Evolutionary tracks of the terrestrial planets

On the basis of the model proposed by Matsui and Abe, we will show that two major factors — distance from the Sun and the efficiency of retention of accretional energy — control the early evolution of the terrestrial planets. A diagram of accretional energy versus the optical depth of a proto-atmosphere provides a means to follow the evolutionary track of surface temperature of the terrestrial planets and an explanation for why the third planet in our solar system is an aqua-planet.     

http://dx.doi.org/10.1016/j.gsf.2016.06.013

The Archean continental crusts account for ca.20% of the present volume,but the thermal history of the Earths' mantle suggests much more continental crusts were formed in the early Archean.Because the Archean continental crust underwent severe metamorphism,it is important to avoid influence by the later thermal events.We carried out a comprehensive geochronological work of Cathodoluminescence(CL) observation and U-Pb dating of zircons from orthogneisses and supracrustal rocks over the Saglek Block to obtain their protolith ages.The zircons were classified into three domains of core,mantle and rims,and the cores were further classified into three groups of inherited,altered and zoned cores based on the zonation on the CL images.We estimated the protolith ages from Pb-Pb ages of the zoned-cores of zircons with low U contents.We made a detailed sketch of a small outcrop in St.John's Harbour South(SJHS) area,and classified the orthogneisses and mafic enclaves into seven generations based on the geologic occurrence.The first and second generations comprise mafic rocks and lack magmatic zircons.We conducted CL imaging and U-Pb dating of zircons from the third,sixth and seventh generation of the orthogneisses to estimate the protolith ages at 3902 L 25,3892 ± 33 and 3897 ± 33 Ma for each,supporting the presence of the over 3.9 Ca Iqaluk Gneiss.The geological occurrence that the mafic rocks occur as enclaves within the 3.9 Ga Iqaluk Gneiss indicates that they are the oldest supracrustal rocks in the world.Our geochronological and geological studies show the Uivak Gneiss is quite varied in lithology and age from 3.6 to 3.9 Ga,and tentatively classified into six groups based on their ages.The oldest Uivak Gneiss components including the Iqaluk Gneiss are present around the SJHS area,and the orthogneisses become young as it is away.The lines of evidence of overprinting of younger granitoid on older granitoid in small outcrops and geological-map scale as well as presence of inherited zircons even in the oldest suite suggests that crustal reworking played an important role on erasing the ancient crusts.     

Trace Element Composition and Classification of the Chinga Iron Meteorite

A complete microelement analysis of the Chinga meteorite was performed, and the possibility of attributing it together with a number of other iron meteorites into the IVC subgroup, which is transition between IVA and IVB is proposed.     

Geochemical and stable isotopic evolution of the Guarani Aquifer System in the state of São Paulo, Brazil

The purpose of this report is to explain geochemical and stable isotopes trends in the Brazilian unit of the Guarani Aquifer System (Botucatu and Piramboia aquifers) in S?o Paulo State, Brazil. Trends of dissolved species concentrations and geochemical modeling indicated a significant role of cation exchange and dissolution of carbonates in downgradient evolution of groundwater chemistry. Loss of calcium by the exchange for sodium drives dissolution of carbonates and results in Na–HCO₃ type of groundwater. The cation-exchange front moves downgradient at probably much slower rate compared to the velocity of groundwater flow and at present is located near to the cities of Sert?ozinho and águas de Santa Barbara (wells PZ-34 and PZ-148, respectively) in a shallow confined area, 50–70 km from the recharge zone. Part of the sodium probably enters the Guarani Aquifer System. together with chloride and sulfate from the underlying Piramboia Formation by diffusion related to the dissolution of evaporates like halite and gypsum. High concentrations of fluorine (up to 13.3 mg/L) can be explained by dissolution of mineral fluoride also driven by cation exchange. However, it is unclear if the dissolution takes place directly in the Guarani Aquifer System or in the overlying basaltic Serra Geral Formation. There is depletion in δ ²H and δ ¹⁸O values in groundwater downgradient. Values of δ ¹³C(DIC) are enriched downgradient, indicating dissolution of calcite under closed system conditions. Values of δ ¹³C(DIC) in deep geothermal wells are very high (>–6.0‰) and probably indicate isotopic exchange with carbonates with δ ¹³C about –3.0‰. Future work should be based on evaluation of vertical fluxes and potential for penetration of contamination to the Guarani Aquifer System. Electronic Publication     

Ca isotopic compositions of dolomite, phosphorite and the oldest animal embryo fossils from the Neoproterozoic in Weng'an, South China

Deciphering the drastic changes of surface environment and the emergence of animals after the Neoproterozoic Snowball Earth event are important for understanding the changes of surface environment and its influence on the evolution of life through geologic time. Especially, the emergence of two types of Metazoan animals such as animal embryo fossils, cnidarians or sponges, and Ediacaran fauna in the late Neoproterozoic was one of the critical turning points in the biological evolution.Calcium is one of the essential elements for the growth of most animals. In this study, in order to evaluate the Ca cycles in the Neoproterozoic, we have measured Ca isotopic ratios (⁴⁴Ca/⁴²Ca and ⁴³Ca/⁴²Ca) for phosphorite, dolostone and phosphatic animal embryo fossils with a multiple collector, inductively coupled plasma-mass spectrometer (MC-ICP-MS). The resulting ⁴⁴Ca/⁴²Ca ratio defined by the relative deviation from the ratio of NIST SRM915a (δ^44/42Ca_NIST915) for phosphorite and dolostone ranges from 0.83 to 0.95‰, demonstrating that the fractionation between phosphorite/dolostone and seawater was very small. This evidence indicates that at the emergence of the Weng'an biota seawater was deficient in Ca probably due to mass deposition of phosphorite/dolostone and to the beginning of Ca-biomineralization.Three phosphatic animal embryo fossils have lower δ^44/42Ca values than the phosphorite and dolomite, implying that the precursor of the phosphatic embryo fossils was able to fractionate Ca isotopes through Ca-biomineralization, consistent with marine gastropods.     

U–Pb zircon geochronology of the North Pole Dome adamellite in the eastern Pilbara Craton

Supracrustal rocks around the North Pole Dome area, Western Australia, provide valuable information regarding early records of the evolution of crustal processes, surface environments, and biosphere. Owing to the occurrence of the oldest known microfossils, the successions at the North Pole Dome area have attracted interest from many researchers. The Paleoarchean successions (Warrawoona Group) mainly comprise mafic‐ultramafic greenstones with intercalated cherts and felsic lavas. Age constraints on the sediments have been mainly based on zircon U–Pb geochronology. However, many zircon grains have suffered from metamictization and contain anomalously high contents of common Pb, which makes interpretation of the U–Pb data complicated. In order to provide more convincing chronological constraints, an U–Pb Concordia age is widely accepted as the best estimate. Most zircons separated from two adamellites also suffered from severe metamictization. In our analyses, less metamictized domains were selected using a pre‐ablation technique in conjunction with elemental mapping, and then their U–Pb isotopic compositions were determined with a laser ablation inductively coupled plasma mass spectrometry. Most analyzed domains contained certain amounts of common Pb (²⁰⁴Pb/²⁰⁶Pb > 0.000 1), whereas three and five U–Pb data points with less common Pb (²⁰⁴Pb/²⁰⁶Pb < 0.000 1) were obtained. These U–Pb datasets yielded U–Pb Concordia ages of ca 3 445 Ma and 3 454 Ma, respectively. These ages represent the timing of the adamellite intrusion, and constrain the minimum depositional age of the Warrawoona Group. In addition, a single xenocrystic zircon grain showed a ²⁰⁷Pb/²⁰⁶Pb age of ca 3 545 Ma, supporting the idea that the sialic basement of the Pilbara Craton existed prior to 3 500 Ma. The in situ U–Pb zircon dating combined with the pre‐ablation technique has the potentials to identify non‐metamictized parts and to yield precise and accurate geochronological data even from partially metamictized zircons.     

Fault system of the 2004 Mid Niigata Prefecture Earthquake and its aftershocks

Data recorded by a seismic network deployed the day after the 2004 Mid Niigata Prefecture Earthquake (M6.8) in central Japan are used to determine the major source faults responsible for the mainshock and major aftershocks. Using this high-resolution seismic data, three major source faults are identified: two parallel faults dipping steeply to the west located 5 km apart, and the other dipping eastward and oriented perpendicular to the west-dipping faults. The analysis also reveals that the lateral variation in seismic velocity observed at the surface extends to a depth of 15 km, encompassing the source area of the mainshock. This strong heterogeneity of the crust, related to the complex geological and tectonic evolution of the area, is considered to be responsible for the prominent aftershock activity following the 2004 Niigata event.     

Effect of canopy openness and meteorological factors on spatial variability of throughfall isotopic composition in a Japanese cypress plantation

Spatial variability of throughfall (TF) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio‐temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip thinning was performed and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre‐thinning (from July to November 2010) and post‐thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ¹⁸O of gross precipitation, whereas the changes in deuterium excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre‐ and post‐thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, that is, significantly negative, significantly positive, and nonsignificant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ¹⁸O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage.