Bonin Arc Research Articles

Uniform processes of melt differentiation in the central Izu Bonin volcanic arc (NW Pacific)

Abstract The intra-oceanic Izu Bonin arc (NW Pacific) has produced a bimodal spectrum of melts with maxima in the basaltic andesitic ( c . 53–54 wt% SiO 2 ) and rhyolitic range ( c . 70–72 wt% SiO 2 ) since arc inception c . 48–49 million years ago. Composition of phenocrysts and accessory minerals from 21 contemporaneous fallout tephras from ODP Site 782A confirm the bimodality and uniformity of the erupted melts. The basaltic andesite melts equilibrated with calcic plagioclase ( c . An 70–95 ), high-Mg# clino- and orthopyroxene and low-Ti titanomagnetite. Dacitic and rhyolitic melts crystallized sodic plagioclase ( c . An 40–60 ), low-Mg# clino- and orthopyroxene, apatite, Ti-rich titanomagnetite in addition to occasional ilmenite and amphibole. The Izu melts are inferred to crystallize at oxygen fugacities between c . 0 to +2.5 log 10 units relative to FMQ, at temperatures between c . 775° and 1100 °C and at pressures between c . 300 and c . 1100 MPa, corresponding to c . 5–35 km lithospheric depth. The compositional uniformity of the tephra layers, which are spaced on average 230 ± 380 ka apart, suggest uniform processes of differentiation since at least c . 42 Ma ago. The tephra record shows no indication of periodic or progressive crustal growth that might correlate with the alternate periods of arc formation, arc rifting or backarc spreading, or would suggest an increasingly efficient ‘crustal filter’ with time. The tephra data tentatively conform to a model where crust grows steadily through intrusions of mafic and evolved melt body batches whereby buoyancy controls the level of solidification. While the tephra compositions demonstrate the uniformity of the processes of melt formation and differentiation through time, the data do not permit the differentiation processes themselves to be constrained. These may comprise fractional crystallization, crustal fusion, fusion of non-peridotitic sub-crustal lithologies, or any combination of these processes.

Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu–Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab

The arc-front volcanoes of Sumisu (31 58N, 1408E) andTorishima (30 58N, 140 38E) in the central Izu^Bonin arc are similar in size and rise as relatively isolated edifices from the seafloor.Together they provide valuable along-arc information about magma generation processes. The volcanoes have erupted low-K basalts originating from both wet and dry parental basaltic magmas (low-Zr basalts and high-Zr basalts, respectively). Based on models involving fluid-immobile incompatible element ratios (La/Sm), the parental basalts appear to result from different degrees of partial melting of the same source mantle ( 20% and 10% for wet and dry basalt magmas, respectively). Assuming that the wet basalts contain greater abundances of slab-derived components than their dry counterparts, geochemical comparison of these two basalt types permits the identification of the specific elements involved in fluid transport from the subducting slab. Using an extensive set of new geochemical data fromTorishima, where the top of the downgoing slab is about 100 km deep, we find that Cs, Pb, and Sr are variably enriched in the low-Zr basalts, which cannot be accounted for by fractional crystallization or by differences in the degree of mantle melting. These elements are interpreted to be selectively concentrated in slab-derived metasomatic fluids. Variations in K, high field strength element and rare earth element concentrations are readily explained by variations in the degree of melting between the lowand high-Zr basalts; these elements are not contained in the slab-derived fluids. Rb and Ba exhibit variable behaviour in the low-Zr basalts, ranging from immobile, similar to K, to mildly enriched in some low-Zr basalts.We suggest that the K-rich mica, phengite, plays an important role in determining the composition of fluids released from the downgoing slab. In arcfront settings, where slab depth is 100 km, phengite is stable, and the fluids released from the slab contain little K. In back-arc settings, however, where the slab is at 100^140 km depth, phengite is unstable, and K-rich fluids are released.We conclude that cross-arc variations in the K content of arc basalts are probably related to differing compositions of released fluids or melts rather than the widely held view that such variations are controlled by the degree of partial melting.

Origin of distinct silicic magma types from the Guachipelín Caldera, NW Costa Rica: Evidence for magma mixing and protracted subvolcanic residence

Lower Pleistocene pyroclastic deposits in NW Costa Rica represent sequential eruptions of distinct dacitic–rhyolitic (69–79% SiO 2) magmas from the Guachipelín Caldera that were emplaced in a period of 0.75 myr. This southern segment of the Central American arc, the Chorotega block, evolved on the modified Caribbean Large igneous province, an overthickened oceanic plateau; silicic magmatism is uncommon in settings such as this, void of continental crust. Previously, models have proposed the generation of silicic magmas in this type of setting by the partial melting or extreme crystal fractionation of intermediate source rocks (e.g. [Tamura, Y., Tatsumi, Y., 2002. Remelting of an andesitic crust as a possible origin for rhyolite magma in oceanic arcs: an example from the Izu–Bonin arc. Journal of Petrology 43, 1029–1047]). In fact, recent melting experiments performed by Sisson et al. [Sisson, T.W., Ratajeski, K., Hankins, W.B., Glazner, A.F., 2005. Voluminous granitic magmas from common basaltic sources. Contributions to Mineralogy and Petrology 148 (5), 542–565] suggest that melting of andesite or high-K basalts, common along the Central American volcanic arc, could produce silicic melts with the characteristic alkali content of these types of eruptives. Geochemical evaluation of the pumice-clasts through the complete sequence of Guachipelín eruptive products allows for subdivision into two major silica groups. First, the dacites are (66–72 wt.% SiO 2 pumice clast; 75.2–76.5 wt.% SiO 2 glass) orthopyroxene-bearing with plagioclase (An 36–65) ± magnesiohornblende, which are found intermittently from the base of the sequence through the uppermost ash-flow deposit. Second, the rhyolites are (72–79 wt.% SiO 2 pumice clast; 74.5–78.6 wt.% SiO 2 glass) biotite and quartz-bearing, with plagioclase (An 24–51) and magnesiohornblende. Chemical heterogeneities were discovered within each of these groups through evaluation of incompatible trace element ratios. Consequently, seven units were defined using the Nb/Ta ratio, which has a significantly wide range, from 3.8 to 29.4, and is interpreted to reflect contributions from more than one intermediate source. Most of these units also display extensive geochemical and petrological evidence of mixing (oxygen isotopes, phenocryst composition, banded pumice clasts, and different glass compositions in single pumice fragments). The distinct geochemical characteristics of these deposits and the stratigraphic occurrence of each flow unit require both limited fractional crystallization within units coupled with melt extractions and extensive mixing. Hence, individual melts would reside in relatively close proximity in subvolcanic zones for extended periods prior to the inception of a particular eruptive event. Considering the temporal (< 0.75 Ma) and spatial (single caldera) constraints of this sequence of eruptions, significant chemical variations of the magmas have occurred.

New seismological constraints on growth of continental crust in the Izu-Bonin intra-oceanic arc

Research Article| November 01, 2007 New seismological constraints on growth of continental crust in the Izu-Bonin intra-oceanic arc Shuichi Kodaira; Shuichi Kodaira 1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Japan Search for other works by this author on: GSW Google Scholar Takeshi Sato; Takeshi Sato 1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Japan Search for other works by this author on: GSW Google Scholar Narumi Takahashi; Narumi Takahashi 1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Japan Search for other works by this author on: GSW Google Scholar Seiichi Miura; Seiichi Miura 1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Japan Search for other works by this author on: GSW Google Scholar Yoshihiko Tamura; Yoshihiko Tamura 2Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Natsushima 2-15, Yokosuka, Japan Search for other works by this author on: GSW Google Scholar Yoshiyuki Tatsumi; Yoshiyuki Tatsumi 2Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Natsushima 2-15, Yokosuka, Japan Search for other works by this author on: GSW Google Scholar Yoshiyuki Kaneda Yoshiyuki Kaneda 2Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Natsushima 2-15, Yokosuka, Japan Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (11): 1031–1034. https://doi.org/10.1130/G23901A.1 Article history received: 19 Mar 2007 rev-recd: 11 May 2007 accepted: 29 Jun 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Shuichi Kodaira, Takeshi Sato, Narumi Takahashi, Seiichi Miura, Yoshihiko Tamura, Yoshiyuki Tatsumi, Yoshiyuki Kaneda; New seismological constraints on growth of continental crust in the Izu-Bonin intra-oceanic arc. Geology 2007;; 35 (11): 1031–1034. doi: https://doi.org/10.1130/G23901A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The process by which continental crust has formed is not well understood, though such crust mostly forms at convergent plate margins today. It is thus imperative to study modern intra-oceanic arcs, such as those common in the western Pacific Ocean. New seismic studies along the representative Izu-Bonin intra-oceanic arc provide unique along-strike images of arc crust and uppermost mantle to complement earlier, cross-arc lithospheric profiles. These reveal two scales (1000–10 km scale) of variations, one at the scale of the Izu versus Bonin (thick versus thin) arc crust, the other at the intervolcano (∼50 km) scale. These images show that: (1) the bulk composition of the Izu-Bonin arc crust is more mafic than typical continental crust, (2) the middle crust with seismic velocities similar to continental crust is predominantly beneath basaltic arc volcanoes, (3) the bulk composition beneath basaltic volcanoes changes little at thick and thin arc segments, and (4) a process to return lower crustal components to the mantle, such as delamination, is required for an arc crust to evolve into continental crust. Continued thickening of the Izu-Bonin crust, accompanied by delamination of lowermost crust, can yield velocity structure of typical continental crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early stages in the evolution of Izu–Bonin arc volcanism: New age, chemical, and isotopic constraints

A remarkable record of early arc volcanism in the Izu–Bonin–Mariana (IBM) arc is exposed in and around the Bonin Islands, an uplifted segment of the IBM forearc. New 40Ar/ 39Ar dating results imply that the boninitic volcanism on Chichijima Island occurred in a brief period during Eocene time, between 46–48 Ma. A slightly younger volcanic succession is identified along the Bonin Ridge, including 44.74 ± 0.23 Ma high-Mg andesite from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, 44.0 ± 0.3 Ma tholeiitic to calcalkaline andesite from Hahajima Island, and 3 samples of andesite collected by the submersible SHINKAI 6500 from the Bonin Ridge Escarpment (BRE) that range in age from 41.84 ± 0.14 to 43.88 ± 0.21 Ma. Four SHINKAI 6500 dives (YK 04–05) on the BRE mapped an elongated constructional volcanic ridge atop the escarpment; we observed steeply west-dipping volcaniclastic debris flows shed from the summit of this ridge into the Ogasawara Trough to the west. These dives recovered fresh andesitic clasts from debris flows along the northern segment of the ridge, and high-Mg andesite lava blocks and Nummulitic limestone of middle Eocene age from the escarpment northwest of Chichijima. Our results also confirm previous inferences that melting of depleted mantle at shallow levels beneath the length of the arc with the aid of hydrous fluids from newly subducted slab to produce boninitic volcanism occurred nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. BRE–Mikazukiyama Formation–Hahajima andesites represent a transitional stage from forearc spreading (represented by ODP site 786-Chichijima boninites) and the stable, mature arc that developed in the Oligocene. These OPX-bearing high-Mg or tholeiitic to calcalkaline andesites were erupted along the BRE, as the arc magmatic axis localized and retreated from the trench.

Geochemical characteristics of Tertiary Sagara oil from an active forearc basin, Shizuoka, Japan

The Sagara oil field is located in the Neogene Kakegawa Basin, close to the Izu collision zone at the junction between the main Japanese Islands and the Izu–Bonin Arc. The Sagara oil field is one of the few oil fields situated in a forearc basin on the Pacific side of Japan and is present in a sedimentary basin with poor oil-generating potential. Several crude oils from Sagara oil field were investigated to infer their origin. Organic geochemical characteristics of Sagara oils showed the influences of light biodegradation, migration-contamination, and migration-fractionation. The maturity levels of Sagara oils evaluated based on abundant alkylnaphthalenes corresponded to 0.9–1.2% vitrinite reflectance. Sagara oils were characterized by significant amounts of higher plant biomarkers, a high pristane/phytane ratio and an absence of organic sulphur compounds, suggesting a siliciclastic source rock deposited under nearshore to fluvial–deltaic environments. Numerous faults and fractures in the active forearc basin provided excellent conduits and facilitated upward migration of light hydrocarbons generated at greater depth in the Kakegawa Basin.

Crustal structure of the ocean-island arc transition at the mid Izu-Ogasawara (Bonin) arc margin

Abstract Wide-angle refraction experiments were conducted to reveal the crustal structure at the transition between the intra-oceanic island arc crust of the mid Izu-Ogasawara (Bonin) arc and the backarc oceanic crust of the Shikoku Basin. The island arc crust consists of an upper crust about 5 km thick with a P-wave velocity &lt;6.0 km/s, a middle crust 5 km thick with a P-wave velocity of 6.0–6.3 km/s, and a lower crust 10 km thick with a P-wave velocity of 6.8–7.2 km/s. The total crustal thickness is about 20 km. The thickness thins to approximately 6 km over a distance of 30 km at the western margin of the Izu-Ogasawara arc (IOA). These features are very similar to those of the northern IOA, which indicates that the crustal structure is relatively constant within 200 km at the northern and mid IOA. The Kinan Escarpment, a 500-km-long fault with a maximum offset of 800 m, characterizes the transition zone between the IOA and Shikoku Basin. The seismic crustal model indicates that the escarpment is a fault which tears the whole oceanic crust along the western margin of the IOA. However, no significant differences exist in the crustal structure on either side of the escarpment, and the Kinan Escarpment seems to be a zone of the structural weakness from its birth.

Wet and dry basalt magma evolution in Torishima volcano, Izu–Bonin arc, Japan: The possible role of phengite

Wet and dry basalt magma evolution in Torishima volcano, Izu–Bonin arc, Japan: The possible role of phengite

Origin of silicic magmas and the compositional gap at Sumisu submarine caldera, Izu–Bonin arc, Japan

Sumisu caldera volcano, Izu–Bonin arc, Japan, is dominated by the bimodal assemblage of basalt–basaltic andesite (< 55 wt.% SiO 2) and dacite–rhyolite (> 66 wt.% SiO 2). Andesites (56–61 wt.% SiO 2) are minor in volume and a compositional gap of ∼ 6 wt.% SiO 2 exists between the andesites and the dacites. Most andesites contain sieve-textured plagioclase, and some dacites have resorbed plagioclase containing many glass inclusions. Some orthopyroxene phenocrysts in andesites and dacites have overgrowth rims with a high Ca content, a feature consistent with crystallization from a higher-temperature magma than their cores. Dacites, rhyolites and glasses in the matrix and plagioclases possess compositions similar to those of partial melts produced by dehydration-melting of calc-alkaline andesite (Tanzawa tonalite) at low pressure (3 kbar). Phenocryst assemblages of these andesites, dacites and rhyolites resemble restite phase assemblages of the dehydration melting. Prolonged fractionation or partial melting of basalts, in contrast, is untenable based on petrography and fractionation calculations. We suggest that the silicic magmas at Sumisu caldera volcano have been produced by dehydration melting of solidified calc-alkaline andesite (protolith) in the upper to middle crust, which segregated efficiently from restite crystals. In rare situations, reheating and en-mass remobilization permitted the same andesites to erupt as andesites (56–61 wt.% SiO 2). Thus, Sumisu has a relatively narrow temperature difference (900–1100 °C) associated with a large compositional range (56–74 wt.% SiO 2).

Variation in the mantle sources of the northern Izu arc with time and space — Constraints from high-precision Pb isotopes

We present new ages and geochemical data for back-arc lavas from the northern Izu–Bonin arc 33–35° N including high-precision double-spike Pb isotope measurements. The northern part of the Izu–Bonin arc is distinct from the rest of the arc as it lacks active rifting behind the volcanic front but it does have Quaternary volcanoes (e.g. Niijima). However, in common with the rest of the arc the northern section has back-arc seamount chains and NE–SW volcanic ridges. 40Ar/ 39Ar dating of volcanic rocks has revealed that Quaternary volcanism is limited to within 40 km of the volcanic front. Miocene and Pliocene volcanism extended as far as 120 km west of the current volcanic front along the back-arc seamounts and ridges. The chemical characteristics of back-arc volcanism are significantly different in the Pliocene–Quaternary compared to the Miocene. Opx–cpx andesite and hornblende andesite are dominant in Miocene volcanic centres, while Pliocene and Quaternary centres are characterized by basalt and rhyolite. Miocene volcanic centres show a marked correlation between Th/Ce and Pb and Nd isotopes. Generally, these lavas have higher Δ7/4 and lower 143Nd/ 144Nd with increasing Th/Ce. In contrast, the Pliocene and Quaternary lavas show little, if any, Th enrichment relative to potential mantle sources and no correlation with isotopes. These correlations suggest that partial melt of sediment from the subducting slab was an important component in the Miocene, whereas, the Pliocene–Quaternary volcanic centres show little evidence of sediment melt and are restricted to a contribution of fluid from altered oceanic crust and fluid from sediment. Quaternary volcanoes at similar distances from the volcanic front are calculated to have similar compositions and amounts of slab-derived fluid in their sources. However, on Pb–Pb isotope plots, they lie closer to the NHRL towards south (i.e., Δ8/4 decreases towards south). Almost parallel but distinct trends on Pb–Pb plots imply differing mantle sources depending on the northward position within the arc. This variation in mantle wedge composition is seen to continue into the central part of the arc [Ishizuka, O., Taylor, R.N., Milton, J.A., Nesbitt, R.W., Fluid–mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes, Earth Planet. Sci. Lett. 211 (2003a) 221–236]. The observed along-arc isotopic variation can be explained by the presence of two mantle components; a MORB source observed in the back-arc basins of the Philippine Sea Plate (e.g. [Hickey-Vargas, R., Origin of the Indian Ocean-type isotopic signature in basalts from Philippine Sea Plate spreading centers: An assessment of local versus large-scale processes, J. Geophys. Res. 103 (1998a) 20963–20979]) and a more Pacific MORB-like source. In the northern part of the Izu–Bonin arc, Miocene to Pliocene volcanism was active over a much wider back-arc region than present, and these lavas show the greatest influence of sediment melt generated at slab depths of > 250 km. However, in the Quaternary active volcanism was constrained to the vicinity of the volcanic front. This Quaternary magmatism can be related to the release of fluid from the slab at relatively shallow depths (110–200 km). These spatial and temporal chemical variations could be related to either the change of the angle of subducting Pacific Plate and/or changes in the regional stress regime (e.g. resulting from the collision of the Izu–Bonin arc with the Honshu arc or in association with initiation of rifting in the south).

Integrated description of deformation modes in a sedimentary basin: A case study around a shallow drilling site in the Mizunami area, eastern part of southwest Japan

Abstract Tectonic episodes in a sedimentary basin are described on the basis of an integrated study combining reflection seismic interpretation, drilling survey and paleomagnetism. A shallow inclined borehole penetrated a fault shown by reflection seismic and geological surveys in the Mizunami area, in the eastern part of southwest Japan. Paleomagnetic measurements were carried out on core samples successfully oriented using side‐wall image logging of structural attitude. At six horizons, stable characteristic remanent magnetization (ChRM) was confirmed through thermal and alternating field demagnetization tests, which were carried by magnetite with minor amounts of high coercivity minerals, as revealed by experiments of stepwise acquisition of isothermal remanent magnetization. After correction of multiphase deformation inferred from borehole structural analysis, ChRM directions were combined with previous data and confirmed an easterly deflection reflecting the coherent clockwise rotation of the arc before the Middle Miocene. Compilation of reliable paleomagnetic data described differential rotation of the eastern part of southwest Japan raised by collision of the Izu–Bonin Arc since the Middle Miocene. The present study suggests that (i) the Mizunami area is adjacent to a highly deformed zone bounded by the Akaishi Tectonic Line (ATL), and (ii) forearc deformation of southwest Japan is localized around the ATL, which is quite different from gradual bending on the back‐arc side without remarkable crustal break related to the collision event.

Stable isotope compositions of serpentinite seamounts in the Mariana forearc: Serpentinization processes, fluid sources and sulfur metasomatism

The Mariana and Izu–Bonin arcs in the western Pacific are characterized by serpentinite seamounts in the forearc that provide unique windows into the mantle wedge. We present stable isotope (O, H, S, and C) data for serpentinites from Conical seamount in the Mariana forearc and S isotope data for Torishima seamount in the Izu–Bonin forearc in order to understand the compositions of fluids and temperatures of serpentinization in the mantle wedge, and to investigate the transport of sulfur from the slab to the mantle wedge. Six serpentine mineral separates have a restricted range of δ18O (6.5–8.5‰). Antigorite separates have δD values of − 29.5‰ to − 45.5‰ that reflect serpentinization within the mantle wedge whereas chrysotile has low δD values (− 51.8‰ to − 84.0‰) as the result of re-equilibration with fluids at low temperatures. Fractionation of oxygen isotopes between serpentine and magnetite indicate serpentinization temperatures of 300–375 °C. Two late cross-fiber chrysotile veins have higher δ18O values of 8.9‰ to 10.8‰ and formed at lower temperatures (as low as ∼100 °C). Aqueous fluids in equilibrium with serpentine at 300–375 °C had δ18O = 6.5–9‰ and δD = − 4‰ to − 26‰, consistent with sediment dehydration reactions at temperatures < 200 °C in the subducting slab rather than a basaltic slab source. Three aragonite veins in metabasalt and siltstone clasts within the serpentinite flows have δ18O = 16.7–24.5‰, consistent with the serpentinizing fluids at temperatures < 250 °C. δ13C values of 0.1–2.5‰ suggest a source in subducting carbonate sediments. The δ34S values of sulfide in serpentinites on Conical Seamount (− 6.7‰ to 9.8‰) result from metasomatism through variable reduction of aqueous sulfate (δ34S = 14‰) derived from slab sediments. Despite sulfur metasomatism, serpentinites have low sulfur contents (generally < 164 ppm) that reflect the highly depleted nature of the mantle wedge. The serpentinites are mostly enriched in 34S (median δ34Ssulfide = 4.5‰), consistent with a 34S-enriched mantle wedge as inferred from arc lavas.

Geochemical discrimination of tectonic setting for Devonian basalts of the Yarrol Province of the New England Orogen, central coastal Queensland: An empirical approach*

Fault blocks and inliers of uppermost Silurian to Middle Devonian strata in the Yarrol Province of central coastal Queensland have been interpreted either as island-arc deposits or as a continental-margin sequence. They can be grouped into four assemblages with different age ranges, stratigraphic successions, geophysical signatures, basalt geochemistry, and coral faunas. Basalt compositions from the Middle Devonian Capella Creek Group at Mt Morgan are remarkably similar to analyses from the modern Kermadec Arc, and are most consistent with an intra-oceanic arc associated with a backarc basin. They cannot be matched with basalts from any modern continental arc, including those with a thin crust (Southern Volcanic Zone of the Andes) or those built on recently accreted juvenile oceanic terranes (Eastern Volcanic Front of Kamchatka). Analyses from the other assemblages also suggest island-arc settings, although some backarc basin basalt compositions could be present. Arguments for a continental-margin setting based on structure, provenance, and palaeogeography are not conclusive, and none excludes an oceanic setting for the uppermost Silurian to Middle Devonian rocks. The Mt Morgan gold–copper orebody is associated with a felsic volcanic centre like those of the modern Izu–Bonin Arc, and may have formed within a submarine caldera. The data are most consistent with formation of the Capella Creek Group as an intra-oceanic arc related to an east-dipping subduction zone, with outboard assemblages to the east representing remnant arc or backarc basin sequences. Collision of these exotic terranes with the continent probably coincided with the Middle–Upper Devonian unconformity at Mt Morgan. An Upper Devonian overlap sequence indicates that all four assemblages had reached essentially their present relative positions early in Late Devonian time. Apart from a small number of samples with compositions typical of spreading backarc basins, Upper Devonian basalts and basaltic andesites of the Lochenbar and Mt Hoopbound Formations and the Three Moon Conglomerate are most like tholeiitic or transitional suites from evolved oceanic arcs such as the Lesser Antilles, Marianas, Vanuatu, and the Aleutians. However, they also match some samples from the Eastern Volcanic Front of Kamchatka. Their rare-earth and high field strength element patterns are also remarkably similar to Upper Devonian island arc tholeiites in the ophiolitic Marlborough terrane, supporting a subduction-related origin and a lack of involvement of continental crust in their genesis. Modern basalts from rifted backarc basins do not match the Yarrol Province rocks as well as those from evolved oceanic arcs, and commonly have consistently higher MgO contents at equivalent levels of rare-earth and high field strength elements. One of the most significant points for any tectonic model is that the Upper Devonian basalts become more arc-like from east to west, with all samples that can be matched most readily with backarc basin basalts located along the eastern edge of the outcrop belt. It is difficult to account for all geochemical variations in the Upper Devonian basalts of the Yarrol Province by any simplistic tectonic model using either a west-dipping or an east-dipping subduction zone. On a regional scale, the Upper Devonian rocks represent a transitional phase in the change from an intra-oceanic setting, epitomised by the Middle Devonian Capella Creek Group, to a continental margin setting in the northern New England Orogen in the Carboniferous, but the tectonic evolution must have been more complex than any of the models published to date. Certainly there are many similarities to the southern New England Orogen, where basalt geochemistry indicates rifting of an intra-oceanic arc in Middle to Late Devonian time.

Stable isotopic compositions of methane and carbon monoxide in the Suiyo hydrothermal plume, Izu–Bonin arc: Tracers for microbial consumption/production

Effluent hydrothermal water samples in and around the water column of the Suiyo seamount caldera (ca. 2 km diameter), Izu–Bonin arc, were taken to determine concentrations and stable carbon isotopic compositions ( δ 13C) of methane (CH 4) and carbon monoxide (CO) in the plume. Venting fluids on the caldera floor (ca. 1370 m depth) were also examined, using the manned submersible Shinkai 2000, to compare the chemical composition and isotopic composition of CH 4 and CO with those in the hydrothermal plume. Strong CH 4 enrichment was detected not only in the water column within the caldera, but also on those outside the caldera at the depth of ca. 1100 m, the sill depth of the caldera wall. Within the plume, we also detected significant CO enrichment. The δ 13C of CO in the plume, however, exhibited highly 13C-depleted values (− 110‰ VPDB to − 60‰ VPDB) compared with those in seafloor venting hydrothermal fluids (around − 31‰ VPDB). Besides, the plume samples exhibited higher CO / CH 4 ratios depending on the distance from the seafloor venting site. We conclude that in situ microbial activity in the plume causes the CO enrichment in the hydrothermal plume. The present results suggest that CO enrichment in deep sea water could act as a new tracer for unidentified seafloor hydrothermal activities, especially for detecting plumes distant from venting sites, as well as for quantifying in situ microbial activity related to CO production in hydrothermal plumes.

Magma differentiation rates from ( 226Ra / 230Th) and the size and power output of magma chambers

We present a mathematical model for the evolution of the ( 226Ra / 230Th) activity ratio during simultaneous fractional crystallization and ageing of magma. The model is applied to published data for four volcanic suites that are independently known to have evolved by fractional crystallization. These are tholeiitic basalt from Ardoukoba, Djibouti, MORB from the East Pacific Rise, alkali basalt to mugearite from Vestmannaeyjar, Iceland, and basaltic andesites from Miyakejima, Izu–Bonin arc. In all cases ( 226Ra / 230Th) correlates with indices of fractional crystallization, such as Th, and the data fall close to model curves of constant fractional crystallization rate. The best fit rates vary from 2 to 6 × 10 − 4 yr − 1 . Consequently, the time required to generate moderately evolved magmas ( F ≤ 0.7) is of the order of 500 to 1500 yrs and closed magma chambers will have lifetimes of 1700 to 5000 yrs. These rates and timescales are argued to depend principally on the specific power output (i.e., power output per unit volume) of the magma chambers that are the sites of fractional crystallization. Equating the heat flux at the EPR to the heat flux from the sub-axial magma chamber that evolves at a rate of ca. 3 × 10 − 4 yr − 1 implies that the magma body is a sill of ca. 100 m thickness, a value which coincides with independent estimates from seismology. The similarity of the four inferred differentiation rates suggests that the specific power output of shallow magma chambers in a range of tectonic settings covers a similarly narrow range of ca. 10 to 50 MW km − 3 . Their differentiation rates are some two orders of magnitude slower than that of the basaltic Makaopuhi lava lake, Hawaii, that cooled to the atmosphere. This is consistent with the two orders of magnitude difference in heat flux between Makaopuhi and the East Pacific Rise. ( 226Ra / 230Th) data for magma suites related by fractional crystallization allow the magma differentiation rate to be estimated and, from this, the thermal budget of the magma chamber addressed, and where an independent measurement of heat flux exists, to place constraints on the size of the magma chamber. Such results have the potential to constrain the likely timescale and size of future eruptions of evolved magmas.

Heterogeneity within the subducting Pacific slab beneath the Izu–Bonin–Mariana arc: Evidence from tomography using 3D ray tracing inversion techniques

Distinct zones of seismic heterogeneity along the Izu–Bonin–Mariana arc have been investigated using complimenting regional bulk sound, shear wave speed, and P-wave tomographic images. The distribution of seismic anomalies and the inferred geometry of the subducting Pacific plate have been modelled in unprecedented detail using joint tomography and new P-wave model using a 3D inversion algorithm. The use of 3D ray tracing techniques and smaller cell parameterizations have greatly enhanced the resolution of gradients, therefore, the models show much more detail about the structure and physical properties of the subduction zone. The well-defined features from the multiple wave speed images are used to elucidate the distinct morphology change between slab beneath the Izu–Bonin (horizontal slab) and Mariana (vertical) arcs and the distribution of physical properties in the mantle and the subducting oceanic lithosphere. Changes in physical properties within the slab tear at the southern end of the Izu–Bonin arc, identified as a “gap or thinning” in the tomographic images, could be the result of the distortion of the Pacific plate as its shape transforms between near horizontal to near vertical, a decrease in the rigidity and strength of the lithosphere, the subduction of the Marcus–Necker Ridge, change in subduction velocity, or a combination of all these factors. Strong, slow anomalies exist both, in the mantle wedge, above and below the slab beneath the Izu–Bonin arc but are not present beneath the Mariana arc. At the junction between the Izu–Bonin and Mariana arcs it appears that the there are two separate pieces of the Pacific plate: the torn slab north of 26° N and a buckled near vertically dipping slab south of the tear. The transition between the two distinct slab morphologies coincides with the location of the Ogasawara Plateau and the trench.

Rapid tectonics of the Late Miocene Boso accretionary prism related to the Izu–Bonin arc collision

Abstract The structure, paleomagnetism and biostratigraphy of the Nishizaki and Kagamigaura formations on the southern Boso Peninsula, central Japan, were investigated to determine the chronographic constraints on the accretion, post‐Late Miocene rotation and regional tectonics in the Izu–Bonin island arc collision zone. The geological structures on the southern Boso Peninsula are characterized by an east–west trending and south‐verging fold and thrust belt that curves toward the northwest–southeast in the northwest extent of the Nishizaki Formation. Two stages of tectonic rotation were revealed by paleomagnetic and structural studies. The first is believed to have occurred after the accretion of the Nishizaki Formation and before the deposition of the Kagamigaura Formation, while the second is confidently correlated with the 1 Ma Izu block collision. The northwest extent of the Nishizaki Formation was rotated clockwise by approximately 65–80°, whereas the rotation was only 25–30° in the east, and 11–13° in the overlying Kagamigaura Formation. Radiolarian biostratigraphy suggests a depositional age of 9.9–6.8 Ma (Upper Miocene period) for the Nishizaki Formation and 4.19‐3.75 Ma (Pliocene period) for the lower Kagamigaura Formation. These results indicate that the age of accretion and first‐stage rotation of the Nishizaki Formation can be constrained to the interval of 6.80–3.75 Ma. This structure most likely represents the northward bending caused by collisions of the Tanzawa and Izu blocks with the Honshu island arc, and suggests rapid processes of accretion, collision, uplift and the formation of new sedimentary basins within a relatively short period of time (2.61–3.05 my).

Are Arc Basalts Dry, Wet, or Both? Evidence from the Sumisu Caldera Volcano, Izu–Bonin Arc, Japan

Basalt–basaltic andesite ( Fo91) in the low-Zr basalts compared with those in high-Zr basalts (<Fo89). The low-Zr basalts contain up to 11 vol. % augite, but many high-Zr basalts are free of augite, which appears only in their more differentiated products. The lowZr basalts are considered to be hydrous magmas in which olivine crystallizes first followed by augite and plagioclase, whereas the highZr basalts are dry. The low-Zr basalts have higher U/Th ratios than the high-Zr basalts. We suggest that both dry and wet primary basalts existed in the Sumisu magmatic system, each having different trace element concentrations, mineral assemblages and mineral chemistry. The lower contents of Zr and light REE and magnesian primary olivines in the wet basalts could have resulted from a higher degree of partial melting ( 20%) of a hydrous source mantle compared with 10% melting of a dry source mantle. The Sr, Nd and Pb isotope compositions of the wet and dry basalts are similar and are limited in range. These lines of evidence indicate that a mantle diapir model might be applicable to satisfy the configuration of such a mantle source region beneath a single volcanic system such as Sumisu.

Evidence of sub-vent biosphere: enzymatic activities in 308 °C deep-sea hydrothermal systems at Suiyo seamount, Izu–Bonin Arc, Western Pacific Ocean

A high-temperature deep-sea hydrothermal system related to dacitic arc-volcanism was drilled using a tethered, submarine rock-drill system as a part of the Archaean Park Project. The benthic multi-coring system (BMS) employed allowed for direct sampling of microorganisms, rocks and fluids beneath hydrothermal vents. The samples examined in this study were from sites APSK 05 and APSK 07 on the Suiyo Seamount of the Izu-Bonin Arc in the Pacific Ocean. Based on the vertical distribution of samples derived from this vigorous sub-vent environment, a model of deep-sea subterranean chemistry and biology was determined detailing optimal microbial activities. Deep-sea hydrothermal sub-vent core samples of dacitic arc-volcanism obtained at the Suiyo Seamount, Izu-Bonin Arc, Western Pacific Ocean were analyzed for acid and alkaline phosphatase enzymatic activities. Useful biomarkers of acid phosphatase (ACP) and alkaline phosphatase (ALP) enzymatic activities were positively correlated against each other and was greatest at the partial middle core sequences; ACP and ALP activities determined were as high as 5.10 and 6.80 nmol/min/g rock, respectively. Biochemical indicators of ACP and ALP were consistent with the origin of biogenic amino acids occupied in the sub-vent region and microbial cell number in the fluid. The significant enzymatic activities demonstrated in this study provides crucial evidence that sub-vent regions represent part of the previously unknown extreme-environment biosphere, extending the known subterranean habitable spaces of, for example, extremophilic microbes. This boring trial was first example of discharging high temperature hydrothermal activities at the frontal arc volcanoes.

Imaging changes in morphology, geometry, and physical properties of the subducting Pacific plate along the Izu–Bonin–Mariana arc

Recent studies on slab structure in the northwest Pacific have imaged variable geometries of the subducted oceanic lithosphere along the plate margin. Tomographic images show portions of the subducted Pacific plate penetrate vertically into the lower mantle below the Mariana arc, whereas below the Izu–Bonin arc, the slab appears to be lain down horizontally on top of the 670-km discontinuity. Using new technology to grid and visualize regional seismic tomography data even more detail about the structure of the subducting plates can be extracted from the three-dimensional images. We investigate the morphology and geometry of the subducting Pacific plate as it changes between horizontal to nearly vertical using P-wave tomographic images. The most profound result from these new models is the presence of a distinct change of seismic property in the downgoing Pacific slab beneath the Izu–Bonin arc at a depth of 350–400 km. The position of this anomaly corresponds to a region north and west of the Ogasawara Plateau. We propose the change in morphology and physical property of the slab is related to the distortion of the Pacific plate geometry as its shape converts from near horizontal to vertical, together with the subduction of the Ogasawara Plateau.