Yukio Isozaki

Accreted oceanic materials in Japan

Abstract The Phanerozoic circum-Pacific orogenic belts contain numerous ocean-derived materials accreted through plate converging processes. Japanese Islands, in particular, display various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone, MORB-like rocks and oceanic mantle, and pelagic sediments. The compilation of these rocks in many subduction complexes of Late Permian to the present, led to following conclusions. Accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust, i.e. Layer 1 and Layer 2. Many fragments of seamount with alkali basalt (600), hot-spot seamount (26), oceanic reef limestone (291), MORB-like basalt (200), and numerous cherts (more than 1000) are recognized as ancient oceanic materials accreted to the Japanese Islands. However, gabbros and mantle materials of Layer 3 and lower parts of the oceanic lithosphere, scarcely occur in subduction-accretion complexes except for a few examples of back-arc basin or fore-arc origin. Accretion occurs episodically. In Southwest Japan, oceanic materials were accreted intermittently in 1. (a) end-Permian, 2. (b) Middle-Late Jurassic, 3. (c) Late Cretaceous times, 4. (d) at ca. 50 Ma, and 5. (e) in Miocene times, while in Northeast Japan and Hokkaido this occurred in (b) Middle-Late Jurassic, (c) Late Cretaceous, and (f) Early Cretaceous times. In contrast to the general belief on accretion of younger oceanic plates, the majority of Japanese subduction-accretion complexes were formed during the subduction of plates, up to 160 Ma old. The accretionary events in end-Permian and Middle-Late Jurassic times coincide with northward collision of ancient island arcs, oceanic rises or seamount chains (of hot-spot origin) with the Asian continent. Accretion relevant to subduction of older plates may be controlled by the collision-subduction process of these topographic reliefs on an oceanic plate. In addition, the chronological coincidence with the continent collision-amalgamation between the Sino-Korean and Siberian platforms and between the Sino-Korean and Yangtze blocks, also implies collision-induced voluminous supply of elastics from back-arc regions and its contribution to the formation of huge accretionary complexes. Accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts. This implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels. With respect to the metamorphic grades for Japanese subduction complexes, oceanic materials have less than 1 vol.% in the zeolite facies, 15–20% in the prehnite-pumpellyite metagraywacke facies, and ca. 30% in the greenschist/glaucophane schist facies and albite-epidote amphibolite fades. This relationship indicates that the major process for landward accretion of oceanic materials is not off-scraping or sedimentary mixing at the trench, but underplating (subcretion) at much deeper levels of a subduction zone.

Well‐documented travel history of Mesozoic pelagic chert in Japan: From remote ocean to subduction zone

The Mino-Tanba belt in southwest Japan, a segment of the Cordilleran-type orogenic chain of Jurassic east Asia, is composed mainly of a Middle-Upper Jurassic subduction-accretion complex in which Triassic and Lower Jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with Middle-Upper Jurassic clastic rocks. High-resolution microfossil (conodont and radiolaria) research has identified very low average sedimentation rates of about 0.5 g/cm²/1000 yr in the chert units, similar to those of modern pelagic sediments accumulated in open ocean environments. Judging from the low average sedimentation rate, high purity of biogenic silica, long duration of continuous deposition (>50 m.y.), and wide along-strike extent (>1000 km), the bedded radiolarian cherts in the Mino-Tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments; accordingly, the alleged origin in smaller marginal basins is untenable. Upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench. The tectonic interlayering of these cherts and coarse-grained terrigenous elastics is a secondary feature that was added through duplexing-underplating in the subduction zone. On the basis of the primary stratigraphy and field occurrence of Triassic bedded chert in the Mino-Tanba belt, newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a Cordilleran-type bedded chert from its birth at a mid-oceanic ridge to its demise at a subduction zone.

Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era

Methanogenic microbes may be one of the most primitive organisms, although it is uncertain when methanogens first appeared on Earth. During the Archaean era (before 2.5 Gyr ago), methanogens may have been important in regulating climate, because they could have provided sufficient amounts of the greenhouse gas methane to mitigate a severely frozen condition that could have resulted from lower solar luminosity during these times. Nevertheless, no direct geological evidence has hitherto been available in support of the existence of methanogens in the Archaean period, although circumstantial evidence is available in the form of ∼2.8-Gyr-old carbon-isotope-depleted kerogen. Here we report crushing extraction and carbon isotope analysis of methane-bearing fluid inclusions in ∼3.5-Gyr-old hydrothermal precipitates from Pilbara craton, Australia. Our results indicate that the extracted fluids contain microbial methane with carbon isotopic compositions of less than -56‰ included within original precipitates. This provides the oldest evidence of methanogen (> 3.46 Gyr ago), pre-dating previous geochemical evidence by about 700 million years.

Carbon Isotopic Signatures of Individual Archean Microfossils(?) from Western Australia

New types of carbonaceous filamentous microstructures have been identified in silica veins at two new localities in the ∼3.5 Ga North Pole area of Western Australia. Their carbon isotopic compositions were measured in situ by secondary-ion mass spectrometry. The carbonaceous filaments are ∼1μm wide, 10 to 100 μm long, and are permineralized in a fine-grained (∼1 μm) silica matrix. They are morphologically divided into three types (i.e., spiral, threadlike, and branched filaments). Their sizes and morphologies resemble modern and previously reported fossil bacteria. These similarities and their complex three-dimensional geometry suggest that they may represent morphologically preserved fossil bacteria. δ13C values of the carbonaceous filaments range from −42 to −32‰, which strongly suggest that they are composed of biologically fixed organic compounds, possibly via the reductive acetyl-CoA pathway or the Calvin cycle. This is consistent with the hypothesis that autotrophs already existed on the Archean Earth.

Carbon isotopes and petrography of kerogens in ∼3.5-Ga hydrothermal silica dikes in the North Pole area, Western Australia

More than 600 specimens of ∼3.5 Ga-old hydrothermal silica dikes from the North Pole area, Pilbara craton, Western Australia, have been studied petrographically. The kerogens in 44 samples have been analyzed isotopically (C and N) and chemically (C, N, and H). The silica dikes are composed mainly of fine-grained silica (modal abundance: >97%) and are classified into two types by minor mineral assemblages: B(black)-type and G(gray)-type. The B-type silica dikes contain kerogen (0.37 to 6.72 mgC/g; average 2.44 mgC/g, n = 21) and disseminated sulfides, dominantly pyrite and Fe-poor sphalerite. In some cases, carbonate and apatite are also present. Their silica-dominated and sulfide-poor mineral assemblages suggest precipitation from low-temperature reducing hydrothermal fluid (likely 100–200°C). On the other hand, the G-type silica dikes are sulfide-free and concentrations of kerogen are relatively low (0.05 to 0.41 mgC/g; average 0.17 mgC/g, n = 13). They typically contain Fe-oxide (mainly hematite) which commonly replaces cubic pyrite and rhombic carbonate. Some G-types occur along secondary quartz veins. These textures indicate that the G-type silica dikes were formed by postdepositional metasomatism (oxidation) of the B-types, and that the B-types probably possess premetasomatic signatures. The δ13C values of kerogen in the B-types are −38.1 to −33.1‰ (average −35.9‰, n = 21), which are ∼4‰ lower than those of the G-types (−34.5 to −30.0‰; average −32.2‰, n = 19), and ∼6‰ lower than bedded chert (−31.2 to −29.4‰; average −30.5‰, n = 4). This indicates the preferential loss of 12C during the metasomatism (estimated fractionation factor: 0.9985). Considering the metasomatic effect on carbon isotopes with probably minor diagenetic and metamorphic overprints, we conclude that the original δ13C values of the kerogen in the silica dikes would have been heterogeneous (∼5‰) and at least some material had initial δ13C values of ≤ −38‰. The inferred 13C-depletions of organic carbon could have been produced by anaerobic chemoautotrophs such as methanogen, but not by aerobic photoautotrophs. This is consistent with the estimated physical and chemical condition of the hydrothermal fluid, which was probably habitable for anaerobic and thermophilic/hyperthermophilic chemoautotrophs. Alternatively, the organic matter may have been possibly produced by abiological reaction such as Fischer-Tropsch Type (FTT) synthesis under the hydrothermal condition. However, the estimated condition is inconsistent with the presence of the effective catalysts for the FTT reaction (i.e., Fe-Ni alloy, magnetite, and hematite). These lines of evidence suggest the possible existence of biosphere in the ∼3.5 Ga sub-seafloor hydrothermal system.

Rare earth element variations in mid-Archean banded iron formations: Implications for the chemistry of ocean and continent and plate tectonics

Abstract Abundances of major and rare earth elements (REEs) are reported for mid-Archean (3.3–3.2 Gyr) sedimentary rocks including banded iron formations (BIFs) and ferruginous/siliceous mudstone from the Cleaverville area in the Pilbara craton, Western Australia. Geological, lithological, and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments, ranging from that typical of mid-oceanic spreading centers to convergent plate boundary settings; a range of environments most likely caused by plate movements. Except for the mudstone, the REE content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons. Europium anomalies decrease up-section (Eu/Eu∗ values normalized to NASC change from 3.5 to 1.1) as the REE contents and LREE/HREE ratios increase. The striking similarity in these REE signatures of BIFs and modern hydrothermal sediments leads us to propose that the BIFs were in situ hydrothermal precipitates near a mid-ocean ridge. Significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon. The observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics, dominated by horizontal movement, was already operating in the mid-Archean. Distal hydrothermal sediments without a Eu anomaly (when normalized to NASC) suggest that mid-Archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no Eu anomaly (i.e, it had a negative Eu anomaly when normalized to chondrite). In addition to an absence of a Eu anomaly, mid-Archean seawater did not have a Ce anomaly, suggesting less oxic conditions in the mid-Archean than in the modern ocean.

Stable carbon isotope signature in mid-Panthalassa shallow-water carbonates across the Permo^Triassic boundary: evidence for 13 C-depleted superocean

The Jurassic accretionary complex in southwest Japan contains exotic blocks of the Permo–Triassic limestone primarily deposited on ancient mid-oceanic seamounts in an ancient Pacific Ocean or superocean Panthalassa. This study examines stable carbon isotope compositions (δ13Ccarb and δ13Corg) of such open-ocean shallow-water limestone across the Permo–Triassic boundary (PTB) at Kamura and Taho in southwest Japan. The results show an almost identical secular change in δ13Ccarb values with a remarkable negative spike across the PTB in both sections. This confirms for the first time that the mid-Panthalassa shallow-water carbonates are bio- and chemo-stratigraphically correlated not with previously studied PTB sections from the peripheries of Pangea. The negative shift in δ13Ccarb occurs parallel to that of δ13Corg in both sections, and the difference (Δ13C=δ13Ccarb−δ13Corg) remains nearly constant throughout the sections. This implies that the 13C-depleted water should have developed widely, probably in a global extent, throughout the superocean Panthalassa across the PTB. These findings suggest that a large input of 12C-enriched carbon into the ocean–atmosphere system has occurred and may have caused a global environment change probably relating to the greatest mass extinction in the Phanerozoic.

Well-preserved underplating structure of the jadeitized Franciscan complex, Pacheco Pass, California

Duplex structures several hundred metres thick bounded by marker beds exhibiting basalt-chert association have been found in the Franciscan complex near Pacheco Pass, central Diablo Range, California. The terrane was regionally metamorphosed and deformed under blueschist facies conditions at 150 °C and 7–8 kbar. Detailed mapping of the basalt-chert sequence has clarified the change of environment from mid-oceanic ridge to subduction zone. Eastward convergence of duplexing indicates eastward subduction of the oceanic plate during Early Cretaceous time. The presence of duplexing and a reconstruction of oceanic plate stratigraphy recorded within the underthrust unit demonstrate the accretionary origin of protoliths of the Franciscan coherent unit, which has long been believed to have an in situ origin, interpreted as either near-trench volcanism or accreted terranes in which “geosynclinal-type” volcanism occurred.

The Story of O2

How did biological, geochemical, and geophysical processes produce an atmosphere that allowed complex animal life to evolve?

Tectonic erosion in a Pacific-type orogen: Detrital zircon response to Cretaceous tectonics in Japan

U-Pb dating of detrital zircons from the Lower Cretaceous Sanbagawa and the recently recognized Upper Cretaceous Shimanto high-pressure (HP) metamorphic rocks in southwestern Japan has revealed the presence of abundant Proterozoic (ca. 1500–2000 Ma) detrital grains. In contrast, coeval non- to weakly metamorphosed accretionary complex (AC) and forearc basin sediments in southwestern Japan lack these older signatures. The only possible source of the Proterozoic detrital grains is the Jurassic AC in southwestern Japan, which structurally overlies the Cretaceous HP units. The Proterozoic grains were incorporated into the protoliths of HP-ACs, without polluting coeval forearc basin to trench sediments, likely by tectonic erosion in the forearc domain. Along the Cretaceous Wadati-Benioff plane, the tectonic erosion peeled off the sole part of the pre-existing forearc crust and mixed it with the subducting trench sediments prior to the peak HP metamorphism. In the Cretaceous subduction-related margin around Japan, the tectonic erosion likely occurred twice.