To understand the timing and mode of crustal production and reworking in the Archean, we performed U-Pb and Hf isotopic analyses of detrital zircon grains from the ca. 2.3 Ga Murmac Bay Group in the Rae Craton, central Canada. The zircon U-Pb ages range from 3.9 to 2.3 Ga with a significant gap interval of 3.6–3.3 Ga, indicating that felsic magmatism has semi-continuously within the craton since the early Archean. The combined U-Pb and Hf isotopic data define three distinct Hf isotope-age arrays that share a similar slope equivalent to that of typical 176Lu/177Hf ratio of continental crust, and the slope intersects the mantle evolution curve at 2.9–2.6, 3.3–3.2, and 3.8–3.6 Ga. The secular trends in zircon Hf isotopes illustrate episodic crust formation from depleted mantle during the three periods with subsequent reworking of pre-existing crusts into younger granitoids. Furthermore, these results infer that granitoid crust was rarely reworked for more than 800 million years after its formation. This finding is well explained by assuming that the Archean Rae block has grown outward from the interior by adding new crusts through subduction-related magmatism and/or by secondary accretion of exotic arc crusts. In such a tectonic framework, younger crusts were likely utilized more preferentially in crustal melting during subduction-related magmatism. These observations suggest that plate subduction has operated already in the early Archean, as early as 3.6 Ga Eoarchean.
The normal alkanes (n-alkanes) and their hydrogen isotopic (δD) values in sediments and aquatic and terrestrial plants from Fuxian Lake area of China were analyzed by gas chromatography-mass spectrometry (GC-MS) and GC-isotope ratio mass spectrometry (GC-IRMS), respectively. According to the δD values of the odd carbon-numbered n-alkanes, the sediment samples were divided into types I and II. The type I samples had a heavier hydrogen isotopic composition (–161 to –155‰ on average) compared to that of the type II samples (–208 to –173‰ on average). This isotopic difference is likely a result of their different biological sources. Similarity in the average n-alkane δD values in the sediments and plants indicated that C17 and C21 to C25 were mainly derived from aquatic submerged plants in the lake, C27 and C29 mainly from woody plants from the surrounding area, and C31 and C33 principally from a mixture of terrestrial herbaceous and woody plants. The results further demonstrate that n-alkane hydrogen isotopic composition can be used as an useful source indicator. The study also suggests that the ecological environment of the lake area is an important factor controlling the hydrogen isotopic composition of lake sedimentary n-alkanes. This study also found that the plots of the sedimentary n-alkane δD values and the average n-alkane chain length (ACL values), and the sedimentary n-alkane δD values and relative contribution of n-alkanes from woody and terrestrial herbaceous plants (Qw values) can distinguish sediment samples from the two different ecological environments of Fuxian Lake and Gahai Lake. Our results also show that the effect of the ecological environment on hydrogen isotopic composition of sedimentary n-alkanes should be considered when reconstructing paleoclimatic and hydrologic conditions using sedimentary n-alkane δD values.
We developed in situ analyses of hydrogen and sulfur isotope ratios of basaltic glass using high-resolution, multi-collection secondary ion mass spectrometry (CAMECA IMS-1280HR). Hydrogen and sulfur isotopes of standard basaltic glasses were determined by a high-temperature conversion elemental analyzer/isotope ratio mass spectrometer (IRMS) and IRMS, respectively. For the in situ analysis of sulfur isotopes, a defocused Cs beam (~0.5 nA; ~10 μm diameter) was used, but for hydrogen isotopes, we used a larger defocused beam (~5 nA; ~15 μm diameter) to decrease the hydrogen background. For analyses of D/H (34S/32S) ratios, 16OH (32S) and 16OD (34S) were measured in multi-detection mode with a Faraday cup and an axial electron multiplier, respectively. Each measurement time was 6–7 minutes. Precisions (2 standard errors) for D/H and 34S/32S ratios were ~6‰ (H2O > 1 wt%) and ~0.6‰ (S > 1000 ppm), respectively. Our developed method for rapid and high spatial resolution analysis can determine multiple elements and isotopes of volatiles in a single small melt inclusion of ~30 μm diameter. Using this method, we analyzed hydrogen and sulfur isotope ratios of submarine basaltic glasses from mid-oceanic ridges and oceanic islands of Hawaii and confirmed that their D/H and S isotope ratios were consistent with reported values.
On June 29, 2015, a small phreatic eruption occurred in the Owakudani geothermal area on Hakone volcano, central Japan. Ashfall was observed on June 29 and 30. In this study, constituent minerals, whole-rock composition, and water-soluble components of the ash sampled at the Owakudani geothermal area on June 30 were analyzed to determine the source depth and hydrothermal condition of this ash. The ash sample included smectite, pyrite, tridymite cristobalite, gypsum, and anhydrite with plagioclase and quartz, indicating that the ash was discharged from a relatively low-temperature (lower than 100–150°C) and a near-neutral part of a hydrothermal alteration zone of the Owakudani geothermal area. The whole-rock chemical composition was characterized by an elevated Al2O3 content, supporting the inference that the source of the ash was in a near-neutral hydrothermal condition. Comparison of the mineral assemblage of the ash with that of the drilling core of the steam well evidently showed that the source depth of this ash was shallower than 350 m, as smectite is a mineral predominantly found at depths shallower than 350 m. In the ash leachate, 12,200 mg/kg of Cl and 6,600 mg/kg of SO4 were detected and their origin was deduced to be thermal water adhesion rather than volcanic gas because of their large adhesion amount and high Cl/S molar ratio. This Cl/S ratio corresponds to that of thermal waters approximately 23–351 m deep. This depth range is consistent with the lower limit depth estimated from the mineral assemblage.
In order to reveal elemental mobility during weathering rind formation, a basalt cobble buried in a fluvial terrace conglomerate layer is examined based on the elemental concentration profiles across the rind to fresh core for major and trace elements including rare earth elements. The weathering rind showed extreme depletion of SiO2, FeO, MgO, CaO, Na2O, Co, Zn, Sr, Ni, Y and LREEs and slight depletion of Al2O3 compared to fresh core. Dissolution of SiO2 together with Al2O3 and Zn can be explained by basification of interstitial water by dissolution of alkali and alkaline earth elements under the groundwater table. While REEs and Y are immobile under the basic condition because of low solubility, dissolution of these elements may have occurred under the oxidative and acidic condition above groundwater table after subsequent uplifting the layer. Thus the dissolution of elements have occurred at two stages during weathering rind formation.