Izu-Bonin-Mariana Research Articles

Age, Mineral Chemistry, and Geochemistry of Metamorphic Basement Rocks from the Southern Yap Arc, Western Pacific

Studying the basement characteristics and tectonic evolution of the proto-Yap Izu–Bonin–Mariana arc system can provide essential clues for understanding the subduction process in the Western Pacific Ocean. The Yap arc is a part of the Izu–Bonin–Mariana (IBM) arc, but the origin and formation time of the metamorphic basement rocks of the Yap arc still need to be determined. In this paper, we present the first systematic and detailed chronological, mineralogical, whole-rock major element and trace element research on metamorphic basement rocks from four stations in the Southern Yap arc and discuss the origin of the metamorphic basement rocks in the Southern Yap arc. The results show that the single mineral U-Pb ages of titanites and 40Ar/39Ar ages of amphiboles in the Southern Yap arc are 19.3 and 19.4 Ma, respectively, which are consistent within the error ranges, and these ages represent the metamorphic age of the basement rocks. The amphibolite is mainly produced via the metamorphism of back-arc basin basalt (BABB) and forearc basalt (FAB). The temperature and pressure conditions of this metamorphism are 446.5–641.6 °C and 0.24–0.73 GPa, the facies series of which is the medium P/T series. This study provides important data for understanding the Yap subduction system and the early evolution of the proto-IBM Arc.

Tectonic and magmatic evolution of the southernmost Mariana convergent margin: Constraints from geochronology and geochemistry of the Challenger Deep sediments

The southernmost Mariana convergent margin (SMCM) is a distinct segment of the Izu-Bonin-Mariana (IBM) subduction system with unique east-west orientation, however, its nature and tectonic history remains enigmatic. Here, for the first time, we present the detailed geochemical data of detrital minerals and UPb ages of accessory minerals from two sediment samples recovered at the deepest part of Southern Marianna Trench including the Challenger Deep. Detrital magmatic zircon UPb ages can be divided into three groups. Group 1 zircons show a weighted mean age of 50.3 ± 1.2 Ma, contemporary to the published boninites from northern IBM. The εHf(t) of these zircons (11.1–16.4) are also similar to the boninites (12.6–16.8) and volcanic arc rocks (4.2–14.2), but lower than forearc basalts (17.8–22.1), suggesting that these zircons are from boninitic or arc magmatism. The apatite UPb dating obtained a similar age of 55.0 ± 5.5 Ma. Therefore, if they are all from boninites, then subduction initiation magmatism ended at SMCM was likely coeval with northern IBM. Group 2 and Group 3 zircons have εHf(t) values similar to Group 1 zircons, but recorded younger 206Pb/238U ages of 16.8 ± 1.8 Ma and 11.0 ± 0.2 Ma, respectively, which are most likely related to the arc volcanism at West Mariana Ridge (WMR). Additionally, a rutile age of 19.0 ± 0.6 Ma is identified, which is also likely related to WMR. The chemical composition and calculated P-T conditions of clinopyroxenes indicate that they are mainly from the Mariana forearc basalts and arc volcanic rocks. In situ Sr isotopic compositions of plagioclases also show strong affinity to the Mariana forearc basalts and arc volcanic rocks. Therefore, our study indicates that sediments at the deep trench can record the long-term tectonic and magmatic evolution of the convergent margin.

Induced subduction initiation near an ultra-slow spreading ridge: A case study from the Beila ophiolite in the north-central Tibetan Plateau

Subduction initiation of oceanic lithosphere is the key scientific problem of the plate tectonics on Earth. When, where, and how an oceanic subduction began remained obscure because of little geological records. Here, we report a newly documented ophiolitic sequence, including ultra-slow spreading ridge fragments (V1) and associated fore-arc lavas (V2) units in the Bangong-Nujiang suture zone, north-central Tibetan Plateau. The V1 units are controlled by detachment fault with peridotite, gabbro and pillow basalt. Peridotite exhibits uniform characteristics of abyssal peridotite, representing residues after low-degree melting of asthenospheric mantle. Mafic rocks display geochemical features of normal mid-ocean ridge basalt (N-MORB). These features, combined with zircon U-Pb dating, indicate that formation of the oceanic lithosphere occurred along an ultra-slow spreading ridge around 177–170 Ma in this area. The V2 units occur as a complete sequence from bottom to top of fore-arc basalt (FAB), basaltic andesite, boninite, and high-Mg arc-related rocks with formation ages of 160–150 Ma, which directly extruded and overlay on the V1 units. This sequence is identical to those of the Izu-Bonin-Mariana (IBM) fore-arc, and represent magmatic response to subduction initiation (SI). Thus, we propose that the Beila ophiolite preserved an entire process of oceanic transition from ultra-slow spreading oceanic ridge to the SI. Subduction initiation in the Beila area was induced by southward subduction transition after the closure of the northern branch of the Bangong-Nujiang Meso-Tethys Ocean and subsequent continental collision. This study provides direct geological evidence for delineating the geodynamic mechanism of an oceanic initial subduction.

Inferring the Paleo‐Location of Proto‐Arc Magmas During Subduction Infancy in the Izu‐Bonin‐Mariana

AbstractSubduction 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.

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

Slab Pull Drives IBM Trench Advance Despite the Weakened Philippine Sea Plate

AbstractThe 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.

Chemostratigraphy of the extrusive sequence of a Late Cretaceous Neotethyan ophiolite in southern Iran and its significance for the mode and tempo of subduction initiation magmatism and melt evolution in forearc tectonic settings

Chemostratigraphy of lavas in extrusive sequences of modern and ancient forearc environments provides significant information on oceanic crust formation through progressive stages of melt evolution from subduction initiation magmatism (SIM) to arc infancy, and arc maturation-related magmatism. We present here new trace element and radiogenic Sr–Nd–Pb isotopic data from a Late Cretaceous ophiolite in the Outer Zagros Belt of southern Iran and discuss its magmatic development through SIM in a Neotethyan forearc setting. Initial volcanism produced the stratigraphically oldest boninitic (BON) lavas in the Haji-Abad ophiolite whose trace element ratios as well as Pb isotopic signatures indicate a refractory mantle source, which was contaminated by subducting slab-derived fluids. The BON lavas change structurally upwards into island-arc tholeiitic (IAT) lavas and their fractionated felsic derivatives that display isotopic evidence for the contribution of sediment-derived melts in their magmatic source(s). This melt evolution pattern of the Haji-Abad ophiolite marks a major difference from the progressive melt evolution trends reported from the Izu–Bonin–Mariana (IBM) forearc lava sequence, which starts at the bottom with forearc basalts (FAB) phasing up-section into BON and IAT affinity lavas. IAT lavas in the IBM are in turn overlain by calc-alkaline (CAL) lavas. Our geochemical modelling points to a probable existence of a less depleted, pre-subduction mantle whose partial melting probably generated primary magmas before the eruption of boninitic lavas, although the record of such magmas is missing in the Haji-Abad ophiolite. Our comparison of the chemostratigraphy of the Outer Zagros ophiolites with the documented record of subduction initiation generated volcanic rocks in the other Neotethyan ophiolites in the region and the IBM forearc setting strongly suggests that there is no single rule or template for SIM. Supplementary material: Details of analytical procedures and supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6837597 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Heavy magnesium isotopic compositions of basalts erupted during arc inception: Implications for the mantle source underlying the nascent Izu-Bonin-Mariana arc

Basalts formed during the early development of an arc are usually buried, but they record critical information relating to the mantle source of the overriding plate prior to the addition of subducting slab components. Basalts recovered at Site U1438 of International Ocean Discovery Program (IODP) Expedition 351, in the Amami Sankaku Basin (ASB), formed during the transition from forearc basalt (FAB) and boninite eruptions to stratovolcano developments in the Izu-Bonin-Mariana (IBM) arc. In this study, we present magnesium (Mg) isotopic data (δ26Mg) for the ASB basalt core samples, formed at arc inception in the absence of down-going slab components, and report the implications of these data for the characteristics of the mantle sources underlying the proto-IBM arc. The δ26Mg values of the ASB basalts range from −0.21‰ to +0.08‰, with an average value of −0.13 ± 0.07‰. These values are systematically higher than those of mid-ocean ridge basalts (MORBs) (−0.25 ± 0.06‰). No obvious effects of post-eruptive alteration, fractional crystallization, partial melting, or subduction component addition can be identified in the Mg isotopic compositions either of the ASB basalts or their mantle source, given the absence of correlations between the δ26Mg values and the proxies of these processes (e.g., K/Nb, Nb/Zr, Ba/La, Nb/Y, etc.). We conclude that the high δ26Mg values of the ASB basalts are inherited from their mantle source that was enriched in heavy Mg isotopes. Melting of an ultra-depleted, refractory spinel peridotite source containing talc-bearing serpentinite components can explain the origin of the heavy Mg isotopic compositions in the ASB basalts. The presence of water-rich serpentinite components in the mantle provides a new perspective on how a refractory mantle source can undergo partial melting at temperatures and pressures similar to the formation of MORBs without the influence of subducted materials, which has important implications for the initiation of plate subduction. Furthermore, the Mg isotopic contributions of subducted additions and mantle source materials should be carefully discussed in future studies due to the heterogeneity of Mg isotopes in the mantle.

The Redox State of Incipient Oceanic Subduction Zones: An Example From the Troodos Ophiolite (Cyprus)

AbstractTo understand the oxygen fugacity (fO2) during subduction initiation, we examined proto‐arc boninites and associated mantle peridotites from the Troodos ophiolite, Cyprus. The Troodos ophiolite represents an exhumed piece of oceanic lithosphere that formed during subduction initiation in Neo‐Tethys in Late Cretaceous. Dunites and surrounding harzburgites in the Kokkinorotsos podiform chromite deposit of the ophiolite were formed by reactions between peridotites and ascending boninitic magmas. The olivine‐chromite oxybarometers show that the fO2 values (∆QFM −0.41 ± 0.37) of the dunites are slightly higher than those (∆QFM −0.85 ± 0.52) of the harzburgites under mantle conditions, indicating that the mantle fO2 was raised limitedly by percolation of the boninitic magmas. Meanwhile, the boninitic upper pillow lavas in the Margi area also recorded relatively reduced fO2 values (∆QFM −0.52 ± 0.20) during olivine‐chromite crystallization, comparable to the fO2 of mid‐ocean ridge basalts. Shallow‐level processes (i.e., magma ascent, crystallization, crustal assimilation, and degassing) appear to have limited influence on the estimated fO2 values in Troodos. The similarly low fO2 values of the mantle peridotites and boninitic magmas suggest that the Troodos primary boninitic magmas were perhaps not as oxidized as the Izu‐Bonin‐Mariana (IBM) proto‐arc boninitic magmas, though both mantle sources have captured the slab fluids released during subduction inception. This difference might be attributed to the different nature of the slab components (i.e., a more reduced subduction input for Troodos). Our study thus suggests that subducted slabs might not always release oxidized fluids, and the oxidation processes might be different for each nascent subduction zones.

Geochemistry of Waziristan Ophiolite Complex, Pakistan: Implications for Petrogenesis and Tectonic Setting

The Waziristan ophiolite complex (WOC), a segment of paleo-suture zone between the Indo-Pakistan plate to the south-east and the Afghan microplate to the north-west, is primarily composed of serpentinized dunites and serpentinites after harzburgites, crosscut by pyroxenites. The crustal portion dominantly comprises basalts and dolerites, with less abundant plagiogranites. Whole-rock XRF and trace element ICP-MS analyses were undertaken to determine the degree of melt extraction and subsequent geochemical depletion or enrichment of peridotite protoliths. The investigated peridotites are extremely refractory and show high Mg# values (0.88–0.92), low Al2O3 (0.19–0.96 wt%), total alkali values (0.02 &lt; Na2O + K2O ≤ 0.20 wt%), CaO (0.04–0.73 wt%), and TiO2 (0.001–0.017 wt%), but are rich in Cr (up to 3550 ppm) and Ni (up to 2340 ppm). Bulk REE modelling suggests that Waziristan peridotites underwent a high degree (15%–25% melting) of melt extraction in the closed system of spinel-bearing peridotite facies at shallow depths above a subduction zone and, in their chemistry (Sc, REE, Al2O3, and MgO), resemble Izu-Bonin-Mariana (IBM) forearc peridotites. Their U-shaped REE patterns and trace elements resemble peridotites derived from supra-subduction zone (SSZ) settings, especially a forearc setting, and also reflect metasomatism by slab-derived fluids. The ultramafic rocks preserve depletion in REE content (0.03 &lt; ΣREECN &lt; 0.60), indicating the highly depleted nature of refractory peridotites. The basalts (Mg#, 0.48-0.68) are similar in composition to typical boninite and show low to moderate MgO (6.2–13.0 wt%), low total alkali content (0.01–4.45 wt%) and TiO2 (0.13–0.17 wt%), but are higher in Al2O3 (≈11.9 wt%). They show a sub-alkaline affinity and possess geochemical signatures that are transitional from island arc basalt (IAB) magmas to boninitic magmas due to the changes from an extensional to a compressional regime during the initiation of subduction. These geochemical signatures suggest the formation of basalts from melts, resulting from the re-melting of the depleted mantle during intra-oceanic subduction initiation. The higher Th/Nb and V/Ti ratios of the studied basalts further confirm their generation in an SSZ setting characterized by subduction-derived fluids under higher oxidizing conditions. The mafic-ultramafic rocks of the WOC were, therefore, likely formed during intra-oceanic subduction initiation (forearc spreading) in the SSZ setting; they were exhumed along a thrust fault, and obducted onto the forearc region as result of the collision between the Indian plate and the Afghan microplate.

Along-strike island-arc crustal growth rate estimation: case study of the Izu–Bonin–Mariana subduction system

SUMMARYThe island-arc crustal growth rate (IACGR) is the island-arc magma production volume per 1 km width along the arc strike within one million years, and its variations are highly related to slab dehydration and mantle wedge melting. A novel method that includes Earth density modelling, gravity forward and inversion, and arc crustal growth thickness integration is designed to estimate the IACGR. This method can not only estimate the IACGR along the entire arc length but also assess the crustal growth of both remnant and active arcs. Therefore, the estimation result has high coverage and low uncertainty. Here, the Izu–Bonin–Mariana (IBM) subduction zone is taken as a case study region. The estimated time-averaged IACGR along the IBM arc changes between 16 and 59 km3 km−1 Myr−1, with a mean value of 40 km3 km−1 Myr−1, and this result matches the findings of previous studies well. The uncertainties due to crustal thickness inversion are relatively larger than those from flow line reconstruction. The rate results of the Mariana part have lower uncertainties than those of the Izu–Bonin parts since the arc boundaries can be delineated more accurately. The IACGR of the region where a plateau approaches the trench tends to be overestimated because the collision of the plateau with the island arc thickens the island arc crust and bias the uniform pre-existing crustal thickness assumption.

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.

Tectono-magmatic evolution of the Philippine Sea Plate: A review

• Geological and geophysical characteristics of the PSP. • Tectonic evolution of the PSP as a key to the west Pacific trench-arc-basin system. • Subduction initiation, origin of basins and intraplate magmatism. • Relationship between subduction input and arc/back-arc output. The West Pacific Margin incorporates 75% of global marginal basins/back-arc basins. These basins and their adjacent trenches and island arcs constitute a huge trench-arc-basin system created by the interactions among three tectonic plates (the Pacific, Eurasian, and Indian plates) since about 50 Ma. Located among these three plates, the Philippine Sea Plate (PSP) is surrounded by a series of subduction zones, e.g., the Philippine Trench to the west, the Ryukyu Trench and the Nankai Trough to the north, the Izu-Bonin-Mariana (IBM) Trench to the east, and the Yap and Palau trenches to the south. Plate reconstruction shows that the PSP was originally located near the equator, and it has gradually moved northward since the Early Cenozoic. In addition, due to multiple subducted slab rollback events and the related multiple rifting of the proto-IBM Arc, several important tectonic units have been formed, including back-arc basins and relic/active arcs. The back-arc basins include the West Philippine Basin (WPB, spreading at 65-35 Ma), the Shikoku Basin (SKB, spreading at 27-15 Ma), the Parece Vela Basin (PVB, spreading at 29-15 Ma), and the Mariana Trough (MT, spreading from 5 Ma to present), and the main arcs include the Relic Kyushu-Palau Arc (ridge) (KPR, termination at 29 Ma), the Relic West Mariana Arc (ridge) (WMR, termination at 6Ma), the Active IBM Arc (since 11 Ma), and the Yap Arc (since 24 Ma). The geochemistry of these main tectonic units is as follows. The WPB has undergone back-arc spreading and intraplate magmatism, producing many rock types such as back-arc basin basalts (BABBs) similar to normal mid-oceanic ridge basalt (NMORB) and/or enriched mid-oceanic ridge basalt (EMORB) in the central basin, and oceanic island basalts (OIBs) in the Benham Rise and the Urdaneta. In addition, several Late Mesozoic arc terranes (e.g., the Amami Plateau, the Daito Ridge, and the Huatun Basin) developed in the WPB. The SKB mainly contains back-arc basin basalts (BABBs). After termination of the spreading in the SKB, two groups of seamounts with ages of 15-7 Ma formed. One group has undergone subduction. The other group (the Kinan Seamount Chain) is characterized by an OIB and mixed mantle source (Indian Ocean MORB and enriched mantle type one (EM1) geochemical signatures. The PVB basement rock is composed of back-arc basin basalts (BABBs). However, the southern PSP basin is very complex. The Mariana Trough mainly contains BABBs, with island arc basalts (IABs) occurring in the spreading center near the island arc. The IBM Arc mainly contains 52-51 Ma fore-arc basalts, 49-45 Ma boninites, and <45 Ma mature island-arc magmatic rocks. The two relic arcs (the KPR and the WMR) mainly contain IABs. The basement rocks in these back-arc/arc tectonic units are isotopically similar to Indian Ocean MORB. Thus, the West Pacific Ocean, e.g., the PSP, is an ideal place for studying several important geological processes, including interactions between large-scale tectonic plates, subduction initiation, back-arc spreading, intra-oceanic subduction, intraplate magmatism in a back-arc basin, and the geochemical cycle of a subduction zone.

Origin and evolution of the slab fluids since subduction inception in the Izu-Bonin-Mariana: A comparison with the southeast Mariana fore-arc rift

Subduction zones have played a central role in exchanging volatiles (H2O, CO2, S, halogens) between the different Earth's reservoirs throughout its history. Fluids that are released as the subducted plates dehydrate are major agents that transfer these volatiles inside the Earth; but the origins and the compositional evolution of the slab fluids as plates begin to sink are yet to be understood. To explore processes that tool place during subduction infancy, here we examine the compositions of proto-arc magmas from the Izu-Bonin-Mariana (IBM) convergent margin that formed during subduction inception; and we compare these to a modern example of near-trench spreading in the southeast Mariana fore-arc rift (SEMFR). There is a temporal and spatial evolution in the slab fluid composition that is accompanied with a change in the fluid reservoirs, as subduction progresses. During the early stages of the subduction zone, dehydration of the serpentinized subducting mantle likely triggered dehydration and melting of the altered oceanic crust in the amphibolite facies to produce boninites. As the subduction zone matured, the volcanic arc front was displaced away from the trench. The arc magmas captured deeper slab fluids released from the subducted oceanic crust, the sediments and the underlying serpentinized mantle. Dehydration and melting of the subducted sediment became more prevalent with time and increasing slab depth (≥ 100 km) to produce arc magmas. This compositional evolution was associated with a deepening of magma generation, which is likely accompanied with the progressive serpentinization of fore-arc mantle and a slab rollback. Hence, fore-arc mantle serpentinization might have facilitated arc maturation and subduction stabilization throughout the IBM history.

Forearc lava stratigraphy of the Beila Ophiolite, north-central Tibetan Plateau: Magmatic response to initiation of subduction of the Bangong-Nujiang Meso-Tethys Ocean

It has been suggested that boninites and associated volcanic rocks found in supra-subduction zone (SSZ) ophiolites form in a forearc setting, meaning that these rocks could provide crucial constraints on understanding the magmatic response to subduction initiation. Here, we report newly documented forearc basalts (FAB) and boninites from the Beila Ophiolite of the Bangong–Nujiang Suture Zone, north-central Tibetan Plateau. The lava stratigraphy, from bottom to up, includes FAB, basaltic boninites, and boninites, similar to lavas from the Izu–Bonin–Mariana (IBM) forearc. The FAB has geochemical characteristics similar to those of IBM forearc basalts, with nearly flat REE patterns, and shows enrichment in Rb, Ba, Pb, and Sr and depletion in Nb, Ta, and Zr, suggesting an origin by decompression melting of a depleted mantle source with minor input of subducting plate materials. The basaltic boninites and boninites show similar petrological features, as well as porphyritic textures with phenocrysts of olivine, clinopyroxene, and minor orthopyroxene. The basaltic boninites and boninites display uniform major- and trace-element characteristics that are similar to those of typical boninites, with high MgO (>10.78 wt%), Cr, and Ni contents, extremely low TiO2 (<0.21 wt%), rare earth element (REE), and high-field-strength element (HFSE) contents, and U-shaped REE patterns. In addition, the basaltic boninites and boninites have lower ɛNd(t) values than those of the FAB and display variable degrees of influence by slab melts and fluids. Geochemical modeling further suggests that the boninites were generated later during residual highly depleted mantle fluxing by the subducting plate. Together with contemporary boninitic and arc-related magmas along the Bangong-Nujiang Suture Zone and adjacent terranes (the North Lhasa and South Qiangtang), it suggests that the whole Meso-Tethys Ocean was subducted since the Middle Jurassic. Moreover, the Beila lava stratigraphy with FAB overlain by boninite is similar to that found in the IBM forearc and in many ophiolites (e.g., Troodos, Oman, and Xigaze), and represents a continuous geochemical transition from MORB-like to boninitic affinities. This similarity suggests that initiation of subduction might be commonly involved in the formation of ophiolites.

The Fuchuan Ophiolite in South China: Evidence for Modern‐Style Plate Tectonics During Rodinia Breakup

AbstractSubduction initiation processes, such as those evidenced in the Izu‐Bonin‐Mariana (IBM) forearc crust are a key feature of modern‐style plate tectonics. However, the evidence for this process in the Precambrian is limited, due to the scarcity of ophiolite suites and in particular a lack of typical complete ophiolitic sequences similar to those observed in Phanerozoic ophiolites. Mineral analyses of olivine, orthopyroxene, and chrome spinel from the harzburgite and two types of clinopyroxene from the cumulate gabbro of the Fuchuan ophiolite, South China suggest formation in a Neoproterozoic subduction system similar to the IBM. High temperature (1,000–1,200°C), low pressure along with a high degree of melt extraction (&gt;18%) have been recorded in the harzburgite. This evidence, in conjunction with magma evolution from forearc basalt to boninite to hydrous calc‐alkaline magmatism, recorded in the crustal section of the Fuchuan ophiolite indicates a transition from initial to mature subduction in a forearc setting. Statistical analysis of global ophiolites through time suggests that IBM‐like subduction systems may have successfully operated since at least the latest Neoproterozoic (ca. 1,000 Ma) and be pervasive on Earth from ca. 850 Ma. Additional evidence for Neoproterozoic ophiolites from India, Siberia, and Arabian‐Nubian Shield, which are also similar to the IBM forearc ophiolite, suggests that cold and deep subduction became much more common in the Neoproterozoic (ca. 850 Ma) around the periphery of the Rodinia supercontinent. This IBM‐like subduction system may have acted as the geodynamic trigger for the break‐up of Rodinia.

Copper recycling and redox evolution through progressive stages of oceanic subduction: Insights from the Izu-Bonin-Mariana forearc

Addition of subducted materials from the slab to the mantle wedge is often thought to elevate the oxygen fugacity of arc magmas and also to fertilize the mantle wedge in metals for subduction-related Cu-Au deposits. However, it remains controversial if slab-driven metal addition is effective and whether it occurs at all stages of subduction. The Izu-Bonin-Mariana (IBM) forearc preserves a full record of arc development from the initiation of subduction of the Pacific plate to mature arc volcanism. Here, we study the ore-forming and redox-sensitive metal Cu and its isotopes (δ65Cu) in the type-locality forearc basalts (FABs), boninites and high-Mg andesites from the IBM forearc, as well as MORBs from the East Pacific rise. Overall, the FABs display variably high Cu contents and MORB-like δ65Cu, consistent with limited Cu isotopic fractionation during partial melting of typical MORB mantle sources. Beginning with the boninites, the magma products from the IBM record strong slab signals, high water contents and high fO2 (ΔFMQ > +1). However, the oxidized, hydrous boninites and subsequent high-Mg andesites display low Cu contents, and mantle-like Cu/Sc ratios and δ65Cu, with no clear indication of the addition of Cu from the slab. The boninites show noticeably lower TiO2, Yb and Cu contents than the FABs, consistent with refractory mantle sources. Combined with available data from boninites and arc basalts worldwide, our results lead to a general conclusion that subducted oceanic slabs from initial to mature subduction contribute little Cu to the mantle wedge. This is further supported by the compiled Cu contents of arc peridotites. Adding Cu-poor, water-rich slab melts to the mantle wedge at any stage of Pacific oceanic subduction causes limited release of Cu, which remains trapped predominantly by reduced sulfides in the subducting slab. This likely reflects the overall reducing nature of slab materials added to the mantle wedge. However, we propose that the subsequent reaction of such reducing, hydrous slab melts with peridotites and flux melting produce oxidized primitive arc magmas deep in the mantle wedge. The reactive process promotes sulfide dissolution and metal release from the mantle wedge itself to oxidized arc magmas with high sulfur solubility, which explains the inheritance of mantle-like δ65Cu and the sulfide-undersaturated early-stage evolution in boninites and arc basalts. This study elucidates the role of subducted oceanic slabs in metal transfer and redox evolution and implies no significant Cu enrichment in the metasomatized mantle sources of magmatic arcs.

Covariation of Slab Tracers, Volatiles, and Oxidation During Subduction Initiation

AbstractSubduction‐related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.

The Boninite‐like Dolerites in the Xigaze Ophiolites, Tibet: Similar to the MORB‐like Dolerites

AbstractThe Early Cretaceous Xigaze ophiolites (XO) exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet, preserved the structure of the upper mantle and oceanic crust, is interpreted as a definite record of Neo‐Tethys oceanic lithosphere. Many evolution models of the XO have been proposed since the 1980s (Nicolas et al., 1981; Hébert et al., 2012; Maffione et al., 2015). However, the geodynamic environment of the XO whether at a MOR or SSZ remains controversial. As key evidence for the identification of the SSZ ophiolite, the boninite‐like dolerites from the XO are still controversial and poorly constraint (Chen et al., 2003; Bao et al., 2013; Dai et al., 2013). According to previous reports, dolerites were subdivided to MORB‐like type and boninite‐like type (Chen et al., 2003; Dai et al., 2013). Apart from high‐SiO2 (&gt; 52 wt%), high MgO (&gt;8 wt%) and low‐TiO2 (&lt;0.5 wt%), we found the boninite‐like and MORB‐like dolerites are indistinguishable in outcrop, mineral and chemical. They are sills invaded into mantle sequence of the XO, with 50.35–56.80 wt% SiO2, 0.32–1.19 wt% TiO2, 7.39–8.89 wt%Fe2O3T, 5.50–10.42 wt% MgO and Mg# of 0.49–0.74. MORB‐like dolerites display trace element and REE patterns similar to those of most fore‐arc or/and back‐arc basalts from Izu‐Bonin‐Mariana (IBM), i.e. enrichment in large ion lithophile elements (LILEs, e.g. Cs, Rb, Ba, Pb, U) and depletion in high‐field strength elements (HFSE, Nb, Ta) (Fig. 1). Boninite‐like dolerites resemble the MORB‐like dolerites in trace elements and REE patterns, excepting for lower concentrations in REE, but distinct from the boninites discovered from the IBM or Troodos ophiolite (Fig. 1c, d). Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs. Nb/Yb diagram (Fig. 2a), suggesting constant subduction component added to the mantle source (Pearce et al., 1995). We chose depleted MORB source Mantle (DMM, Workman and Hart, 2005) as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean (2012). The results show that MORB‐like and boninite‐like dolerites can be explained by non‐batch fractional melting (≥15%) of a depleted spinel peridotite with few subduction‐derived components (Fig. 2b). Therefore, the boninite‐like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab‐derived fluid and the XO cannot be used as analogs for the subduction initiation.