Large-scale structures in the Earth’s interior: Top-down hemispherical dynamics constrained by geochemical and geophysical approaches

Geochemical and geophysical observations for large-scale structures in the Earth’s interior, particularly horizontal variations of long wavelengths such as degree-1 and degree-2 structures, are reviewed with special attention to the cause of hemispherical mantle structure. Seismic velocity, electrical conductivity, and basalt geochemistry are used for mapping the large-scale structures to discuss thermal and compositional heterogeneities and their relations to dynamics of the Earth’s interior. Seismic velocity structure is the major source of information on the Earth’s interior and provides the best spatial resolution, while electrical conductivity is sensitive to water/hydrogen contents. The composition of young basalts reflects the mantle composition, and the formation age of large-scale structures can be inferred based on the radiogenic isotopes. Thus, these different research disciplines and methods complement each other and can be combined to more concretely constrain the structures and their origins. This paper aims to integrate observations from these different approaches to obtain a better understanding of geodynamics. Together with numerical modeling results of convection in the mantle and the core, “top-down hemispherical dynamics” model of the crust-mantle-core system is examined. The results suggest that a top-down link between the supercontinents, mantle geochemical hemisphere, and inner core seismic velocity hemisphere played an essential role in formation of the large-scale structures and dynamics of the Earth’s interior

Сучасні стратегії розвитку науки

Робоча програма з дисципліни «Стратегії розвитку науки» за спеціальністю 6.030302 «Реклама та зв’язки з громадськістю» галузі знань 0303 «Журналістика та інформація», освітньо-кваліфікаційного рівня «бакалавр». – 2016. – 48 с

Progressive evolution of whole-rock composition during metamorphism revealed by multivariate statistical analyses

The geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study applies k‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO₂ and Na₂O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the same P–T conditions; (2) with higher‐grade whole‐rock compositions, more H₂O is retained. These results provide insight into the mechanism suppressing dehydration under high‐P metamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed

A cold seep triggered by a hot ridge subduction

The Chile Triple Junction, where the hot active spreading centre of the Chile Rise system subducts beneath the South American plate, offers a unique opportunity to understand the influence of the anomalous thermal regime on an otherwise cold continental margin. Integrated analysis of various geophysical and geological datasets, such as bathymetry, heat flow measured directly by thermal probes and calculated from gas hydrate distribution limits, thermal conductivities, and piston cores, have improved the knowledge about the hydrogeological system. In addition, rock dredging has evidenced the volcanism associated with ridge subduction. Here, we argue that the localized high heat flow over the toe of the accretionary prism results from fluid advection promoted by pressure-driven discharge (i.e., dewatering/discharge caused by horizontal compression of accreted sediments) as reported previously. However, by computing the new heat flow values with legacy data in the study area, we raise the assumption that these anomalous heat flow values are also promoted by the eastern flank of the currently subducting Chile Rise. Part of the rift axis is located just below the toe of the wedge, where active deformation and vigorous fluid advection are most intense, enhanced by the proximity of the young volcanic chain. Our results provide valuable information to current and future studies related to hydrothermal circulation, seismicity, volcanism, gas hydrate stability, and fluid venting in this natural laboratory

Temporal evolution of proto-Izu–Bonin–Mariana arc volcanism over 10 Myr: Constraints from statistical analysis of melt inclusion compositions

International Ocean Discovery Program (IODP) Expedition 351 ‘Izu–Bonin–Mariana (IBM) Arc Origins’ drilled Site U1438, situated in the northwestern region of the Philippine Sea. Here volcaniclastic sediments and the igneous basement of the proto-IBM volcanic arc were recovered. To gain a better understanding of the magmatic processes and evolution of the proto-IBM arc, we studied melt inclusions hosted in fresh igneous minerals and sampled from 30–40 Myr old deposits, reflecting the maturation of arc volcanism following subduction initiation at 52 Ma. We performed a novel statistical analysis on the major element composition of 237 representative melt inclusions selected from a previously published dataset, covering the full age range between 30 and 40 Ma. In addition, we analysed volatiles (H2O, S, F and Cl) and P2O5 by secondary ion mass spectrometry for a subset of 47 melt inclusions selected from the dataset. Based on statistical analysis of the major element composition of melt inclusions and by considering their trace and volatile element compositions, we distinguished five main clusters of melt inclusions, which can be further separated into a total of eight subclusters. Among the eight subclusters, we identified three major magma types: (1) enriched medium-K magmas, which form a tholeiitic trend (30–38 Ma); (2) enriched medium-K magmas, which form a calc-alkaline trend (30–39 Ma); (3) depleted low-K magmas, which form a calc-alkaline trend (35–40 Ma). We demonstrate the following: (1) the eruption of depleted low-K calc-alkaline magmas occurred prior to 40 Ma and ceased sharply at 35 Ma; (2) the eruption of depleted low-K calc-alkaline magmas, enriched medium-K calc-alkaline magmas and enriched medium-K tholeiitic magmas overlapped between 35 and 38–39 Ma; (3) the eruption of enriched medium-K tholeiitic and enriched medium-K calc-alkaline magmas became predominant thereafter at the proto-IBM arc. Identification of three major magma types is distinct from the previous work, in which enriched medium-K calc-alkaline magmas and depleted low-K calc-alkaline magmas were not identified. This indicates the usefulness of our statistical analysis as a powerful tool to partition a mixture of multivariable geochemical datasets, such as the composition of melt inclusions in this case. Our data suggest that a depleted mantle source had been replaced by an enriched mantle source owing to convection beneath the proto-IBM arc from >40 to 35 Ma. Finally, thermodynamic modelling indicates that the overall geochemical variation of melt inclusions assigned to each cluster can be broadly reproduced either by crystallization differentiation assuming P = 50 MPa (∼2 km deep) and ∼2 wt% H2O (almost saturated H2O content at 50 MPa) or P = 300 MPa (∼15 km deep) and ∼6 wt% H2O (almost saturated H2O content at 300 MPa). Assuming oxygen fugacity (fO2) of log fO2 equal to +1 relative to the nickel–nickel oxide (NNO) buffer best reproduces the overall geochemical variation of melt inclusions, but assuming more oxidizing conditions (log fO2 = +1 to +2 NNO) probably reproduces the geochemical variation of enriched medium-K and calc-alkaline melt inclusions (30–39 Ma)

Deep subduction of H 2 O and de£ection of volcanic chain towards backarc near triple junction due to lower temperature

Abstract In central Japan near the triple junction of the Pacific, Philippine Sea and North American (or Ohotsuku) plates, the volcanic chain deflects towards backarc compared to the adjacent northeast Japan and Izu^Bonin arcs, and lies V2003 00 km above the Wadati^Benioff zone. Numerical modeling shows that thermal recovery of the subducting Pacific plate is slow, due to the overlapping Philippine sea plate, which shifts the dehydration reactions to greater depths along the Pacific plate and causes the magmatism above the deep Wadati^Benioff zone. The low geothermal gradient along the subducting Pacific plate also implies that a considerable amount of H 2 O is carried further down by phase A which is formed just above the subducting plate, without being released for magmatism. Central Japan can be regarded as an entrance for H 2 O into the deep mantle.

Transportation of H2O and Melting beneath the Japan Arcs

Recent knowledge concerning the phase relationships of hydrous peridotitic and basaltic systems allows us to model fluid generation and migration in subduction zones. Here I present: (1) numerical models for the transportation of H2O and melting beneath the Japanese islands, in which generation and migration of aqueous fluid, its interaction with the convecting solid, and melting are considered, based on the phase relationships; (2) predictions of the corresponding seismic structures based on the calculated distribution of the fluids (aqueous fluids and melts); (3) 3-D seismic tomographic images beneath the islands; (4) analyses of the distribution of the volcanoes and the volcanic chains; and (5) comparisons between the model predictions and the observations. The model calculation suggests that in northeast Japan, nearly all of the H2O expelled from the subducted Pacific plate is hosted by serpentine and chlorite just above the plate, and is brought down to～150 km. Breakdown of serpentine and chlorite at these depths results in the formation of a fluid column through which H2O is transported upwards, and results in the initiation of melting in the mantle wedge beneath the backarc. Seismic tomographic studies suggest the existence of such a melting region beneath the backarc. In central Japan, the subducted Philippine Sea plate overlaps the subducted Pacific plate. This geometry causes slow thermal recovery of the subducted Pacific plate, resulting in dehydration reactions at levels (200-300 km) deeper than in northeast Japan, and deflection of the volcanic chain towards the backarc side. In contrast, in southwest Japan, where a relatively hot part of the Philippine sea plate (Shikoku basin) subducts, the dehydration reactions are predicted to occur at relatively shallow levels (< 100 km in depth). The seismic tomographic image supports well the predicted distribution of the fluids beneath the volcanic front to the forearc region. These comparisons between the model predictions and the observations suggest that the thermal structure (determined by age and subduction velocity) of the subducting plate strongly controls the distribution of the aqueous fluids and melts in subduction zones through the position of the dehydration reactions