ALBERT

Description: We review the evolution of late Cenozoic magmatism in the NE Japan arc, and examine the relationship between the magmatism and the crust–mantle structure. Recent studies reveal secular changes in the mode of magmatic activity, the magma plumbing system, erupted volumes and magmatic composition associated with the evolution of crust–mantle structures related to the tectonic evolution of the arc. The evolution of Cenozoic magmatism in the arc can be divided into three periods: the continental margin (66–21 Ma), the back-arc basin (21–13.5 Ma) and the island-arc period (13.5–0 Ma). Magmatic evolution in the back-arc basin and the island-arc periods appears to be related to the 2D to 3D change in the convection pattern of the mantle wedge related to the asthenosphere upwelling and subsequent cooling of the mantle. Geodynamic changes in the mantle caused back-arc basin basalt eruptions during the back-arc basin opening (basalt phase) followed by crustal heating and re-melting, which generated many felsic plutons and calderas (rhyolite/granite phase) in the early stage of the island-arc period. This was followed by crustal cooling and strong compression, which ensured vent connections and mixing between deeper mafic and shallower felsic magmas, erupting large volumes of Quaternary andesites (andesite phase).

Description: Primitive basalts are rarely found in arcs. The active NW Rota-1 volcano in the Mariana arc has erupted near-primitive lavas, which we have sampled with ROV Hyper-Dolphin (HPD). Samples from the summit (HPD480) and eastern flank (HPD488) include 17 magnesian basalts (51–52 wt % SiO 2 ) with 7·5–9·5 wt % MgO and Mg-number of 61–67, indicating little fractionation. Olivine phenocrysts are as magnesian as Fo 93 and contain 0·4 wt % NiO; the Cr/(Cr + Al) values of spinels are mostly 0·5–0·8, indicating equilibrium with depleted mantle. There are three petrographic groups, based on phenocryst populations: (1) cpx–olivine basalt (COB); (2) plagioclase–olivine basalt (POB); (3) porphyritic basalt. Zr/Y and Nb/Yb are higher in POB (3·1–3·2 and 1·2–1·5, respectively) than in COB (Zr/Y = 2·8–3·0 and Nb/Yb = 0·7–0·9), suggesting that POB formed from lower degrees of mantle melting, or that the COB mantle source was more depleted. On the other hand, COB have Ba/Nb (70–80) and Th/Nb (0·4–0·5) that are higher than for POB (Ba/Nb = 30–35 and Th/Nb = 0·1–0·2), and also have steeper light rare earth element (LREE)-enriched patterns. Moreover, COB have enriched 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, and higher Pb isotope values, suggesting that COB has a greater subduction component than POB. 176 Hf/ 177 Hf between COB and POB are similar and Hf behavior in COB and POB is similar to that of Zr, Y and HREE, suggesting that Hf is not included in the subduction component, which produced the differences between COB and POB. The calculated primary basaltic magmas of NW Rota-1 volcano (primary COB and POB magmas) indicate segregation pressures of 2–1·5 GPa (equivalent to 65–50 km depth). These magmas formed by 24–18% melting of mantle peridotite having Mg-number ~89·5. Diapiric ascent of hydrous peridotite mixed heterogeneously with sediment melts may be responsible for the NW Rota-1 basalts. These two basalt magma types are similar to those found at Sumisu and Torishima volcanoes in the Izu–Bonin arc, with COB representing wetter and POB representing drier magmas, where subduction zone-derived melt components are coupled with the water contents.

Description: The youngest geomagnetic polarity reversal, the Matuyama-Brunhes boundary (MBB), provides an important datum plane for sediments, ice cores, and lavas. Its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records, and is supported by 40 Ar/ 39 Ar dating of Hawaiian lavas using recent age calibrations. Challenging this age, however, are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high-sedimentation-rate marine records, and cosmogenic nuclides in marine sediments and an Antarctic ice core. Here, we present a U-Pb zircon age of 772.7 ± 7.2 ka from a marine-deposited tephra just below the MBB in a forearc basin in Japan. U-Pb dating has a distinct advantage over 40 Ar/ 39 Ar dating in that it is relatively free from assumptions regarding standardization and decay constants. This U-Pb zircon age, coupled with a newly obtained oxygen isotope chronology, yields an MBB age of 770.2 ± 7.3 ka. Our MBB age is consistent with those based on the latest orbitally tuned marine sediment records and on an Antarctic ice core. We provide the first direct comparison between orbital tuning, U-Pb dating, and magnetostratigraphy for the MBB, fulfilling a key requirement in calibrating the geological time scale.

Description: A reactive flow geochemical model based on pMELTS thermodynamic calculations explains the observed modal, major, and trace element variations in the Red Hills peridotite, New Zealand. The model also reproduces the major and trace element chemical variations in mid-ocean ridge basalts (MORB) observed in present-day spreading ridges. The Red Hills peridotite is thought to originate from palaeo-MOR magmatic processes in the mantle–Moho transition zone. The peridotite body consists of a harzburgite matrix and dunite channels. The harzburgite forms the Lower Unit and is intruded by replacive dunite channels in the Upper Unit. This lithology gradually turns into a massive dunite zone in which disseminated to lenticular clinopyroxene aggregates are present. The rare earth element (REE) abundances in the peridotite samples vary greatly depending on their lithologies. In the Lower Unit, REE are extremely depleted, whereas in the Upper Unit they are relatively enriched, in contradiction to the depleted lithologies. Our model consists of two stages. The first stage assumes melting of depleted MORB source mantle in the garnet stability field, and the second assumes reactions between residual solids and the melts from the first stage in the spinel stability field in an open system. The model explains the formation of depleted harzburgite and the formation of dunite channels in the harzburgite matrix well. The major and trace element compositions of the melts calculated by the model vary from ultra-depleted MOR melts in harzburgite to normal MORB in dunite, suggesting that these lithologies are residues of a palaeo-MOR. The model also explains the origins of the local and global geochemical trends observed in MORB and the geochemical variation in abyssal peridotite samples. Our model confirms the important role of reactive flow in the mantle–Moho transition zone beneath MORs.

Description: We present major element, trace element, and petrographic data on alkali basalts from St. Helena, and examine the geochemical characteristics of a recycled component involved in the source of HIMU ( 206 Pb/ 204 Pb 〉20·5) ocean island basalts. Petrographic and compositional variations in the St. Helena basalts are best explained by the combined effect of fractional crystallization and accumulation of phenocrysts. Primary melt compositions are estimated by correcting for the effects of crystal–liquid differentiation by reconstructing the order of crystallization and the relative amount of fractionated phases. This calculation indicates that the St. Helena alkali basalts are derived from a common primary magma with 14–20 wt % MgO. Simple partial melting of fertile mantle peridotite, depleted mid-ocean ridge basalt (MORB)-source mantle, or garnet pyroxenite fails to produce the St. Helena primary melt. Instead, this primary melt can be reproduced if there are contributions from ancient recycled oceanic crust and depleted peridotite [(Rb/Nb) PM = 0·38–0·80]. Subducted sediment can be excluded to explain the low (Rb, Ba, U)/Nb and Ce/Pb of St. Helena basalts. Geochemical modeling using major and trace element abundances, together with Sr, Nd, Pb, and Hf isotope ratios, indicates that the St. Helena primary melt can be formed by 1–2% melting of a peridotitic source that was refertilized by a small amount (8–18%) of melt derived from recycled oceanic crust. This source has a similar trace element pattern to modern normal (N)-MORB, but element abundances are 0·1–0·2 times N-MORB values. The calculated recycled crust has a wide range of present-day Pb isotopic ratios ( 206 Pb/ 204 Pb of 21·7–79·3 and 208 Pb/ 204 Pb of 40·8–89·3), 87 Sr/ 86 Sr of 0·7018–0·7028, 143 Nd/ 144 Nd of 0·51274–0·51285, and 176 Hf/ 177 Hf of 0·28262–0·28293 after a residence time of 1·2–2·8 Gyr. Rb, Ba, Pb, Sr, and light rare earth element abundances in the recycled crust are depleted compared with modern N-MORB, whereas Th, U, Sm, and Nd abundances fall within the range of compositional variations in modern N-MORB. The trace element compositions of the recycled oceanic crust can be explained by element behavior during seafloor alteration and subduction zone dehydration of oceanic crust. Therefore, recycling of ancient subducted oceanic crust is a potential process for producing the St. Helena HIMU basalts.

Description: Pagan is one of the largest volcanoes along the Mariana arc volcanic front. It has a maximum elevation of 570 m (Mt. Pagan), but its submarine flanks descend to 2000–3000 m below sea level, and are unexplored. Bathymetric mapping and ROV Hyper-Dolphin dives (HPD1147 and HPD1148) on the submarine NE and SW flanks of Pagan were carried out during cruise NT10-12 of R.V. Natsushima in July 2010. There are no systematic compositional differences between subaerial lavas reported in the literature and differentiated submarine lavas collected in HPD1148, with 〈7 wt % MgO, suggesting they are derived from the same magmatic system. However, these differentiated lavas show complexities including magma mixing; thus we concentrate on magnesian submarine lavas (〉7 wt % MgO). Twenty least-fractionated basalts (48·5–50 wt % SiO 2 ) collected during HPD1147 extend to higher MgO (10–11 wt %) and Mg# (66–70) than the subaerial lavas. Olivine (up to Fo 94 ) and spinel (Cr# up to 0·8) compositions suggest that these Pagan primitive magmas formed from high degrees of mantle melting. Two basalt types can be distinguished based on their geochemistry at similar (10–11 wt %) MgO; these erupted recently, 500 m apart. Both contain clinopyroxene and olivine phenocrysts and are referred to as COB1 and COB2. Lower TiO 2 , FeO, Na 2 O, K 2 O, incompatible trace element abundances, and Nb/Yb suggest that COB1 formed from higher degrees of mantle melting. In addition, light rare earth element (LREE) enrichment and higher Th/Nb in COB2 contrast with LREE depletion and lower Th/Nb in COB1. Higher Ba/Th and Ba/Nb and lower Th/Nb indicate that the main subduction addition in COB1 was dominated by hydrous fluid, whereas that in COB2 was dominated by sediment melt. Sr–Nd–Pb–Hf isotopes are also consistent with this interpretation. These observations suggest that the subduction component responsible for the greater degree of melting of the COB1 source was mostly hydrous fluid. The origin of such different metasomatic agents resulted in different primary magmas forming in the same volcano. Both hydrous fluid and sediment melt components may have unmixed from an originally homogeneous supercritical fluid in or above the subducting slab below the volcanic front. These may have been added separately to the mantle wedge peridotite (mantle diapir) and resulted in two neighboring but completely different primary magmas from the same diapir. Moreover, these primitive lavas suggest that even for intra-oceanic arcs assimilation–fractional crystallization is inevitable when these magmas evolve in the crust and, in addition, that phlogopite is present in their mantle residue and thus played an important role in their genesis.

Description: Pleistocene basalts from Daisen and Mengameyama in the SW Japan volcanic arc of western Honshu are characterized by an abundance of olivine crystals with Fe-rich rims. At Daisen, these have previously been interpreted to have formed from their host melt by equilibrium crystal fractionation and by disequilibrium fractionation during supercooling. Here we use combined electron probe microanalysis, isotopography, transmission electron microscopy and selected area electron diffraction to show that crystal rims are significantly enriched in aluminium (up to c . 1 wt%) and hydrogen (up to c . 10 000 ppm) hosted in oriented low-density amorphous domains. These domains are interpreted to have formed by melting of deuteric and/or post-deuteric metasomatic alteration minerals upon uptake of older olivine crystals into fresh, initially aphyric host melts up to a few hours prior to eruption. It is argued that uptake of variably altered crystals into initially aphyric or sparsely phyric melts may be a common process at subduction zones, and can account for typical disequilibrium textures displayed by arc magmas erupted in SW Japan and elsewhere. Analyses of the altered crystal cargo in arc volcanic rocks therefore provides an important tool for understanding subvolcanic hydrothermal systems and the interaction of ascending melts with such systems. Supplementary material: Olivine mineral chemistry data from two typical Daisen basalts and one typical Mengameyama basalt, and a figure showing the locations of all focussed ion beam (FIB) sections studied here, are available at http://www.geolsoc.org.uk/SUP18760 .

Description: Initial subduction-related boninitic magmatism occurred between 48 and 44 Ma in the Izu–Bonin–Mariana (IBM) arc. High-Mg adakites and low-Ca boninites have been dredged from the Bonin Ridge fore-arc seamount. Whole-rock 40 Ar/ 39 Ar ages suggest that the boninite (44·0 ± 1·4 Ma) and adakite (43·1 ± 1·0 and 40·8 ± 0·8 Ma) magmatism overlapped, or that the adakite magmatism occurred slightly later than the boninite magmatism. The low-Ca boninites are high-Mg andesites and exhibit U-shaped rare earth element (REE) patterns with an elevated average Mg# of 0·78 [Mg# = Mg/(Mg + Fe) molar ratio] and Ni content of 667 ppm. The high-Mg adakites are andesitic to dacitic in composition; they exhibit markedly high Sr contents and low Y contents and are highly enriched in light REE but depleted in heavy REE, with an average Mg# of 0·79 and Ni content of 433 ppm. A geochemical mass-balance model (Arc Basalt Simulator Version 3) indicates that both magma types could be generated by partial melting of a depleted mantle source fluxed by water-rich slab-derived melts in a hot subduction environment, comparable with the present-day South Chile (ridge subduction) or Southwest Japan (young slab subduction) arcs. An extremely high slab melt flux of 22% is required for the formation of the high-Mg adakite, whereas a low flux of 3% is sufficient for the low-Ca boninite. The low-Ca boninite requires a high-temperature shallow slab (854°C, 2·7 GPa on average), consisting of altered oceanic crust of the Pacific plate and volcaniclastic sediments from HIMU seamounts, and high-temperature shallow mantle melting (1216°C, 0·8 GPa) of depleted Indian mid-ocean ridge basalt (MORB)-type mantle. These modelled conditions are consistent with the occurrence of hot shallow mantle wedge melting in the initial subduction zone at the boundary between Pacific- and Indian-type mantle domains, as suggested by previous studies. In contrast, high-Mg adakite requires a higher temperature and deeper slab (929°C, 4·1 GPa), with the same slab components and slightly deeper but less hot melting (1130°C, 1·1 GPa) of HIMU-type depleted mantle, to satisfy the low Hf isotope ratios. This may occur because of the subsequent cooling of the mantle wedge by the establishment of the subduction system after the boninite magmatism and involvement of a small volume of an isotopically enriched mantle source embedded in the Indian-type mantle. The petrogenetic conditions provide constraints for reconstructing the tectonic settings of the early IBM arc. The hot subduction model would be consistent with the tectonic models with regard to the initiation of subduction associated with fore-arc spreading; this allowed the upwelling of the asthenospheric mantle to generate slab melts from the old Pacific plate slab and hot shallow mantle melting by slab melt fluxing for both boninite and adakite activities.

Description: It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derived from a common primary TH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the ‘primary’ low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr ( 87 Sr/ 86 Sr = 0·7058–0·7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr ( 87 Sr/ 86 Sr = 0·7039–0·7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr ( 87 Sr/ 86 Sr = 0·7044–0·7055) medium-K andesites, all of which have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneath Azuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-K CA basalt has a basaltic groundmass with Mg-rich olivine (Fo 89 ) and calcic plagioclase phenocrysts (An 90 ) and the most unradiogenic Sr ( 87 Sr/ 86 Sr = 0·7037–0·7038), suggesting that it originated from a primary mantle melt. Major and trace element microanalysis of the basaltic groundmass indicates that the primary magma composition is close to high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-K TH basalts and simultaneously requires a reappraisal of the origin of the across-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc.