Koichi Mimura

3He/4He ratios in well gases in the Kinki district, SW Japan: surface appearance of slab-derived fluids in a non-volcanic area in Kii Peninsula

Abstract A total of 27 well gas samples collected from SW Japan were subjected to precise analysis of He, Ne and Ar isotopic compositions. The sampling area covers localities where mantle-derived helium emanates in the fore-arc of the Kii Peninsula, and is known as the ‘Kinki spot’ [e.g., Sano and Wakita, J. Geophys. Res. 90 (1985) 8728–8741]. The Kinki spot apparently is located within the fore-arc with no obvious magmatic activity, which contrasts to the occurrence of similarly high 3 He/ 4 He ratios along the volcanic front of NE Japan. Our high density sampling revealed more clearly that the high 3 He/ 4 He ratio wells are distributed in the Kii Peninsula above the area where the young and hot Philippine Sea Plate is subducting with a relatively steeper dip than in the adjacent area; this situation favors the near-trench side of the subducting slab entering eclogite facies conditions involving slab dehydration, without inducing melting of the mantle wedge. The aqueous fluids derived from the slab acquire mantle He during their passage through the mantle wedge. Thus, the observed association of mantle He and the fore-arc dehydration of the Philippine Sea Plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle He in well gases of the Kii Peninsula. The high 3 He/ 4 He region of Kii Peninsula also coincides with the occurrence of long-period tremors. These are caused by the movement of fluids derived from the subducting slab. Localized stresses cause significant uplift in the area, interpreted as indicating that intra-crustal fractures provide pathways through the crust for the fluids derived from depth. Thus, magma is not necessarily required as the carrier of mantle-derived helium to the surface of the fore-arc region of Kii Peninsula.

Biogenicity of Morphologically Diverse Carbonaceous Microstructures from the ca. 3400 Ma Strelley Pool Formation, in the Pilbara Craton, Western Australia

Morphologically diverse structures that may constitute organic microfossils are reported from three remote and widely separated localities assigned to the ca. 3400 Ma Strelley Pool Formation in the Pilbara Craton, Western Australia. These localities include the Panorama, Warralong, and Goldsworthy greenstone belts. From the Panorama greenstone belt, large (> 40 μm) lenticular to spindle-like structures, spheroidal structures, and mat-forming thread-like structures are found. Similar assemblages of carbonaceous structures have been identified from the Warralong and Goldsworthy greenstone belts, though these assemblages lack the thread-like structures but contain film-like structures. All structures are syngenetic with their host sedimentary black chert, which is associated with stromatolites and evaporites. The host chert is considered to have been deposited in a shallow water environment. Rigorous assessment of biogenicity (considering composition, size range, abundance, taphonomic features, and spatial distributions) suggests that cluster-forming small (<15 μm) spheroids, lenticular to spindle-like structures, and film-like structures with small spheroids are probable microfossils. Thread-like structures are more likely fossilized fibrils of biofilm, rather than microfossils. The biogenicity of solitary large (>15 μm) spheroids and simple film-like structures is less certain. Although further investigations are required to confirm the biogenicity of carbonaceous structures from the Strelley Pool Formation, this study presents evidence for the existence of morphologically complex and large microfossils at 3400 Ma in the Pilbara Craton, which can be correlated to the contemporaneous, possible microfossils reported from South Africa. Although there is still much to be learned, they should provide us with new insights into the early evolution of life and shallow water ecosystems.

Stratigraphy and sedimentary petrology of an Archean volcanic–sedimentary succession at Mt. Goldsworthy in the Pilbara Block, Western Australia: implications of evaporite (nahcolite) and barite deposition

Abstract A approximately 3.3 Ga sedimentary succession at Mt. Goldsworthy in the northeastern Pilbara Block, Australia contains beds composed of silicified pseudomorphs of nahcolite (NaHCO3) or barite (BaSO4). This sedimentary succession correlates with the Corboy Formation and consists of lower, middle and upper sedimentary units, which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks. The lower and middle units are predominantly siliciclastic, whereas the upper unit is characterized by ferruginous cherts and banded iron-formations with minor proportions of intercalated mature sandstone. The succession defines an overall upward fining and deepening trend. The detrital materials were derived from older greenstone successions, volcaniclastics and reworked precipitative beds. Silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit. The formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from Na+-HCO3− brines in a closed or semi-closed evaporitic basin. Bladed barite deposits occur mainly in the middle unit, which was deposited in a sub-aerial to shallow marine environment. Barite may have precipitated as a result of mixing of SO42−-rich seawater and Ba2+-rich hydrothermal fluids. The Mt. Goldsworthy sedimentary succession records an Archean shallow to sub-aerial sedimentary environment that probably developed in a continental margin setting.

Synthesis of polycyclic aromatic hydrocarbons from benzene by impact shock: Its reaction mechanism and cosmochemical significance

Abstract The synthesis of polycyclic aromatic hydrocarbons (PAHs) from benzene by shock waves was studied in order to search for a novel possibility of PAH formation under cosmochemical conditions. Shock waves generated by projectile impacts were transmitted into pure benzene, and then the shocked samples were analyzed by FID gas chromatography and gas chromatography-mass spectrometry. The projectile velocity ranged from 100 to 1200 m/s. The physical conditions of shocked benzene in liquid form were estimated by the Hugoniot data. The shock waves caused reactions between benzene molecules to produce PAHs with high-molecular weights ranging from 128 (naphthalene) to 306 (quaterphenyl). Major products were naphthalene, biphenyl, fluorene, trans-stilbene, phenanthrene, and chrysene. Striking aspects emerge from the experiments: (1) the molar yields of products were enhanced exponentially with increasing projectile velocity, (2) the composition of products remained constant independent of the projectile velocity, and (3) the mutual ratios between structural isomers and the ratios of various products to chrysene showed definite values independent of the projectile velocity. These results were identical for experiments at two different temperatures, 77 K (benzene in solid form) and 290K (benzene in liquid form). I propose in this study that thermochemical reactions of ground states play a major role in the shock synthesis, although reactions of excited states cannot be ruled out. Examination of the yield relationships among structural isomers in products suggests that concerted cycloaddition reactions controlled by Woodward-Hoffmann rules explain the formation of some products better than do radical addition reactions. Most species of PAHs reported to be present in carbonaceous chondrites and interplanetary dust particles were synthesized during the present experiment. Furthermore, abundance ratios between some structural isomers in shock-induced PAHs are approximately the same as those in carbonaceous chondrites such as the Murchison meteorite. Shock synthesis must have operated during shock events in cosmochemical environments, and the shock-induced PAHs may be present in the interstellar medium, in atmospheres of Jovian planets, and in carbonaceous chondrites.

Early evolution of large micro-organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic-walled microfossils

The Strelley Pool Formation (SPF) is widely distributed in the East Pilbara Terrane (EPT) of the Pilbara Craton, Western Australia, and represents a Paleoarchean shallow-water to subaerial environment. It was deposited ~3.4 billion years ago and displays well-documented carbonate stromatolites. Diverse putative microfossils (SPF microfossils) were recently reported from several localities in the East Strelley, Panorama, Warralong, and Goldsworthy greenstone belts. Thus, the SPF provides unparalleled opportunities to gain insights into a shallow-water to subaerial ecosystem on the early Earth. Our new micro- to nanoscale ultrastructural and microchemical studies of the SPF microfossils show that large (20-70 μm) lenticular organic-walled flanged microfossils retain their structural integrity, morphology, and chain-like arrangements after acid (HF-HCl) extraction (palynology). Scanning and transmitted electron microscopy of extracted microfossils revealed that the central lenticular body is either alveolar or hollow, and the wall is continuous with the surrounding smooth to reticulated discoidal flange. These features demonstrate the evolution of large micro-organisms able to form an acid-resistant recalcitrant envelope or cell wall with complex morphology and to form colonial chains in the Paleoarchean era. This study provides evidence of the evolution of very early and remarkable biological innovations, well before the presumed late emergence of complex cells.

Carbon isotopic analyses of ca. 3.0 Ga microstructures imply planktonic autotrophs inhabited Earth’s early oceans

The ca. 3 Ga Farrel Quartzite (FQ, Western Australia) contains possible organic microfossils of unusual spindle-like morphology that are surprisingly large and complex, preserved along with spheroids. The unusual nature of the possible fossils, coupled with their antiquity, makes their interpretation as biogenic difficult and debatable. Here, we report 32 in situ carbon isotopic analyses of 15 individual FQ specimens. The spheroids and the spindle-like forms have a weighted mean δ 13 C value of –37‰, an isotopic composition that is quite consistent with a biogenic origin. Both the spheroids and the spindle-like structures are isotopically distinct from the background organic matter in the same thin section (weighted mean δ 13 C value of –33‰), which shows that the preserved microstructures are not pseudofossils formed from physical reprocessing of the bulk sedimentary organic material. When considered along with published morphological and chemical studies, these results indicate that the FQ microstructures are bona fide microfossils, and support the interpretation that the spindles were planktonic. Our results also provide metabolic constraints that imply most of these preserved microorganisms were autotrophic. The existence of similar spindles in the ca. 3.4 Ga Strelley Pool Formation of Australia and the ca. 3.4 Ga Onverwacht Group of South Africa suggests that the spindle-containing microbiota may be one of the oldest, morphologically preserved examples of life. If this is the case, then the FQ structures represent the remains of a cosmopolitan biological experiment that appears to have lasted for several hundred million years, starting in the Paleoarchean.

Three-Dimensional Morphological and Textural Complexity of Archean Putative Microfossils from the Northeastern Pilbara Craton: Indications of Biogenicity of Large (>15 μm) Spheroidal and Spindle-Like Structures

We recently reported a diverse assemblage of carbonaceous structures (thread-like, film-like, spheroidal, and spindle-like) from chert in the ca. 3.0 Ga Farrel Quartzite of the Gorge Creek Group in the Pilbara Craton, Western Australia. Results from a rigorous examination of occurrence, composition, morphological complexity, size distributions, and taphonomy provided presumptive evidence for biogenicity. In this study, we present new data of morphological and textural complexity of large (>15 microm) spheroidal and spindle-like structures, using an in-focus, 3-D image reconstruction system, which further raises the scale of credibility that these structures are microfossils. While many of the large spheroids are single-walled, and the wall is irregularly folded, a few specimens are partially blistered, double walled, or have a dimpled wall. The wall-surface texture varies from smooth and homogeneous (hyaline) to patchy, granular or reticulate. Such variation is best explained as resulting from taphonomic processes. Additionally, an inner solitary body, present in some large spheroids, is hollow and partially broken, which indicates a primary origin for this substructure. Spindle-like structures have two types of flange-like appendage; one is attached at the equatorial plane of the body, whereas the other appears to be attached peripherally. In both cases, the appendage tends to have a flat geometry, a tapering thickness, and constancy in shape, proportions, and dimensions. Spindle-wall surfaces are variously textured and heterogeneous. These morphological and textural complexities and heterogeneity refute potential abiogenic formation models for these structures, such as crystals coated with organic matter, fenestrae, and the diagenetic redistribution of carbonaceous matter. When coupled with other data from Raman spectroscopy, NanoSIMS analysis, and palynology, the evidence that these large carbonaceous structures are biogenic appears compelling, though it is still equivocal as to whether they are cells or outer envelopes of colonies of smaller cells.

Redox change in sedimentary environments of Triassic bedded cherts, central Japan: possible reflection of sea-level change

Middle Triassic radiolarian bedded cherts in the Mino Belt, central Japan, include a sequence showing an abrupt facies change from the lower to the upper, where grey-black bedded cherts enriched in carbonaceous matter and framboidal pyrite are overlain by brick-red hematitic bed- ded cherts. Brownish-yellow chert enriched in goethite and purple-red chert occur at the boundary between the grey-black bedded cherts and the brick-red bedded cherts. This facies change is in accor- dance with stratigraphic variations of geochemical characteristics; the lower section grey-black bedded cherts, compared with the upper section brick-red bedded cherts, are enriched in Ctot and Stot, and are characterized by lower MnO/TiO 2 , higher FeO/Fe 2 O 3 * (total iron as Fe 2 O 3 ) and more variable Fe2O3*/TiO2 values. Some of the lower section samples, in addition, are characterized by an enrich- ment in some transition metals (Ni, Cu, and Zn). The covariation of mineralogical and geochemical characteristics indicates that sedimentary environments and diagenetic processes were different between the lower and the upper section bedded cherts. During the deposition of the lower section bedded cherts, the sedimentary environment was anoxic and bacterial sulphate reduction occurred during the early diagenetic stage. In contrast, the upper section bedded cherts were subjected to less reducing diagenetic processes; active sulphate reduction did not occur. The change of sedimentary environment and diagenetic process at the site of deposition is likely to be attributed to the fluctuated concentration of dissolved oxygen in the water mass of a semi-closed marginal ocean basin, which was potentially caused by sea-level change that occurred during Middle Triassic time.

Hydrocarbon gases and aromatic hydrocarbons produced by impact shock from frozen benzene: Cosmochemical significance

Abstract Shock waves trigger reactions in solid benzene at 77 K to produce hydrogen, light alkanes from C1 to C3, light alkenes from C2 to C3, acetylene, aromatic hydrocarbons with high-molecular weights ranging from 102 (phenylacetylene) to 306 (quaterphenyl), and unknown carbonaceous materials. These products are similar to those yielded by rapidly quenched pyrolysis. Shock synthesis favours the formation of compounds with metapositions in these structural isomers. The reaction mechanism producing aromatic hydrocarbons with low molecular weights is vague at present, but it cannot be a simple radical reaction. Most of shock-derived products are detected in carbonaceous chondrites. These results suggest that many sorts of hydrocarbons in solar materials such as meteorites have been produced by frequent and violent shocks throughout the history of the solar system. In particular, this process may be responsible for the formation of extraterrestrial unsaturated hydrocarbon gases.

Pressure-induced oligomerization of benzene at room temperature as a precursory reaction of amorphization

Oligomerization of benzene at high pressures up to 16 GPa was investigated at room temperature using an opposed-anvil type pressure apparatus. The recovered samples were analyzed using GC-MS to identify and quantify the products after the high-pressure experiments. Some structural isomers of benzene dimer as well as biphenyl, naphthalene, and terphenyl isomers were detected at pressures higher than 13 GPa. The molar yield of the polycyclic aromatic hydrocarbons increased concomitantly with increasing pressure, although benzene still remained. The oligomerization is likely to occur when the neighbor distance of the benzene molecules exceeds the threshold of the reaction distance. The oligomerization is regarded as a precursory phenomenon of the amorphization that occurs at higher pressure.