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Central aims of IODP Expedition 352 were to delineate and characterize the magmatic stratigraphy in the Bonin forearc to define key magmatic processes associated with subduction initiation and their potential links to ophiolites. Expedition 352 penetrated 1.2 km of magmatic basement at four sites and recovered three principal lithologies: tholeiitic forearc basalt (FAB), high-Mg andesite, and boninite, with subordinate andesite. Boninites are subdivided into basaltic, low-Si, and high-Si varieties. The purpose of this study is to determine conditions of crystal growth and differentiation for Expedition 352 lavas and compare and contrast these conditions with those recorded in lavas from mid-ocean ridges, forearcs, and ophiolites. Cr# (cationic Cr/Cr+Al) vs. TiO relations in spinel and clinopyroxene demonstrate a trend of source depletion with time for the Expedition 352 forearc basalt to boninite sequence that is similar to sequences in the Oman and other suprasubduction zone ophiolites. Clinopyroxene thermobarometry results indicate that FAB crystallized at temperatures (1142–1190 °C) within the range of MORB (1133–1240 °C). When taking into consideration liquid lines of descent of boninite, orthopyroxene barometry and olivine thermometry of Expedition 352 boninites demonstrate that they crystallized at temperatures marginally lower than those of FAB, between ~1119 and ~1202 °C and at relatively lower pressure (~0.2–0.4 vs. 0.5–4.6 kbar for FAB). Elevated temperatures of boninite orthopyroxene (~1214 °C for low-Si boninite and 1231–1264 °C for high-Si boninite) may suggest latent heat produced by the rapid crystallization of orthopyroxene. The lower pressure of crystallization of the boninite may be explained by their lower density and hence higher ascent rate, and shorter distance of travel from place of magma formation to site of crystallization, which allowed the more buoyant and faster ascending boninites to rise to shallower levels before crystallizing, thus preserving their high temperatures.

The Kyushu‐Palau Ridge (KPR) is a 2600 km long remnant island arc that is separated from the active Izu‐Bonin‐Mariana (IBM) arc by a series of spreading and rift basins. We present 40Ar/39Ar ages and geochemical data for the entire length of the Kyushu‐Palau arc as well as for the conjugate arc which is stranded within the IBM fore arc. New 40Ar/39Ar ages indicate that the KPR was active between 25 and 48 Ma, but the majority of the exposed volcanism occurred in the final phase, between 25 and 28 Ma. Rifting of the Kyushu‐Palau arc to form the Shikoku and Parece Vela basins occurred simultaneously along the length of the arc (circa 25 Ma), and at a similar distance from the trench. Unlike the IBM, the KPR has only limited systematic along‐arc geochemical trends. Two geochemical components within the KPR indicate an origin in the suprasubduction mantle. First, EM‐1‐like lavas are identified in a restricted section of the arc, suggesting a localized heterogeneity. Second, EM‐2‐like arc volcanoes formed on juvenile West Philippine Basin crust, potentially reflecting ingress of mantle from the then active EM‐2 province which lies in the west. Another geochemical heterogeneity is found at the KPR–Daito Ridge intersection where the arc developed on preexisting Cretaceous Daito Ridge crust. The geochemical characteristics at this intersection likely result from the involvement of sub–Daito Ridge lithospheric mantle. Subduction flux beneath the KPR generally matches post–45 Ma Eocene/Oligocene lavas in the IBM fore arc, involving fluids and melts derived from altered igneous crust. Key Points Evaluation of the effects of long‐lived mantle heterogeneity on arc geochemistry Comprehensive geochemical data set on a remnant oceanic island arc Crustal and mantle process at the infant oceanic arc and subsequent maturation

Subduction zones are one of the principal drivers of the modern‐day plate tectonics, but the processes that develop as incipient subduction zones mature are yet to be understood. Finding modern analogs for different stages of subduction infancy remains an outstanding challenge to answer questions on how and when an oceanic subduction zone reaches a quasi‐steady state to become long‐lived. Here, we compare the southern Mariana intra‐oceanic arc, in which near‐trench spreading and infant arc magmatism developed ∼3–4 Myr ago, with the Izu‐Bonin‐Mariana (IBM) proto‐arc magmas that formed during subduction infancy ∼52 Myr ago. Building upon this comparison, we propose that the Southern Mariana may be considered as a modern analog to subduction infancy. The similarities between the Southern Mariana arc magmas and the IBM Eocene proto‐arc magmas suggest that the IBM proto‐arc crust might have formed within 80–90 km from the paleo‐trench (slab at < 90 km paleo‐depth), while the Eocene infant arc was likely located at ∼100 km from the paleo‐trench (∼100–125 km slab paleo‐depth). We use our constraints further to propose a new conceptual model of subduction evolution for IBM. In this model, development of the subduction channel during arc infancy facilitated downward slab penetration and intensified corner flow in the sub‐arc asthenosphere of the mantle wedge. As such, cooling and serpentinization of the fore‐arc mantle might be considered as the principal drivers of subduction maturation and stabilization over the IBM lifetime. Key Points We use the Southern Mariana to place constraints onto the petrogenesis of the Izu‐Bonin‐Mariana (IBM) proto‐arc crust The IBM proto‐arc magmas developed within 90 km from the paleo‐trench and overlaid a paleo‐slab within 90 km depth The subduction channel likely played an important role in the evolution and stabilization of the IBM convergent margin

The Philippine Sea Plate in the West Pacific is a unique natural laboratory to study subduction dynamics and the evolution of upper plate magmatism following subduction initiation. To investigate these processes, International Ocean Discovery Program Expedition 351 recovered at Site U1438, located in a rear‐arc position, a complete sedimentary sequence from recent age to the early Eocene and the top of the underlying volcanic basement. The recovered cores offered the opportunity to study for the first time and in unprecedented detail the styles, products, and timing of the volcanic events that marked the emplacement, growth, and demise of the Kyushu‐Palau volcanic arc following the inception of the Izu‐Bonin‐Mariana subduction. Here, we report a magnetostratigraphy for Site U1438 based on ∼60,000 remanence directions isolated from 1,063 archive half core sections and 429 discrete specimens. We identified 142 magnetic reversals and correlated 115 of them with the geomagnetic polarity timescale. When combined with additional biostratigraphic and geochronological constraints, our magnetostratigraphy allowed construction of a high‐resolution age model for Site U1438 and the determination of changes in sedimentation rates. We show that following subduction initiation at 52–50 Ma and the emplacement of basalts in the rear‐arc at 48.7 Ma, a diffuse volcanism in the rear‐arc (48.4–45.6 Ma) preceded the true emplacement of the Kyushu‐Palau arc at 40.2 Ma, which then grew through four compositionally distinct eruptive phases until 28.8 Ma. Subsequent rollback of the Pacific slab triggered rifting of the arc (28.8–24.3 Ma) and ultimately back‐arc spreading in the Shikoku and Parece Vela basins. Plain Language Summary During 2014 International Ocean Discovery Program Expedition 351 recovered a unique ∼1,200‐m‐thick sedimentary and volcanic sequence at Site U1438 within the Philippine Sea Plate, which forms the upper plate of the famous Izu‐Bonin‐Mariana (IBM) subduction system of West Pacific. The recovered rocks recorded the beginning of the subduction, and the emplacement, growth, and death of the Kyushu‐Palau volcanic arc that developed upon initiation of the IBM subduction, and may therefore hold the key to unravel the complex geological evolution of the entire West Pacific region. In this study, we reconstructed the polarity reversals of the geomagnetic field recorded at Site U1438 and produced a detailed magnetostratigraphy for this sedimentary sequence. We correlated 115 out of 142 identified magnetic reversals from our magnetostratigraphy to the reference geomagnetic polarity timescale to build a high‐resolution age model of Site U1438. This age model helped us to constrain in unprecedented detail the timing of the tectonic and magmatic events that followed IBM subduction initiation at 52–50 Ma. We show that the Kyushu‐Palau volcanic arc established in the upper plate 40.2 Ma, approximately 10–12 million years after the beginning of the Izu‐Bonin‐Mariana subduction, and then required another ∼11 million years until 28.8 Ma to grew to a full size through four compositionally distinct eruptive phases. Subsequent change in the subduction dynamics at 28.8 Ma yielded extension in the upper plate, causing the volcanic arc to split and ultimately shut down at 24.3 Ma and a new ocean basin (Shikoku and Parece Vela basins) to form within the Philippine Sea Plate. Key Points A magnetostratigraphy for the 1,600‐m‐thick sedimentary and volcanic sequence at International Ocean Discovery Program Site U1438 is presented We produced a high‐resolution age model and sedimentation rates for Site U1438 We reconstructed the ages of the emplacement, rise, and death of the Kyushu‐Palau arc in the Philippine Sea Plate

How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts. How the subducted oceanic lithosphere provides fluids and melts to flux the subarc mantle source of arc magmas is controversial. Here the authors use Mo and other isotopes to show serpentinites formed in both the forearc mantle and the subducted lithosphere contribute to generating arc magmas.

Plate tectonics requires the formation of plate boundaries. Particularly important is the enigmatic initiation of subduction: the sliding of one plate below the other, and the primary driver of plate tectonics. A continuous, in situ record of subduction initiation was recovered by the International Ocean Discovery Program Expedition 352, which drilled a segment of the fore-arc of the Izu-Bonin-Mariana subduction system, revealing a distinct magmatic progression with a rapid timescale (approximately 1 million years). Here, using numerical models, we demonstrate that these observations cannot be produced by previously proposed horizontal external forcing. Instead a geodynamic evolution that is dominated by internal, vertical forces produces both the temporal and spatial distribution of magmatic products, and progresses to self-sustained subduction. Such a primarily internally driven initiation event is necessarily whole-plate scale and the rock sequence generated (also found along the Tethyan margin) may be considered as a smoking gun for this type of event. The magmatic progression produced during the initiation of the Izu-Bonin-Marianas subduction zone took place rapidly over 1 million years, but it has been unclear why. Here, using numerical models, the authors show that subduction initiation was dominated by vertical forces, internal to the system itself, progressing to self-sustained subduction.

The mechanism behind the significant Izu‐Bonin‐Mariana (IBM) trench advance is still controversial. We conduct slab subduction numerical models that reproduce the spatio‐temporal tectonic evolution of the Philippine Sea region to investigate whether slab pull from the Ryukyu subduction zone can cross the weakened Philippine Sea Plate and act on the IBM trench. Model results show that the lithospheric strengthening and weakening effects cancel out each other during the rift stage so that the slab pull from the Ryukyu Trench can transmit through the weak fossil spreading centers and intra‐arc rifts and drive the Izu‐Bonin Trench's advance. In contrast, lithospheric weakening overwhelms lithospheric strengthening and impedes stress transfer in the back‐arc spreading stage, suggesting that the slab pull cannot directly pull the Mariana Trench to advance at present. Our study indicates that extreme rheological parameters are not needed for the IBM trench advance, despite the Philippine Sea Plate is weakened. Plain Language Summary Understanding why IBM (Izu‐Bonin‐Mariana) has the globe's most significant trench advance affects not only our understanding of subduction dynamics and lithospheric deformation mechanisms, but also our knowledge on slab rheology, which is key to understanding many fundamental geodynamic questions. A number of models have been proposed to explain the IBM trench advance. These models can be broadly classified into two categories: one is single‐slab subduction, and the other is double‐slab subduction. Single‐slab subduction models usually require extreme rheological parameters that seem inconsistent with those inferred from slab curvature and gravity observations, while double‐slab subduction models need to consider whether the slab‐pull force can transmit through the weak Philippine Sea Plate. The Philippine Sea Plate is strongly weakened due to extensive back‐arc extensional deformations. We conduct numerical models that reproduce the first‐order tectonic evolution of the IBM Subduction Zone to solve this dispute. The model results show that slab pull from the Ryukyu Trench can pass through the weakened Philippine Sea Plate and act on the IBM Trench. Thus, the IBM Trench advance does not require special rheological parameters as suggested by single‐slab models. Key Points Slab pull can pass through the fossil young spreading centers and active intra‐arc rifts, but not active spreading centers Extremely strong slabs or strong subduction‐interface coupling are not needed to explain the IBM trench advance The Mariana Trench advance may be driven by the advance of the Izu‐Bonin Trench at its north

Oceanic plateaus inevitably interact with trench–arc systems during oceanic subduction, but the subduction mechanisms are poorly understood. The Yap trench–arc system is an important part of the western Pacific subduction zone, and is undergoing ultra-slow subduction of the Caroline Plateau. As such, it is ideal for investigating the magmatic–tectonic processes of oceanic plateau subduction. In this study, we document the mineralogy and geochemistry of serpentinized peridotites from the Yap–Mariana Junction. These peridotites can be subdivided into two groups. Group I samples have intermediate to high whole-rock CaO contents and Al2O3/SiO2 values, low chromite Cr# (as low as 0.16) and high chromite Ga/Fe3+# values, and low pyroxene Mg# values relative to Mariana Forearc peridotites. These features are consistent with those of abyssal peridotites. Based on La/Sm ratios, two types of clinopyroxene are identified in group 1 samples. Melting models show that the light rare earth element (LREE)-depleted patterns of type 1 clinopyroxene can be produced by ~  5% fractional melting or ~  6% open-system melting of depleted upper mantle, while the nearly flat REE patterns of type 2 clinopyroxene are related to LREE-enriched melts. These clinopyroxene compositions record melt impregnation and reactions in the mantle beneath the Yap–Mariana Junction. In contrast, group 2 samples have similar compositions to forearc peridotites, such as low whole-rock Al2O3/SiO2 values, and high chromite Cr# (up to 0.73) and low chromite Ga/Fe3+# values. The different trends in plots of chromite Cr#–TiO2 and 100Ti/Fe3+#–Ga/Fe3+# indicate that the group 1 samples experienced reactions with mid-ocean ridge basalt-like melts, whereas the group 2 samples experienced reactions with island arc tholeiite-like melts. The Mg#, TiO2, and Al2O3 values of the melts in equilibrium with chromite in the group 1 samples are consistent with those of the Parece Vela Basin basalts, while those from group 2 samples define a compositional trend similar to arc-related volcanic rocks. Therefore, this region experienced a tectonic transition from a back-arc spreading ridge to a subduction zone. Compared with the Mariana Trench that is undergoing normal oceanic crustal subduction, the absence of volcanic rocks at the Yap Trench are the response to ultra-slow subduction of the Caroline Plateau.

New whole‐rock major and trace element geochemistry from the Leka Ophiolite Complex in Norway is presented and compared to the geochemical evolution and proposed tectonomagmatic processes recorded in the Izu‐Bonin‐Mariana system. These data demonstrate that the Leka Ophiolite Complex formed as forearc lithosphere during subduction initiation. A new high‐precision zircon U‐Pb date on forearc basalt constrains the timing of subduction initiation in the “Leka sector” of the Iapetus Ocean to 491.36 ± 0.17 Ma. The tectonomagmatic record of the Leka Ophiolite Complex captures only the earliest stages of subduction initiation and is thereby distinct from some other Appalachian–Caledonian ophiolites of similar age. The diversity of Appalachian–Caledonian ophiolite records may represent differing preservation and exposure of a variable forearc lithosphere. Plain Language Summary The Leka Ophiolite Complex (LOC) represents a preserved fragment of oceanic crust that formed during subduction in the Iapetus Ocean. Geochemical information recorded in the LOC rocks shows that it formed during the initial phase of subduction. The age of subduction initiation in the Iapetus Ocean is estimated at 491.36 million years ago based on isotopic dating of minerals within the LOC rocks. Other fragments of preserved oceanic crust with similar ages are found in the Appalachian–Caledonian mountains; however, their geochemical information suggests that they may have formed during different stages of the subduction zone development. We consider the variations in the oceanic crustal record to reflect selective preservation of different parts of the variable oceanic crust formed during the development of a subduction zone in the Iapetus Ocean. Key Points The Leka Ophiolite Complex (LOC) preserves a record of geochemical variation from forearc basaltic to boninitic magmatism, reflecting formation during initiation and early evolution of a subduction zone A 491.36 ± 0.17 Ma U–Pb zircon date for an LOC forearc basalt is considered to date subduction initiation in the “Leka sector” of the Iapetus Ocean Differences between the LOC pseudostratigraphy and the model Izu‐Bonin‐Mariana forearc may result from selective preservation of the spatially variable forearc lithosphere in addition to the specific history of formation, obduction, deformation, and uplift/erosion records

Understanding the age and dynamics of the overriding plates allows an assessment of competing subduction initiation hypotheses. The Izu‐Bonin‐Mariana margin in the Western Pacific is a key example of initiation and hence it is important to constrain the age and origin of the oldest igneous crust of the supra‐subduction Philippine Sea Plate. We present geochronological and geochemical data of igneous rocks from the oldest ocean basins of the Philippine Sea Plate: the West Philippine and Palau Basins. Basalts from these basins have enriched geochemical characteristics similar to the EM‐2‐like mantle component found in OIB‐like basalts associated with the Oki‐Daito mantle plume. Ages of basalts from the northernmost West Philippine Basin (WPB) and the Palau Basin range from 43.5 to 50.5 Ma, which is similar to the oldest samples associated with the Oki‐Daito mantle plume (48–50 Ma). This implies that the plume contributed to magmatism from the onset of basin formation. It also provides support for the proposition that rifting of the Mesozoic arc terrane and subsequent seafloor spreading of the WPB was triggered by the arrival of the Oki‐Daito mantle plume at the base of the lithosphere. The age of these Philippine Sea Basins implies that only the Mesozoic Daito Ridge Group and the Gagua Ridge existed as Philippine Sea Plate crust before subduction initiation. A major fault activity after 37 Ma in the northernmost WPB demonstrates that careful reconstruction of the Eocene Philippine Sea Plate is critical to understanding plate dynamics during subduction initiation in the Western Pacific. Plain Language Summary This study investigates the formation of the crust beneath the Philippine Sea Plate. An outcome of this research is a clearer understanding of the plate tectonic configuration in the Western Pacific at the point when volcanic activity started. We collected rock samples from what is thought to be the oldest sections of the West Philippine Basin and the Palau Basin, and determined their age and chemical composition. These rocks were erupted between 43.5 and 50.5 million years ago, and are of a similar age and composition to the volcanic rocks known to be associated with the Oki‐Daito mantle plume (48–50 million years old). These similarities imply that the initial volcanism on the Philippine Sea Plate was triggered by the arrival of the upwelling Oki‐Daito mantle plume. A consequence of the hot and buoyant mantle plume was to stretch the existing crust and develop a new ocean basin by volcanic activity. The new ages for the oldest ocean basins in the Philippine Sea Plate reveal that only the Mesozoic arc terrane existed as the Philippine Sea Plate crust before subduction initiation. This provides new constraints on the conditions under which subduction can initiate between the two plates. Key Points New dating indicates that a major part of the oldest Philippine Sea Plate basins formed after c. 51 Ma Ocean crust of the oldest Philippine Basins is affected by the Oki‐Daito mantle plume Onset of spreading of the West Philippine Basin could have been triggered when the Oki‐Daito plume hit the base of preexisting lithosphere