths of a weak layer below 660 km and weaker asthenosphere and slabs. Our results suggest that slab structures at different subduction zones are sensitive to the viscosity of the asthenosphere, strength of slabs, values of Clapeyron slope and the density and viscosity of the thermochemical piles, while different internal heat generation rates do not affect the slab structures. We find that with a moderately weak asthenosphere ($10^{20}$ Pa·s) and strong slabs, the predicted slab structures are consistent with the tomography models, and the observed geoid is also matched well. Moreover, our models successfully reproduce the degree-2 structure of the lower mantle beneath Africa and the Pacific, also known as Large Low Shear Velocity provinces (LLSVPs). A moderate Clapeyron slope of −2.5 MPa K−1 at 660 km aids in slab stagnation while higher values result in massive slab accumulation at that depth, ultimately leading to slab avalanches. We also find that the convective patterns in the thermal and thermochemical cases with slightly denser LLSVPs are similar, although the geoid amplitudes are lower for the latter. However, with more dense LLSVPs, the slabs cannot perturb them and no plumes are generated. Plumes arise as thermal instabilities from the edges of the LLSVPs, when cold and viscous slabs perturb them. While our predicted plume locations are consistent with the observed hotspot locations, matching the plume structures in tomography models is difficult. These plumes are essential in fitting the finer features of the observed geoid. In longer-duration models, more voluminous subducted material reaches the CMB, which tends to erode the LLSVPs significantly, and yields a poor fit to the observed geoid. Our results suggest that with the presence of a thin, moderately weak layer below 660 km, a slightly dense LLSVP, and Clapeyron slope of −2.5 MPa K−1, the velocity anomalies in seismic tomography and the long-wavelength geoid can be matched well. One of the limitations of our models is that the assimilated plate motion history may be too short to overcome arbitrary initial conditions effects. Also, assimilated true plate velocities in our models may not represent the true convective vigour of the Earth.

Deep‐Learning‐Based Prediction of the Tetragonal → Cubic Transition in Davemaoite

AbstractDavemaoite, that is, CaSiO3 perovskite (CaPv), is the third most abundant phase in the lower mantle and exhibits a tetragonal‐cubic phase transition at high pressures and temperatures. The phase boundary in CaPv has recently been proposed to be close to the cold slab adiabat and cause mid‐mantle seismic wave speed anomalies (Thomson et al., 2019, https://doi.org/10.1038/s41586‐019‐1483‐x). This study utilized accurate deep‐learning‐based simulations and thermodynamic integration techniques to compute free energies at temperatures ranging from 300 to 3,000 K and pressures up to 130 GPa. Our results indicate that CaPv exhibits a single cubic phase throughout lower‐mantle conditions. This suggests that the phase diagram proposed by Thomson et al. requires revision, and mid‐mantle seismic anomalies are likely attributable to other mechanisms.

Elasticity of Single‐Crystal Clinohumite at High Pressures and Temperatures: Implication for the H2O and F Circulation in the Earth's Mantle

AbstractIn this study, we have determined the single‐crystal elasticity of clinohumite [Mg8.85Ti0.19Si3.93O16(OH1.11F0.89)] using Brillouin measurement up to 21 GPa at 300 K and 1 bar at 750 K, respectively. The elasticity of clinohumite was determined to be KS0 = 126.2(3) GPa, G0 = 79.7(2) GPa with pressure derivatives KS′ = 4.2(1), G′ = 1.3(1), pressure derivatives ∂KS/∂T = −0.024(1) GPa/K, and ∂G/∂T = −0.011(1) GPa/K). We comprehensively examined the effects of varying H2O, fluorine content and thermal states, on the velocity and density structures of the subducted harzburgite layer. Assuming a typical H2O content of 2 wt.% within harzburgite, our modeling has shown that hydrous harzburgite with clinohumite as the decomposition product of serpentine along a hot slab geotherm even has the VP and VS 0.4–0.8(6)% greater than it dry counterpart at 250–380 km depth. Yet in the top transition zone, the addition of H2O and F can effectively lower the sound velocities and density. The F‐bearing hydrous harzburgite has the VP and VS 1.1(5)–1.3(3)% lower than its dry counterpart, and only 0.6(5)% and 2.3(5)% greater than the pyrolitic mantle. Along cold slab geotherm, phase A will replace clinohumite as the dominant hydrous phase in the harzburgite, the VP and VS are 4.8(5)–5.3(3)% and 5.9(5)–6.0(3)% greater than the pyrolitic mantle in the upper mantle. In the top transition zone, the difference is approximately 3% in VP and 5% in VS. Our results provide crucial experimental evidence for future assessments of the seismic signals of subducted slabs with different hydrous minerals and thermal states.

Research on Continuous Casting–Hot Rolling Scheduling Model Involving Reheating Furnace Conversion Mode and Improved Bat Optimization Solution Algorithm

As the main energy-consuming equipment, the reheating furnace plays an important role in the scheduling of continuous casting and hot rolling for a steel mill. This paper studies the energy consumption of cold and hot slab conversion mode in a reheating furnace, forming four conversion relationships, with the goal of minimizing the coupling function between the maximum completion time of the hot rolling and the energy waste of the reheating furnace, and modelling the scheduling of a continuous casting–hot rolling process. An improved ordered pair bat algorithm is used to solve for special process conditions of the proposed scheduling model. The effectiveness of the proposed model and algorithm is verified by solving the different slab charging sequence and conversion mode. Moreover, a comparative study of different types of batches is also conducted, and it is found that the model can save more than 5000 GJ per month for the steel mill, which can achieve the goal of effective energy saving and increasing enterprise profit.

Seismic evidence for melt-rich lithosphere-asthenosphere boundary beneath young slab at Cascadia

The Lithosphere-Asthenosphere Boundary (LAB) beneath oceanic plates is generally imaged as a sharp seismic velocity reduction, suggesting the presence of partial melts. However, the fate of a melt-rich LAB is unclear after these plates descend into the mantle at subduction zones. Recent geophysical studies suggest its persistence with down-going old and cold slabs, but whether or not it is commonly present remains unclear, especially for young and warm slabs such as in the Cascadia subduction zone. Here we provide evidence for its presence at Cascadia in the form of a large (9.8±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document}1.5%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\%$$\\end{document}) decrease in shear-wave velocity over a very small (<3 km) depth interval. Similarly large and sharp seismic velocity reduction at the bottom of both old and young slabs, as well as along the base of oceanic plates before subduction, possibly represents widespread presence of melts. The melt-rich sub-slab LAB may strongly influence subduction dynamics and viscoelastic earthquake cycles.

Sublithospheric Diamonds: Plate Tectonics from Earth's Deepest Mantle Samples

Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. ▪Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples.▪Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds.▪Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes.▪Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget.

Arc volcanism initiated on the eastern margin of Eurasia during the Early Cretaceous: Geochemistry of the Takanokura volcanic rocks in the Abukuma Mountains, Northeast Japan

AbstractThe Early Cretaceous Takanokura Formation in the eastern part of the Abukuma Mountains consists of a lower felsic ignimbrite and upper intermediate lavas and volcaniclastic rocks, representing the initial arc‐type in northeast Japan. In this study, I analyzed the major and trace element contents and Sr‐Nd‐Pb isotopic ratios of these eruptive products; then, I discussed their magma genesis based on their geochemical properties. Although igneous rocks of the same period in other localities of northeast Japan are characterized by the occurrence of adakites, these volcanism are composed of non‐adakitic high‐ to medium‐K andesite to rhyolite that are rich in large‐ion lithophile elements and poor in high‐field‐strength elements and have low Sr/Y values and flat heavy rare earth element patterns. Furthermore, these rocks have high radiogenic Pb isotopic ratios. The rhyolite and dacite have been thought to derive from crustal melting, whereas the andesite formed by the mixing of crustal felsic melts and mafic magmas generated by melting of the lithospheric mantle. Although previous studies attributed the formation of the Early Cretaceous adakites to the hot subduction of a mid‐ocean ridge, recent global plate motion reconstructions reject this model. To generate magma from a cold slab and lithospheric mantle that does not spontaneously melt, the mantle wedge under northeast Japan must have experienced heating. During this period, the volcanic province along the eastern margin of Eurasia expanded rapidly toward the trench, forming grabens. Therefore, I concluded that the advance of the hot asthenosphere into the forearc region that led to this expansion, which caused the retreat of the subduction boundary of the paleo‐Pacific plate to retreat and ultimately converted northeast Japan from an accretionary complex into a volcanically active region.

In situ study of the reaction phase A plus high-P clinoenstatite to forsterite plus water at reduced water activity

Abstract. We examined the reaction phase A plus high-P clinoenstatite to forsterite plus water (Reaction R1) by means of in situ X-ray diffraction measurements with the large volume press at the synchrotron PETRA III, Hamburg. Contrary to the study of Lathe et al. (2022), in which all experiments on Reaction (R1) were performed at a water activity of 1, the reversed experiments presented in this study were performed at reduced water activity with mole fractions of about XH2O= XCO2=0.5. The intention of this investigation was to test the observation made by Perrillat et al. (2005), which was that dehydration reactions are kinetically faster at reduced than under water-saturated conditions. The position of Reaction (R1) at the reduced conditions was determined by reversal brackets at 9.1 and 9.5 GPa (630 and 700 ∘C), at 9.7 and 10.0 GPa (725 and 700 ∘C), at 9.8 and 10.2 GPa (675 and 750 ∘C), and at 10.5 GPa (675 and 740 ∘C). Additionally, we performed two offline experiments with brackets at 10.0 and 10.6 GPa (750 and 700 ∘C, respectively) that are in agreement with the results of the in situ experiments. We do not observe any “intermediate” precursor phase in our experiments. The equilibrium of Reaction (R1) is shifted by about 100 ∘C to lower temperature compared to the results under water-saturated conditions. Thus, at a water activity (aH2O) below 1 the phase A plus clinoenstatite dehydration reaction can only occur in extremely cold subduction slabs. The kinetics of Reaction (R1) dehydration at reduced water activity is slower than that determined previously by Lathe et al. (2022) under water-saturated conditions. Thus, the above-mentioned hypothesis of Perrillat et al. (2005) could not be confirmed. However, in both of our studies on Reaction (R1), the newly formed dehydration product forsterite was of nanometer size, which supports earlier experimental observations, which is that product phases of dehydration reactions are generally very fine-grained and might promote the concept that intermediate-depth earthquakes in subduction zones are initiated by mechanical instabilities from extremely fine-grained materials formed during dehydration reactions.

The sound velocity of garnet at high pressure: Implications for high velocity anomalies in cold subducting slabs

The origin of the high velocity anomalies in cold subduction slabs revealed by seismological tomography remains unknown. Garnet is a major component of the pyrolitic mantle and subduction slabs and crucially affects their physical properties, such as densities and velocities. Based on previous studies, the predicted velocities of garnets with various compositions are much different from the measured values. Here, we measured the velocities of four hot-pressed natural garnet samples, namely, Alm65.1Grs17.5Prp8.5Sps2.0And6.9, Alm60.9Grs25.9Prp8.2Sps2.0And3.0, Alm49.6Grs5.3Prp37.8Sps1.7And5.6, and Grs89.0Prp0.4Sps6.3And4.3, at pressures up to 11 GPa using the ultrasonic interference method in conjunction with a Walker-type multi anvil, to further constrain the composition dependence on sound velocities. Our results show that the velocities and the bulk and shear moduli monotonically increase with pressure but display a complicated dependence on composition. Accordingly, we have determined the dependences of P- and S-wave velocities of garnets on pressure and composition and reconstructed the velocity profiles of pyrolite, eclogite, harzburgite, and a garnet layer at depths of 90–300 km in a cold subducted slab. Our model proposes that the presence of a garnet layer with a thickness of 5.9–15.9 km atop the slab could cause velocity contrasts of 4.0–6.0% for VP and 4.1–7.7% for VS between the slab and the overlying pyrolite. The presence of a layer with a high proportion of garnet could contribute to observed high-velocity anomalies in cold subduction zones.

Equation of state for Mg3Al2Si3O12 pyrope: Implications for post-garnet transitions and mantle dynamics

Mantle convection is the dominant process driving the upwelling of hot materials and subduction of cold slabs. These processes are density-dependent and influenced by post-spinel and post-garnet transitions in the mantle transition zone (MTZ)/ lower mantle (LM). Knowledge of the Clapeyron slope of these transformations is used to express the dynamics of mantle blocks and is necessary for understanding the deep Earth. Here, we provide the Kunc-Einstein equation of state (EoS) for the pyrope Mg3Al2Si3O12 (Prp) calculated from a joint analysis of the experimentally measured isobaric heat capacity, bulk moduli, thermal expansion, pressure (P), unit cell volume (V), temperature (T) data. Based on our model, the bulk modulus and its pressure derivatives were K0,T0 = 168.5 GPa, K′0,T = 4.77, and V0 = 1501.7 Å. The optimised parameters include two Einstein temperatures, i.e., θ1 = 331 and θ2 = 1093 K, Grüneisen parameter at ambient condition γ0 = 1.77, infinite compression γ∞ = 0 with β = 1.12 and an intrinsic anharmonicity parameter a0 = –20. The value for the thermal expansion coefficient was calculated to be α = 2.33·10−5 K−1, and the thermodynamic Grüneisen parameter was estimated as γth = 1.42. The obtained EoS for Prp and the preliminarily fitted EoS for Al-bearing akimotoite (Al-Aki), in combination with literature data on bridgmanite (Bdm) and corundum (Crn), allowed the calculation of the phase diagram of the system with 75 mol% MgSiO3 + 25 mol% Al2O3 under LM conditions. The transformation from Prp to Bdm + Crn was computed at 24 GPa and 1570 K and exhibited a slightly positive Clapeyron slope (dP/dT = 2.1 MPa/K). The stability field of Al-Aki was detected at T = 1250–1570 K and P = 23–27 GPa. At P values higher than 24–27 GPa, the Al-Aki transforms into a Bdm + Crn assemblage with a highly negative Clapeyron slope. Calculations of sound velocities for the studied phases showed that the transformation from Prp to Bdm + Crn increased Vp and Vs by up to 9 and 20%, respectively. Such a big jump in the sound velocities indicates that the post-garnet transition is a better candidate than the post-Aki transition for producing a double discontinuity at the base of the MTZ, in combination with the transformation of ringwoodite into the Bdm + ferropericlase assemblage. The increase in sound velocities associated with the formation and dissolution of Al-Aki is unlikely to be sensitive to the MTZ. The combination of post-garnet, post-spinel, and post-Aki transitions near 660–720 km depths may have sluggished both the upwelling hot mantle and the subducted cold plates. The stagnant, hot LM material at the base of MTZ warmed the harzburgite and eclogite layers stacked during subduction. The heated MTZ may have been involved in further upwelling.

A Truncated Transient Slab Model for a Reheating Furnace

This paper considers the steel reheating process, in which cold slabs are reheated to a temperature suitable for hot rolling operations. Temperature uniformity within the slab is very important during reheating, as non-uniformities often lead to quality concerns in the later stages of the production process. Attaining slab temperature uniformity is quite challenging, therefore, a thorough understanding of the reheating process becomes very important. Due to the extreme conditions inside the furnace, numerical tools like computational fluid dynamics are necessary for such situations. However, when applied to these complex installations, it often leads to very lengthy computational times. This paper develops a computationally efficient transient model to simulate the environment around a slab. This Truncated Transient Slab Model is a coupled model: it uses the temperature and flow profiles from a full-scale steady-state simulation and imposes those on a smaller domain that represents the immediate environment around the slab. The efficiency of the model is compared to the state of the art. With the simulations available in the literature, it was found that the proposed model can simulate a furnace seven times larger (grid size), with much higher grid and time resolutions, using only a third of the computational resources.

A revisit to the phase transition behavior of K-feldspar at high-pressure and high-temperature: Implications on metastable K-feldspar in cold subduction

Abstract Felsic gneiss is a major type of ultrahigh-pressure metamorphic rock in continental subduction zones. As an important mineral of felsic gneiss, the phase behavior of K-feldspar is of great significance for studying subducting seismic activities, crust-mantle interactions, and plate tectonics in the Earth’s interior. In this study, we investigated the phase relations in K-feldspar using Raman spectroscopy combined with externally heating diamond-anvil cell (DAC) under high-pressure (28 GPa), high-temperature (400 °C), and simultaneous high pressure-temperature (P-T) (14 GPa, 430 °C) conditions. The phase diagram of K-feldspar at relatively low temperatures has been constructed, and together with previous results, a more comprehensive P-T phase diagram of K-feldspar is established. K-feldspar undergoes a reversible phase transition from C1 to P1 (metastable K-feldspar) symmetry at 10.3 GPa. Metastable K-feldspar is an intermediate phase from K-feldspar to K-holl-I (KAlSi3O8 with a hollandite-I structure) in extremely cold subduction slabs (&amp;lt;2 °C/km) or the center of some old, cold, and rapidly subducting slabs. Metastable K-feldspar is stable even at 11.4 GPa and 400 °C in enriched hydroxyl group (OH) environments, which shows greater high-P-T stability than K-feldspar. Thus metastable K-feldspar could withstand subduction-zone fluids, low-temperature metamorphism, and survive to deeper than previously expected. These results enhance our understanding of the formation paths and conditions of K-holl-I, the subducted depth of K-feldspar, the effect of subduction-zone fluids on continental subduction, and provide a possible reason for the origin of intermediate- and deep-focused seismicity.

Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity

The upper boundary of the mantle transition zone, known as the “410-km discontinuity”, is attributed to the phase transformation of the mineral olivine (α) to wadsleyite (β olivine). Here we present observations of triplicated P-waves from dense seismic arrays that constrain the structure of the subducting Pacific slab near the 410-km discontinuity beneath the northern Sea of Japan. Our analysis of P-wave travel times and waveforms at periods as short as 2 s indicates the presence of an ultra-low-velocity layer within the cold slab, with a P-wave velocity that is at least ≈20% lower than in the ambient mantle and an apparent thickness of ≈20 km along the wave path. This ultra-low-velocity layer could contain unstable material (e.g., poirierite) with reduced grain size where diffusionless transformations are favored.

A View from Northern Mexico: Abortions before Roe v. Wade.

A View from Northern Mexico: Abortions before Roe v. Wade Lina-Maria Murillo (bio) On October 31, 1968, Accion: Diaro Independiente’s front page seared with the headline “Murió una Norteamericana al provocarle un aborto,” a North American woman dies after an abortion. Alongside the headline, the newspaper published a picture of the eighteen-year-old woman, Jo Anne Homann. Lying on a steel counter at the local mortuary, Homann’s long dark hair adrift on the mortician’s table, eyes closed, pale youthful face turning, her features revealed the onset of rigor mortis, her mouth slightly agape.1 Mexican newspapers in Nogales, Sonora, offered neither Homann nor her family any privacy or decorum in its reporting. Anonymity and privacy worked along a complicated continuum in Mexico. Doctors, attorneys, and abortion referral services sought to protect the identity of people providing and seeking services. Mexican news media and law enforcement demanded the opposite. Mexican authorities and politicians pushed back against racialist images of Mexico as a sanctuary for “back alley” abortionists by exposing those who sought abortions and those who provided them. No person’s identity was above publicity. The death of Jo Anne Homann, from Hemet, California, brought this issue into stark relief.2 Early reports printed the macabre picture of Homann’s corpse lying “on a cold slab” in the Nogales funeral home, as other news media printed her full name and home address.3 Meanwhile, journalists sought to present law enforcement efforts in the best light possible, stating that Mexican officials counted on the “support [End Page 30] of the brave police offers from the United States, [and] agents from the Department of Investigations, directed by Chief Rafael Torres Montaño, [who were able] to identify the deceased.”4 Highlighting the collaboration of transborder law enforcement efforts to end the scourge of illegal abortion provision in the region, Mexican newspapers made clear the local Mexican government’s position that it was neither lenient nor negligent in its stance on abortion in the borderlands. The history of abortion before Roe has been central to the history of reproduction in the twentieth-century United States. Roe v. Wade became a watershed moment in U.S.-centered reproductive rights legislation. This U.S.-centric view, however, has for too long obscured how criminalization in the United States pushed abortion across national borders before the landmark ruling.5 In other places, I have argued that U.S.-based activists and doctors deployed the back-alley butcher myth to refer to Mexican abortion providers, racializing Mexico as an inherently dangerous place and Mexican providers as innately dangerous people. This tactic hastened U.S. courts’ liberalization of abortion laws in places like California and New Mexico, setting the foundations for Roe, in the name of protecting U.S. women from potential butchery in Mexico.6 In this essay, I examine the view from Mexico of Mexican providers and of U.S. women crossing the border for abortions. While historians have studied at length the stigma women confronted in a U.S. context before Roe, there remains a dearth of information about the ways in which Mexican and other borderland periodicals described those in search of reproductive health care in Mexico.7 Given the rhetoric of danger and filth associated with medical care in Mexico, stories like the one of Jo Anne Homann reinforced narratives of Mexico as a dangerous place to seek [End Page 31] abortion care or any kind of health care. Mexicans, however, held their own strong views about the thousands of American women seeking abortions in Mexican clinics and those who provided the procedure. Assessing Mexicans’ perspective helps us view Mexico not merely as a passive extension of the U.S. reproductive landscape but as an important and active player in reproductive health care in the borderlands. In addition, the underground abortion business changed “traditional” streams of migration as mostly American women traveled south beyond U.S. borders for access to reproductive services in the 1950s and 1960s. While scholars of abortion history have foregrounded the denial of human rights of those forced to migrate for reproductive health care, the politics of specific borders should not go unrecognized in such...

High-pressure experimental and thermodynamic constraints on the solubility of carbonates in subduction zone fluids

Carbon on the Earth's surface can be carried into the mantle by subducting slabs in the form of carbonates and organics. However, not all the subducting carbon could be recycled into the Earth's deep mantle because parts of them will be released from the subducting slabs by decarbonation processes (e.g., metamorphism and dissolution). In comparison with metamorphic decarbonation, the dissolution of carbonates in subduction zone fluids is more essential, but not well constrained. In this study, we systematically constrained the solubility of different carbonates (e.g., calcite/aragonite, dolomite, and magnesite) under high-pressure and high-temperature conditions relevant to subduction zones by combining experimental and thermodynamic simulations. We simultaneously compared the dissolution processes of calcite/aragonite, dolomite, and magnesite in aqueous fluids under the same P–T conditions using the four-hole gasket technique in diamond anvil cell (DAC) experiments. In situ Raman spectra of fluids in the DAC clearly showed that dolomite had a similar solubility to aragonite, while the solubility of magnesite was much lower than that of aragonite and dolomite under the same P–T conditions. Notably, we also found that the dissolution of dolomite was an incongruent process via the reaction CaMg(CO3)2(solid) in H2O = MgCO3(solid) + Ca2+(aqueous) + CO32−(aqueous). Based on the experimental results, we reasonably suppressed some aqueous species in the DEW model and quantitatively calculated the speciation and solubility of calcite/aragonite, dolomite, and magnesite in aqueous fluids at pressures up to 5 GPa and temperatures up to 800°C. Then, we modeled and compared the decarbonation fluxes between metamorphism and dissolution from typical marine sediment and altered oceanic crust in subduction zones. In the cold or intermediate subducting slabs, decarbonation fluxes by the dissolution of carbonates are about three times larger than that from metamorphism. Considering the total decarbonation fluxes of metamorphism and dissolution, the cold subducting slabs could release a similar magnitude of carbon as the hot ones.

An Intelligent Logic-Based Mold Breakout Prediction System Algorithm for the Continuous Casting Process of Steel: A Novel Study.

Mold breakout is one of the significant problems in a continuous casting machine (caster). It represents one of the key areas within the steel production facilities of a steel plant. A breakout event on a caster will always cause safety hazards, high repair costs, loss of production, and shutdown of the caster for a short while. In this paper, a logic-judgment-based mold breakout prediction system has been developed for a continuous casting machine. This system developed new algorithms to detect the different sticker behaviors. With more algorithms running, each algorithm is more specialized in the other behaviors of stickers. This new logic-based breakout prediction system (BOPS) not only detects sticker breakouts but also detects breakouts that takes place due to variations in casting speed, mold level fluctuation, and taper/mold problems. This system also finds the exact location of the breakout in the mold and reduces the number of false alarms. The task of the system is to recognize a sticker and prevent a breakout. Moreover, the breakout prediction system uses an online thermal map of the mold for process visualization and assisting breakout prediction. This is done by alerting the operating staff or automatically reducing the cast speed according to the location of alarmed thermocouples, the type of steel, the tundish temperature, and the size of the cold slab width. By applying the proposed model in an actual steel plant, field application results show that it could timely detect all 13 breakouts with a detection ratio of 100%, and the frequency of false alarms was less than 0.056% times/heat. It has the additional advantage of not needing a lot of learning data, as most neural networks do. Thus, this new logical BOPS system should not only detect the sticker breakouts but also detect breakouts taking place due to variations in casting speed and mold level fluctuation.