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The International Ocean Discovery Program Expedition 350 drilled between two Izu rear‐arc seamount chains at Site U1437 and recovered the first complete succession of rear‐arc rocks. The drilling reached 1806.5 m below seafloor. In situ hyaloclastites, which had erupted before the rear‐arc seamounts came into existence at this site, were recovered in the deepest part of the hole (∼15–16 Ma). Here it is found that the composition of the oldest rocks recovered does not have rear‐arc seamount chain geochemical signatures, but instead shows affinities with volcanic front or some of the extensional zone basalts between the present volcanic front and the rear‐arc seamount chains. It is suggested that following the opening of the Shikoku back‐arc Basin, Site U1437 was a volcanic front or a rifting zone just behind the volcanic front, and was followed at ~ 9 Ma by the start of rear‐arc seamount chains volcanism. This geochemical change records variations in the subduction components with time, which might have followed eastward moving of hot fingers in the mantle wedge and deepening of the subducting slab below Site U1437 after the cessation of Shikoku back‐arc Basin opening. [ABSTRACT FROM AUTHOR]

Santorini volcano sits ~145 km above the Aegean Wadati-Benioff zone, where the African plate subducts northward beneath Eurasia. There are only a few localities in the whole Aegean where basaltic lavas primitive enough to constrain mantle processes beneath the Aegean arc can be found; in this work we analyzed one such locality, a basalt lava from the southern part of Santorini. We apply a suite of petrological tools (PRIMACALC2 and ABS5) in sequence to estimate magma chamber conditions, primary magma composition, mantle melting conditions, and slab dehydration conditions. Back-calculation modeling based on major-element chemistry yields shallow magma chamber conditions of P  = 0.02 GPa, fO 2  = QFM + 2, and ~1 wt% H 2 O in the primary magma. The estimated major element composition of this primary magma then leads to estimated mantle melting conditions of 2.1 GPa, 1353 °C, and F  = 8%; whereas a more precise estimate derived from trace element modeling implies 1.7 GPa, 1323 °C, and F  = 18%. Furthermore, the trace element model implies a slab flux derived from 4.6 GPa (~150 km slab depth). The estimated slab depth, magma segregation conditions, and magma chamber depth are all consistent with seismic observations, supporting slab dehydration in the seismically imaged steep slab interval and flux melting in a relatively hot mantle wedge. [ABSTRACT FROM AUTHOR]

The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonatebearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate-silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source. [ABSTRACT FROM AUTHOR]

Intra-plate basalts of ~ 35–0 Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature ( T p ), while the Shandong basalts have the lowest T p . We infer that a deep high- T p plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low- T p plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~ 35 Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation. [ABSTRACT FROM AUTHOR]

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 % SiO2) collected during HPD1147 extend to higher MgO (10–11 wt %) and Mg# (66–70) than the subaerial lavas. Olivine (up to Fo94) 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 TiO2, FeO, Na2O, K2O, 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. [ABSTRACT FROM PUBLISHER]

Abstract: The origin of historic potassic basalts (from the 1719 to 1721AD eruption) from the Wudalianchi volcanic field, northeast China, is investigated. The samples examined in this study are aphyric, and contain ~5vol.% olivine microphenocrysts. Geochemical analyses of the samples, along with published data for material from the 1719 to 1721AD eruption, show that the basalts exhibit linear compositional trends in whole-rock variation diagrams, and are characterized by K2O enrichment (>4wt.%) and an EM1-like isotopic signature. Using thermodynamic calculations and the observed petrological features of the basalts, the temperature of the magmas shortly before eruption is estimated to have been ~1250°C, and the water content of the magma at depth is estimated to have been >~1.1wt.%. Because the estimated temperatures are significantly higher than the projected maximum temperature of the lithospheric mantle beneath the Wudalianchi volcanic field, the magmas were likely derived from the asthenospheric mantle. We suggest that both the potassic- and EM1-like natures of the basalts originated from the mantle transition zone, metasomatized by K-rich sediment fluids ~1.5Ga ago through a stagnation of an ancient slab. The linear whole-rock compositional trends of the basalts primarily reflect the geochemical heterogeneity in the mantle transition zone, with variable but coupled contributions of ~1.5Ga sediments in the ancient stagnant slab, and recent sediments and peridotites in the stagnant Pacific slab. We infer that the Wudalianchi magmatism was caused by an upwelling of a hydrous mantle plume from the mantle transition zone, which was hydrated through the stagnation of the ancient subducted slab and the recent Pacific slab. [Copyright &y& Elsevier]

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 (€87Sr/€86Sr = 0·7058–0·7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (€87Sr/€86Sr = 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 (€87Sr/€86Sr = 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 (Fo89) and calcic plagioclase phenocrysts (An90) and the most unradiogenic Sr (€87Sr/€86Sr = 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. [ABSTRACT FROM PUBLISHER]

The Miocene northeast Honshu magmatic arc, Japan, formed at a terrestrial continental margin via a stage of spreading in a back-arc basin (23-17 Ma) followed by multiple stages of submarine rifting (19-13 Ma). The Kuroko deposits formed during this period, with most forming during the youngest rifting stage. The mode of magma eruption changed from submarine basalt lava flows during back-arc basin spreading to submarine bimodal basalt lava flows and abundant rhyolitic effusive rocks during the rifting stage. The basalts produced during the stage of back-arc basin spreading are geochemically similar to mid-ocean ridge basalt, with a depleted Sr- Nd mantle source, whereas those produced during the rifting stage possess arc signatures with an enriched mantle source. The Nb/Zr ratios of the volcanic rocks show an increase over time, indicating a temporal increase in the fertility of the source. The Nb/Zr ratios are similar in basalts and rhyolites from a given rift zone, whereas the Nd isotopic compositions of the rhyolites are less radiogenic than those of the basalts. These data suggest that the rhyolites were derived from a basaltic magma via crystal fractionation and crustal assimilation. The rhyolites associated with the Kuroko deposits are aphyric and have higher concentrations of incompatible elements than do post- Kuroko quartz-phyric rhyolites. These observations suggest that the aphyric rhyolite magma was derived from a relatively deep magma chamber with strong fractional crystallization. Almost all of the Kuroko deposits formed in close temporal relation to the aphyric rhyolite indicating a genetic link between the Kuroko deposits and highly differentiated rhyolitic magma. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The northern Kyushu-Palau Ridge (KPR), remnant conjugate arc of the Izu-Ogasawara (Bonin)-Mariana (IBM) active arc, is dominated by basalt-andesite except for the Komahashi-Daini Seamount where acidic plutonic rocks of 38 Ma were recovered. These mafic to intermediate volcanics are produced by the rifting volcanism in the proto-IBM arc associated with spreading of the Shikoku Basin. The HFSE and HREE contents and ratios of these volcanics indicate enriched source mantle composition compared to recent volcanic front. The LILE ratios exhibit similar characteristics to reararc volcanism of the recent Izu arc, and some enriched volcanics exhibit high abundance of sediment melt inputs. Based on these observations and compilations of the published data set, the replacement event of the wedge mantle under the IBM arc occurred two times. The first event occurred between 45 and 38 Ma, with Pacific type mantle being replaced by depleted Indian type mantle. The second event occurred between 36 and 25 Ma, enriched mantle flowed from reararc side. The slab component during the proto-IBM arc rifting was a similar characteristic to recent reararc volcanism of the Izu arc, and sediment melt added in a local area. [ABSTRACT FROM AUTHOR]

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 (206Pb/204Pb >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 (206Pb/204Pb of 21·7–79·3 and 208Pb/204Pb of 40·8–89·3), 87Sr/86Sr of 0·7018–0·7028, 143Nd/144Nd of 0·51274–0·51285, and 176Hf/177Hf 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. [ABSTRACT FROM AUTHOR]

Abstract: The mechanisms of continental intraplate magmatism were investigated with respect to Cenozoic basaltic shield lavas from the Changbaishan area, northeast China, using a comprehensive data set of major and trace elements, Sr, Nd, and Pb isotopic compositions, and K–Ar ages. The shield lavas show a significant variation in TiO2 content (1–4 wt.%), which exhibits a positive covariance with K2O and a negative covariance with SiO2 for most samples. The compositional variation of the lavas cannot be explained solely by magma chamber processes, such as fractional crystallization and crustal assimilation, and may in part be attributed to processes in the magmatic source region. The variation in TiO2 is considered to mainly reflect variations in the degree of partial melting of the source mantle, and the negative covariance of SiO2 and TiO2 suggests that lower-degree melts were produced at greater depths. On the basis of the observed negative covariance of TiO2 with K–Ar ages in the main lavas, it would appear that magmas segregated at progressively greater depths with time, since at least 4.2 Ma. This feature cannot be explained in terms of temporal changes in the mantle potential temperature and/or bulk source composition. Instead, it appears likely that the rate of asthenosphere upwelling beneath the Changbaishan area decreased with time, whereby the melting depth gradually increased, resulting from a reduction in the heat flux from the upwelling mantle to the overlying lithosphere. The inferred deceleration of the asthenospheric mantle is considered to have been related to a change from an extensional to compressive tectonic stress regime during the late Pliocene. Our study suggests that long-term volcanic activity in the Changbaishan area will continue to decrease unless there is an increase in the upwelling rate of the asthenospheric mantle. [Copyright &y& Elsevier]

The Sunda Arc of Indonesia developed along the convergent margin between the Eurasian and the Australian Plates. More than 100 Quaternary volcanic centers occur along the arc. The West Java Arc is a segment of the Sunda Arc in which more than 10 volcanic centers are located, corresponding to the 120 to 200 km depth contours of the Wadati–Benioff zone. The geochemistry of 207 Quaternary lavas from six centers across the arc was investigated. The lavas range from basalt to dacite. Incompatible element abundances increase from the volcanic front to the rear-arc in response to a change from low-K to high-K suites. Nd–Sr isotope compositions of the basalts scatter between mid-ocean ridge basalt (MORB) source mantle and Indian Ocean sediment (SED) compositions, with volcanic front low-K basalts having more radiogenic Nd than the rear-arc basalts. It is suggested that mixing between slab-derived fluids mainly from the SED and melt from MORB source mantle played a significant role in determining the geochemistry of the West Java basalts. Incompatible element patterns in primitive mantle normalized multi-element plots are almost identical across the arc, except for greater inclination and weaker positive Sr spikes in the rear-arc basalts. This suggests a lower degree of partial melting in the rear-arc mantle, accompanied by change in SED fluid composition between the volcanic front and the rear-arc. The latter is confirmed by fluid-fluxed melting model calculations using multiple trace elements and Nd and Sr isotopes. All the West Java Arc lavas require deficit of Sr from the slab SED. This may occur due to selective breakdown of Sr-rich hydrous silicate minerals, such as zoisite, at shallower depths before the SED component reaches the depth of dehydration effective for magma genesis. The rear-arc basalts need further Sr deficits along with lesser fluid. These features are commonly observed in many arc basalts, and are likely attributable to the same mechanism. [ABSTRACT FROM AUTHOR]

Quaternary lavas from the NE Japan arc show geochemical evidence of mixing between mantle-derived basalts and crustal melts at the magmatic front, whereas significant crustal signals are not detected in the rear-arc lavas. The along-arc chemical variations in lavas from the magmatic front are attributable almost entirely to geochemical variations in the crustal melts that were mixed with a common mantle-derived basalt. The mantle-derived basalts have slightly enriched Sr–Pb and depleted Nd isotopic compositions relative to the rear-arc lavas, but the variation is less pronounced if crustal contributions are eliminated. Therefore, the source mantle compositions and slab-derived fluxes are relatively uniform, both across and along the arc. Despite this, incompatible element concentrations are significantly higher in the rear-arc basalts. We examine an open-system, fluid-fluxed melting model, assuming that depleted mid-ocean ridge basalt (MORB)-source mantle melted by the addition of fluids derived from subducted oceanic crust (MORB) and sediment (SED) hybrids at mixing proportions of 7% and 3% SED in the frontal- and rear-arc sources, respectively. The results reproduce the chemical variations found across the NE Japan arc with the conditions: 0·2% fluid flux with degree of melting F = 3% at 2 GPa in the garnet peridotite field for the rear arc, and 0·7% fluid flux with F = 20% at 1 GPa in the spinel peridotite field beneath the magmatic front. The chemical process operating in the mantle wedge requires: (1) various SED–MORB hybrid slab fluid sources; (2) variable amounts of fluid; (3) a common depleted mantle source; (4) different melting parameters to explain across-arc chemical variations. [ABSTRACT FROM PUBLISHER]

Abstract: Submarine lavas recovered from the Hilina bench region, offshore Kilauea, Hawaii Island provide information on ancient Kilauea volcano and the geochemical components of the Hawaiian hotspot. Alkalic lavas, including nephelinite, basanite, hawaiite, and alkali basalt, dominate the earliest stage of Kilauea magmatism. Transitional basalt pillow lavas are an intermediate phase, preceding development of the voluminous tholeiitic subaerial shield and submarine Puna Ridge. Most alkalic through transitional lavas are quite uniform in Sr–Nd–Pb isotopes, supporting the interpretation that variable extent partial melting of a relatively homogeneous source was responsible for much of the geochemical diversity of early Kilauea magmas (). These samples are among the highest 206Pb/204Pb known from Hawaii and may represent melts from a distinct geochemical and isotopic end-member involved in the generation of most Hawaiian tholeiites. This end-member is similar to the postulated literature Kea component, but we propose that it should be renamed Hilina, to avoid confusion with the geographically defined Kea-trend volcanoes. Isotopic compositions of some shield-stage Kilauea tholeiites overlap the Hilina end-member but most deviate far into the interior of the isotopic field defined by magmas from other Hawaiian volcanoes, reflecting the introduction of melt contributions from both “Koolau” (high 87Sr/86Sr, low 206Pb/204Pb) and depleted (low 87Sr/86Sr, intermediate 206Pb/204Pb) source materials. This shift in isotopic character from nearly uniform, end-member, and alkalic, to diverse and tholeiitic corresponds with the major increase in Kilauea''s magmatic productivity. Two popular geodynamic models can account for these relations: (1) The upwelling mantle source could be concentrically zoned in both chemical/isotopic composition, and in speed/extent of upwelling, with Hilina (and Loihi) components situated in the weakly ascending margins and the Koolau component in the interior. The depleted component could be refractory and spread throughout or scavenged from the overlying lithosphere. (2) The Hilina (and Loihi) components could be a more fertile material (lower melting temperature) spread irregularly throughout the Hawaiian source in a matrix of more refractory depleted and Koolau compositions. Modest upwelling along the leading hotspot margin melts the fertile domains predominantly, while the refractory matrix also partially melts in the more vigorously upwelling hotspot interior, diluting the Hilina and Loihi components and yielding voluminous isotopically diverse tholeiitic magmas. [Copyright &y& Elsevier]

Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite–hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37–38 Ma, indicating that felsic plutonic activity occurred during the early stages of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO2, except for an increasingly strong negative Eu anomaly at higher SiO2 levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the same arc from the Tanzawa Complex (10–5 Ma), central Japan, considered to represent the lower–middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO2 plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex. [ABSTRACT FROM AUTHOR]

The Ueno Basalts of central Japan comprise a monogenetic volcanic cone complex that was active between 2·76 and 1·34 Ma. Basalts were erupted at more than 14 centers scattered over a region 40 km in diameter. Alkali basalt was erupted first, followed by sub-alkaline basalt. Quasi-concentric expansion of eruption centers coinciding with uplift and with decreasing alkalinity of the lavas suggests that Ueno magmatism originated from a mantle diapir as it mushroomed at the base of the lithosphere. Depleted asthenospheric mantle (alkali basalt), enriched lithospheric mantle (sub-alkaline basalt), and crustal components are identified as chemical end-members in the petrogenesis of the Ueno Basalts. Incompatible trace element abundances indicate that the Ueno alkali basalts are typical within-plate basalts, whereas the sub-alkaline basalts show strong affinities with normal arc lavas. Sr–Nd–Pb isotopic compositions indicate that the mantle source of the alkali basalts was more depleted than that of the sub-alkaline basalts. About 7% melting of asthenospheric mantle in the garnet-lherzolite stability field produced the primitive alkali basalts and 12% melting of spinel lherzolite within the subcontinental lithosphere produced the primitive sub-alkaline basalts. Isotopic compositions and fluid mobile/immobile element ratios broadly covary with SiO2 contents in the sub-alkaline suite, and increasing silica content is associated with stronger EMII (Enriched Mantle II) isotope affinities and fluid mobile element abundances. A progressive AFC (assimilation–fractional crystallization) model assuming assimilation of a low-K silicic melt reproduces the chemical variations observed in the sub-alkaline suite. Melting of a flattening mantle diapir at the base of the lithosphere is the dominant cause of Ueno magmatism, accompanied by the assimilation of older arc crust. [ABSTRACT FROM PUBLISHER]

We investigated the Sr–Nd–Hf–Pb isotope compositions of clinopyroxene separates and strongly leached bulk rock powders from the basalt lavas on Raivavae Island in the South Pacific. These data allowed us to explore the magma chamber processes of the two basalt groups: the high-μ-type (μ = 238 U/ 204 Pb, HIMU) Rairua basalts erupted at ~ 9 Ma, and the depleted mantle (DM)-type Anatonu basalts erupted at ~ 6 Ma. We assessed different analytical techniques for clinopyroxene sample treatment, including leaching, sample digestion, chemical separation, and determination of parent/daughter ratios for use in age corrections. The age-corrected initial Sr–Nd–Hf isotope compositions of clinopyroxene samples solely overlap on the bulk rock compositions of each basalt type. The initial Pb isotope compositions show significant differences between the clinopyroxenes and bulk rock samples in both the Rairua and Anatonu basalts. The clinopyroxenes have less radiogenic Pb than their hosts, reflecting the magma chamber processes. The overall variations in the bulk rock Pb isotopes are explained by mixing of melts from the HIMU- and DM-type sources in the plume. In detail, early stage Rairua magma was more influenced by DM melts from which Rairua clinopyroxene was crystallized. Subsequent supply of dominant HIMU melts drove the bulk rock magma chemistry more HIMU-like. The Anatonu DM magma was largely from depleted mantle that differs from the Pacific MORB source. Subordinate contribution of a Pacific alkali-basalt-like component is suggested from the clinopyroxene chemistry of Anatonu perhaps from impregnated pyroxenite in the lithospheric mantle. [ABSTRACT FROM AUTHOR]

Abstract: The feasibility of oceanic igneous crust melting in stagnant slabs has previously been proposed from experimental petrological research. However, geochemical evidence for such melting has not yet been found from igneous rocks. We present evidence to suggest that melts from the igneous layer in the stagnant Pacific slab contributed to the source composition of basalts erupted in eastern China. Fe-rich (>13wt.%), Si-poor (<43wt.%) basalts occur only above the leading edge of the stagnant Pacific slab in eastern China, ~2000km west of the Pacific Plate trench. The source of these basalts has Nd–Hf and Sr–Nd–Pb isotopic compositions akin to those of the igneous layer in the Pacific slab, and their extremely low Rb and Pb contents suggest that their source material was modified by subduction processes. Together with forward modeling calculation on trace elements and isotope compositions, these geochemical characteristics imply that they received a contribution from fluid released from hydrated transition zone and dehydrated carbonate-bearing oceanic crust in the stagnant slab, without a long time-integrated ingrowth of Sr–Nd–Hf–Pb isotope systems, almost at the leading edge of the stagnant Pacific slab. [Copyright &y& Elsevier]

Abstract: We present a new set of He–Ne–Ar isotopic compositions for HIMU and EM1 lavas from the Cook–Austral Islands in the south Pacific. 3He/4He of the HIMU lavas are lower than MORB values, as previously demonstrated, but a coherent variation in 3He/4He with Pb isotopes indicates two-component mixing to form the lavas. One component is the HIMU reservoir with 3He/4He of 6 Ra or lower, and the other is the local lithosphere, which is commonly involved in EM1 lavas. Relative abundances of radiogenic and nucleogenic 4He, 21Ne and 40Ar show systematic variations. In 4He/40Ar*–4He/21Ne* space, the HIMU lavas define a trend that is parallel to, but offset from the trend previously observed for other OIBs. Using 4He/21Ne* as a monitor of elemental fractionation of noble gasses, fractionation-corrected 4He/40Ar* is higher than the 4He/40Ar* production ratio in the mantle, reflecting a HIMU reservoir with a lower K/U (approximately 3000) than canonical mantle value (13000). Radiogenic 3He/4He and the low K/U are best explained by a model where the HIMU reservoir was formed by direct accumulation of, or deep mantle metasomatism with, ancient subducted oceanic crust modified by hydrothermal alteration and dehydration during its subduction. If the subducted oceanic crust with fractionated K/U forms a large isolated reservoir in the mantle, previous estimates of K/U and K concentrations for the bulk silicate Earth, that did not take this reservoir into consideration, will be too high. Moreover, the mass balance calculation indicates that the subducted oceanic crust may make a significant contribution to the U mantle budget, requiring there to be less in the primitive mantle. However, a contribution from the primitive mantle is not ruled out entirely, unless the subducted oceanic crust had a relatively high U concentration and low K/U over geologic time and was totally preserved in a convecting mantle. [Copyright &y& Elsevier]

Abstract: We report Sr, Nd, and Pb isotopic data of young alkaline basalt lava from a new type of volcano (petit-spot) on the northwestern Pacific Plate. Petit-spot lavas show Dupal, or extremely EM-1-like, Sr–Nd–Pb isotopic compositions. The data cannot be explained by contamination of pelagic sediment, in spite of the prediction on the basis of geological observation. We thus consider that the geochemistry of petit-spot lava indicates the existence of recycled fertile plate materials, not only the Dupal isotopic signature, in the northern hemisphere Pacific upper mantle unrelated to one or more active plumes. In consideration of published experimental results for fertile plate materials, selective melting of recycled material is a process critical in generating petit-spot lava. Moreover, the small volume of the volcano and low degree of melting in the mantle source needed to form strongly alkalic lavas suggest that petit-spot volcanism is originated from small-scale heterogeneities of recycled material. This idea consistently explains the geochemistry and noble gas isotopic composition of petit-spot lava, and also suggests small-scale heterogeneity widespread in the upper mantle of the Pacific Ocean. Together with a revised view of upper mantle heterogeneity, we propose that gross upper mantle composition is controlled by abundances and scales of regions of recycled material that correspond to differences in the relative position to the Pangea supercontinent, suggesting the link to the tectonic origin of the global scale heterogeneity. [Copyright &y& Elsevier]

Basaltic glass grains from the submarine south flank of Kı¯lauea, Hawai′i, have Cl concentrations of 0.01–1.68 wt%, the latter being the highest Cl content yet recorded for a Hawaiian glass. The high-Cl glass grains are products of brine assimilation by tholeiite magma. The glasses are grains in a sandstone clast from bedded breccias draping the southwestern margin of Kı¯lauea’s submarine midslope bench. The clast contains two distinct suites of glass grains: abundant degassed tholeiites, perhaps derived from subaerial lavas of Mauna Loa that shattered upon ocean entry, and a smaller population of Kea-type tholeiite (n=17 analyzed) that erupted subaqueously, based on elevated S (780–1050 ppm), H2O (0.42–1.27 wt%), and CO2 (<30–120 ppm), probably early in Kı¯lauea’s shield-building stage. Ten grains in this group have Cl>1000 ppm, six >5000 ppm, and two grains have >10 000 ppm dissolved Cl. Abundances of H2O, Na2O, K2O, and several trace elements increase regularly with Cl concentration, and we estimate that Cl enrichment was due to up to 13 wt% addition of a brine consisting of 78% H2O (wt), 13% Cl, 4.4% Na, 2.6% K, 2.6% Ca, 620 ppm Ba, 360 ppm Sr, 65 ppm Rb, and 7 ppm Pb. The large amounts of brine addition argue against bulk assimilation of low-porosity brine-bearing rock. The brine’s composition is appropriate for a seawater-derived hydrothermal fluid that reacted with basaltic wall rocks at T>100°C, losing Mg and S and gaining K, Ca, Rb, Ba, Sr, and Pb, followed by phase separation near 500°C and ∼50 MPa (5 km below sea level at hydrostatic pressure). Brine was assimilated at or near the depth it formed, as estimated on petrologic grounds, but under lithostatic conditions. The highest extents of assimilation either forced volatile saturation of the magma or enriched already coexisting magmatic vapor in H2O. Possible mechanisms for assimilation are: (1) forcible injection of brine into magma during bursting of overpressured pockets heated by new dikes, or (2) intrusion of magma into lenses or sills occupied by trapped brine. [Copyright &y& Elsevier]

Many seamounts on the Ontong Java Plateau (OJP) occur near the Stewart Arch, a topographic high that extends parallel to the North Solomon Trench along the southern margins of the plateau. Despite the thick sediment cover, several volcanic cones with strong acoustic reflection were discovered on the submarine flank of the Nuugurigia Seamount. From such volcanic cones, basalts were successfully sampled by dredging. Radiometric dating of basalts and ferromanganese encrustation indicate eruption age of 20–25 Ma, significantly younger than the ~ 122 Ma main OJP plateau and post-plateau basalts. The age range coincides with the collision of the OJP with the Solomon Arc. The Nuugurigia basalts geochemically differ from any other rocks sampled on the OJP so far. They are alkali basalts with elevated Sr, low Zr and Hf, and Enriched Mantle-I (EMI)-like isotopic composition. Parental magmas of these alkali basalts may have formed by small-degree melting of peridotitic mantle impregnated with recycled pyroxenite material having enriched geochemical composition in the OJP's mantle root. We conclude that small-volume alkali basalts from the enriched mantle root migrated through faults or fractures caused by the collision along the Stewart Arch to form the seamount. Our results suggest that the collision of the OJP with the Solomon arc played an important role in the origin of similar post-plateau seamounts along the Stewart Arch. [ABSTRACT FROM AUTHOR]

Numerous large igneous provinces formed in the Pacific Ocean during Early Cretaceous time, but their origins and relations are poorly understood. We present new geochronological and geochemical data on rocks from the Manihiki Plateau and compare these results to those for other Cretaceous Pacific plateaus. A dredged Manihiki basalt gives an 40Ar-39Ar age of 117.9 ± 3.5 Ma (2σ), essentially contemporaneous with the Ontong Java Plateau ∼2500 km to the west, and the possibly related Hikurangi Plateau ∼3000 km to the south. Drilled Manihiki lavas are tholeiitic with incompatible trace element abundances similar to those of Ontong Java basalts. These lavas may result from high degrees of partial melting during the main eruptive phase of plateau formation. There are two categories of dredged lavas from the Danger Islands Troughs, which bisect the plateau. The first is alkalic lavas having strong enrichments in light rare earth and largeion lithophile elements; these lavas may represent late-stage activity, as one sample yields an 40Ar-39Ar age of 99.5 ± 0.7 Ma. The second category consists of tholeiitic basalts with U-shaped incompatible element patterns and unusually low abundances of several elements; these basalts record a mantle component not previously observed in Manihiki, Ontong Java, or Hikurangi lavas. Their trace element characteristics may result from extensive melting of depleted mantle wedge material mixed with small amounts of volcaniclastic sediment. We are unaware of comparable basalts elsewhere. [ABSTRACT FROM AUTHOR]

Abstract: Two manned submersible dives examined the Hualalai Northwest rift zone and an elongate ridge cresting at 3900 mbsl during a 2002 JAMSTEC cruise. The rift zone flank at dive site S690 (water depth 3412–2104 m) is draped by elongated and truncated pillow lavas. These olivine-rich tholeiitic lavas are compositionally indistinguishable from those examined further south along the bench, except that they span a wider range in dissolved sulfur content (200–1400 ppm). The elongate ridge investigated in dive S692, located at the base of the bench, is a package of distinct lithologic units containing volcaniclastic materials, glassy pillow breccias, and lava blocks; these units contain a range of compositions including tholeiitic basalt, transitional basalt, and hawaiite. The textures, compositions, and stratigraphic relationships of materials within the elongate ridge require that a variety of transport mechanisms juxtaposed materials from multiple eruptions into individual beds, compacted them into a coherent package of units, and brought the package to its present depth 10 km from the edge of the North Kona slump bench. Sulfur-rich hawaiite glasses at the base of the elongate ridge may represent the first extant representatives of juvenile alkalic volcanism at Hualalai. They are geochemically distinct from shield tholeiite and post-shield alkalic magmas, but may be related to transitional basalt by high-pressure crystal fractionation of clinopyroxene. Tholeiitic glasses that compose the majority of the exposed outcrop are similar to Mauna Kea tholeiites and other Hualalai tholeiites, but they differ from younger basalts in having greater incompatible element enrichments and higher CaO for a given MgO. These differences could arise from small extents of partial melting during the transition from alkalic to shield stage magmatism. Low sulfur contents of most of the volcaniclastic tholeiites point to early emergence of Hualalai above sea level relative to the development of the midslope slump bench. [Copyright &y& Elsevier]

Many groups or chains of seamounts border, or are associated with, oceanic plateaus but their origin and relationship with plateau volcanism are debated. An example is Ojin Rise Seamounts, a group of ~80 seamounts located northeast of the 147–134 Ma Shatsky Rise, an oceanic plateau that was formed along the trace of a triple junction in the northwest Pacific Ocean. A notable topographic feature of Ojin Rise Seamounts is that they are distributed in such a way that they appear to be connected to another oceanic plateau in the east, the ~100 Ma Hess Rise. Many seamounts are arranged in a NW–SE direction (almost in the same absolute plate motion direction) and some are organized into seamount chains consisting of several volcanic cones connected by a common platform, reflecting a complex volcanic history. Here we report petrography as well as major and trace element, Sr, Nd, Pb, and Hf isotope, and 40Ar-39Ar age data for basalts dredged from five seamounts in the western part of Ojin Rise Seamounts. We found that the 40Ar-39Ar ages of the cones (123.7–120.7 Ma) are distinctly younger than the magnetic lineation age (M9-M7; 134–132 Ma) around the chain. The younger age suggests either a protracted development for the seamount chain, previously proposed to have formed near-ridge, or age difference between the platform and cones. The morphological and age differences are supported by petrography and geochemistry: platform basalts are aphyric, whereas cone basalts are slightly to highly plagioclase-phyric (up to 48 vol%); platform basalts are depleted in trace element and isotope composition, but cone basalts are enriched with high Nb/Ti and 87Sr/86Sr, and low 143Nd/144Nd and 176Hf/177Hf ratios. The depleted signature of the platform basalts is similar to that of the abundant normal type basalts of Shatsky Rise with an isotope composition that overlaps with that of normal mid-ocean ridge basalts (N-MORB), whereas trace elements are enriched (e.g., higher Nb/Zr and Nb/Ti than N-MORB). However, slight geochemical differences between the Ojin platform basalts and Shatsky normal type basalts exist and are expressed in higher 208Pb/204Pb isotope ratios of the former. The depleted signature implies shallow mantle dilution of the (enriched) Shatsky plume tail near a ridge. In contrast, the enriched signature of the cone basalts indicates little contribution from N-MORB source mantle as (enriched) plume volcanism moved away from the ridge. The enriched features (Nb/Zr and 176Hf/177Hf) of the Ojin Rise cone basalts increase with increasing distance from Shatsky Rise and conform with a geochemical evolution model that involves increasing contribution from a highly enriched Hess Rise source component with time. Unlabelled Image • Several Ojin Rise seamounts consist of lower platform and upper volcanic cones. • Geochemical data show that platform lavas are depleted but cone lavas are enriched. • Platform lavas were probably produced by near-ridge plume tail volcanism. • Cone lavas may have been formed by plume tail volcanism away from the ridge. [ABSTRACT FROM AUTHOR]