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 ( ...super(206)Pb/ super(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) sub(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 ( super(206)Pb/ super(204)Pb of 21.7-79.3 and super(208)Pb/ super(204)Pb of 40.8-89.3), super(87)Sr/ super(86)Sr of 0.7018-0.7028, super(143)Nd/ super(144)Nd of 0.51274-0.51285, and super(176)Hf/ super(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.
Pb isotope compositions of melt inclusions provide unique information about the composition of primary magmas and their source. In this study, we have developed a method for measuring Pb isotopes in ...small olivine-hosted melt inclusions (>40 μm) from young and old volcanoes by LA-MC-ICP-MS. We used a new interface cone assemblage consisting of a Jet sample cone and X skimmer cone. A small flow of N 2 gas was added to the carrier gas and passed through the assemblage to enhance the signal intensity. In addition the energy and repetition rate of the laser conditions were reduced and the signal integration time was shortened in order to lengthen the laser ablation time and to collect enough data. Mass bias and instrument drift were corrected using a standard–sample–standard bracketing method. The analysis routine employed eight ion counters to receive 238 U, 235 U, 232 Th, 208 Pb, 207 Pb, 206 Pb, 204 Pb and 202 Hg signals simultaneously, which allowed Hg interference to be corrected on 204 Pb, and in old samples U–Th decay to be age-corrected. Using the Jet and X cones, under the same laser ablation conditions, the precisions for almost all the measured standard glasses are improved by at least a factor of two compared to using standard cones. At 208 Pb signal intensity >200 000 cps, external precisions of ratios involving 204 Pb are better than 1.3% (2RSD) and precisions of 208 Pb/ 206 Pb and 207 Pb/ 206 Pb are better than 0.23% (2RSD). The results of Pb isotopes in olivine-hosted melt inclusions, using 45 μm laser spots, show that the internal precisions of 208 Pb/ 206 Pb and 207 Pb/ 206 Pb for most analyzed melt inclusions are better than 0.2% (2RSE) and for ratios involving 204 Pb are better than 0.8% (2RSE). We are able to present the first ever Pb isotope data from ∼260 Ma Emeishan flood basalt olivine-hosted melt inclusions. They show the importance to do age correction which results in the reduction of the spread of data in old samples. The mean values of age-corrected 208 Pb/ 206 Pb and 207 Pb/ 206 Pb have 1.2% and 2.8% deviations from the uncorrected mean values, respectively. The method developed here provides a fast, precise and accurate in situ Pb isotopic composition analysis, applicable not only to melt inclusions from young basalts, but also from old samples that require correction for U–Th decay.
Basaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global ...mantle dynamics. In this study, we present new whole‐rock major and trace element abundances, Sr‐Nd‐Mo‐Os isotopes, water contents and He isotopes of volcanic glasses, U‐Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid‐ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine‐dominated (peridotitic) mantle source. There is currently no evidence for a high‐3He/4He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes.
Key Points
Late Cenozoic lavas are predominantly derived from a peridotitic source
Recycled young crust and sediment were involved in the mantle source
Late Cenozoic magmatism originated from upwelling of the mantle transition zone
Nishinoshima, a submarine volcano in the Ogasawara Arc, approximately 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Olivine‐bearing phenocryst‐poor ...andesites found in older submarine lavas from the flanks of the volcano have been used to develop a model for the genesis of andesitic lavas from Nishinoshima. In this model, primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. Thus, it only operates beneath Nishinoshima and submarine volcanoes in the Ogasawara Arc and other oceanic arcs, where the crust is thin. The primary magma compositions have changed from basalt, produced at considerable depth, to andesite, produced beneath the existing thinner crust at this location in the arc. This reflects the thermal and mechanical evolution of the mantle wedge and the overlying lithosphere. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past. Andesites produced by this shallow and hydrous melting of the mantle could accumulate through collisions of plates to generate continental crust.
Nishinoshima, a submarine volcano in the Ogasawara Arc, ~ 1 000 km south of Tokyo, Japan, suddenly erupted in November 2013, after 40 years of dormancy. Primary andesite magmas originate directly from the mantle as a result of shallow and hydrous melting of plagioclase peridotites. It is suggested that continental crust‐like andesitic magma builds up beneath submarine volcanoes on thin arc lithosphere today, and has built up beneath such volcanoes in the past.
西之島は東京から 1 000 km 南に位置する, 小笠原弧の海底火山である. 2013 年 11 月, 西之島は 40 年ぶりに噴火した. 陸上と海底の調査により, 西之島海底火山の本体が安山岩であることが明らかとなり, 海底から採取された溶岩 (斑晶に乏しく, かんらん石を含有する安山岩) の分析からは, 安山岩マグマの新しい成因モデルが提示された. マントルの浅い部分 (30 km 以浅) は斜長石かんらん岩で構成され, この低圧マントルの含水下の部分融解により, 初生安山岩マグマが生成した. このマグマから, 主にかんらん石が結晶分別することによって, 西之島の安山岩マグマがつくられた. 一方, 西之島付近の古い時代の小海丘は玄武岩でできており, 初生マグマの組成が, 玄武岩質から安山岩質へと, 時間とともに変化したことも示された. すなわち, マントルの融解深度が地殻直下へと浅くなってきたことが示された. 地殻の薄い海洋島弧において生成された安山岩は, プレートの衝突帯で集積することにより, 大陸へと成長していく, と考えられる.
It is common knowledge that the Moho is the boundary between the crust and the Earth's mantle. Here we show along several seismic profiles through the Pacific Plate that a correlation exists between ...the strength of Moho reflections, crustal thickness, and water depth. Where the Moho can be detected clearly, the overlying oceanic crust is systematically thicker and the water depths are shallower. We suggest that two end‐members of oceanic crust exist in fast spreading environments: one thick, underlain by a clear Moho; the other thinner, without a Moho; with all intermediate situations. In the Oman ophiolite, the best‐preserved on‐land analogue of fossil oceanic lithosphere created by fast‐spreading, the boundary between the mantle peridotites and the lower crustal gabbros mainly consists of a dunitic transition zone (DTZ) ranging from a few meters to a few hundred meters in thickness. A sudden influx of seawater down to the base of the crust at the mid‐ocean ridge (MOR) results in the hydrous (re‐)melting of mantle peridotites, producing a dunitic residue at the crust–mantle boundary that represents the most reflective Moho. At the same time, the hydrous melting, in addition to the normal decompression melting, beneath the MOR, increases the thickness of the oceanic crust by enhancing magma production. In the absence of hydrous melting, the DTZ is thin or absent at the crust–mantle boundary, and instead the uppermost mantle harzburgite is intruded by gabbros, and/or the overlying crustal gabbro is intruded by numerous wehrlite bodies, which will be seismically gradational.
Numerous geochemical studies have indicated that the Hawaiian mantle plume consists of several distinct components. However, their origin remains controversial, with a number of different ...interpretations having been proposed. We present new major element, trace element and high-precision Sr–Nd–Pb–He isotope data for a suite of fresh submarine lavas erupted by the Koolau, Kilauea and Loihi volcanoes, which are widely believed to have sampled three distinct Hawaiian plume components. The Sr and Nd isotope compositions of the Loihi lavas are similar to those of Kilauea lavas. However, our double-spike Pb isotopic data show that Loihi lavas have both Kilauea-like and Loihi-like compositions. This discovery implies that the Loihi source region contains a Kilauea-like (‘Kea’) mantle component. Our new data support the existence of three major types of intrinsic plume component: a Loihi component, an ‘enriched’ (Koolau) component and a ‘depleted’ (Kea) component. We propose that the Loihi component is a common component, forming the matrix in the Hawaiian mantle plume, and that the isotopic differences between the various shield lavas reflect different mixing proportions of the Loihi component and recycled oceanic crust components (EM-1-like and HIMU-like). The Koolau component contains a higher proportion of EM-1, whereas the Kea component contains a higher proportion of HIMU. EM-1- and HIMU-like recycled oceanic crust components are distributed on a fine scale throughout the peridotitic matrix within the Hawaiian plume. Both components are present in the sources beneath Kea- and Loa-trend volcanoes. We infer that the thermal structure and spatially distributed compositional heterogeneity of the plume are important in controlling the isotopic composition of lavas from a given Hawaiian volcano.
Silicic caldera volcanoes present major volcanic and seismic hazards but also host dynamic hydrothermal and groundwater systems and a rich but largely unexplored subsurface biosphere. Many of these ...volcanoes are hosted in rift settings. The intricate connections and feedbacks among magmatism, rifting, hydrothermal processes, and the biosphere in these complex systems remain poorly understood, necessitating subsurface joint observations that are only enabled by scientific drilling. The CALDERA (Connections Among Life, geo-Dynamics and Eruptions in a Rifting Arc caldera) project workshop funded by the International Continental Scientific Drilling Program (ICDP) gathered multi-disciplinary international experts in January 2023 to advance planning of a scientific drilling project within one of these dynamic, rift-hosted calderas, the Okataina Volcanic Centre (OVC), Aotearoa New Zealand.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Understanding the physicochemical conditions extant and mechanisms operative during explosive volcanism is essential for reliable forecasting and mitigation of volcanic events. Rhyolitic pumices ...reflect highly vesiculated magma whose bubbles can serve as a strain indicator for inferring the state of stress operative immediately prior to eruptive fragmentation. Obtaining the full kinematic picture reflected in bubble population geometry has been extremely difficult, involving dissection of a small number of delicate samples. The advent of reliable high-resolution tomography has changed this situation radically. Here we demonstrate via the use of tomography how a statistically powerful picture of the shapes and connectivity of thousands of individual bubbles within a single sample of tube pumice emerges. The strain record of tube pumice is modelled using empirical models of bubble geometry and liquid rheology, reliant on a constraint of magmatic water concentration. FTIR analysis reveals an imbalance in water speciation, suggesting post-eruption hydration, further supported by hydrogen and oxygen isotope measurements. Our work demonstrates that the strain recorded in the tube pumice dominated by simple shear (not pure shear) in the late deformational history of vesicular magma before eruption. This constraint in turn implies that magma ascent is conditioned by a velocity gradient (across the conduit) at the point of origin of tube pumice. Magma ascent accompanied by simple shear should enhance high eruption rates inferred independently for these highly viscous systems.
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