Abstract
How serpentinites in the forearc mantle and subducted lithosphere become involved in enriching the subarc mantle source of arc magmas is controversial. Here we report molybdenum isotopes for ...primitive submarine lavas and serpentinites from active volcanoes and serpentinite mud volcanoes in the Mariana arc. These data, in combination with radiogenic isotopes and elemental ratios, allow development of a model whereby shallow, partially serpentinized and subducted forearc mantle transfers fluid and melt from the subducted slab into the subarc mantle. These entrained forearc mantle fragments are further metasomatized by slab fluids/melts derived from the dehydration of serpentinites in the subducted lithospheric slab. Multistage breakdown of serpentinites in the subduction channel ultimately releases fluids/melts that trigger Mariana volcanic front volcanism. Serpentinites dragged down from the forearc mantle are likely exhausted at >200 km depth, after which slab-derived serpentinites are responsible for generating slab melts.
Advent of Continents: A New Hypothesis Tamura, Yoshihiko; Sato, Takeshi; Fujiwara, Toshiya ...
Scientific reports,
09/2016, Letnik:
6, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The straightforward but unexpected relationship presented here relates crustal thickness to magma type in the Izu-Ogasawara (Bonin) and Aleutian oceanic arcs. Volcanoes along the southern segment of ...the Izu-Ogasawara arc and the western Aleutian arc (west of Adak) are underlain by thin crust (10-20 km). In contrast those along the northern segment of the Izu-Ogasawara arc and eastern Aleutian arc are underlain by crust ~35 km thick. Interestingly, andesite magmas dominate eruptive products from the former volcanoes and mostly basaltic lavas erupt from the latter. According to the hypothesis presented here, rising mantle diapirs stall near the base of the oceanic crust at depths controlled by the thickness of the overlying crust. Where the crust is thin, melting occurs at relatively low pressures in the mantle wedge producing andesitic magmas. Where the crust is thick, melting pressures are higher and only basaltic magmas tend to be produced. The implications of this hypothesis are: (1) the rate of continental crust accumulation, which is andesitic in composition, would have been greatest soon after subduction initiated on Earth, when most crust was thin; and (2) most andesite magmas erupted on continental crust could be recycled from "primary" andesite originally produced in oceanic arcs.
The 2021 eruption of Fukutoku-Oka-no-Ba (FOB) in the northwest Pacific on 13 August 2021 produced a large volume of pumice that drifted westward for ~1300 km to the Nansei Islands, Japan, and some ...extent. In February 2022, pumice with similar characteristics to the FOB pumice was deposited along the Gulf of Thailand. The pumice clasts deposited in Songkhla Province, Thailand, were fine-grained with <4 cm in size and rounded. Most of the clasts consisted of clinopyroxene, plagioclase (andesine), and olivine phenocrysts in a vesiculated grey groundmass, with black-coloured spots exhibiting signatures of a basaltic magma. The whole-rock compositions of the pumice are trachytic, with 61 mass% SiO2 and 9 mass% total alkali (Na2O + K2O). The overall characteristics in the pumice from Thailand are similar to those in FOB pumice. These pumice in Thailand were from the 2021 FOB eruption, and drifted >2800 km south-westward across the South China Sea.
Understanding the age and dynamics of the overriding plates allows an assessment of competing subduction initiation hypotheses. The Izu‐Bonin‐Mariana margin in the Western Pacific is a key example of ...initiation and hence it is important to constrain the age and origin of the oldest igneous crust of the supra‐subduction Philippine Sea Plate. We present geochronological and geochemical data of igneous rocks from the oldest ocean basins of the Philippine Sea Plate: the West Philippine and Palau Basins. Basalts from these basins have enriched geochemical characteristics similar to the EM‐2‐like mantle component found in OIB‐like basalts associated with the Oki‐Daito mantle plume. Ages of basalts from the northernmost West Philippine Basin (WPB) and the Palau Basin range from 43.5 to 50.5 Ma, which is similar to the oldest samples associated with the Oki‐Daito mantle plume (48–50 Ma). This implies that the plume contributed to magmatism from the onset of basin formation. It also provides support for the proposition that rifting of the Mesozoic arc terrane and subsequent seafloor spreading of the WPB was triggered by the arrival of the Oki‐Daito mantle plume at the base of the lithosphere. The age of these Philippine Sea Basins implies that only the Mesozoic Daito Ridge Group and the Gagua Ridge existed as Philippine Sea Plate crust before subduction initiation. A major fault activity after 37 Ma in the northernmost WPB demonstrates that careful reconstruction of the Eocene Philippine Sea Plate is critical to understanding plate dynamics during subduction initiation in the Western Pacific.
Plain Language Summary
This study investigates the formation of the crust beneath the Philippine Sea Plate. An outcome of this research is a clearer understanding of the plate tectonic configuration in the Western Pacific at the point when volcanic activity started. We collected rock samples from what is thought to be the oldest sections of the West Philippine Basin and the Palau Basin, and determined their age and chemical composition. These rocks were erupted between 43.5 and 50.5 million years ago, and are of a similar age and composition to the volcanic rocks known to be associated with the Oki‐Daito mantle plume (48–50 million years old). These similarities imply that the initial volcanism on the Philippine Sea Plate was triggered by the arrival of the upwelling Oki‐Daito mantle plume. A consequence of the hot and buoyant mantle plume was to stretch the existing crust and develop a new ocean basin by volcanic activity. The new ages for the oldest ocean basins in the Philippine Sea Plate reveal that only the Mesozoic arc terrane existed as the Philippine Sea Plate crust before subduction initiation. This provides new constraints on the conditions under which subduction can initiate between the two plates.
Key Points
New dating indicates that a major part of the oldest Philippine Sea Plate basins formed after c. 51 Ma
Ocean crust of the oldest Philippine Basins is affected by the Oki‐Daito mantle plume
Onset of spreading of the West Philippine Basin could have been triggered when the Oki‐Daito plume hit the base of preexisting lithosphere
New Sr, Nd, Hf, and Pb isotope and trace element data are presented for basalts erupted in the Izu back arc. We propose that across‐arc differences in the geochemistry of Izu‐Bonin arc basalts are ...controlled by the addition of aqueous slab fluids to the volcanic front and hydrous partial melt of the slab to the back arc. The volcanic front has the lowest concentrations of incompatible elements, the strongest relative enrichments of fluid‐mobile elements, and the most radiogenic Sr, Nd, Hf, and Pb, suggesting the volcanic front is the result of high degrees of partial melting of a previously depleted mantle source caused by an aqueous fluid flux from the slab. Relative to the volcanic front, the back arc has higher concentrations of incompatible elements and elevated La/Yb and Nb/Zr, suggesting lower degrees of partial melting of a less depleted or even enriched mantle source. Positive linear correlations between fluid‐immobile element concentrations and the estimated degree of mantle melting suggest the slab contribution added to the mantle wedge in the Izu back arc is a supercritical melt. Pb, Nd, and Hf isotopes and Th/La systematics of back‐arc basalts are consistent with a slab melt composed of >90% altered oceanic crust and <10% sediment; that is, altered oceanic crust, not subducted sediment, dominates the slab contribution. High field strength element systematics require supercritical melts to be in equilibrium with residual rutile and zircon.
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.
Nanometer-sized crystals (nanolites) play an important role in controlling eruptions by affecting the viscosity of magmas and inducing bubble nucleation. We present detailed microscopic and ...nanoscopic petrographic analyses of nanolite-bearing and nanolite-free pumice from the 2021 eruption of Fukutoku-Oka-no-Ba, Japan. The nanolite mineral assemblage includes biotite, which is absent from the phenocryst mineral assemblage, and magnetite and clinopyroxene, which are observed as phenocrysts. The boundary between the nanolite-bearing brown glass and nanolite-free colorless glass is either sharp or gradational, and the sharp boundaries also appear sharp under the transmitted electron microscope. X-ray absorption fine structure (XAFS) analysis of the volcanic glass revealed that the nanolite-free colorless glass records an oxygen fugacity of QFM + 0.98 (log units), whereas the nanolite-bearing brown glass records a higher apparent oxygen fugacity (~ QFM + 2). Thermodynamic modelling using MELTS indicates that higher oxygen fugacities increase the liquidus temperature and thus induced the crystallization of magnetite nanolites. The hydrous nanolite mineral assemblage and glass oxygen fugacity estimates suggest that an oxidizing fluid supplied by a hot mafic magma induced nanolite crystallization in the magma reservoir, before the magma fragmentation. The oxidation-induced nanolite crystallization then enhanced heterogeneous bubble nucleation, resulting in convection in the magma reservoir and triggering the eruption.
The processes that create continental crust in an intraoceanic arc setting are a matter of debate. To address this issue, we conducted an active source wide‐angle seismic study to examine along‐arc ...structural variations of the Izu intraoceanic arc. The data used were acquired over a 550‐km‐long profile along the volcanic front from Sagami Bay to Tori‐shima. The obtained structural model showed the existence of felsic to intermediate composition middle crust with a P wave velocity (Vp) of 6.0–6.5 km s−1 in its upper part and 6.5–6.8 km s−1 in its lower part. The thickness of the middle crust varied markedly from 3 to 13 km. The underlying lower crust also consisted of two layers (Vp of 6.8–7.2 km s−1 in the upper part and Vp of 7.2–7.6 km s−1 in the lower part). The upper of these layers was interpreted to consist of plutonic gabbro, and the lower layer was interpreted to be mafic to ultramafic cumulates. Average crustal velocities calculated from our model showed remarkable lateral variation, which correlated well with arc volcanism. Low average crustal seismic velocities (∼6.7 km s−1), due to thick middle crust, were obtained beneath basaltic volcanoes (e.g., O‐shima, Miyake‐jima, Hachijo‐jima, Aoga‐shima), while higher average velocities (∼7.1 km s−1) were obtained beneath rhyolitic volcanoes (e.g., Nii‐jima, Kurose, South Hachijo caldera, Myoji knoll, and South Sumisu caldera). We concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the Izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution, or remelting of preexisting continental crust, or both.
This study reports the preliminary results of the first analysis of benthic foraminifera of seafloor surface sediments from the Nishinoshima volcanic area. The samples were collected in 2015 during ...several DEEP TOW (deep ocean floor towed survey system) dredges on the summit and flank of Nishinoshima-Minami Knoll, a knoll ∼8 km southeast of Nishinoshima Island that forms part of the same submarine volcanic edifice. The two stations DT-1170 (N9) and DT-1173 (N12) were sampled from water depths of 1062–1015 m and 516–203 m, respectively, and were located at distances of 10 and 8 km from the main Nishinoshima edifice, respectively. The two stations displayed a typical faunal structure for deep-sea environments, with low foraminiferal densities but highly diversified assemblages. A total of 131 species, excluding undetermined porcelaneous and hyaline species groups, were found, and are recent deep-sea benthic foraminifera previously identified in the bathyal depths around Izu-Bonin volcanic arc, or species found in the Neogene land-based strata in the western Pacific region. We conclude that the bathyal benthic foraminiferal assemblages in the studied area are likely to be free from recent volcanic activity. We provide detailed taxonomic descriptions of the 21 commonly occurring species. This exploratory study therefore provides crucial basic information about benthic foraminiferal faunas in the Nishinoshima area, which could be used in future environmental analysis of this highly dynamic region.
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