Mantle In Subduction Zones Research Articles

The origin and evolution of Earth's nitrogen.

Nitrogen is a vital element for life on Earth. Its cycling between the surface (atmosphere+crust) and the mantle has a profound influence on the atmosphere and climate. However, our understanding of the origin and evolution of Earth's nitrogen is still incomplete. This review presents an overview of the current understanding of Earth's nitrogen budget and the isotope composition of different reservoirs, laboratory constraints on deep nitrogen geochemistry, and our understanding of the origin of Earth's nitrogen and the deep nitrogen cycle through plate subduction and volcanism. The Earth may have acquired its nitrogen heterogeneously during the main accretion phase, initially from reduced, enstatite-chondrite-like impactors, and subsequently from increasingly oxidized impactors and minimal CI-chondrite-like materials. Like Earth's surface, the mantle and core are also significant nitrogen reservoirs. The nitrogen abundance and isotope composition of these three reservoirs may have been fundamentally established during the main accretion phase and have been insignificantly modified afterwards by the deep nitrogen cycle, although there is a net nitrogen ingassing into Earth's mantle in modern subduction zones. However, it is estimated that the early atmosphere of Earth may have contained ∼1.4 times the present-day atmospheric nitrogen (PAN), with ∼0.4 PAN being sequestered into the crust via biotic nitrogen fixation. In order to gain a better understanding of the origin and evolution of Earth's nitrogen, directions for future research are suggested.

Interaction of upwelling asthenosphere with oceanic lithospheric mantle in Bangong-Nujiang subduction zone: A new mechanism for the petrogenesis of Nb-enriched basalts

Interaction of upwelling asthenosphere with oceanic lithospheric mantle in Bangong-Nujiang subduction zone: A new mechanism for the petrogenesis of Nb-enriched basalts

Segmentation and Radial Anisotropy of the Deep Crustal Magmatic System Beneath the Cascades Arc

AbstractVolcanic arcs consist of many distinct vents that are ultimately fueled by the common melting processes in the subduction zone mantle wedge. Seismic imaging of crustal‐scale magmatic systems can provide insight into how melt is organized in the deep crust and eventually focused beneath distinct vents as it ascends and evolves. Here, we investigate the crustal‐scale structure beneath a section of the Cascades arc spanning four major stratovolcanoes: Mt. Hood, Mt. St. Helens (MSH), Mt. Adams (MA), and Mt. Rainier, based on ambient noise data from 234 seismographs. Simultaneous inversion of Rayleigh and Love wave dispersion constrains the isotropic shear velocity (Vs) and identifies radially anisotropic structures. Isotropic Vs shows two sub‐parallel low‐Vs zones (∼3.45–3.55 km/s) at ∼15–30 km depth with one connecting Mt. Rainier to MA, and another connecting MSH to Mt. Hood, which are interpreted as deep crustal magma reservoirs containing up to ∼2.5%–6% melt, assuming near‐equilibrium melt geometry. Negative radial anisotropy, from vertical fractures like dikes, is prevalent in this part of the Cascadia, but is interrupted by positive radial anisotropy, from subhorizontal features like sills, extending vertically beneath MA and Mt. Rainier at ∼10–30 km depth and weaker and west‐dipping positive anisotropy beneath MSH. The positive anisotropy regions are adjacent to rather than co‐located with the isotropic low‐Vs anomalies. Ascending melt that stalled and mostly crystallized in sills with possible compositional differences from the country rock may explain the near‐average Vs and positive radial anisotropy adjacent to the active deep crustal magma reservoirs.

Highly siderophile element behavior in the subduction zone mantle: Constraints from data on the Yap Trench peridotites

The highly siderophile elements (HSEs) in peridotites from the Yap Trench are investigated to understand their behavior in the subduction zone mantle, which is characterized by high melt depletion and metasomatism. Results reveal that the abundance of HSEs in Yap Trench peridotites is extremely low compared with that in oceanic peridotites from the mid-ocean ridge and other subduction-zone environments, indicating the extensive extraction of the sulfide melt. Chondrite-normalized HSE patterns are heterogeneous and exhibit strong fractionation of platinum-group elements (PGEs) and slight enrichment of Re and Au. The Ru/Ir (1.64–16.12), Os/Ir (0.15–7.19), and Pt/Pd (0.53–13.17) ratios in most of the studied samples are higher than those in CI chondrite (Ru/Ir: 1.49; Os/Ir: 1.06; and Pt/Pd: 1.67). These suprachondritic PGE ratios result from the exhaustion of the base metal sulfide (BMS) and the formation of platinum-group minerals and alloys, such as Os–Ru-rich sulfides and PtFe alloys, that are selectively enriched with certain HSEs. Mantle metasomatism appears to have no systematic influence on PGEs, although it may increase the abundance of Re and Au in the Yap Trench peridotites. Moreover, compared to abyssal peridotites, subduction-related peridotites have fractionated PGE patterns, particularly for the iridium group, suggesting that subduction conditions may promote the fractionation of the PGEs. However, suprachondritic PGE ratios are generally observed in subduction-related peridotites with a low Al2O3 content (<1 wt%) instead of subduction-related fertile samples. This suggests that highly depleted and subduction-related conditions are required for the fractionation of PGEs. We recommend that the high oxygen fugacity, hydrous melting, and high degree melting of the subduction-zone mantle may result in the exhaustion of the BMS in highly depleted peridotites, promoting the formation of platinum-group minerals and HSE-rich alloys, in which the HSEs are significantly fractionated.

Nitrogen enrichments in sheeted dikes and gabbros from DSDP/ODP/IODP Hole 504B and 1256D: Insights into nitrogen recycling in Central America and global subduction zones

The fate of subducted crustal nitrogen (N) in modern-style subduction zones remains hotly debated from nearly complete return to the surface via arc volcanoes to net ingassing into the deep mantle beyond the sub-arc depth. One of the major obstacles for this controversy is the lack of constraint on the input N flux from altered oceanic crust (AOC). While notable N enrichment by low-temperature hydrothermal alteration has been widely reported in the upper volcanic section of AOC, N behavior during high-temperature alteration of the underlying intrusive oceanic crust (i.e., sheeted dikes and gabbros) remains poorly understood. Here we examined the bulk-rock N concentrations and isotopic compositions of the AOC (with a main focus on sheeted dikes and gabbros, yet including basalts from sections that have not been examined before) from ODP/IODP Hole 1256D and DSDP/ODP Hole 504B, which are two reference sites for subducting material into the warm Central America (CA) subduction zone. The results show that the intrusive sections have comparable N concentrations and isotopic compositions to their overlying upper volcanic rocks at 1256D (sheeted dikes: 16.4±7.2 ppm and −1.3±1.5‰; gabbros: 11.3±5.6 ppm and +1.3±1.0‰; basalts: 11.8±3.8 ppm and +0.9±1.7‰) and 504B (sheeted dikes: 7.6±2.1 ppm and +2.1±1.7‰; basalts: 8.0±3.8 ppm and +2.4±2.9‰). These data can be readily explained by mixing between inherited mantle N and secondary N mostly derived from seawater/sediments with a minor contribution from abiotic N2 reduction. The secondary N in sheeted dikes and gabbros was mainly incorporated at moderate- to high-temperature (>250 °C) alteration stages and likely resides in chlorite, secondary plagioclase and amphibole. These new data clearly indicate that the intrusive section of AOC is a non-negligible N reservoir. Using these data, together with previously published N concentration data of basalts from Holes 1256D and 504B and of subducting sediments from Hole 1039B offboard the CA trench, we obtained a total N input flux of 3.1±0.6 × 109 mol‧yr−1 to 4.0±0.8 × 109 mol‧yr−1 into the CA trench. Integrating recently published N output flux of 0.58 × 109 to 1.4±0.6 × 109 mol‧yr−1 at the CA arc, an N recycling efficiency of 15±3–45±5% was yielded for the CA margin, indicating that up to 55±5–85±3% of subducted slab N is possibly transported beyond the sub-arc depth in this warm subduction zone. Based on the new finding of comparable N enrichment between basalts and intrusive oceanic crust, our modeling also gave a revised N recycling efficiency of 12±10–17±12% for the cold Izu-Bonin-Mariana (IBM) subduction zone, which is consistent with previous estimate of 4–17%. While these new estimates do not provide a conclusive constraint on the effect of subduction-zone thermal structure on N recycling, they consistently suggest that a large fraction of slab N (>50%) can survive the sub-arc filter and be delivered to the deeper mantle in global subduction zones.

Equilibrium boron isotope fractionation during serpentinization and applications in understanding subduction zone processes

Serpentinites entering subduction zones show selective enrichment in trace elements and are the most important carriers of B in this setting. The evolving B isotopic composition (δ11B) of serpentinite is used in studying water-mediated interactions between mantle and crust in subduction zones, but we lack a full understanding of the processes controlling B isotope fractionation during serpentinization that would allow this tracer to be better utilized. Boron isotope fractionation between lizardite/forsterite/diopside and aqueous fluids is investigated using quantum mechanics (density functional theory, DFT) and the first-principles molecular dynamics simulation (FPMD) here. The 11B enrichment sequence in minerals follows the order of forsterite > clinopyroxene (diopside) > white mica > phlogopite > lizardite. On the basis of boron configurations in aqueous fluid acquired from FPMD simulations based on DFT, the reduced isotopic partition function ratio (β-factor) of aqueous B(OH)3 and B(OH)4− are 1000lnβB(OH)3 = −20.1 + 37.07(1000/T) + 8.2(1000/T)2 and 1000lnβB(OH)4- = −13.94 + 24.43(1000/T) + 9.12(1000/T)2 at the P-T range of 0–500 MPa and 298–1000 K. This theoretical approach gives an equilibrium B isotope fractionation of Δ11Blizardite-fluid = 15.89–22.88(1000/T) - 0.19(1000/T)2 between lizardite and neutral serpentinization fluid in the temperature range from 298 to 673 K. By contrast, the boron isotope fractionation between minerals formed as the slab descends (e.g., forsterite, diopside) and fluid released from subducted crustal lithologies is limited, indicating that secondary olivine minerals may inherit the δ11B signatures of the dehydrated serpentinites. This study provides new insights into how B isotopes can constrain boron sources and pathways in subduction zones, and trace phases recycled into the deep mantle and their later recycling in ocean island basalts.

Oxygen isotope (δ18O, Δ′17O) insights into continental mantle evolution since the Archean

Oxygen isotopic ratios are largely homogenous in the bulk of Earth’s mantle but are strongly fractionated near the Earth’s surface, thus these are robust indicators of recycling of surface materials to the mantle. Here we document a subtle but significant ~0.2‰ temporal decrease in δ18O in the shallowest continental lithospheric mantle since the Archean, no change in Δ′17O is observed. Younger samples document a decrease and greater heterogeneity of δ18O due to the development and progression of plate tectonics and subduction. We posit that δ18O in the oldest Archean samples provides the best δ18O estimate for the Earth of 5.37‰ for olivine and 5.57‰ for bulk peridotite, values that are comparable to lunar rocks as the moon did not have plate tectonics. Given the large volume of the continental lithospheric mantle, even small decreases in its δ18O may explain the increasing δ18O of the continental crust since oxygen is progressively redistributed by fluids between these reservoirs via high-δ18O sediment accretion and low-δ18O mantle in subduction zones.

Epidote as a conveyor of water into the Earth’s deep mantle in subduction zones: Insights from coupled high-pressure and high-temperature experiments

Abstract Epidote is a major hydrous mineral in subducted mafic oceanic crust. Understanding its stability in the subduction zone environment is important for evaluating its role as a conveyor of water into the deep Earth. Here we report experimental results on epidote by simulating the high-pressure-temperature (P-T) conditions of the plate subduction environment. We used a diamond-anvil cell with an external resistance heating system, combined with in situ X-ray diffraction (XRD) and Raman spectroscopy techniques. Experiments at ambient pressure and high temperatures indicate that epidote starts to decompose at 1223 K and breaks down completely at 1373 K. In situ XRD analyses show no phase transition at temperatures up to 1272 K and pressures up to 14.0 GPa. Raman spectra indicate that epidote is stable at 1272 K and 14.0 GPa, but the energies of two Si-O bonds (ν2,ν5) and one M-O bond (ν3) increase with increasing temperature. The cation H+ moves for a distance when the P-T is increased to 13.0 GPa and 1123 K. Based on the thermal structure of subducted slabs in typical hot and cold subduction zones, we infer that epidote can convey water downward into the mantle transition zone through subducted mafic oceanic crust.

Evidence of intraplate magmatism and subduction magmatism during the formation of Nagaland–Manipur Ophiolites, Indo–Myanmar Orogenic Belt, north‐east India

Mafic extrusive rocks (basalts) and intrusive rocks (gabbros) from the Nagaland–Manipur Ophiolite (NMO) of the Indo–Myanmar Orogenic Belt (IMOB), north‐east India, are investigated to understand their magmatic evolution in diverse tectonic environments. Basalts are distinguished into two types: basalt‐I and basalt‐II. Basalt‐I type shows the sub‐alkaline character with Nb/Y &lt; 0.50, low Nb/Th (2.36–7.94), and low to moderate La/Sm (1.00–4.12) indicating derivation from a slightly enriched mantle source and also supported by their enriched LREE pattern with flat HREE. They are depleted in HFSEs (Nb and Ti) but enriched in U and Pb, which is indicative of a typical subduction origin derived from an MORB‐type mantle source. Investigated samples of basalt‐II and gabbros have an equal composition with alkaline characteristics. They have Nb/Y &gt; 0.50, high Nb/Th (8.38–13.37), and highly enriched LREE (La/Sm = 4.41–6.35) pattern. They show typical Ocean Island Basalt (OIB) characters of a plume source. The two sets of basalts and gabbros found in this study have no sign of genetic relationship, and therefore, it strongly suggests that they were derived from two different mantle sources of a plume and a subduction zone mantle wedge. Our study supports the theory that the NMO has records of different magmatic episodes produced ranging from plume‐related magmatism, to divergent and convergent plate magmatism that were generated at diverse tectonic settings.

Postseismic geodetic signature of cold forearc mantle in subduction zones

A sharp thermal contrast between the cold forearc and the hot arc and backarc is considered fundamental to various subduction-zone processes. However, direct observational evidence for this contrast is rather limited. If this contrast is present, it must cause a rheological contrast in the mantle wedge: elastic in the forearc and viscoelastic in the arc and backarc for the timescale of earthquake cycles. Here we demonstrate that postseismic deformation following large subduction earthquakes provides independent evidence for the thermally controlled rheological contrast. Specifically, we show that seaward postseismic motion is deflected upward at the edge of the cold forearc mantle wedge, causing diagnostic uplift just seaward of the volcanic arc. From numerical simulations of postseismic deformation following the 2011 moment magnitude (Mw) 9 Tohoku-oki, 2010 Mw 8.8 Maule, 2007 Mw 8.4 Bengkulu and 1960 Mw 9.5 Chile earthquakes, together with a global synthesis of postseismic uplift measurements, we find that cold forearc mantle is present irrespective of the diversity in tectonic settings. Our findings also indicate that field surveys eight years after the 1960 Chile earthquake provided some of the earliest evidence for viscoelastic postseismic deformation. We suggest that the established link between long-term thermal processes and short-term earthquake cycle deformation is important to understanding subduction-zone dynamics. Deformation after large subduction earthquakes reflects the thermal contrast between the mantle wedge and its nose, according to numerical simulations and a synthesis of postseismic uplift data from subduction zones.

Phase transitions in CaCO3 under hydrous and anhydrous conditions: Implications for the structural transformations of CaCO3 during subduction processes

AbstractThe transport of calcium carbonate (CaCO3) into the Earth’s interior through subduction is one of the key processes in the global cycling of carbon. To develop a better understanding of the CaCO3 structural stability during subduction processes, the phase transitions among CaCO3-I (calcite), CaCO3-II, -III/IIIb, and aragonite under pressure-temperature (P-T) conditions up to 2.5 GPa and 600 °C, in hydrous and anhydrous environments, were investigated using a hydrothermal diamond-anvil cell. One displacive and two reconstructive processes during the phase transitions among CaCO3 polymorphs were confirmed from the results obtained from in situ observations and Raman spectroscopic measurements. Meanwhile, the effect of Ca-substitutional metal cations (e.g., Mg2+) in CaCO3 and the presence of an aqueous fluid on the phase transition processes have been determined. Specifically, the CaCO3-I ↔ -II phase transition is a displacive process, occurring instantly at pressures varying from 1.6 GPa at room temperature to 1.5 GPa at 500 °C with the phase equilibrium boundary having a minimum P-T point at ~1.4 GPa at 300 °C, and is completely reversible upon cooling and decompression. The CaCO3-II → -III phase transition is a reconstructive process, observed at P-T conditions from 2.0 GPa at room temperature to 2.5 GPa at 150 °C, and is accomplished by solid recrystallization starting from CaCO3-II, transitioning through an intermediate CaCO3-IIIb, and ending at the CaCO3-III structure. The phase transition between CaCO3-I or -II and aragonite, which is also a reconstructive process, was found to occur by progressive solid recrystallization under high P-T hydrous and anhydrous conditions, or alternatively, via dissolution-precipitation under low-P-T hydrous conditions, depending on the presence of aqueous fluids and the heating rate of the system. The substitution for Ca2+ by other metal cations (e.g., Mg2+, Mn2+, Fe2+) in CaCO3 results in a significant increase in the pressures for the displacive and solid recrystallization reconstructive phase transitions, but has no detectable influence on the CaCO3-I/II ↔ aragonite transformation via a dissolution-precipitation process under hydrous conditions. Our results show that the presence of Ca-substitutional metal cations in CaCO3 is a key factor controlling the phase stability of CaCO3 under high P-T conditions, and suggest that aragonite should be the predominant phase in the upper mantle in subduction zones where the heating rate is very low and slab dehydration is prevalent.

Magnesium-carbon isotopes trace carbon recycling in continental subduction zone

Subduction of the oceanic crust is an important mechanism for transporting the surface carbon into the deep mantle. Subduction of the continental crust is generally preceded by subduction of the oceanic slab, which may carry different sources of crustal carbon into the same subduction zone with enhanced carbon recycling at the convergent plate boundary. Thus, it is significant to identify whether crustal carbon was recycled into the mantle along continental subduction zones. In this contribution, we report for the first time the Mg and C isotope compositions of post-collisional mafic igneous rocks from the Hong'an and Dabie orogens in east-central China. There are relatively low δ26Mg values of −0.77‰ to −0.24‰ and high δ13C values of −4.0‰ to −1.8‰ in association with very high C contents of 9712–10,177 ppm for the Hong'an rocks, indicating recycling of the early subducted, carbonated oceanic crust into the mantle. In contrast, there are relatively high δ26Mg values of −0.23‰ to −0.18‰ and low δ13C values of −5.9‰ to −4.1‰ in association with relatively low C contents of 711–2313 ppm for the Dabie rocks, demonstrating recycling of the crustal carbon from the subsequently subducted continental lithosphere. These interpretations are verified by quantitative modeling of the geochemical data for the two orogens of mafic igneous rocks. The results suggest that addition of 30–40% carbonated oceanic crust-derived materials and 10–20% continental crust-derived materials to the mantle sources can produce the Mg–C–Sr–Nd–Hf isotope compositions of mafic igneous rocks in the Hong'an and Dabie orogens, respectively. Therefore, the different types of crustal carbon were incorporated into the mantle wedge at different depths by the successive subduction of oceanic and continental slabs. In particular, the carbon recycling is evident in the continental subduction zone. As such, carbon fluxes in and out of the mantle in continental subduction zones also need to be taken into account when constraining the global C cycle.

Experimental constraints on the partial melting of sediment-metasomatized lithospheric mantle in subduction zones

AbstractSedimentary diapirs can be relaminated to the base of the lithosphere during slab subduction, where they can interact with the ambient lithospheric mantle to form variably metasomatized zones. Here, high-pressure experiments in sediment-harzburgite systems were conducted at 1.5–2.5 GPa and 800–1300 °C to investigate the interaction between relaminated sediment diapirs and lithospheric mantle. Two end-member processes of mixed experiments and layered (reaction) experiments were explored. In the first end-member, sediment and harzburgite powders were mixed to a homogeneous proportion (1:3), whereas in the second, the two powders were juxtaposed as separate layers. In the first series of experiments, the run products were mainly composed of olivine + orthopyroxene + clinopyroxene + phlogopite in subsolidus experiments, while the phase assemblages were then replaced by olivine + orthopyroxene + melt (or trace phlogopite) in supersolidus experiments. Basaltic and foiditic melts were observed in all supersolidus mixed experiments (~44–52 wt% SiO2 at 1.5 GPa, ~35–43 wt% SiO2 at 2.5 GPa). In the phlogopite-rich experiment (PC431, 1.5 GPa and 1100 °C), the formed melts had low alkali contents (~&amp;lt;2 wt%) and K2O/Na2O ratios (~0.4–1.1). In contrast, the quenched melt in phlogopite-free/poor experiments showed relatively higher alkali contents (~4–8 wt%) and K2O/Na2O ratios (~2–5). Therefore, the stability of phlogopite could control the bulk K2O and K2O/Na2O ratios of magmas derived from the sediment-metasomatized lithospheric mantle. In layered experiments, a reaction zone dominated by clinopyroxene + amphibole (or orthopyroxene) was formed because of the reaction between harzburgite and bottom sediment-derived melts (~62.5–67 wt% SiO2). The total alkali contents and K2O/Na2O ratios of the formed melts were about 6–8 wt% and 1.5–3, respectively. Experimentally formed melts from both mixed and reaction experiments were rich in large ion lithosphile elements and displayed similar patterns with natural potassium-rich arc lavas from oceanic subduction zones (i.e., Mexican, Sunda, Central American, and Aleutian). The experimental results demonstrated that bulk sediment diapirs, in addition to sediment melt, may be another possible mechanism to transfer material from a subducting slab to an upper mantle wedge or lithospheric mantle. On the other hand, the breakdown of phlogopite may play an important role in the mantle source that produces potassium-rich arc lavas in subduction zones.

Raman spectroscopic constraints on compression and metastability of the amphibole tremolite at high pressures and temperatures

Amphiboles, containing up to 2 wt% water in the form of hydroxyl units, contribute significantly to water transport into the deep lithosphere and upper mantle in subduction zones, where dehydration reactions make the water available for metasomatic processes and flux melting of the overlying mantle wedge. Here, we investigate the high-pressure and temperature behavior of the calcic end-member amphibole Ca2Mg5Si8O22(OH)2-tremolite. Tremolite is characterized by Raman spectroscopy at pressures to ~ 49 GPa at room temperature, and at temperatures up to 540 K at ambient pressure. The behavior of the hydroxyl stretching vibration, concordant with previous infrared spectroscopic results, implies that the role of Davydov splitting in this amphibole is small, and the monotonically increasing mode shift of the hydroxyl peak under pressure indicates no approach toward hydrogen-bond symmetrization. As with a range of other hydroxyl-bearing minerals, the hydroxyl librations have slightly negative pressure-induced mode shifts. Intensity trends of Raman peaks that involve Ca–O displacements suggest changes in the calcium cation bonding environment above 10 GPa. The peak shifts of 18 modes are used to determine their isothermal and isobaric mode-Gruneisen parameters (γiT and γiP). The intrinsic mode anharmonicities, calculated from the Gruneisen parameters, indicate largely quasi-harmonic internal mode vibrations while the lower frequency vibrations associated with cations display significant anharmonicity. The general topology of the tremolite structure remains metastable to ~ 50 GPa, and our work provides constraints on the metastability of amphiboles in subduction zones and the upper mantle, as well as the intrinsic crystallographic stability of the monoclinic amphibole structure.

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.

Impact of a Larger Fore‐Arc Region on Earthquake Ground Motions in South‐Central Alaska Including the 2018 M 7.1 Anchorage Inslab Earthquake

AbstractThe thermal state of the crust and mantle in subduction zones is controlled by the depth of the subducting plate. With low‐angle subduction, like at the eastern end of the Alaska subduction zone, the less attenuating fore‐arc is extended farther from the trench and can effect ground motions in addition to source and site effects. Recent crustal and subduction earthquakes in south‐central Alaska, including the 2018 M 7.1 Anchorage event, demonstrate these effects. Inslab earthquake waves in the subducting plate can propagate up the slab to the fore‐arc region with less attenuation, causing an increase in observed ground motions. Long‐period ground motions from the 2018 M 7.1 Anchorage earthquake are significantly higher than predicted ground motions from current subduction ground‐motion models within 50–100 km of the epicenter. At short periods, ground motions show reduced amplitudes due to nonlinear sediment effects in the Anchorage area, reducing the damage potential of the earthquake. At long periods, ground motions are little affected by sediment nonlinearity and remain higher than expected. The duration of shaking was too short for widespread liquefaction effects, unlike during the 1964 M 9.2 earthquake.Other historical earthquakes have produced similar increases in ground motions in the Cook Inlet and Kenai Peninsula region. At both short and long periods, ground motions from the 2016 Iniskin M 7.1 inslab earthquake are higher than expected in the Cook Inlet region. The 2015 Redoubt M 6.3 inslab earthquake also shows increased ground motions in the Cook Inlet region at all periods. Crustal Q estimates from Lg waves show less attenuation in south‐central Alaska at longer periods. In the larger south‐central Alaska region crustal Q(f)=336f0.34 compared to Q(f)=217f0.84 for all of Alaska with most of the decrease in attenuation at frequencies below 2 Hz.

Ancient Melt Depletion and Metasomatic History of the Subduction Zone Mantle: Osmium Isotope Evidence of Peridotites from the Yap Trench, Western Pacific

Highly depleted peridotites from the Yap Trench in the western Pacific Ocean have been studied for Re-Os elements and Re-Os isotopes. These peridotites have a low Re-Os content and variable 187Os/188Os ratios (0.12043–0.14867). The highest 187Os/188Os ratio is far higher than that of the primitive upper mantle and the lowest 187Os/188Os ratio is comparable to the most unradiogenic 187Os/188Os ratio (0.11933) discovered in subduction zone peridotites. The suprachondritic 187Os/188Os ratios of the Yap Trench peridotites results from modification of the mantle wedge by slab-derived fluid and melt. This is consistent with the observation that high 187Os/188Os ratios generally occur in oceanic peridotites with low Os content (&lt;2 ppb) since Os may be reduced during late processes such as fluid alteration and melt refertilization. The sub-chondritic 187Os/188Os ratios of the Yap Trench peridotites correspond to a Re depletion age of 0.24–1.16 billion years, which means that these peridotites represent old mantle residue of ancient melting events. This ancient melting, combined with probable back-arc melting and forearc melting during subduction initiation, indicates that the Yap Trench mantle has a complex evolutionary history. The amount of old mantle residue in the oceanic asthenosphere was underestimated because the 187Os/188Os ratio in mantle peridotites is elevated during late processes. Therefore, old depleted mantle fragments may contribute substantially to the chemical heterogeneity of the oceanic mantle.

Ophiolitic Pyroxenites Record Boninite Percolation in Subduction Zone Mantle

The peridotite section of supra-subduction zone ophiolites is often crosscut by pyroxenite veins, reflecting the variety of melts that percolate through the mantle wedge, react, and eventually crystallize in the shallow lithospheric mantle. Understanding the nature of parental melts and the timing of formation of these pyroxenites provides unique constraints on melt infiltration processes that may occur in active subduction zones. This study deciphers the processes of orthopyroxenite and clinopyroxenite formation in the Josephine ophiolite (USA), using new trace and major element analyses of pyroxenite minerals, closure temperatures, elemental profiles, diffusion modeling, and equilibrium melt calculations. We show that multiple melt percolation events are required to explain the variable chemistry of peridotite-hosted pyroxenite veins, consistent with previous observations in the xenolith record. We argue that the Josephine ophiolite evolved in conditions intermediate between back-arc and sub-arc. Clinopyroxenites formed at an early stage of ophiolite formation from percolation of high-Ca boninites. Several million years later, and shortly before exhumation, orthopyroxenites formed through remelting of the Josephine harzburgites through percolation of ultra-depleted low-Ca boninites. Thus, we support the hypothesis that multiple types of boninites can be created at different stages of arc formation and that ophiolitic pyroxenites uniquely record the timing of boninite percolation in subduction zone mantle.

Origin of pre-mesozoic xenocrystic zircons in cretaceous sub-volcanic rocks of the northern Andes (Colombia): paleogeographic implications for the region

This paper reports the presence of xenocrystic zircons in sub-volcanic rocks (Guarco Andesite and Porphyritic Intrusives units) exposed in the northeast of the Western Cordillera of Colombia (WCC) and provides a new contribution to the understanding of the tectono-magmatic evolution of the area during the Upper Cretaceous associated with the geological history of the Caribbean Large Igneous Province (CLIP). The study area is located in an accretionary terrane constituted by a basement of oceanic affinity (Barroso Formation and San José de Urama Diabases unit), amalgamated against the northwestern South American margin and associated with the development of CLIP. The elemental and isotopic geochemical data (with an initial ratio of 87Sr/86Sr between 0.70334 and 0.70543 and εΝd between +6.25 and + 7.42) indicate an island arc environment for the formation of sub-volcanic rocks and a mantle wedge source area (and/or the arc itself), which presents subordinate levels of crustal material. U-Pb zircon ages by SHRIMP IIe and LA-ICP-MS show ages from the Mesoarchean to the Upper Cretaceous, with representative peaks of approximately 2700, 2100, 600 and 90 Ma. The younger age population is interpreted as the crystallization age of the studied rocks and indicates a spatial and temporal relationship with the basement rocks associated with the CLIP. The older zircons are interpreted as xenocrysts coming from terrigenous sediments that were dragged down into the subduction channel (reflecting crustal recycling processes through the mantle in subduction zones). Alternatively, the xenocrystic zircons could have come from the assimilation of crustal sediments that were previously deposited over the primitive arc. We suggest that the source areas of zircon xenocrysts probably are located close to the Arequipa Massif and the Marañon Complex inliers, and close to the ensialic Basement of the Central Cordillera of Colombia (CCCB). Using these source areas and paleogeographic/paleomagnetic models published by other authors, we propose two possible paleogeographic scenarios differentiated by the polarity of the subduction zone.

East‐West Differential Underthrusting of the Indian Lithospheric Plate Beneath Central Tibet Revealed by Imaging VP/VS

AbstractAlthough numerous research studies have already provided basic information on deeper lithospheric structure under Tibet, the northernmost edge and the geometry of the northward underthrusting of the Indian mantle lithosphere are yet to be understood. The P wave to S wave velocity ratio (VP/VS) can provide more insight into the mantle physical state than either VP or VS alone, owing to their different sensitivities to partial melting, temperature, and composition. High VP/VS usually indicates that partial melting exists in the upper mantle, while anomalously low VP/VS in the upper mantle reflects the enrichment of orthopyroxene in the subduction zone mantle wedge. Using teleseismic records of 164 broadband stations deployed in central Tibet, we obtained a high‐resolution velocity ratio image of the upper mantle. A striking difference is seen between western and eastern Tibet. A distinct high VP/VS zone exists at relatively a shallow depth (&lt;200 km) to the west of the Yadong‐Gulu Rift, while the eastern part exhibits a shallow region (south of the Bangong‐Nujiang suture in the depth range of 120–160 km) with abnormally low VP/VS. Moreover, there are high VP/VS areas existing below 270‐km depth, and it appears further east at depth of 340 km than that at depth of 270 km. We propose that this difference reflects different geometries of the underthrusting Indian mantle lithosphere from west to east. Slab tearing under eastern Tibet and lithospheric delamination under western Tibet are likely good candidates to explain the observation.