1,721,009 research outputs found
Using high-resolution Pb isotopes to unravel the petrogenesis of Sakurajima Volcano, Japan
No full textSakurajima volcano on the rim of Aira caldera erupts daily, threatening the major nearby population centre of Kagoshima. Before 1955, eruptions have typically consisted of intermittent plinian and effusive activity, but since 1955 frequent vulcanian explosions have occurred, indicating a change in pre-eruptive processes. High resolution Pb isotopes are used here to constrain the components, including crustal assimilants, and petrogenetic processes contributing to the composition of both pre and post 1955 magmas. Sakurajima eruptive products have more radiogenic Pb (206Pb/204Pb = 18.40-18.42) than nearby Sumiyoshiike basalts (206Pb/204Pb = 18.24); a proxy for the primitive magma feeding the Sakurajima-Aira system. Sakurajima samples lie along a mixing line between these basalts and locally exposed crustal compositions. Their Pb isotopes are consistent with addition of ~5 % average crustal melt to the primitive basaltic magma. The narrow range of Pb isotope ratios, despite variable SiO2 contents (56.6 – 72.3 wt %), suggests that the final erupted magmas are derived from fractional crystallization of a mafic precursory magma displaying relatively consistent levels of crustal contamination. Andesites erupted between 4-3.7 ka and the 1995 eruption are contaminated to a greater extent, indicating that magmas with distinct compositions can feed phases of activity or individual eruptions. Post 1955 andesitic pyroclastics have lower SiO2 and higher MgO than older lavas, yet equivalent Pb isotope ratios. The more mafic composition of post 1955 eruptive products can be attributed to increased throughput ofmafic magma to the system
Two contrasting magmatic types coexist after the cessation of back-arc spreading
No full textAmongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300 km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC).Chemically, enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that the back-arc seamount chains and the East Shikoku Basin seamounts have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism and distinct from the MORB-like lavas of the Shikoku back-arc basin. 40Ar/39Ar ages range from 15.7 to 9.6 Ma for the East Shikoku Basin seamounts, indicating this arc magmatism started immediately after the Shikoku Basin stopped spreading.Although the KSC volcanoes are found to be contemporaneous with the seamount chains and East Shikoku Basin seamounts, their chemical characteristics are very different. Unlike the calc-alkaline seamount chains, the KSC lavas range from medium-K to shoshonitic alkaline basalt. Their trace element characteristics indicate the absence of a subduction influence and their radiogenic isotope systematics reflect a mantle source combining a Philippine Sea MORB composition and an enriched mantle component (EM-1). One of the most remarkable features of the KSC is that their geochemistry has a distinct temporal variation. Element ratios such as Nb/Zr and concentrations of incompatible elements such as K2O increase with decreasing age and reach a maximum at ca. 7 Ma when the KSC ceased activity.<br/
Triggering of major eruptions recorded by actively forming cumulates
No full textMajor overturn within a magma chamber can bring together felsic and mafic magmas, prompting de-volatilisation and acting as the driver for Plinian eruptions. Until now identification of mixing has been limited to analysis of lavas or individual crystals ejected during eruptions. We have recovered partially developed cumulate material (‘live’ cumulate mush) from pyroclastic deposits of major eruptions on Tenerife. These samples represent “frozen” clumps of diverse crystalline deposits from all levels in the developing reservoir, which are permeated with the final magma immediately before eruptions. Such events therefore record the complete disintegration of the magma chamber, leading to caldera collapse. Chemical variation across developing cumulus crystals records changes in melt composition. Apart from fluctuations reflecting periodic influxes of mafic melt, crystal edges consistently record the presence of more felsic magmas. The prevalence of this felsic liquid implies it was able to infiltrate the entire cumulate pile immediately before each eruption
Geochemical evolution of arc and slab following subduction initiation: a record from the Bonin Islands, Japan
No full textVolcanism following the initiation of subduction is vital to our understanding of this specific magmageneration environment. This setting is represented by the first development of the Izu–Bonin–Mariana arc system as subduction commenced along the Western Pacific margin in the Eocene. A new collectionof volcanic rocks recovered from the islands and exposed crustal sections of the Bonin Ridge spans the first 10Myr of arc evolution. An elemental and radiogenic isotope dataset from this material is presented in conjuction with new 40Ar/39Ar ages and a stratigraphic framework developed by a detailed mapping campaign through the volcanic sections of the Bonin Islands. The dating results reveal that both the locus and type of magmatism systematically changed with time in response to the progressive sinking of the slab until the establishment of steady-state subduction at around 7–8Ma.Following initial mid-ocean ridge basalt (MORB)-like spreading-related basalt magmatism, volcanic centres migrated away from the trench and changed from high-Si boninite to low-Si boninite or high-Mg andesite, then finally tholeiitic or calcalkaline arc magma. Subducting pelagic sediment combined with Pacific-type igneous ocean crust dominates the slab input to the shallow source of high-Si boninites at 49 Ma, but high-precision Pb isotope data show that this sediment varies in composition along the subducting plate. At around 45Ma, volcanism switched to low-Si boninite and the pelagic sediment signature was almost entirely replaced by volcanic or volcaniclastic material originating from a HIMUocean island source. These low-Si boninites are isotopically consistent with a slab component comprising variable proportions of HIMU volcaniclastic rocks and Pacific MORB. In turn, this signature was replaced by a Pacific MORB-dominated flux in the post 45Ma tholeiite and calcalkaline volcanic rocks.Notably, each change in slab-derived flux coincided with a change in the magma type. Fluctuations in the slab-derived geochemical signature were superimposed on a change in the mantle wedge source from highly depleted harzburgite to a depleted MORB-type mantle-type source. In turn, this may correspondto the increasing depth of the leading edge of the slab through this 5Myr period
Late seafloor carbonate precipitation in serpentinites from the Rainbow and Saldanha sites (Mid-Atlantic Ridge)
No full textSerpentinized ultramafic rocks recovered during several recent oceanographic missions (1997-2002) on the Rainbow hydrothermal field and on the Saldanha seamount (36°14'N and 36°34'N, Mid-Atlantic Ridge) often exhibit late-stage carbonatization associated to secondary oxidation effects. These carbonate occurrences, mostly vein-filling aragonite, occasionally form dense webs almost completely engulfing and replacing the serpentinite itself.The 87Sr/86Sr (approx. 0.709) and stable isotope signatures (?13CPDB = 0.2-3.3 ‰; ?18OV-SMOW = 32.2-35.2 ‰) of the carbonate fraction in these serpentinites indicate carbonate precipitation from unmodified seawater, under abiotic conditions, and very low temperatures, close to bottom-water temperatures measured at the sampling stations. These analytical data imply that, unlike the serpentinite-hosted carbonate chimneys in the Lost City hydrothermal field (Ludwig et al., 2006), the vein-filling aragonite in the Rainbow and Saldanha serpentinites has a non-hydrothermal, low-temperature seawater origin. <br/
Large-volume lateral magma transport from the Mull volcano: An insight to magma chamber processes
No full textLong-distance lateral magma transport within the crust has been inferred for various magmatic systems including oceanic island volcanoes, mid-oceanic ridges, and large igneous provinces. However, studying the physical and chemical properties of active fissure systems is difficult. Hence, this study investigates the movement of magma away from the Mull volcano in the North Atlantic Igneous Province, where erosion has exposed its upper crustal dike networks. Magmatic lineations within dikes indicate that the magma flow in the Mull dike suite changed from near vertical to horizontal within 30 km of the volcanic center. This implies that distal dikes were fed by lateral magma transport from Mull. Geochemical characteristics indicate that many <50 km long dikes have deep crustal signatures, reflecting storage and assimilation in Lewisian basement. Following crystallization and assimilation in the lower crust, magma fed an upper crustal reservoir, where further fractionation and incorporation of Moinian rocks generated felsic compositions. Distal dikes are andesitic and reflect events in which large volumes of mafic and felsic magma were combined by mixing between lower and upper crustal reservoirs to generate the 30–80 km3 required to supply the long-distance dikes. Once propagated, compositions along dikes were not significantly affected by assimilation and crystallization. Supplying the distal dikes with magma would have required a large-scale evacuation of the crustal reservoirs that acted as a potential trigger for explosive volcanism and the caldera formation recorded in Mull central complex
Isotopic characteristics of subduction fluids in an intra-oceanic setting, Izu–Bonin Arc, Japan
No full textNew radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active Izu–Bonin arc, the Oligocene Izu arc, and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene–Recent Izu–Bonin frontal-arc lavas are highly depleted in Zr, Nb and the rare-earth elements relative to typical mid-ocean ridge basalt (MORB), indicating that the mantle wedge source has undergone a previous episode of melting. Ratios between these elements (such as Nb/Zr and La/Sm), as well as 143Nd/144Nd, do not vary significantly along the length of the frontal-arc. These parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and 87Sr/86Sr increase along the frontal-arc to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimposed on the mantle wedge. Sr–Nd and Pb–Nd isotope variation indicate that both Sr and Pb become more radiogenic after fluid addition. However, Pb isotope ratios do not correlate with increases in Pb concentration or ratios such as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the arc lavas appears to be independent of the amount of Pb introduced by subduction fluids into the mantle source. This buffering of Pb isotopes along the frontal-arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid. Isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge has isotopic characteristics similar to Indian Ocean MORB, and that the subduction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu arc and fore-arc basin were derived from a mantle wedge of similar composition to the active arc. Despite levels of Pb enrichment comparable to those of the modern arc, the Pb isotopes of the Oligocene volcanics indicate a lower sediment input into the melting region
Magmatic evolution during proto-oceanic rifting at Alu, Dalafilla and Borale Volcanoes (Afar) determined by trace element and Sr-Nd-Pb isotope geochemistry
No full textContinental rifting and associated magmatism can eventually result in the formation of new ocean basins. However, the characteristics of magmatism in the latest stages of rifting are poorly understood. The Erta-Ale volcanic segment (EAVS) in the Danakil Depression of Afar, Ethiopia, provides a unique natural laboratory in which to investigate how magma generation evolves during the shift from continental rifting to oceanic spreading. Here we present new trace element data combined with Sr-Nd-Pb isotope ratios for three volcanoes, Alu, Dalafilla and Borale, in the north of the EAVS. These data shed light on the changes in melt production and storage that occur at this late stage in the rifting cycle. Elevated Ce/Pb and ΔNb (33–48, 0.25–0.47 respectively) of the basalts, alongside Sr-Nd-Pb isotope geochemistry indicate the presence of a HIMU component, supplied by the Afar plume, together with contamination by the crust. Melting conditions, estimated using the trace element ratios, Smn/Ybn, Dyn/Ybn and Cen/Smn, indicate that magmas were primarily derived from spinel lherzolite (85–90%) with minor garnet lherzolite (10–15%) with a melt fraction of ~4%. Melt-mantle equilibrium depths are estimated to be on the order of 64 to 83 km, shallower than that previously inferred within Afar. We suggest that this is likely a result of the more plate thinning beneath the EAVS compared to other parts of Afar. Basaltic volcanics are found to have heterogeneous Sr-Nd-Pb isotope compositions whilst those more evolved rocks (i.e., SiO2 ≥52 wt%) exhibit consistent radiogenic compositions. This indicates that homogenisation of all melt compositions occurs prior-to or during melt differentiation, with the latter process occurring rapidly in upper crust with minimal crustal contamination. Overall whilst the Afar plume appears to be the dominant mantle component in the volcanic rocks, the melt characteristics and magmatic storage conditions beneath the EAVS shows variability that is likely controlled by a dynamic interplay between rifting and mantle processes
Making and breaking an island arc: A new perspective from the Oligocene Kyushu-Palau arc, Philippine Sea
No full textThe Kyushu-Palau Ridge (KPR) is a 2600 km long remnant island arc that is separated from the active Izu-Bonin-Mariana (IBM) arc by a series of spreading and rift basins. We present 40Ar/39Ar ages and geochemical data for the entire length of the Kyushu-Palau arc as well as for the conjugate arc which is stranded within the IBM fore arc. New 40Ar/39Ar ages indicate that the KPR was active between 25 and 48 Ma, but the majority of the exposed volcanism occurred in the final phase, between 25 and 28 Ma. Rifting of the Kyushu-Palau arc to form the Shikoku and Parece Vela basins occurred simultaneously along the length of the arc (circa 25 Ma), and at a similar distance from the trench. Unlike the IBM, the KPR has only limited systematic along-arc geochemical trends. Two geochemical components within the KPR indicate an origin in the suprasubduction mantle. First, EM-1-like lavas are identified in a restricted section of the arc, suggesting a localized heterogeneity. Second, EM-2-like arc volcanoes formed on juvenile West Philippine Basin crust, potentially reflecting ingress of mantle from the then active EM-2 province which lies in the west. Another geochemical heterogeneity is found at the KPR–Daito Ridge intersection where the arc developed on preexisting Cretaceous Daito Ridge crust. The geochemical characteristics at this intersection likely result from the involvement of sub–Daito Ridge lithospheric mantle. Subduction flux beneath the KPR generally matches post–45 Ma Eocene/Oligocene lavas in the IBM fore arc, involving fluids and melts derived from altered igneous crust
Melt generation and evolution in the Adda’do rift segment of the Afar rift from trace elements and petrography
No full textAlong-rift variations in the stage of continental separation are observed in the northern East African Rift System (EARS), from magma-assisted continental rifting in the Main Ethiopian Rift (MER) to nascent oceanic spreading in Afar. However, the implications on spatial and temporal changes in mantle melting and melt evolution remain poorly understood. Given that the EARS is the longest and best-exposed example of continental rifting in the world, the MER and Afar are an ideal place to investigate magmatism and volcanism in late-stage continental rifts. Here, we focus on the Adda’do Magmatic Segment (AMS) in the northernmost sector of the MER, that has experienced the most prolonged lithospheric thinning. We present new trace element data and petrographic observations from around 50 samples, and combine these with geochemical modelling to investigate depth of melt origin and melt evolution, in the AMS. Using mixing modelling of garnet lherzolite and spinel lherzolite mantle peridotite sources, we show that the AMS magmas are produced from a relatively deep source with 10%–60% garnet lherzolite, corresponding to depths of around 85 km, and generated by approximately 4%–9.5% partial melting of the mantle. We find no significant variation of these characteristics with either sample age or sample location at the AMS, suggesting no systematic temporal variations occurred in either the depth or the degree of melting within a single magmatic segment. However, on a regional scale, depth of melting is between that interpreted for the MER and northern Afar, implicating the stage of rift evolution and consequent degree of lithospheric thinning as a major control. MELTS modelling of the samples indicates that the observed variations in sample compositions in the AMS can be explained predominantly by fractional crystallisation, with negligible crustal contamination in the basaltic samples. Crustal contamination may play a greater role in the composition of intermediate and evolved samples in the AMS.</p
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