Deep Subduction Zone Research Articles

Numerical modeling the process of deep slab dehydration and magmatism

This study uses a 2D high-resolution thermo-mechanical coupled model to investigate the dynamic processes of deep plate hydration, dehydration, and subsequent magmatic activity in ocean-continent subduction zones. We reveal the pathways and temporal evolution of water transport to the deep mantle during the subduction process. Plate dehydration plays a critical role in triggering partial melting of the deep mantle and related magmatic activity. Our study shows significant differences in the volumes of melt produced at different depths, with dehydration reactions in deeper regions being weaker compared to shallower ones. It takes a longer time to reach the suitable P-T conditions for hydrous melting in the deep mantle. The results highlight the geophysical significance of water transport in deep subduction zones and its role in magmatic processes, particularly in the formation of magma chambers beneath continental plates.

Detection of Hidden Low-Frequency Earthquakes in Southern Vancouver Island with Deep Learning

Low-frequency earthquakes (LFEs) are small-magnitude earthquakes that are depleted in high-frequency content relative to traditional earthquakes of the same magnitude. These events occur in conjunction with slow slip events (SSEs) and can be used to infer the space and time evolution of SSEs. However, because LFEs have weak signals, and the methods used to identify them are computationally expensive, LFEs are not routinely cataloged in most places. Here, we develop a deep-learning model that learns from an existing LFE catalog to detect LFEs in 14 years of continuous waveform data in southern Vancouver Island. The result shows significant increases in detection rates at individual stations. We associate the detections and locate them using a grid search approach in a 3D regional velocity model, resulting in over 1 million LFEs during the performing period. Our resulting catalog is consistent with a widely used tremor catalog during periods of large-magnitude SSEs. However, there are time periods where it registers far more LFEs than the tremor catalog. We highlight a 16-day period in May 2010, when our model detects nearly 3,000 LFEs, whereas the tremor catalog contains only one tremor detection in the same region. This suggests the possibility of hidden small-magnitude SSEs that are undetected by current approaches. Our approach improves the temporal and spatial resolution of the LFE activities and provides new opportunities to understand deep subduction zone processes in this region.

Multi‐Stage Magmatism During Slab Exhumation Drives the Geochemical Evolution of Continental Crust: Insights From Paleozoic Granitoids in South Altyn, Western China

AbstractThe present continental crust is characterized by a felsic upper crust and a mafic lower crust, resulting from significant geochemical differentiation over geological time. While various processes have been proposed to explain this differentiation, subduction zones remain pivotal regions for understanding the compositional evolution of continental crust. This study focuses on the South Altyn (SA) continental subduction‐collision belt in western China, a unique setting that experienced ultra‐deep (>300 km) continental subduction followed by multi‐stage exhumation. We present a comprehensive study of four granitoid suites from Tatelekebulake (TTLK) area in SA: biotite granite (BG), monzogranite (MG), K‐feldspar granite (KG), and leucogranite (LG). Comprehensive studies on petrology, geochemistry and zircon U‐Pb dating show that these granitoids formed at 494, 451, 414, and 418 Ma, respectively, and originated from protoliths with affinity to the subducted continental crust in SA. Phase equilibrium modeling suggests that BG formed at ∼800°C and 0.6 GPa, while the MG, KG, and LG formed by differentiation crystallization of the BG magma under progressively decreasing temperature and pressure conditions (750°C, 0.5 GPa; 740–700°C, 0.2 GPa; and 700–640°C, 0.1 GPa, respectively). These results, combined with previous studies, allow us to reconstruct the tectonic processes of continental exhumation and subsequent orogenic collapse in SA during the Early Paleozoic. Importantly, our findings reveal that magmatism derived from partial melting of subducted continental crust can promote the geochemical evolution of continental crust toward more felsic compositions, even in the absence of significant crustal growth or mantle‐derived magmatism. This study provides a valuable case for understanding the compositional evolution of continental crust in deep subduction zones and challenges conventional models that rely heavily on arc magmatism for crustal differentiation. Moreover, our results contribute to a broader understanding of crustal evolution processes in collisional orogens worldwide and highlight the importance of recycling and differentiation of subducted continental material in shaping crustal compositions.

Mineral precipitation sequence from multi-stage fluids released by eclogite during high-pressure metamorphism

Abstract Arc magmas above subduction zones hold abundant fluid-mobile elements attributed to fluids released from the dehydrating subducted oceanic crust. However, the quantity of trace elements in the fluids and their evolution with the metamorphic processes during subduction and exhumation are still unclear. The precipitation sequence of vein minerals preserves the nature of multi-stage high-pressure (HP) metamorphic fluids and the fingerprint of mass exchange in deep subduction zones. In this contribution, we conducted detailed petrological studies and phase equilibria modeling on a unique HP omphacite-rich vein and its host eclogite from the Chinese southwestern Tianshan. The host eclogite consists mainly of garnet, omphacite, epidote, glaucophane, phengite, quartz, and rutile. Garnet in the eclogite records prograde subduction and early exhumation characterized by decompression heating at P-T conditions of ~2.4–2.6 GPa and 460–540 °C. The embedded omphacite-rich vein has similar mineral assemblage to the host eclogite. Garnet grains in this vein are predominantly distributed along or intersect the vein wall, which record similar eclogite-facies metamorphic conditions to that of the host eclogite. Omphacite is dominant in the vein, while epidote and glaucophane occur interstitially. Phase equilibria modeling reveals a sequential growth of garnet-dominated, omphacite-dominated, and epidote-dominated assemblages from fluids originating from the breakdown of different hydrous minerals. These lines of evidence suggest that the formation of multi-stage HP fluids are a continuous long-term process with a spontaneously short-distance transport and sequential mineral precipitation. Calculated fluid compositions demonstrate that the fluids released by lawsonite breakdown during exhumation have great potential to modify the trace element systematics of arc magmas. Our findings reveal the nature and evolution of multi-stage HP metamorphic fluids from internal sources during subduction and exhumation of oceanic crust, providing valuable insights into the chemical compositions of arc magmas.

Multiphase-solid fluid inclusions in HP-LT eclogite facies rock (Zavkhan Terrane, Western Mongolia): evidence for the evolution from saline to hypersaline fluids during metamorphism in subduction zone

Fluid inclusions in high- and ultrahigh-pressure metamorphic rocks provide direct information on the composition of the fluids that evolved during metamorphism and fluid-rock interactions in deep subduction zones. We investigate the fluid inclusions in the Khungui eclogite of the Zavkhan Terrane, Central Asian Orogenic Belt. Fluid inclusions are observed in garnet and quartz in the eclogite samples that underwent metamorphism during subduction. The primary fluid inclusions in quartz are composed of liquid and vapor with high salinities (15.7–16.4 wt.% NaCl eq.), whereas the secondary fluid inclusions in quartz are classified as: relatively high salinity (Type I:12.5–16.3 wt.% NaCl eq.) and low salinity (Type II:6.7–10.6 wt.% NaCl eq.). The garnet shows compositional zoning from Ca-poor cores to Ca-rich rims, and the rims that grew during the eclogite-stage metamorphism (2.1–2.2 GPa at 580–610 °C) preferentially contain numerous primary fluid inclusions. The primary fluid inclusions in garnet are commonly bi-phases (liquid and vapor); however, some are multiphase-solid fluid inclusions composed of fluids (liquid and vapor) and combinations of several minerals (halite, quartz, apatite, calcite, biotite, chlorite, and actinolite). Bi-phase fluid inclusions preferentially occur in the inner parts of the Ca-rich garnet rim, whereas multiphase-solid fluid inclusions occur along the margins of the Ca-rich rim. We hypothesize that the multiphase-solid fluid inclusions are formed via interactions between trapped fluids, trapped minerals, and the host garnet during exhumation. By combination of FIB–SEM and synchrotron X-ray CT analyses, the detailed occurrences, volumes, and compositions of the solid phases in the fluid inclusion was analyzed. We then conduct mass balance analysis to reconstruct accurate fluid compositions using data from the FIB–SEM and synchrotron X-ray CT images of the multiphase-solid fluid inclusion. The results of these analyses reveal that (1) fluid changed from an H2O-dominated saline fluid (13–16 wt. % NaCl eq.) at the prograde to the earlier eclogite stage to H2O–CO2-dominated hypersaline fluid at later eclogite stage (~ 32 wt. % NaCl eq., 7.3 wt. % CO2 and ~ 19 molal dissolved cations); (2) a variety of mineral assemblages in multiphase-solid fluid inclusions are produced by post-entrapment reactions between the trapped hypersaline fluid, trapped minerals and the fluid host mineral. The evolution of fluids from saline to hypersaline during the eclogite facies stage is probably caused by the formation of hydrous minerals (i.e., barroisite) under a near-closed system.

Supercritical fluid in deep subduction zones as revealed by multiphase fluid inclusions in an ultrahigh-pressure metamorphic vein

Due to their low viscosity, high mobility, and high element contents, supercritical fluids are important agents in the cycling of elements. However, the chemical composition of supercritical fluids in natural rocks is poorly understood. Here, we investigate well-preserved primary multiphase fluid inclusions (MFIs) from an ultrahigh-pressure (UHP) metamorphic vein of the Bixiling eclogite in Dabieshan, China, thus providing direct evidence for the components of supercritical fluid occurring in a natural system. Via the 3D modeling of MFIs by Raman scanning, we quantitatively determined the major composition of the fluid trapped in the MFIs. Combined with the peak-metamorphic pressure-temperature conditions and the cooccurrence of coesite, rutile, and garnet, we suggest that the trapped fluids in the MFIs represent supercritical fluids in a deep subduction zone. The strong mobility of the supercritical fluids with respect to carbon and sulfur suggests that such fluids have profound effects on global carbon and sulfur cycling.

Cycling of CO2 and H2O constrained by experimental investigation of a model ophicarbonate at deep subduction zone conditions

Ophicarbonates are carbonate-bearing serpentinites and therefore have high concentrations of CO2 and H2O. Their high concentrations of CO2 and H2O potentially make them important contributors to recycling of CO2 and H2O between Earth's surface and interior. However, the phase relations of ophicarbonates under deep subduction zone conditions remain poorly constrained, especially under conditions where melting may occur. Here, we present laboratory experiments on a model ophicarbonate at 3-7 GPa and 700-1100°C. We find that chlorite is stable at 3 GPa and ≤ 800°C, and phase A and balangeroite are stable at 7 GPa and ≤ 700°C. At all other conditions there is no stable residual H2O-bearing phase, providing support for the existence on an “anhydrous nose” in ultramafic rocks between 4-6 GPa in relatively warm subduction zones. We find that the solidus decreases with increasing pressure from between 900 and 1000°C at 3 GPa to between 700 and 800°C at 7 GPa. We find that the near-solidus melt is carbonate-rich and this melt coexists with a residual carbonate phase over a small temperature range. Using mass balance calculations, we estimate phase proportions as function of P and T to estimate mass fractions of CO2 remaining in the solid lithology. When compared to P-T paths of subducting slabs our results suggest that significant amounts of CO2 will be released from the slab in hot subduction zones, while significant amounts of CO2 will be retained in the slab in cold subduction zones of the modern Earth. In ancient subduction zones, ophicarbonate may have been more important to deep C fluxes due to the absence of thick carbonate sequences, and like modern Earth can either deliver significant amounts of CO2 to the mantle wedge or to the deep mantle depending on the P-T trajectories of subducting slabs.

Direct surface-wave tomography under Northeast China: New insights into 3-D crustal S-wave velocity structure and dynamics of intraplate volcanism

A new three-dimensional shear-wave velocity structure of the crust beneath Northeast China is constructed by inversion of phase velocity dispersion curves between 5 and 40 s using the direct surface-wave ambient noise tomography method. In addition to 127 NECESSArray (NorthEast China Extended SeiSmic Array) seismic stations, 19 SCVDSZ (Study of Changbai Volcanoes and Deep Subduction Zone) plus 3 CDSN (China Digital Seismograph Network) stations are added in the data collection. The addition of these data greatly improves the ray path coverage and imaging results around the Zhangguangcai range and the Changbaishan mountain. Our results show an obvious low-velocity anomaly extending from the surface down to uppermost mantle under the Changbaishan, suggesting that the volcano formation could be related to mantle upwelling. One more striking feature is a widely distributed mid-lower crustal low-velocity layer under the entire Northeast China, which is connected to those in the upper crust under the volcanoes, indicating that these volcanoes could have exchanges of material and energy in the mid-lower crust. In addition, under the Shuangliao and Erke volcanoes, low-velocity anomalies are visible in the upper and middle crust but high-velocity anomalies in the lower crust, whereas under the Abaga and Halaha volcanic groups, the low-velocity anomalies extend from the crust down to uppermost mantle, suggesting their continuous mantle magma provision. Integrating with previous global and regional body-wave tomographic models, our results suggest that the volcanoes in Northeast China could be related to hot and wet mantle upwelling in the big mantle wedge caused by the deep subduction of the subducted Pacific slab in the mantle transition zone.

In-situ Raman spectroscopic analysis of dissolved silica structures in Na2CO3 and NaOH solutions at high pressure and temperature

The dissolved silica structures in quartz-saturated 0.50 and 1.50 m [mol kg H2O–1] Na2CO3 and 0.47 m NaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na2CO3 and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than those in pure H2O indicated the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q1 (silicate center having a single bridging oxygen atom) in the spectra of the Na2CO3 solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na2CO3 and NaOH to fluids, corresponding to an elevation of the measured quartz solubilities. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions, because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.

Synthetic Co-Existing Wadsleyite β-(Mg,Fe)2SiO4 and Ringwoodite γ-(Mg,Fe)2SiO4: an Optical Absorption Spectroscopy Study

The synthetic high-pressure α- and β-modification of (Mg1–xFex)2SiO4, wadsleyite and ringwoodite, respectively, were studied by optical absorption spectroscopy at ambient and hydrostatic high-pressure conditions. In addition, the effects of thermal annealing on the crystals were investigated. Under hydrostatic compression up to ~13 GPa and then consequent released to atmospheric pressure there were changes in the spectra and related changes in the crystal color. This is a clear indication that some Fe2+ was oxidized to Fe3+. The spectra of both ringwoodite and wadsleyite change after annealing in air at temperatures up to 300 °C. The intensities of electronic spin-allowed bands of Fe2+ decrease and the intensity of the charge-transfer electronic transition O2– → Fe3+, as given by the low-energy absorption edge in the UV region, increases. These crystal-chemical changes are shown by a weakening of the blue (ringwoodite) and green (wadsleyite) colors and a concomitant increase in yellowish tints. The effects of Fe2+ oxidation to Fe3+, upon decompression from high pressures as well as through annealing at relatively low temperatures, can cause the disintegration of both phases. Thus, both minerals have not yet been reliably identified at near surface Earth conditions after originating from deep-seated volcanism or deep subduction zone processes.

Uncovering the physical controls of deep subduction zone slow slip using supervised classification of subducting plate features

Summary Deep slow slip events (SSEs) at subduction zones have significantly contributed to refining our understanding of the megathrust earthquake cycle at the brittle-ductile transition. However, the specific combination of factors that determine their occurrence has not yet been fully explored. Here we evaluate the contribution of several of these characteristics using globally mapped geophysical data that are used as proxies for physical properties of the subducting plate. This is performed by classifying 25 km-wide, trench-parallel segments into binary classes based on the observation (or lack thereof) of deep, short- or long-term SSEs. The five characteristics explored here include subducting plate age, sediment thickness, relative plate velocity, slab dip, and plate surface roughness. We use these characteristics to train six Machine Learning models based on different learning algorithms: Gaussian Naïve Bayes, Logistic Regression, Linear Discriminant Analysis, Random Forest, Support Vector Machine, and K-Nearest Neighbour. Short-term SSE models show that subducting plate age, relative velocity, and sediment thickness have the strongest predictive power with the first two characteristics negatively correlating and sediment thickness positively correlating with SSE occurrence, respectively. These results are consistent with a conceptual model where slow slip is controlled by conditions favoring the enduring release (and possible storage) of fluids near the source region. However, the relationship between these features and elevated pore fluid pressures is not established here and further evidence is needed to validate this hypothesis. We then use a final model constructed as a weighted average of the best performing models to make predictions on the probability of SSE occurrence, with predicted short-term SSE occurrence in South America, the Aleutians, Sumatra, Vanuatu and Solomon, as well as long-term SSE occurrence in the Aleutians, Izu-Bonin, Kuril-Kamchatka, Mariana, and Tonga-Kermadec. Overall, long-term SSE models do not perform as well as the short-term SSE models which may indicate that long-term SSEs are controlled by a different and/or extended set of physical characteristics than the short-term SSEs.

Aqueous fluids are effective oxidizing agents of the mantle in subduction zones

Aqueous fluids produced by dehydration of the downgoing slab facilitate chemical exchange in subduction zones, but the efficiency of fluid-mediated redox transfer as a mechanism to deliver oxidized material from the slab to the sub-arc mantle remains hotly debated. Here we report the first direct measurements of the oxidation state of experimentally produced slab fluids using in situ redox sensors. Our experiments show that the dehydration of natural antigorite serpentinite at shallow subduction zone conditions (1 GPa, 800 °C) produces moderately oxidizing fluids (QFM + 2) with elevated concentrations of Na, K, Ca, and Mg. The composition and redox of the experimental fluids are then used to parameterize a thermodynamic reactive transport model to investigate the interaction of slab fluid with the sub-arc mantle from 1–4 GPa and 700–900 °C. Recently determined equation of state parameters for aqueous fluids at high pressures now enables thermodynamic modeling of aqueous fluid–rock interactions at conditions relevant to deep subduction zones for the first time. Our thermodynamic modeling demonstrates that aqueous fluid can efficiently oxidize Fe in mantle minerals via the reduction of H+ to H2 in the fluid. We estimate that < 1–3 kg of serpentinite-derived fluid at 850–900 °C is required to increase the Fe3+/ΣFe in 1 kg of sub-arc mantle from MORB-like values (0.15) to those of primitive arc basalts (0.2–0.3). We calculate that a slab fluid flux of 1.4 × 109–1.4 × 1014 kg year−1 is required to oxidize sufficient sub-arc mantle to produce the average annual flux of magmas at arcs, which overlaps with the estimated range of H2O flux in subduction zones.

Serpentinite-derived low δ7Li fluids in continental subduction zones: Constraints from the fluid metasomatic rocks (whiteschist) from the Dora-Maira Massif, Western Alps

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior.

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

AbstractDating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.

Constraints from the dehydration of antigorite on high-conductivity anomalies in subduction zones

Regions with high electrical conductivities in subduction zones have attracted a great deal of attention. Determining the exact origin of these anomalies could provide critical information about the water storage and cycling processes during subduction. Antigorite is the most important hydrous mineral within deep subduction zones. The dehydration of antigorite is believed to cause high-conductivity anomalies. To date, the effects of dehydration on the electrical conductivity of antigorite remain poorly understood. Here, we report new measurements of the electrical conductivity of both natural and hot-pressed antigorite at pressures of 4 and 3 GPa, respectively, and at temperatures reaching 1073 K. We observed significantly enhanced conductivities when the antigorite was heated to temperatures beyond its thermodynamic stability field. Sharp increases in the electrical conductivity occurred at approximately 848 and 898 K following the decomposition of antigorite to forsterite, enstatite and aqueous fluids. High electrical conductivities reaching 1 S/m can be explained by the presence of an interconnected network of conductive aqueous fluids. Based on these results for the electrical conductivity of antigorite, we conclude that high-conductivity regions associated with subduction zones can be attributed to dehydration-induced fluids and the formation of interconnected networks of aqueous fluids during the dehydration of antigorite.

Lithium Isotopic Geochemistry in Subduction Zones: Retrospects and Prospects

AbstractSubduction zones involve many complex geological processes, including the release of slab‐derived fluids, fluid/rock interactions, partial melting, isotopic fractionations, elemental transporting, and crust/mantle interactions. Lithium (Li) isotopes (6Li and 7Li) have relative mass difference up to 16%, being the largest among metal elements. Thus, Li isotopes have advantage to interprete trace various geological processes. Most importantly, during crust/mantle interactions in deep subduction zones, surface materials and mantle rocks usually have distinct Li isotopic compositions. Li isotopes can be potential tracer for subduction processes, from the onset of subduction to the release of Li from subducted slabs and interaction with mantle wedge, as well as the fate of Li in slab‐derived fluids and residual slabs. Moreover, the Li isotopic composition of subducting output materials can provide useful information for understanding global Li circulation. With developments in measurement and expansion of Li isotopic database, Li isotopic geochemistry will provide more inference and be a powerful tracer for understanding subduction‐related processes. This work retrospected the application of Li isotopes in tracing successive subduction processes, and made some prospects for further studies of Li isotopes.

Dependence of the Brittle Ductile Transition on Strain-Rate-Dependent Critical Homologous Temperature

Earthquakes mainly occur in crust or mantle that is below a critical temperature for the tectonic strain-rate, |$\dot{e}_t$|⁠, such that stress builds up to the breaking point before it can relax due to creep. Then long-range stress correlation gives rise to power law seismicity including large events. The limiting temperature depends on pressure, which is taken into account by finding a critical homologous temperature THc = T/TM above which earthquakes are rarely observed (where T, TM are temperature and average melting temperature of constituent minerals). We find that THc for ocean plates is ∼0.55. For California earthquakes, it is also close to 0.55. The uppermost mantle layer of oceanic plates of thickness ∼50 km is composed of harzburgite and depleted peridotite from which basalt has been removed to form ocean crust. Thus it has a higher melting temperature than the peridotite of the surrounding mantle, or the lower halves of plates. Thicknesses of seismicity in deep subduction zones, determined from 2-D polynomial fits to a relocated catalogue, are ∼50 km, which suggests that the earthquake channel is confined to this layer. We construct models to find homologous temperatures in slabs, and find that seismicity thicknesses are also, on average, confined to TH ≤ 0.55 ± 0.05. The associated rheology is compared with that obtained from flexure models of ocean lithosphere. The brittle-ductile transition occurs where viscosity drops from high values in the cold cores of slabs to values of 1022–1023 Pa s, that is, where creep strain-rates become comparable to tectonic rates. The cut-off for deep earthquakes is not sharp. However they appear unlikely to occur if homologous temperature is high TH > 0.55. Exceptions to the rule are anomalously deep earthquakes such as those beneath the Iceland and the Hawaiian hotspots, and the Newport Inglewood Fault. These are smaller events with short-range stress correlation, and can be explained if strain-rates are two to three orders of magnitude higher than those associated with earthquakes located where TH ≤ 0.55. We conclude that the brittle-ductile transition corresponds to the transition from long-range (regional) to short-range (localized on asperities) stress correlation.

1909 Taipei Earthquake Ground Motion Simulation

The 1909 Taipei earthquake (M 7.3) occurred beneath the Taipei metropolitan area (TMA) causing substantial damage according to the historical literature. According to the hypocenter relocation and tectonic implications provided in a previous study, we simulated ground motions within the TMA using a hybrid simulation method involving the spectral-element method (SEM) and the empirical Green's function method (EGFM). We used the SEM for simulating low-frequency components and the EGFM for simulating high-frequency components. These high and low frequency components were subsequently combined. For the EGFM we used the records from a recent M_L 4.9 earthquake (11 October 2013, depth = 143.8 km) in the Taipei area as the empirical Green's function. According to the historical literature, the observed PGA (peak ground acceleration) values are 59.2 and 67.0 gal at ancient stations TAP and KEE, with periods of 1.21 and 1.34 s, respectively. By comparing the simulated PGA values at modern stations TAPB and WFSB to the historical documented ones for 12 different models, our result suggests that the 1909 Taipei earthquake was an event with a magnitude of about M_w 7.3 and stress drop of approximately 30 bars, or a smaller equivalent magnitude between M_w 6.8 - 7.3 but with much higher average stress drop of more than 100 bars. For a deep event beneath TMA a larger vertical P-wave motion and longer period shaking wave, as addressed in the historical literature, might be expected with prolonged shaking as found in the simulation. A seismic hazard assessment is necessary for metropolitan Taipei to better understand the long period shaking from deep subduction zone intra plate events.

Formation of multiple high-pressure veins in ultrahigh-pressure eclogite (Hualiangting, Dabie terrane, China): Fluid source, element transfer, and closed-system metamorphic veining

Multiple high-pressure (HP) metamorphic veins and their host ultrahigh-pressure (UHP) eclogites in the Hualiangting area (Dabie terrane, China) were investigated to constrain the provenance of vein-forming fluids, element transfer during UHP fluid–rock interaction, and “closed-system” vein crystallization in a deep subduction zone. The HP veins consist of significantly different mineral assemblages and can be classified as omphacite–epidote (Omp–Ep), epidote-quartz (Ep–Qz), and kyanite–epidote–quartz (Ky–Ep–Qz) veins from the vein-eclogite boundary to the vein interior. Towards the veins, the abundances of coesite-enclosing epidote, kyanite, and quartz in the eclogite systematically decrease, indicating an intensive fluid-rock interaction between the eclogite and vein-forming fluid. Mass-balance calculations indicate that the interaction resulted in strong depletion of Sr and Pb (up to −80% to −90%); moderate depletion of U (up to −80%), Th (up to −50%), light rare earth elements (LREEs) (up to 40–60%), and transition metal elements (TMEs) (Cr, V, Sc, Co, and Ni) (up to −40%); and low depletion of heavy rare earth elements (HREEs) (<−25%) in the eclogites near the vein. The mass losses of these trace elements gradually increase towards the vein and are largest in the eclogite nearest the vein.Whole-rock and epidote in situ Sr isotopic analyses indicate that the fluids responsible for all types of HP veins originated from the eclogites themselves rather than from external gneisses (i.e., “internal” fluids). Petrologic studies and phase equilibrium modeling indicate that the vein-forming fluids were primarily produced by the breakdown of UHP lawsonite in the host eclogite during exhumation at P–T conditions of ~2.7–3.0GPa and 660–720°C. Therefore, the formation of multiple HP veins and the relevant dissolution-transport-precipitation processes were isolated from the large-scale infiltration of external fluid (i.e., in a geochemically closed system). The contents of the fluid-mobile elements (Cs, Rb, U, Th, and LREEs) in the veins gradually increase from the vein-eclogite boundary towards the vein interior, whereas the contents of the sparingly soluble elements (Mg, Ti, TMEs, and HREEs) and the δEu values gradually decrease, suggesting multi-stage crystallization of a solute-rich UHP fluid. The geochemical characteristics (La, Cr, and δEu values) of the veins and vein epidote, coupled with the modeled fluid compositions, indicate that the Omp–Ep veins first crystallized from the solute-rich vein-forming fluid and that the Ep–Qz and Ky–Ep–Qz veins formed by the crystallization of residual fluids after the formation of Omp–Ep and Ep–Qz veins, respectively. This study indicates that internally derived UHP fluids in eclogite are key agents for efficiently extracting important trace elements from a deeply subducted continental slab through fluid–rock interactions and that the multi-stage crystallization of a highly restricted fluid could produce multiple HP veins and significant trace element fractionations during slab exhumation.

Reduced sediment melting at 7.5–12 GPa: phase relations, geochemical signals and diamond nucleation

Melting of carbonated sediment in the presence of graphite or diamond was experimentally investigated at 7.5–12 GPa and 800–1600 °C in a multianvil apparatus. Two starting materials similar to GLOSS of Plank and Langmuir (Chem Geol 145:325–394, 1998) were prepared from oxides, carbonates, hydroxides and graphite. One mixture (Na-gloss) was identical in major element composition to GLOSS, and the other was poorer in Na and richer in K (K-gloss). Both starting mixtures contained ~6 wt% CO2 and 7 wt% H2O and were doped at a ~100 ppm level with a number of trace elements, including REE, LILE and HFSE. The near-solidus mineral assemblage contained a silica polymorph (coesite or stishovite), garnet, kyanite, clinopyroxene, carbonates (aragonite and magnesite–siderite solid solution), zircon, rutile, bearthite and hydrous phases (phengite and lawsonite at 10 GPa). Hydrous phases disappear at ~900 °C, and carbonates persist up to 1000–1100 °C. At temperatures >1200 °C, the mineral assemblage consists of coesite or stishovite, kyanite and garnet. Clinopyroxene stability depends strongly on the Na content in the starting mixture; it remains in the Na-gloss composition up to 1600 °C at 12 GPa, but was not observed in K-gloss experiments above 1200 °C. The composition of melt or fluid changes gradually with increasing temperature from hydrous carbonate-rich ( 1). Aragonite and Fe–Mg carbonate have very different REE partition coefficients (DMst-Sd/L ~ 0.01 and DArg/L ~ 1). Nb, Ta, Zr and Hf are strongly incompatible in both carbonates. The bearthite/melt partition coefficients are very high for LREE (>10) and decrease to ~1 for HREE. All HFSE are strongly incompatible in bearthite. In contrast, Ta, Nb, Zr and Hf are moderately to strongly compatible in ZrSiO4 and TiO2 phases. Based on the obtained partition coefficients, the composition of a mobile phase derived by sediment melting in deep subduction zones was calculated. This phase is strongly enriched in incompatible elements and displays a pronounced negative Ta–Nb anomaly but no Zr–Hf anomaly. Although all experiments were conducted in the diamond stability field, only graphite was observed in low-temperature experiments. Spontaneous diamond nucleation and the complete transformation of graphite to diamond were observed at temperatures above 1200–1300 °C. We speculate that the observed character of graphite–diamond transformation is controlled by relationships between the kinetics of metastable graphite dissolution and diamond nucleation in a hydrous silicocarbonate melt that is oversaturated in C.