Possible sub-arc origin of podiform chromitites

Abstract The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc-related primitive magmas. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high-field strength elements (HFSE) to large-ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.     

Isotopic compositions of potassium and calcium in magnetic spherules from marine sediments

Isotopic compositions of potassium and calcium in individual magnetic spherules were determined. No significant anomaly was observed for potassium within twice the statistical error (2σ), although for calcium isotopes enrichments of⁴⁶Ca,⁴⁴Ca and⁴²Ca were observed in one spherule. The relative excess of⁴⁶Ca,⁴⁴Ca and⁴²Ca in the spherule agrees with the relative yield of spallogenic calcium isotopes observed in iron meteorites. This fact indicates that the enrichment in the calcium isotopes was caused by cosmic ray irradiation of the spherule in outer space.     

Late Cretaceous dacitic dykes swarm from Central Iran,a trace for amphibolite melting in a subduction zone

Late Cretaceous Bayazeh dyke swarm is situated in the western part of the Central-East Iranian Microcontinent (CEIM). These dykes with a dominant northeast-southwest trend occur in the Eastern margin of the Yazd block. They cross cut the Lower Cretaceous sedimentary rocks. The length of the Bayazeh dykes occasionally reaches up to the 2 km. Rock forming minerals of these dykes are plagioclase (andesine and oligoclase), amphibole (magnesio-hastingsitic hornblende, magnesio-hornblende and tschermakitic hornblende), quartz, K-feldspar (orthoclase), zircon and apatite. Secondary minerals are chlorite (pycnochlorite), albite, magnetite and calcite. The main textures are porphyritic, glomeroporphyritic and poikilitic. The felsic character of the Bayazeh dacitic dykes is shown by their high SiO₂ (62.70 to 64.60 wt %) and low [Fe₂O₃* + MgO + MnO + TiO₂] (average 4.64 wt %) contents. These dykes represent the peraluminous to metaluminous nature and their Na₂O and K₂O values are 5.20–7.14 and 1.51–2.59 wt %, respectively, which reveal their sodic chemistry. The trace element characteristics are the LREE enrichment relative to HREE, [La/Yb]_CN = 13.27–22.99, and slightly negative or positive Eu anomaly. These geochemical characteristics associated with low Nb/La (0.16–0.25), Yb/Nd (0.04–0.05) and high Zr/Sm (37.60–58.25) ratios indicate that the melting of a metamorphosed subducted oceanic crust is occurred where the residual mineral assemblage is dominated by garnet amphibolite. The chemical compositions of the Bayazeh dykes resemble those of slab-derived tonalite-trondhjemite-granodiorite (TTG) series. They were formed by subduction of Mesozoic Neo-Tethys -related Nain and Ashin oceanic crusts.     

Shock-wave compression of silica gel as a model material for comets

A shock-wave compression experiment using synthesized silica gel was investigated as a model for a comet impact event on the Earth’s surface. The sample shocked at 20.7 GPa showed considerable structural changes, a release of water molecules, and the dehydration of silanol (Si–OH) that led to the formation of a new Si–O–Si network structure containing larger rings (e.g., six-membered ring of SiO₄ tetrahedra). The high aftershock temperature at 20.7 GPa, which could be close to 800 °C, influenced the sample structure. However, some silanols, which were presumed to be the mutually hydrogen-bonded silanol group, remained at pressures >20.7 GPa. This type of silanol along with a small number of water molecules may remain even after shock compression at 30.9 GPa, although the intermediate structure of the sample recovered was similar to that of silica glass.     

High-K Calc-Alkaline Eocene Volcanic Rocks from the Anarak Area (Central Iran): A Key Structure for the Early Stages of Oceanic Basin Closure and the Beginning of Collision

Geotectonics - The combination of geochemical analyses and geochronological data of Eocene volcanic rocks reveals a gradual shift in the dominant magmatic series from calc-alkaline to high-K...     

http://dx.doi.org/10.1016/j.gsf.2016.07.005

The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H₂O, CO₂, N₂ as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics.     

Tectonic implication of Lower Cretaceous chromian spinel-bearing sandstones in Japan and Korea

Abstract In Japan and Korea, some Lower Cretaceous terrigenous clastic rocks yield detrital chromian spinels. These chromian spinels are divided into two groups: low-Ti and high-Ti. The Sanchu Group and the Yuno Formation in Japan have both groups, whereas the Nagashiba Formation in Japan and the Jinju Formation in Korea have only the low-Ti spinels. High-Ti spinels are thought to have originated in intraplate-type basalt. Low-Ti spinels (higher than 0.6 Cr#) were probably derived from peridotites, which are highly correlated with an arc setting derivation and possibly with a forearc setting derivation. Low-Ti spinels are seen in the Sanchu Group, the Nagashiba Formation and the Jinju Formation. Low-Ti spinels from the Yuno Formation are characterized by low Cr# (less than 0.6) and these chromian spinels appear to have been derived from oceanic mantle-type peridotite, including backarc. According to maps reconstructing the pre-Sea of Japan configuration of the Japanese Islands and the Korean Peninsula, the Korean Cretaceous basin was comparatively close to the Southwest Japan depositional basins. It is possible that these Lower Cretaceous systems were sediments mainly in the forearc and partly in the backarc regions. The peridotite might have infiltrated along major tectonic zones such as the Kurosegawa Tectonic Zone (= serpentinite melange zone) in which left lateral movement prevailed during the Early Cretaceous.     

A multi‐geochronological study of the Hakusan volcano,central Japan

The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko‐hakusan, and Shin‐Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. ²³⁸U(²³⁴U)–²³⁰Th disequilibrium and ²⁰⁶Pb/²³⁸U methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko‐Hakusan, ca 50 ka for Shin‐Hakusan I, and <10 ka for Shin‐Hakusan II. They are concordant with previous reports based on K–Ar dating. Some of the pyroclastic rocks, possibly originating from Shin‐Hakusan II activities, are dated to be ca 36 ka or 50 ka, and belong to the Shin‐Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin‐Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change.     

Production and degradation of fluorescent dissolved organic matter derived from bacteria

Dynamics of fluorescent dissolved organic matter (FDOM) in ocean environments has received attention over the past few decades. Although it has appeared that in situ production of oceanic FDOM is mainly due to bacteria, the production and bio- and photodegradation processes of bacterial FDOM have not been elucidated. In this study, a culture experiment with bacteria was carried out to assess the production and biodegradation processes of bacterial FDOM. Photodegradation of bacterial FDOM and dissolved organic carbon (DOC) was also examined by exposure to a solar simulator. Bacterial FDOM consists of six components which were determined by parallel factor analysis (PARAFAC). Fluorescence intensities of protein-like FDOM increased with the bacterial biomass, but the increases of humic-like FDOM lagged behind the protein-like FDOM by 5–10 days. Exposure to simulated sunlight caused significant decreases in fluorescence intensities of all components; 52–94% of the initial intensities were lost during 24 h. While, the DOC concentration exhibited a small decrease through the experiment (1.9–11.1%). These results showed that photodegradability of bacteria derived DOC was much less than the fluorescence, indicating that the lifetime of bacteria-derived DOC is much longer than the length estimated by the fluorescence. The role of photobleached FDOM derived from bacteria may be significant in the biogeochemical cycle at the surface layer.     

Mantle hornblendites of Naein ophiolite (Central Iran): Evidence of deep high temperature hydrothermal metasomatism in an upper mantle section

The Naein ophiolite is the most complete ophiolitic exposure in Cental Iran and considered as a remnant of the Mesozoic Central East Iranian microcontinent (CEIM) confining oceanic crust. In the northeastern part of this ophiolite (Darreh Deh area) within the mantle peridotites, a few hundred meters below the top of the Moho transition zone (MTZ), the hornblendites are present as dykes (former cracks and joints) from a few millimeters to nearly 50 cm wide. They have sharp boundaries with the surrounding mantle harzburgites and dunites. These hornblendites are pale green and coarse-grained in hand specimen and composed of magnesio-hornblende (Mg# = 0.93), chlorite (penninite and clinochlore, Mg# = 0.95), Cr-spinel (chromite, Cr# = 0.67 and Mg# = 0.55), tremolite, calcite and dolomite. Tremolites were formed by retrograde metamorphism of hornblendes. Calcite and dolomite occur as late-stage veins. Very high amount of primary hydrous phases (~94 vol % hornblende and chlorite), as well as peculiar mineralogical and chemical characteristics of the Naein ophiolite mantle hornblendites, do not match a magmatic origin. They are possibly products of the reaction between mantle peridotites and seawater-originated supercritical fluids, rich in silicate components. The presence of primary hydrous phases (hornblende and chlorite) may reveal high activity of H₂O in the involved solution. The chemical composition of chromite in the hornblendites is near to the average chromite composition from the surrounding harzburgite and dunite. This suggests that the main source of Cr should be chromites of nearby peridotites, which were totally or partly dissolved by hydrothermal fluids. The positive anomaly of Eu in the chondrite-normalized REE patterns of hornblendes, high modal abundance of Ca-rich hornblende, as well as presence of calcite and dolomite, point to seawater ingression through the gabbros in to the uppermost mantle peridotites. The higher value of MgO than CaO, presence of high-Cr chromite and Cr-enrichment of hornblendes and chlorites indicate a higher contribution of peridotites rather than gabbros to the chemical characteristics of the involved fluids. This study shows that circulation of possibly seawater-derived high temperature hydrous fluids in the upper mantle can leach and provide necessary elements to form hornblendite in joints and cracks of the uppermost mantle.