Abstract Mesosiderites are unique stony‐iron meteorites, composed of eucrite‐like silicates and Fe‐Ni metals. Their formation, including silicate‐metal mixing and metamorphisms, provides important insights into early planetary processes in the inner solar system. This report describes the first in situ U‐Pb and Hf‐W dating of zircon in a mesosiderite Asuka 882023. The U‐Pb (4502 ± 75 Ma) and Hf‐W (4532.8 + 5.7/−10.5 Ma) ages may represent timing of the zircon formation, which is considerably younger than crustal differentiation of the parent body. This evidence, combined with earlier studies of chronology, implies that mesosiderites were reheated at 4530–4520 Ma, clearly after the silicate‐metal mixing.
Abstract Geochemical monitoring of groundwater and soil gas emission pointed out precursor and/or coseismic anomalies of noble gases associated with earthquakes, but there was lack of plausible physico-chemical basis. A laboratory experiment of rock fracturing and noble gas emission was conducted, but there is no quantitative connection between the laboratory results and observation in field. We report here deep groundwater helium anomalies related to the 2016 Kumamoto earthquake, which is an inland crustal earthquake with a strike-slip fault and a shallow hypocenter (10 km depth) close to highly populated areas in Southwest Japan. The observed helium isotope changes, soon after the earthquake, are quantitatively coupled with volumetric strain changes estimated from a fault model, which can be explained by experimental studies of helium degassing during compressional loading of rock samples. Groundwater helium is considered as an effective strain gauge. This suggests the first quantitative linkage between geochemical and seismological observations and may open the possibility to develop a new monitoring system to detect a possible strain change prior to a hazardous earthquake in regions where conventional borehole strain meter is not available.
Understanding the origin and evolution of near-Earth asteroids (NEAs) is an issue of scientific interest and practical importance because NEAs are potentially hazardous to the Earth. However, when and how NEAs formed and their evolutionary history remain enigmas. Here, we report the U-Pb systematics of Itokawa particles for the first time. Ion microprobe analyses of seven phosphate grains from a single particle provide an isochron age of 4.64 ± 0.18 billion years (1σ). This ancient phosphate age is thought to represent the thermal metamorphism of Itokawa's parent body, which is identical to that of typical LL chondrites. In addition, the incorporation of other particles suggests that a significant shock event might have occurred 1.51 ± 0.85 billion years ago (1σ), which is significantly different from the shock ages of 4.2 billion years of the majority of shocked LL chondrites and similar to that of the Chelyabinsk meteorite. Combining these data with recent Ar-Ar studies on particles from a different landing site, we conclude that a globally intense impact, possibly a catastrophic event, occurred ca. 1.4 Ga ago. This conclusion enables us to establish constraints on the timescale of asteroid disruption frequency, the validity of the crater chronology and the mean lifetime of small NEAs.
Remarkable numbers of microbial cells have been observed in global shallow to deep subseafloor sediments. Accumulating evidence indicates that deep and ancient sediments harbor living microbial life, where the flux of nutrients and energy are extremely low. However, their physiology and energy requirements remain largely unknown. We used stable isotope tracer incubation and nanometer-scale secondary ion MS to investigate the dynamics of carbon and nitrogen assimilation activities in individual microbial cells from 219-m-deep lower Pleistocene (460,000 y old) sediments from the northwestern Pacific off the Shimokita Peninsula of Japan. Sediment samples were incubated in vitro with 13 C- and/or 15 N-labeled glucose, pyruvate, acetate, bicarbonate, methane, ammonium, and amino acids. Significant incorporation of 13 C and/or 15 N and growth occurred in response to glucose, pyruvate, and amino acids (∼76% of total cells), whereas acetate and bicarbonate were incorporated without fostering growth. Among those substrates, a maximum substrate assimilation rate was observed at 67 × 10 −18 mol/cell per d with bicarbonate. Neither carbon assimilation nor growth was evident in response to methane. The atomic ratios between nitrogen incorporated from ammonium and the total cellular nitrogen consistently exceeded the ratios of carbon, suggesting that subseafloor microbes preferentially require nitrogen assimilation for the recovery in vitro. Our results showed that the most deeply buried subseafloor sedimentary microbes maintain potentials for metabolic activities and that growth is generally limited by energy but not by the availability of C and N compounds.
Abstract A small hydrovolcanic eruption occurred on Mt. Hakone in central Japan on 29 June 2015, ejecting small amounts of volcanic ash and gas. We have conducted continuous surveys of He isotopes in the Owakudani geothermal area close to Mt. Komagatake, the central cone of Mt. Hakone. Long‐term (decadal‐scale) data from the Black‐egg site show generally constant 3 He/ 4 He ratios with 6.37 ± 0.06 R a , where R a is the atmospheric 3 He/ 4 He ratio of 1.382 × 10 ‐6 , after air correction. Short‐term (monthly) data of two fumarole sites indicate a rapid increase from May through August 2015 and a gradual decrease through February 2018, whereas seismic activity began to intensify in March 2015, reached the maximum in May, and decreased faster than the decrease in He isotopes. We also measured N and Ar isotopes in fumarole samples. A δ 15 N – N 2 / 36 Ar diagram shows that they are well explained by mixing of atmosphere and volcanic gas source components. Monthly variations of air‐corrected δ 15 N values shows a rapid decrease followed by a slow increase, contrary to the air‐corrected 3 He/ 4 He variations, suggesting a common mechanism of mixing between magma and crust during migration of fluids and gases. The time sequence of observed phenomena shows inflation of the volcanic edifice, maximum swarm activity, phreatic eruption, and maximum He and N isotope anomaly. Temporal variations of He and N isotopes were correlated with activation of the Owakudani geothermal system and with termination of volcanic unrest, which may provide useful information for observing and assessing geothermal activity around the world.
The isotopic composition of nitrogen bears important information concerning the fate of this volatile in the mantle‐crust‐atmosphere system. Based on the isotopic compositions of mid‐ocean ridge basalts and pristine diamonds, a δ 15 N value of about ‐ 5±2‰ (where δ 15 N = [( 15 N/ 14 ) sample / ( 15 N/ 14 N) air − 1] × 1000) is assigned to the upper mantle. In contrast, the origin of nitrogen has not been well documented in subduction zones. We report here δ 15 N values, N 2 / 36 Ar, and 40 Ar/ 36 Ar ratios of a comprehensive suite of samples from along convergent plate boundaries measured using a newly developed static mass spectrometry method. The δ 15 N values and the N 2 / 36 Ar ratios vary significantly from −2.7‰ to 5.0‰ and from 2.28×10 4 to 7.15×l0 5 , respectively. Taking a typical δ 15 N value for sediments, assuming that the N 2 / 36 Ar ratio of mid‐ocean ridge basalts is representative of the upper mantle, and that later isotopic and elemental fractionation are not significant, it is possible to deconvolve each fraction of nitrogen on the basis of simple mixing equations. About 50% of nitrogen in back‐arc basin basalts originated from the upper mantle, whereas island arc samples contain only about 15% of the mantle‐derived nitrogen, the major fraction being derived from recycled sedimentary nitrogen.
