This paper contains a series of essays which form a synopsis of my research career, which has been dedicated to orogenic and metamorphic processes in plate boundaries. It also presents my future ...research directions and latest investigations into simplifying the complexity of metamorphic rocks, their field observations, and associated convergent plate margin dynamics. Slab dehydration beneath forearcs and arc has been one of the most exciting problems in the field of convergent plate margin dynamics in recent decades. Orogenic and metamorphic processes are closely related. In the subduction zones, the fluid-mediated processes primarily control the crust-to-mantle transfer of volatiles, redox states of the wedge mantle peridotite and arc magmas, flux-melting to generate arc magmas, and seismicity. Although the methods of classical metamorphic petrology, based on phase equilibrium calculations, and the application of conventional geochronology continue to provide the opportunities to link high-pressure and ultra-high-pressure metamorphism to geophysical observations today, advances in geochemical techniques have great potential for key geological markers in global convergent margins. For example, recent analytical techniques and applications used in studies of metamorphic rocks, such as in-situ Li-B-Sr-Pb spot isotope analyses of jadeite, serpentinite, lawsonite, etc., evaluate hydration and dehydration along the subduction channels and subsequent slab-mantle interaction, and visualize more realistic ancient Pacific-type convergent margins. The scientific value of subduction-zone rocks and crustal rocks are not limited to their worth for geochemistry: Some metamorphic rocks and minerals are important for all realms of geological sciences, from nano-scale kinetics to the scale of mountain building events. Consequently, collaborative exchange among geoscientists, through the application of applying different approaches, tests, and challenges, to address problems related to plate boundaries in the future.
Paleozoic jadeitite–bearing serpentinite–matrix mélange represents the oldest mantle wedge record of a Pacific–type subduction zone of proto–Japan. Most jadeitites are fluid precipitates (P–type), ...but some jadeitites are metasomatic replacement (R–type) which preserve relict minerals and protolith textures. The beauty and preciousness of some gem–quality, semi–translucent varieties of jadeitites in the Itoigawa–Omi area led to the designation of jadeitite as the national stone of Japan by the Japan Association of Mineralogical Sciences. Zircon geochronology indicates jadeitite formed prior to Late Paleozoic Renge metamorphism that formed blueschist and rare eclogite. For example, in the Itoigawa–Omi and Osayama localities, older jadeitites and younger high–pressure/ low–temperature metamorphic rocks in a single mélange complex imply different histories for the subduction channel and jadeite–bearing serpentinite–matrix mélange. This suggests that the jadeitite–hosted mélange (or serpentinized peridotite) can stay within the mantle wedge for a considerable time; thus recrystallization, resorption, and re–precipitation of jadeitite can continue in the mantle wedge environment. Therefore, studies of Paleozoic jadeitites in Japan have great potential to elucidate the earliest stages of orogenic growth (oceanward–accretion and landward–erosion) associated with the subduction of the paleo–Pacific oceanic plates, and to test geophysical observations of modern analogues from a mixture of fossilized mantle wedges and subduction channels.
The dissolved silica structures in quartz-saturated 0.50 and 1.50
m
mol kg H
2
O
–1
Na
2
CO
3
and 0.47
m
NaOH solutions at up to 750 °C and 1.5 GPa were investigated by in-situ Raman spectroscopy ...using a Bassett-type hydrothermal diamond anvil cell. The solubility of quartz in the solutions was determined by in-situ observations of the complete dissolution of the grain. The Raman spectra of the quartz-saturated Na
2
CO
3
and NaOH solutions at high pressures and temperatures exhibited the tetrahedral symmetric stretching band of silica monomers. The lower frequency and broader width of the band than those in pure H
2
O indicated the presence of both neutral and deprotonated monomers. In addition, we newly confirmed the intense bridging oxygen band and the tetrahedral symmetric stretching band of Q
1
(silicate center having a single bridging oxygen atom) in the spectra of the Na
2
CO
3
solutions. The integrated intensity ratios of the bridging oxygen band to the monomer band increased with the addition of Na
2
CO
3
and NaOH to fluids, corresponding to an elevation of the measured quartz solubilities. These observations indicate that the formation of silica oligomers in addition to neutral and deprotonated monomers explains the high dissolved silica concentrations in the solutions. The presence of deprotonated monomers under the experimental conditions suggests that deprotonated oligomers exist in the solutions, because the production of the latter more significantly reduces the Gibbs free energy. The anionic silica species and oligomers formed in alkaline silicate fluids may act as effective ligands for certain metal ions or complexes in deep subduction zones.
Abiotic synthesis of hydrocarbon-bearing fluids during geological processes has a significant impact on the evolution of both the Earth's biosphere and the solid Earth. Aqueous alteration of ...ultramafic rocks, i.e., serpentinization, which forms serpentinite, is one of the geological processes generating abiotic methane (CH
4
). However, abiotic CH
4
generation is not limited to the serpentinization of mafic and ultramafic lithologies. Metasedimentary dolomitic marble from the Hida Belt, Japan, is characterized by the presence of forsterite-rich olivine (Fo
~89–93
), and olivine crystals contain abundant fluid inclusions (<1 to 10 μm in size). Raman spectroscopic analyses of olivine-hosted fluid inclusions found that both primary and secondary fluid inclusions contain CH
4
, lizardite/chrysotile, and brucite. This indicates that micro-scale interactions between COH fluid and host olivine produced CH
4
through the reduction of CO
2
by H
2
released during local serpentinization within inclusions. Our observation implies that the dolomitic marble has the potential to be a key lithology for the synthesis and storage of abiotic CH
4
in a shallower crustal portion of orogenic belts.
Low-temperature omphacite has peculiar microstructures called "antiphase domains (APDs)," which can be formed via phase transition from disordered C2/c to ordered P2/n structure during cooling. Hence ...morphological analyses of the APDs of undeformed omphacite have a potential to unravel the temperature-time (T-t) histories of the eclogite. We investigated five omphacite inclusions in a euhedral garnet porphyroblast obtained from low-temperature eclogite in Syros. The garnet (∼6 mm in size) exhibits a distinct prograde chemical zoning and contains abundant mineral inclusions. Transmission electron microscope (TEM) observations of the focused ion beam (FIB) foils confirmed a heterogeneous distribution of equiaxed APDs (10-280 nm in diameter) and columnar APDs. Size distributions of the equiaxed APDs are characterized by kurtosis values of -0.45-3.91, which are larger than those in the matrix omphacite. The columnar APDs are subdivided into two types: dislocation-related (Type I) and inclusion-host interfacial (Type II). The presence of Type I APDs may suggest the inclusions were deformed prior to the host garnet growth. In contrast, Type II APDs, which are characterized by a bundle of stripe-like APDs (∼40 nm in width) aligned perpendicular to the host garnet, imply the simultaneous growth of omphacite and garnet in a non-deformation state. The presence of these two contrasting APDs of omphacite inclusions in the single prograde-zoned garnet prevents a simple application of geospeedometry based on APD sizes. Nevertheless, our observations demonstrate that APDs are keys to understanding thermodynamic equilibrium states and the mineral growth kinetics during eclogitization.
The Hida Belt, central Japan, is a continental fragment, which was once a part of the crustal basement of the East Asian continental margin. It consists mainly of Permo-Triassic granite-gneiss ...complexes with both syn-to-late-metamorphic migmatite or granite bodies. Clinopyroxene-bearing leucogranite, locally called as ‘Inishi’-type migmatite, occurs characteristically in the migmatite zone associated with amphibolite and marble. The leucogranite is characterized by the presence of coarse-grained diopside–hedenbergite series clinopyroxene and titanite in plagioclase-dominated matrix. Clinopyroxene contains abundant calcite inclusions. Euhedral titanite with high Th/U ratios (~2.8–7.8) and REE contents (~4514–14,069 μg/g) contains polycrystalline ‘granitic’ inclusions. Those mineralogical features indicate the involvement of carbonate during anatexis. Considering a nominal pressure of 0.4–0.7 GPa of the Hida gneiss, Zr-in-titanite thermometry yields a temperature of 730–810 °C and 770–850 °C at aTiO2 = 0.5 and 1, respectively. The titanites show highly variable U/Pb (238U/206Pb = 15.0–24.0) and Pb (207Pb/206Pb = 0.172–0.419) isotope ratios, and the scattered trend follows a discordia line with a lower intercept at 225.4 ± 1.9 Ma. This titanite age would represent the timing of regional anatexis that have occurred in a later stage of the regional metamorphism of the Hida Belt. A high apparent thermal gradient at middle crustal levels might have been caused by regional extension.
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•Titanite-bearing leucogranite of the Hida Belt records an regional anatactic event.•The leucogranite occurs in migmatite associated with amphibolite and marble.•Titanite containing ‘granitic’ inclusions yieldsU--Pb age of 225.4 ± 1.9 Ma.•Zr-in-titanite thermometry yields a high temperature (>700 °C) crystallization.•A high apparent thermal gradient might have been caused by regional extension.
In the Iratsu and the quartz-bearing eclogite bodies of the Sanbagawa high-pressure type metamorphic belt, southwest Japan, zircon U–Pb dating and trace-element analysis of the mafic gneiss combined ...with its geologic structure revealed that the protolith basaltic rock constituted the topographic high on a seafloor in relation to intra-oceanic arc magmatism at ca. 195 Ma. Moreover, the metamorphic zircon U–Pb data and the rare-earth element patterns obtained from the subordinated metasedimentary rocks of the Iratsu and the quartz-bearing eclogite bodies indicate that both bodies were subducted from a trench at ca. 120 Ma and underwent the eclogite facies metamorphism between ca. 120 and 90 Ma. This study, combined with previous studies for the Sanbagawa rocks and the Jurassic-Cretaceous accretionary complexes in Japan, identifies the following constraints that led to the tectonic evolution of the Sanbagawa eclogites: 1) the metamorphic unit including the Iratsu and the quartz-bearing eclogite bodies (the Besshi unit) was subducted from a trench at ca. 120 Ma. 2) This unit was stagnated at the depth of the eclogite–facies condition between ca. 120 and 90 Ma. 3) The eclogites in the Besshi unit was exhumed with the younger metamorphic rocks which were subducted at ca. 100–90 Ma (Asemi-gawa unit). 4) The Besshi unit is a high-pressure metamorphic equivalent of the non- or weakly metamorphic Sanbosan accretionary complex and the Mikabu greenstones from a standpoint of age similarity on accretion. The probable mechanism for the stagnation of the Besshi unit at the depth of the eclogite–acies condition needs 1) the detachment of oceanic-arc material from the subducting slab, driven by the resistance against the subduction of the topographic-high part underneath the forearc, and 2) the oceanward movement of the entire arc-trench system, which might have depressed the subduction of the Besshi unit into a deeper depth than its eclogite depth.
•LA-ICP-MS zircon analysis was applied to the Sanbagawa eclogites in SW Japan.•Protolith of the mafic eclogites was formed by ca. 195 Ma intra-oceanic arc magmatism.•The rocks were stagnated at the eclogite–facies depth during 120–90 Ma.•This stagnation was related to the subduction of intra-oceanic arc.•Oceanward movement of trench was also related the stagnation.
Mafic and ultramafic clasts (mostly ~1–5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition ...366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite–pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite–blueschist-facies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation.
•Mafic and ultramafic clasts were recovered from Marina serpentinite mud volcanoes.•Metamorphic conditions increase with the distances of mud volcanoes from the trench.•Mineralogical changes reflect the cooling of forearc mantle at subduction initiation.
Jadeitites and Plate Tectonics Harlow, George E; Tsujimori, Tatsuki; Sorensen, Sorena S
Annual review of earth and planetary sciences,
05/2015, Letnik:
43, Številka:
1
Journal Article
Recenzirano
Jadeitite is a relatively rare, very tough rock composed predominantly of jadeite and typically found associated with tectonic blocks of high-pressure low-temperature metabasaltic rocks (e.g., ...eclogite, blueschist) in exhumed serpentinite-matrix mélanges. Studies over the past ∼20 years have interpreted jadeitite either as the direct hydrous fluid precipitate from subduction channel dewatering into the overlying mantle wedge or as the metasomatic replacement by such fluids of oceanic plagiogranite, graywacke, or metabasite along the channel margin. Thus, jadeitites directly sample and record fluid transport in the subduction factory and provide a window into this geochemical process that is critical to a major process in the Earth system. They record the remarkable transport of large ion lithophile elements, such as Li, Ba, Sr, and Pb, as well as elements generally considered more refractory, such as U, Th, Zr, and Hf. Jadeitite is also the precious form of jade, utilized since antiquity in the form of tools, adornments, and symbols of prestige.
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