Subduction Factory Research Articles

Syn‐Drift Plate Tectonics

AbstractThe paradigm of plate tectonics holds that ocean plates are rigid during drift and only experience tectonic deformation at subduction zones, but new findings from the Pacific challenge this idea. Geological and geophysical evidence from the Ontong Java, Shatsky, Hess, and Manihiki oceanic plateaux indicates that extensional deformation during plate drift is a widespread phenomenon across the Pacific plate. These anomalously thick oceanic plateaux are weaker regions of the ocean lithosphere and more prone to tectonic deformation. Numerical geodynamic models demonstrate that a slab pull force from distant subduction plate boundaries can be effectively transmitted to oceanic plateaux through strong ocean lithosphere and cause substantial extension during plate drift. Our findings reveal that a wide expanse of the Pacific has experienced syn‐drift plate tectonics linked to pull from the western Pacific subduction factory.

High-field-strength-element enriched arc rocks witness subducted mélange recycling in upwelling asthenosphere

Arc rocks, as the most common product of plate subduction factory, are generally characterized by high field strength element (HFSE) depletions. However, HFSE-enriched arc rocks are also identified, and their origin is still controversial. We conduct combined radiogenic and stable isotope analyses on two ocean island basalt (OIB)-associated HFSE-enriched mafic intrusions in the Yunnan-Burma region. Median incompatible trace element compositions argue for a binary mixing between OIB-like asthenosphere dominated mantle source and a complex arc-like component. The high radiogenic Pb [(206Pb/204Pb)i = 17.99–18.85, (207Pb/204Pb)i = 15.53–15.71 and (208Pb/204Pb)i = 38.16–39.20] and heavy Mg (δ26Mg = −0.115‰ to 0.357‰) isotopes could only be inherited from subducted marine sediments. Altered oceanic crust (often abbreviated to AOC) is also involved in the source as evidenced by light Li isotopes (δ7Li = −2.54‰ to −1.31‰). Therefore, we argue that this arc-like component is inherited from subducted mélange (i.e., bulk marine sediment and AOC) recycling, rather than the more common melt and/or fluid metasomatism. In summary, the OIB associated HFSEs-enriched arc rocks are products of and witnessed the subducted mélange recycling and its interaction with the upwelling asthenosphere.

Recycling of eroded crust by mélange diapirs evidenced from the Early Cretaceous arc magmatism in southern Tibet

Crustal material recycling in a subduction factory increases the diversity and complexity of arc magma. However, the exact origin and mechanism of its contribution to arc magma are not fully understood. Here, we present new data on the Molang basaltic rocks from the southern Lhasa terrane to gain an in-depth-understanding of the mechanisms of crustal recycling. The zircon UPb ages indicate that these rocks were emplaced at 128 Ma. They display characteristics ranging from forearc basalts to tholeiitic arc basalts with depleted mantle isotope compositions (εNd(t) = +4.58 to +4.86) and zircon ɛHf(t) values (+7.3 to +8.7). The uniform isotopic composition (SrNd) and fractionation of trace elements show that crustal materials could have been added to the magma source in the form of bulk sediment. Therefore, we have proposed an argument supporting the mélange diapirs model. The age spectra of the inherited zircons are consistent with those of the detrital zircons from the subduction hanging wall. This implies that the inherited zircons may be unmolten residues of eroded forearc crust that survived erosion and subduction from the slab in well-mixed mélange diapirs that passed through a comparatively hot mantle wedge and occur as key evidence for crustal recycling in the arc magma. Based on a synthesis of earlier research, we suggest that the Molang basaltic rocks were derived from the partial melting of the mélange diapirs in a steep subduction setting during a new subduction initiation event involving the Neo-Tethyan oceanic slab.

Arc volcanic suite from a Miocene subduction system in SW Japan: A geochemical and zircon U-Pb-Lu-Hf study

Arc volcanic rocks are important products of the subduction factory in convergent plate margins and contribute significantly to crust building in the evolving Earth. Southwest Japan provides a window to gain insights into the ongoing subduction-related processes and the early stages of crust building. In this study, we investigate a suite of volcanic rocks from the central segment of the Setouchi volcanic belt (SVB) including dacites and andesite from northeast Shikoku island in southwestern Japan based on petrologic, geochemical, zircon U--Pb, REE (rare-earth element), and Lu--Hf analyses. The dacites and andesite from this region are peraluminous, medium-K, and calc-alkalic, with LREEs (light rare-earth elements) enrichment and depleted in HREEs (heavy rare-earth elements) and HFSEs (high field-strength elements). Two groups of zircon grains occur in these volcanic rocks as revealed from U--Pb studies. The older magmatic zircon grains are of late Cretaceous age and show typical oscillatory zoning yielding 206Pb/238U age peaks at 83.6 Ma with dominantly negative εHf(t) values (−3.7 to +0.9), indicating older reworked crustal sources. The second group is represented by thin elongated prismatic grains with banded structure, which yield 206Pb/238U age peaks at 13.8 Ma and displaying more positive εHf(t) values (+2.4 to +7.7), indicating juvenile sources. A compilation and comparison of the available geochemical and geochronological data on the volcanic suites in the SVB reveal a common source for the coeval island-arc volcanic rocks (andesite-dacites-rhyolite) and HMA (high magnesium andesites). Compared with the HMA, the other volcanics are more evolved through crustal contamination and fractionation.

Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based onP-T-tpaths of the eclogite-bearing Baqing metamorphic complex

AbstractSubduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex.The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

Construction of an island arc and back-arc basin system in eastern Central Asian Orogenic belt: Insights from contrasting Late Carboniferous intermediate intrusions in Central Inner Mongolia, North China

Characterizing island arc and back arc basin system through detailed magmatic profile remains a pivotal task in any regional paleotectonic and supercontinental reconstructions along ancient convergent plate margins, as is of paramount importance for the northern China–southern Mongolia (NCSM) tract along the eastern Central Asian Orogenic belt (CAOB). Here we document episodic Late Carboniferous intermediate to felsic intrusive suites in the West Ujimqin region of central Inner Mongolia, spanning from ca. 315 Ma granodiorites through ca. 310 Ma hornblende diorites to ca. 305–301 Ma diorites. The earlier granodiorites exhibit dominant magnesian-tholeiitic affinity, enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs), and possess whole-rock ISr(t) ratios of 0.707387–0.708799, negative ƐNd(t) of −2.3 to −2.4 and zircon ƐHf(t) values of −0.69 to +4.87 and δ18O of +6.33 to +8.40‰. These features are consistent with the magmatic derivation from dehydration melting of tholeiitic mafic to intermediate lower crustal underplates with obvious high δ18O supracrustal inputs. The ca. 310 Ma pod-like hornblende diorites display high-Ca boninite-like elemental traits and MORB-like isotopic compositions (ISr(t) = 0.704190 to 0.704203, ƐNd(t) = +7.9 to +8.3, zircon ƐHf(t) = +14.7 to +17.5 and δ18O = +4.88 to +6.16‰), indicative of a prior melt-depleted then fluid-enriched mantle source. By contrast, subsequent (301–305 Ma) diorites show arc-like elemental hallmarks, with notable enriched LILEs and depleted HFSEs as well as variable isotopic values (ISr(t) = 0.703690–0.704065, ƐNd(t) = +3.5 to +8.4, zircon ƐHf(t) = +7.10 to +16.4 and δ18O = +3.75 to +6.83‰). They are interpreted to represent the evolved products of hornblende-dominated fractional crystallization from depleted basaltic parental magmas. Given their coincidence with regional back-arc-basin type ophiolitic stratigraphy and detrital zircon age spectra, these intermediate to felsic intrusive suites tend to document the timeline of a back arc basin formation from initial rifting to maturation. In conjunction with Early Carboniferous forearc-type ophiolitic complexes and forearc magmatic records at West Ujimqin and the adjacent region, a Carboniferous island arc and back arc system can be reconstructed from Xilinhot to West Ujimqin. The characterization of this northward (present day coordinates) subduction system in terms of its temporal evolution not only provides overwhelming evidence that modern Pacific-like, archipelago-type arc configuration had prevailed in central Inner Mongolia during the Carboniferous, but also offers vivid insights into the diversity of crustal generation mechanism in a subduction factory that ushered in seminal continental crustal construction in the eastern CAOB.

Recycling of heterogeneous material in the subduction factory: evidence from the sedimentary mélange of the Internal Ligurian Units, Italy

In the Northern Apennine (Italy), the Internal Ligurian Units consist of Middle–Late Jurassic ophiolites covered by thick sedimentary deposits whose top is represented by the Early Paleocene Bocco Shale. This formation is characterized by mass-transport deposits interlayered with thin-bedded siliciclastic turbidites. The sedimentological and structural features of these mass-transport deposits reveal a long-lived history of recycling of heterogeneous material in a subduction setting. This history started with the frontal accretion of a fragment of oceanic crust into an accretionary prism whose lower slope was subsequently affected by tectonic erosion and consequent instability, leading to the production of mass-transport deposits and the transfer of material to the lower plate. These mass-transport deposits were subsequently underthrust and then again transferred to the base of the accretionary prism by coherent underplating, before their exhumation to the surface. The Bocco Shale is thus representative of a subduction setting where both accretionary and erosive events occurred, depending on changing boundary conditions. The reconstructed history for the Bocco Shale indicates that the sedimentary and gravitational processes both at the prism front and on the prism slope, possibly induced by alternating accretion and erosion events, are the most efficient mechanisms of lithological mixing and recycling in subduction margins.

Interaction between oceanic slab and metasomatized mantle wedge: Constraints from sodic lavas from the Qilian Orogen, NW China

Generation of arc volcanic rocks is a complicated process, potentially involving some or all of oceanic slab subduction, mantle wedge metasomatism, and melting of both the subducted slab and the mantle wedge. Interaction between melts of different sources plays important roles in the subduction factory and the formation of magmatic arcs and is still controversial. To elucidate the mantle-melts interaction process, we present a study of the sodic adakite-like lavas (445-444 Ma) and basalt-basaltic andesite (basaltic) enclaves (∼453 Ma) in the early Paleozoic intra-oceanic arc system from the South Qilian Accretionary Belt, Qilian Orogen. Basaltic enclaves in the Muli adakite-like lavas have high Mg# (62–69) high Cr and Ni, and radiogenic Sr and unradiogenic Nd isotopic compositions (ISr = 0.706327–0.706497; εNd(t) = −3.2∼ − 2.0). They are considered to represent primitive magmas derived from a highly metasomatized mantle wedge. The adakite-like lavas are dacitic with adakite-like compositions, enriched SrNd isotopic compositions (ISr = 0.705597–0.706747; εNd(t) = −1.7∼ + 0.4) and significantly variable zircon Hf isotopic compositions (εHf(t) = +0.9∼ + 17.8), most likely a hybrid magma produced by mixing primitive basaltic melts of metasomatized mantle wedge and melts of subducted oceanic crust.The Muli adakite-like lavas and basaltic enclaves occurred in an intra-oceanic island arc setting with an unusually hot thermal structure, which induced melting of subducted slab (including sediments and mafic oceanic crust) and metasomatized mantle wedge. The geodynamic mechanism associated with slab melting is plausibly either or both of the juvenile nature of South Qilian Oceanic Crust, and/or the heating of a slab edge by upwelling hot asthenospheric mantle during subduction initiation.

Geochemistry and zircon U–Pb geochronology constrains late cretaceous plagiogranite intrusions in Mersin ophiolite complex (southern Turkey).

In this study, LA-ICP-MS U–Pb zircon dating were used to determine the age of the newly discovered plagiogranite suite intruding gabbro and volcanic units of Mersin ophiolites from the Inner Tauride Belt. Obtained U–Pb zircon ages from the plagiogranite yielded crystallization ages of 93.0 ± 1.5 to 94.2 ± 2.4 Ma (Turonian–Cenomanian) supporting the idea of Late Cretaceous active subduction factory in the Tauride Suture Zone. The plagiogranites are mainly granodioritic, and tonalitic in composition, and contain mafic microgranular enclaves (MME) ranging from 10 to 45 cm in size. The plagiogranites are geochemically defined by low K2O (0.02–1.03 wt%) and TiO2 (0.17–1.88 wt%) and comparatively high Na2O (2.3–10.2 wt%) and SiO2 (70–78 wt%) compositions together with depletion in Ti, Ta, and Nb. The tectonomagmatic discrimination diagrams, trace, and REE-normalized multi-element patterns indicate that the plagiogranites are distinctive calc-alkaline, I-type volcanic arc granites. Plagiogranites are furthermore characterized by the diffuse presence of isotropic pseudomorphic growth of secondary calcic-amphibole (edenite and actinolite) over a pristine not preserved Ca-inosilicate. Inverse geothermobarometry models indicate a secondary amphibole genesis at ca. 600 °C and 1.5–1.7 kbar, suggesting HT-metasomatism affecting the already intruded plagiogranites. While it is already accepted that Mersin ophiolite complex is generated in a supra-subduction zone, this study represents a new contribution on the evolution of the Mersin ophiolite during the Late Cretaceous Neotethys subduction and could shed light on the genesis of plagiogranites in arc-environments.

Boninite volcanic rocks from the mélange of NW Dinaric-Vardar ophiolite zone (Mt. Medvednica, Croatia) – record of Middle to Late Jurassic arc-forearc system in the Tethyan subduction factory

In the Late Jurassic to Early Cretaceous ophiolite melange from the Mt. Medvednica (Vardar Ocean) blocks of boninite rocks have been documented. They emerge as massive lavas made of augite, spinel, albite and secondary hydrous silicates (e.g., chlorite, epidote, prehnite, and pumpellyite). An established crystallization sequence (spinel→clinopyroxene→plagioclase±Fe-Ti oxides) was found to be typical for the boninite series from the suprasubduction zones (SSZ). Augite crystallization temperatures and low pressures of ~1048 to 1260 °C and ~0.24 to 0.77 GPa, respectively, delineated the SSZ mantle wedge as a plausible source of boninite parental lavas. Their whole-rock geochemistry is characterised by low Ti, P2O5, Zr, Y, high-silica, and high Mg# and Cr# values. Low and U-shaped REE profiles are consistent with the negative Nb-Ta, P and Ti anomalies indicative for SSZ. Thorium and LILE enrichment, and very low initial Nd-isotopic values (eNd(T = 150 Ma) + 0.49 to +1.27) act as vestiges of mantle-wedge metasomatism. The mantle source was likely depleted by the MORB and IAT melt extraction and was contemporaneously affected by subduction fluids, prior to the large-scale adiabatic melting of the mantle hanging wall. This eventually gave rise to boninite lavas and an ultra-refractory harzburgite residiuum. The genesis of boninites is related to the Tithonian mature forearc setting that evolved from an intra-oceanic, Callovian to Oxfordian, island-arc environment. The Mt. Medvednica boninite rocks stand for the youngest SSZ-related Jurassic oceanic crust from the NW segment of the Dinaric-Vardar Tethys that are nowadays obducted onto the passive margins of Adria. Taking into account the existence of similar rocks in the ophiolite zones of Serbia, Albania and Greece, the boninites of Mt. Medvednica strongly favours the single Tethyan oceanic basin that existed in this part of Europe during the Late Jurassic.

Sedimentary inputs to the Nankai subduction zone: The importance of dispersed ash

We examine the importance of dispersed volcanic ash as a critical component of the aluminosilicate sediment entering the Nankai Trough, located south of Japan’s island of Honshu, via the subducting Philippine Sea plate. Multivariate statistical analyses of an extensive major, trace, and rare earth element data set from bulk sediment and discrete ash layers at Integrated Ocean Drilling Program (IODP) Sites C0011 and C0012 quantitatively determine the abundance and accumulation of multiple aluminosilicate inputs to the Nankai subduction zone. We identify the eolian input of continental material to both sites, and we further find that there are an additional three ash sources from Kyushu and Honshu, Japan and other regions. Some of these ash sources may themselves represent mixtures of ash inputs, although the final compositions appear statistically distinct. The dispersed ash comprises 38 ± 7 weight percent (wt%) of the bulk sediment at Site C0011, and 34 ± 4 wt% at Site C0012. When considering the entire sediment thickness at Site C0011, the dispersed ash component supplies 38000 ± 7000 g/cm2 of material to the Nankai subduction system, whereas Site C0012 supplies 20000 ± 3000 g/cm2. These values are enormous compared to the ~2500 g/cm2 (C0011) and ~1200 g/cm2 (C0012) of ash in the discrete ash layers. Therefore, the mass of volcanic ash and chemically equivalent alteration products (e.g., smectite) that are dispersed throughout the stratigraphic succession of bulk sediment appears to be up to 15–17 times greater than the mass of discrete ash layers. The composition of the dispersed ash component at Site C0011 appears linked to that of the discrete layers, and the mass accumulation rate for dispersed ash correlates best with discrete ash layer thickness. In contrast, at Site C0012 the mass accumulation rate for dispersed ash correlates better with the number of ash layers. Together, the discrete ash layers, dispersed ash, and clay-mineral assemblages present a complete record of volcanism and erosion of volcanic sources; and indicate that mass balances and subduction factory budgets should include the mass of dispersed ash for a more accurate assessment of volcanic contributions to large-scale geochemical cycling.

Subduction factory in an ampoule: Experiments on sediment–peridotite interaction under temperature gradient conditions

To better understand processes above subducted oceanic slabs, we have undertaken experiments with juxtaposed sediment and peridotite layers at pressures of 7.5 and 10.5 GPa at a controlled temperature gradient from ∼100 to ∼500 °C per a sample length of ∼3 mm. The sediment starting material contains H2O (6.9 wt%) and CO2 (5.9 wt%) and has a major-element composition similar to GLOSS (Plank and Langmuir, 1998) doped with trace elements at 10–100 ppm levels. Several experiments were conducted with ∼0.5 wt% Cl or F. The peridotite layer is composed of natural olivine (66 wt%), orthopyroxene (27 wt%) and garnet (7 wt%) mixed with ∼15 wt% graphite. Several experimental configurations were investigated, but the “basic” setup has the sediment layer at the bottom in the cold zone (400–1200 °C) overlain by peridotite at 900–1500 °C. The temperature distribution was determined by two thermocouples and orthopyroxene–garnet thermometry.Features common to many experiments are (1) the development of multiple layers of various lithologies and a pool of hydrous silicate or carbonate–silicate melt in the hottest part of the capsule; (2) replacement of olivine by orthopyroxene in the metaperidotite; (3) preservation and growth of garnet and local development of magnesite in the metaperidotite layer; (4) enrichment in garnet within the metasediment layer at the contact with the metaperidotite; (5) formation of a clinopyroxene–garnet assemblage at the bottom (the coldest part); (6) presence of K-bearing phases (phlogopite or phengite) and carbonates in the metasediment layer only at temperatures <700 °C; and (7) occurrence of accessory zircon, rutile and phosphates in the coldest regions. In terms of element redistribution, the peridotite becomes strongly enriched in SiO2 compared to the starting composition, and the sediment gains MgO, FeO and Cr2O3. Potassium is fully extracted into the melt, while Na and Ca are largely retained in the coldest part of the metasediment layer in clinopyroxene, Ca-rich garnet and aragonite. The melt is a product of interaction between partial melt or fluid from the sediment and peridotite. It has a silico-carbonatite composition with variable SiO2, MgO, FeO and CaO contents and low Al2O3. The addition of Cl has almost no effect on element distribution, whereas the addition of F results in the appearance of humite-group minerals containing significant amounts of Ti. Trace-element distribution is controlled by pressure, temperature and mineral assemblages. At low temperatures in the sediment layer (<700 °C) Ba, Rb, Sr and Li are much more mobile than REE and HFSE, which results in high Ba/La, Ba/Nb, Sr/Nb etc. (fluid metasomatism). At higher temperatures in the sediment layer, the melt is markedly enriched in Ba, Rb, Sr, LREE and U relative to Ti, MREE and HREE. Negative Nb–Ta and Zr–Hf anomalies in melts are caused by the retention of rutile, zircon and humite-group minerals in the solid residue. Thermodiffusion may affect the ratios of some highly incompatible elements (e.g., Ta/La). Possible applications of the results to natural deep subduction are discussed in view of variations in mineral assemblages and trace element ratios.

The Role of Recycled Oceanic Crust in the Generation of Alkaline A‐Type Granites

AbstractMelting and dehydration of subducted oceanic slabs dominate element recycling within the subduction factory, but the role of recycled oceanic crust in the source of intraplate magmas is poorly understood. In situ zircon O‐Hf isotope data from two Early Cretaceous alkaline A‐type granites that are genetically related to large‐scale extension of eastern China in the Late Mesozoic (circa 125 Ma) yield low δ18O (1.8 ± 0.3‰ to 5.1 ± 0.3‰, 2σ) and positive εHf(t) (1.5 ± 1.2 to 17 ± 1.2, 2σ). This suggests the contribution of altered oceanic crust and an enriched mantle component in the source region. Elevated initial 87Sr/86Sr and 206Pb/204Pb ratios, and εHf(t) whole rock values with relatively constant εNd(t) values beyond the normal mantle array, require a component that underwent seawater interaction in the source of protolith. The geochemical data require a complex source region for the alkaline A‐type granites in NE China involving more than 40% recycled oceanic crust. This altered oceanic crust beneath the Late Mesozoic lithospheric mantle likely represents remnants of multiple subduction and collision events between microblocks from the Late Paleozoic to Early Mesozoic in northeastern China. Recycling of subducted oceanic crust represents a novel exotic source for the origin of alkaline A‐type granites in intraplate extensional settings.

Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

AbstractVolatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid‐oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA‐type basalts (H2O/Ce = 215 ± 30, δDSMOW = −45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = −30 to −40 ‰). Basalts with EM‐type signatures have regionally variable volatile compositions. Northern Atlantic EM‐type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = −57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM‐type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (−68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM‐type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = −94 ± 3 ‰). A multistage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration, and re‐equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of &lt;1% carbonated eclogite ± sediment‐derived C‐O‐H‐Cl fluids to depleted mantle at 180–280 km (EM) or within the transition zone (PREMA).

Recycling in the subduction factory: Archaean to Permian zircons in the oceanic Cretaceous Caribbean island-arc (Hispaniola)

Little mineralogical evidence is left of the recycling of continental and oceanic crust into the mantle at subduction zones. Zircon, because of its exceptional robustness, is probably the only surviving phase and the best mineral tracer of this global-scale process. This article combines new in-situ U-Pb dating and O and Hf isotope analyses on Cretaceous (co-magmatic) and pre-Cretaceous (inherited) zircons separated from Albian-Aptian arc-related igneous rocks from the Dominican Republic. The O and Hf systematics of Cretaceous zircons reflect derivation from predominantly juvenile sources and variable mixing with evolved melts, as expected for an oceanic island-arc. Inherited zircons yield U-Pb ages between 256 and 2923Ma (n=219). Most studied inherited zircons are Permian-Carboniferous, peaking at ca. 300Ma, and formed in continental crust to judge from mineral inclusions (including quartz, orthoclase, muscovite and apatite), δ18O (6.16 to 12.67‰) and εHf(t) (−11.9 to 1.12) values. Ordovician to Proterozoic zircons yield δ18O between 4.93 and 9.21‰ and εHf(t) between −19.9 and 8.5 and hence crystallized in equilibrium with either mantle-derived (juvenile) magmas or magmas derived from melting of supracrustal rocks. For Archean zircons, δ18O between 6.98 and 7.94‰ and εHf(t) between −18.5 and −1.6 might reflect contribution of supracrustal materials in the parent magmas. We suggest that arc magmas picked up the inherited zircon cargo from their mantle sources. Accounting paleogeographic and paleo-tectonic constraints, Mexican and Colombian terranes are identified as the potential primary (magmatic) and secondary (sedimentary) sources for the studied inherited zircons. We envisage that inherited zircons were transported downward to the mantle beneath the Caribbean plate as detrital grains on top of the subducting Proto-Caribbean and/or Pacific slabs.

A Revised Subduction Inception Model to Explain the Late Cretaceous, Double‐Vergent Orogen in the Precollisional Western Tethys: Evidence From the Northern Apennines

AbstractThe Meso‐Cenozoic alpine belts of the Mediterranean area are characterized by complex architectures, result of a complex subduction and collision evolution that preserve also a legacy of the rifting‐related configuration of the continental margins. The Northern Apennines is a segment of these belts originated during closure of the Ligure‐Piemontese ocean and collision between the Europe and Adria plates. The different configuration of the Adria and Europe margins, inherited from asymmetric rifting, is recorded in the Ligurian Units that preserve incorporation into the subduction factory of fragments of the oceanic domain (Internal Ligurian Units) and portions of the Ocean‐Continent Transition Zone (OCTZ) toward Adria (External Ligurian Units). We provide here unpublished data on the stratigraphy and sedimentology of these units, together with a review of what is already established in literature. Both data sets combined testify that at 80 Ma, an accretionary prism was growing between the deposition basins of the two groups of units and fed both basins with clasts from the ocean realm, the continental crust, and the subcontinental mantle. We propose that closure of the Ligure‐Piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned Adria margin and developed a double‐vergent prism by accreting oceanic material and continental extensional allochthons from the OCTZ. We believe the revised site of subduction initiation, and the precollisional architecture, inherited from the rifting and spreading phases, allows reconciling most of the discrepancies between the various interpretations proposed in literature for the precollisional evolution of the Apennines.

Chemical interactions in the subduction factory: New insights from an in situ trace element and hydrogen study of the Ichinomegata and Oki-Dogo mantle xenoliths (Japan)

The uppermost mantle in back arc regions is the site of complex interactions between partial melting, melt percolation, and fluid migration. To constrain these interactions and evaluate their consequences on geochemical cycles, we carried out an in situ trace element and water study of a suite of spinel peridotite xenoliths from two regions of the Japan back arc system, Ichinomegata (NE Japan) and Oki-Dogo (SW Japan), using LA-ICPMS and FTIR spectrometry, respectively. This study provides the first full dataset of trace element and hydrogen compositions in peridotites including analyses of all their main constitutive silicate minerals: olivine, orthopyroxene and clinopyroxene. The Ichinomegata peridotites sample a LREE-depleted refractory mantle (Mg# olivine=0.90; Cr# spinel=0.07–0.23; Yb clinopyroxene=7.8–13.3×C1-chondrite, and La/Yb clinopyroxene=0.003–0.086×C1-chondrite), characterized by Th-U positive anomalies and constant values of Nb/Ta. The composition of the studied Ichinomegata samples is consistent with that of an oceanic mantle lithosphere affected by cryptic metasomatic interactions with hydrous/aqueous fluids (crypto-hydrous metasomatism). In contrast, the Oki-Dogo peridotites have low Mg# olivine (0.86–0.93) and a broad range of compositions with clinopyroxene showing “spoon-shaped” to flat, and LREE-enriched patterns. They are also characterized by their homogeneous compositions in the most incompatible LILE (e.g., Rb clinopyroxene=0.01–0.05×primitive mantle) and HFSE (e.g., Nb clinopyroxene=0.01–2.16×primitive mantle). This characteristic is interpreted as resulting from various degrees of melting and extensive melt-rock interactions. FTIR spectroscopy shows that olivine in both Ichinomegata and Oki-Dogo samples has low water contents ranging from 2 to 7ppm wt. H2O. In contrast, the water contents of pyroxenes from Ichinomegata peridotites (113–271ppm wt. H2O for orthopyroxene, and 292–347ppm wt. H2O for clinopyroxene) are significantly higher than in Oki-Dogo peridotites (9–35ppm wt. H2O for orthopyroxene, and 15–98ppm wt. H2O for clinopyroxene). This indicates a relationship between melt-rock interaction and water concentrations in pyroxenes. Our study suggests that the water content of the Japan mantle wedge is controlled by the late melt/fluid/rock interactions evidenced by trace element geochemistry: a mechanism triggered by magma-rock interactions may have acted as an efficient dehydrating process in the Oki-Dogo region while the Ichinomegata mantle water content is controlled by slab-derived crypto-hydrous metasomatism.

The distribution of fluid mobile and other incompatible trace elements in orthopyroxene from mantle wedge peridotites

Orthopyroxene is especially suited to decode and testify to the behavior of highly immobile elements during hydrous mantle melting. Laser ablation ICP-MS analyses from orthopyroxene hosted within peridotite from the Coast Range ophiolite (CRO) demonstrates that Group A peridotites (lherzolites) have similar compositions to mid-ocean-ridge abyssal peridotite, whereas other peridotites (Groups B and C; harzburgites) retain depleted signatures, but display ‘spoon-shaped’ enrichments for the light-REE. These patterns are consistent with variable degrees of partial melting of MORB-source asthenosphere initiated within the garnet stability field (<10%) and continuing into the spinel stability field (<15%). A few samples may have been subjected to subsequent melt/rock interaction. The supra-subduction zone (SSZ) environment represented by the CRO is illustrated by enriched fluid mobile elements (Li, Be, B, Pb) in all samples - up to 200× depleted-MORB mantle (DMM). New applications of trace-element addition calculations [Shervais J. and Jean M.M. (2012) Inside the subduction factory: Modeling fluid mobile element enrichment in the mantle wedge above a subduction zone. GCA 95, 270–285] modified for orthopyroxene reveals that tens to hundreds of ppm were added to the DMM-source region. Our purpose is to demonstrate that orthopyroxene, in the absence of clinopyroxene, can be a constructive (and perhaps better) indicator of tectonic environment and magmatic processes that occurred within the North American Cordillera mantle wedge. Through this investigation we have captured all three stages of Coast Range ophiolite petrogenesis: starting with initial SSZ-coupled forearc spreading dominated by decompression melting, to a mature subduction zone with fluid-assisted partial melting, and the transition between the two.

The missing half of the subduction factory: shipboard results from the Izu rear arc, IODP Expedition 350

ABSTRACTIODP Expedition 350 was the first to be drilled in the rear part of the Izu-Bonin, although several sites had been drilled in the arc axis to fore-arc region; the scientific objective was to understand the evolution of the Izu rear arc, by drilling a deep-water volcaniclastic section with a long temporal record (Site U1437). The Izu rear arc is dominated by a series of basaltic to dacitic seamount chains up to ~100-km long roughly perpendicular to the arc front. Dredge samples from these are geochemically distinct from arc front rocks, and drilling was undertaken to understand this arc asymmetry. Site U1437 lies in an ~20-km-wide basin between two rear arc seamount chains, ~90-km west of the arc front, and was drilled to 1804 m below the sea floor (mbsf) with excellent recovery. We expected to drill a volcaniclastic apron, but the section is much more mud-rich than expected (~60%), and the remaining fraction of the section is much finer-grained than predicted from its position within the Izu arc, composed half of ashes/tuffs, and half of lapilli tuffs of fine grain size (clasts <3 cm). Volcanic blocks (>6.4 cm) are only sparsely scattered through the lowermost 25% of the section, and only one igneous unit was encountered, a rhyolite peperite intrusion at ~1390 mbsf. The lowest biostratigaphic datum is at 867 mbsf (~6.5 Ma), the lowest palaeomagnetic datum is at ~1300 mbsf (~9 Ma), and the rhyolite peperite at ~1390 mbsf has yielded a U–Pb zircon concordia intercept age of (13.6 + 1.6/−1.7) Ma. Both arc front and rear arc sources contributed to the fine-grained (distal) tephras of the upper 1320 m, but the coarse-grained (proximal) volcaniclastics in the lowest 25% of the section are geochemically similar to the arc front, suggesting arc asymmetry is not recorded in rocks older than ~13 Ma.

Water and fabric in an ophiolitic peridotite from a supra-subduction zone

Mantle peridotites from modern supra-subduction zones (SSZs) are important windows through which we can investigate the geodynamic processes active in the subduction factory, but they are unfortunately rare and hard to access. Most ophiolitic peridotites stem from SSZ settings and are therefore good candidates to explore the water budget and deformation in mantle wedges. We present here an integrated study of the geochemistry, deformation microstructures and water contents of olivine and orthopyroxene from the Dongqiao harzburgites, central Tibet. These peridotites are characterized by an absence of interstitial clinopyroxene, the partial replacement of orthopyroxene by olivine, the highly magnesian olivine and chromium spinel, and remarkable LREE enrichments. These features suggest that the Dongqiao harzburgites are highly depleted and have undergone a high degree (~40 %) of partial melting, followed by infiltration of and interaction with melt while they were part of the mantle wedge. Olivine and orthopyroxene show prominent plastic deformation microstructures and have developed significant crystallographic preferred orientations (CPOs), suggestive of dislocation creep deformation. Fourier transform infrared analyses show that olivine is essentially dry, while orthopyroxene contains an average water content of 70 ± 14 wt ppm. We propose that orthopyroxene largely retains its in-situ water content from the mantle source, while olivine completely loses its water during emplacement. The orthopyroxene water contents fall into the lower end of the range observed in SSZ peridotites. We consider that the high degree of partial melting and the interaction with a water-undersaturated melt contribute to the relatively lower water contents in orthopyroxene from the Dongqiao harzburgites. Based on experimentally determined hydrogen partition coefficients between olivine and orthopyroxene, the water contents of olivine in the mantle source are calculated to be 7–9 wt ppm (or 114–147 ppm H/Si), which is consistent with the observed A-type CPOs in olivine.