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Nature of the source regions for post-collisional, potassic magmatism in southern and northern Tibet from geochemical variations and inverse trace element modelling
No full textNeogene potassic lavas in northern and southern Tibet have different isotopic (εNd(i) north, -5·5 to -10·3; south -8·8 to -18·1) and major element signatures suggesting derivation from separate sub-continenial lithospheric mantle (SCLM) sources. Inverse trace-element modelling shows that the southern Tibet magmas were derived by 1-2 partial melting of a phlogopite and amphibole peridotite, and that the northern samples were derived by 3-4 partial melting of a phlogopite peridotite. In both cases, melting is inferred to take place in the spinel stabilily field. Both sources show large ion lithophile element (LILE) enrichment relative to the high field strength elements (HFSE), and heavy rare earth element (HREE) depletion relative to primitive mantle. LILE/HFSE enrichment suggests subduction-related metasomatism; HREE depletion is indicative of prior melt extraction. Extension postdates the earliest magmatism in southern and north-central Tibet by 7 Myr and 5 Myr, respectively, which, in combination with the shallow depths of melting inferred for the Tibetan samples, supports geodynamic models invoking thinning of the SCLM. The northern Tibetan magmatism and extension can be explained by convective removal of the lower SCLM; the older ages and arcuate distribution of the southern magmas are most consistent with the SCLM erosion following slab break-off
Crystal form selectivity by humidity control: the case of the ionic co-crystals of nicotinamide and CaCl2
No full textPost-synthesis (de)hydration techniques were used here to explore further hydrated forms of ionic co-crystals (ICCs) of nicotinamide with CaCl2. Humidity is shown to be a crucial factor for ICCs, which cannot be ignored for a complete polymorph screening of this class of compounds. The exposure of nicotinamide·CaCl2·H2O obtained by kneading reaction to a controlled relative humidity of 75% led to the formation of a new hydrated phase: nicotinamide·CaCl2·4H2O. When nicotinamide·CaCl2·H2O was exposed to a relative humidity between 32% and 54%, nicotinamide2·CaCl2·2H2O was obtained. The anhydrous form was achieved as the result of overnight dehydration of nicotinamide·CaCl2·H2O at 150 °C. The tetrahydrate form cannot be obtained as a first product for kinetic (or thermodynamic) reasons. Control of the relative humidity has proven to be an efficient way to selectively isolate and stabilize powders of pure hydrous ICC phases, which is fundamental for industrial applications. Crystalline structures of nicotinamide·CaCl2·4H2O and anhydrous nicotinamide·CaCl2 were determined by powder diffraction
Strong influence of water vapor source dynamics on stable isotopes in precipitation observed in Southern Meghalaya, NE India
No full textTo calibrate δ18O time-series from speleothems in the eastern Indian summer monsoon (ISM) region of India, and to understand the moisture regime over the northern Bay of Bengal (BoB) we analyze the δ18O and δD of rainwater, collected in 2007 and 2008 near Cherrapunji, India. δD values range from + 18.5‰ to − 144.4‰, while δ18O varies between + 0.8‰ and − 18.8‰. The Local Meteoric Water Line (LMWL) is found to be indistinguishable from the Global Meteoric Water Line (GMWL). Late ISM (September–October) rainfall exhibits lowest δ18O and δD values, with little relationship to the local precipitation amount. There is a trend to lighter isotope values over the course of the ISM, but it does not correlate with the patterns of temperature and rainfall amount. δ18O and δD time-series have to be interpreted with caution in terms of the ‘amount effect’ in this subtropical region. We find that the temporal trend in δ18O reflects increasing transport distance during the ISM, isotopic changes in the northern BoB surface waters during late ISM, and vapor re-equilibration with rain droplets. Using an isotope box model for surface ocean waters, we quantify the potential influence of river runoff on the isotopic composition of the seasonal freshwater plume in the northern BoB. Temporal variations in this source can contribute up to 25% of the observed changes in stable isotopes of precipitation in NE India. To delineate other moisture sources, we use backward trajectory computations and find a strong correlation between source region and isotopic composition. Palaeoclimatic stable isotope time-series from northeast Indian speleothems likely reflect changes in moisture source and transport pathway, as well as the isotopic composition of the BoB surface water, all of which in turn reflect ISM strength. Stalagmite records from the region can therefore be interpreted as integrated measures of the ISM strength
Precise and accurate isotope fractionation factors (α17O, α18O and αD) for water and CaSO4·2H2O (gypsum)
No full textGypsum (CaSO4·2H2O) is a hydrated mineral containing crystallization water, also known as gypsum hydration water (GHW). We determined isotope fractionation factors (α17O, α18O and αD) between GHW and free water of the mother solution in the temperature range from 3 °C to 55 °C at different salinities and precipitation rates. The hydrogen isotope fractionation factor (αDgypsum-water) increases by 0.0001 units per °C between 3 °C and 55 °C and salinities <150 g/L of NaCl. The αDgypsum-water is 0.9812 ± 0.0007 at 20 °C, which is in good agreement with previous estimates of 0.981 ± 0.001 at the same temperature. The α18Ogypsum-water slightly decreases with temperature by 0.00001 per °C, which is not significant over much of the temperature range considered for paleoclimate applications. Between 3 °C and 55 °C, α18Ogypsum-water averages 1.0035 ± 0.0002. This value is more precise than that reported previously (e.g. 1.0041 ± 0.0004 at 25 °C) and lower than the commonly accepted value of 1.004. We found that NaCl concentrations below 150 g/L do not significantly affect α18Ogypsum-water, but αDgypsum-water increases linearly with NaCl concentrations even at relatively low salinities, suggesting a salt correction is necessary for gypsum formed from brines. Unlike oxygen isotopes, the αDgypsum-water is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for 17O and 18O follows the theoretical mass-dependent fractionation on Earth (θ = 0.529 ± 0.001). We provide specific examples of the importance of using the revised fractionation factors when calculating the isotopic composition of the fluids
Age and anatomy of the Gongga Shan batholith, eastern Tibetan Plateau, and its relationship to the active Xianshui-he fault
No full textThe Gongga Shan batholith of eastern Tibet, previously documented as a ca. 32–12.8 Ma granite pluton, shows some of the youngest U-Pb granite crystallization ages recorded from the Tibetan Plateau, with major implications for the tectonothermal history of the region. Field observations indicate that the batholith is composite; some localities show at least seven crosscutting phases of granitoids that range in composition from diorite to leucocratic monzogranite. In this study we present U-Pb ages of zircon and allanite dated by laser ablation–inductively coupled plasma–mass spectrometry on seven samples, to further investigate the chronology of the batholith. The age data constrain two striking tectonic-plutonic events: a complex Triassic–Jurassic (ca. 215–159 Ma) record of biotite-hornblende granodiorite, K-feldspar megacrystic granite and leucogranitic plutonism, and a Miocene (ca. 14–5 Ma) record of monzonite-leucogranite emplacement. The former age range is attributed to widespread Indosinian tectonism, related to Paleo-Tethyan subduction zone magmatism along the western Yangtze block of south China. The younger component may be related to localized partial melting (muscovite dehydration) of thickened Triassic flysch-type sediments in the Songpan-Ganze terrane, and are among the youngest crustal melt granites exposed on the Tibetan Plateau. Zircon and allanite ages reflect multiple crustal remelting events; the youngest, ca. 5 Ma, resulted in dissolution and crystallization of zircons and growth and/or resetting of allanites. The young garnet, muscovite, and biotite leucogranites occur mainly in the central part of the batholith and adjacent to the eastern margin of the batholith at Kangding, where they are cut by the left-lateral Xianshui-he fault. The Xianshui-he fault is the most seismically active strike-slip fault in Tibet and is thought to record the eastward extrusion of the central part of the Tibetan Plateau. The fault obliquely cuts all granites of the Gongga Shan massif and has a major transpressional component in the Kangding-Moxi region. The course of the Xianshui Jiang river is offset by ∼62 km along the Xianshui-he fault and in the Kangding area granites as young as ca. 5 Ma are cut by the fault. Our new geochronological data show that only a part of the Gongga Shan granite batholith is composed of young (Miocene) melt, and we surmise that as most of eastern Tibet is composed of Precambrian–Triassic Indosinian rocks, there is no geological evidence to support regional Cenozoic internal thickening or metamorphism and no evidence for eastward-directed lower crustal flow away from Tibet. We suggest that underthrusting of Indian lower crust north as far as the Xianshui-he fault resulted in Cenozoic uplift of the eastern plateau
Comment on ‘Relict basin closure and crustal shortening budgets during continental collision: An example from Caucasus sediment provenance’ by Cowgill et al. [2016]
No full textAline Saintot, Jon Mosar, Aral I. Okay and Anatoly M.
Nikishin
Atmospheric methane variability: Centennial-scale signals in the Last Glacial Period
Get PDFIn order to understand atmospheric methane (CH4) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh-resolution CH4 record of the West Antarctic Ice Sheet Divide ice core 67.2–9.8 ka and find novel, atmospheric CH4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80–500 year range, but we find that age-scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial-scale oscillations in CH4 concentration—the relative amplitude change is constant. We propose that the centennial CH4 signal is related to tropical climate variability that influences predominantly low-latitude wetland CH4 emissions
Defect dynamics and strain coupling to magnetization in the cubic helimagnet Cu2OSeO3
No full textSmall but significant static and dynamic strain coupling effects have been detected in Cu2OSeO3 through elastic and anelastic anomalies associated with magnetic phase transitions observed as a function of temperature (1.5–150 K) and magnetic field (0–300 mT). The magnetic transition near 60 K is accompanied by a small increase in single-crystal elastic constants which can be understood in terms of biquadratic coupling between shear strain and the magnetic order parameter, even though the shear strain itself is almost negligibly small. The conical-collinear transition is associated with distinct minima in the elastic properties, while weaker anomalies at lower fields may be related to changes in the configuration of magnetic domains. A distinctive acoustic loss peak at ∼42K, independent of magnetic field, is attributed to freezing of a defect which is coupled with shear strain, has an associated activation energy of ∼5kJmol−1, and may play a role in pinning the magnetic microstructures. Anomalies below ∼10K indicate the presence of some additional relaxation process which could signify a change in magnetic structure
Climate dependent contrast in surface mass balance in East Antarctica over the past 216 ka
No full textDocumenting past changes in the East Antarctic surface mass balance is important to improve ice core chronologies and to constrain the ice-sheet contribution to global mean sea-level change. Here we reconstruct past changes in the ratio of surface mass balance (SMB ratio) between the EPICA Dome C (EDC) and Dome Fuji (DF) East Antarctica ice core sites, based on a precise volcanic synchronization of the two ice cores and on corrections for the vertical thinning of layers. During the past 216 000 a, this SMB ratio, denoted SMBEDC/SMBDF, varied between 0.7 and 1.1, being small during cold periods and large during warm periods. Our results therefore reveal larger amplitudes of changes in SMB at EDC compared with DF, consistent with previous results showing larger amplitudes of changes in water stable isotopes and estimated surface temperature at EDC compared with DF. Within the last glacial inception (Marine Isotope Stages, MIS-5c and MIS-5d), the SMB ratio deviates by up to 0.2 from what is expected based on differences in water stable isotope records. Moreover, the SMB ratio is constant throughout the late parts of the current and last interglacial periods, despite contrasting isotopic trends
Methods for biogeochemical studies of sea ice: The state of the art, caveats, and recommendations
No full textOver the past two decades, with recognition that the ocean’s sea-ice cover is neither insensitive to climate change nor a barrier to light and matter, research in sea-ice biogeochemistry has accelerated significantly, bringing together a multi-disciplinary community from a variety of fields. This disciplinary diversity has contributed a wide range of methodological techniques and approaches to sea-ice studies, complicating comparisons of the results and the development of conceptual and numerical models to describe the important biogeochemical processes occurring in sea ice. Almost all chemical elements, compounds, and biogeochemical processes relevant to Earth system science are measured in sea ice, with published methods available for determining biomass, pigments, net community production, primary production, bacterial activity, macronutrients, numerous natural and anthropogenic organic compounds, trace elements, reactive and inert gases, sulfur species, the carbon dioxide system parameters, stable isotopes, and water-ice-atmosphere fluxes of gases, liquids, and solids. For most of these measurements, multiple sampling and processing techniques are available, but to date there has been little intercomparison or intercalibration between methods. In addition, researchers collect different types of ancillary data and document their samples differently, further confounding comparisons between studies. These problems are compounded by the heterogeneity of sea ice, in which even adjacent cores can have dramatically different biogeochemical compositions. We recommend that, in future investigations, researchers design their programs based on nested sampling patterns, collect a core suite of ancillary measurements, and employ a standard approach for sample identification and documentation. In addition, intercalibration exercises are most critically needed for measurements of biomass, primary production, nutrients, dissolved and particulate organic matter (including exopolymers), the CO2 system, air-ice gas fluxes, and aerosol production. We also encourage the development of in situ probes robust enough for long-term deployment in sea ice, particularly for biological parameters, the CO2 system, and other gases