Gold solubility in oxidized and reduced, water-saturated mafic melt

We have performed experiments to evaluate Au solubility in natural, water-saturated basaltic melts as a function of oxygen fugacity. Experiments were carried out at 1000 °C and 200 MPa, and oxygen fugacity was controlled at the fayalite-magnetite-quartz (FMQ) oxygen fugacity buffer and FMQ + 4. All experiments were saturated with a metal-chloride aqueous solution loaded initially as a 10 wt% NaCl eq. fluid. The stable phase assemblage at FMQ consists of basalt melt, olivine, clinopyroxene, a single-phase aqueous fluid, and metallic Au. The stable phase assemblage at FMQ + 4 consists of basalt melt, clinopyroxene, magnetite-spinel solid solution, a single-phase aqueous fluid, and metallic Au. Silicate glasses (i.e., quenched melt) and their contained crystalline material were analyzed by using both electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Measured Au concentrations in the quenched melt range from 4.8 μg g⁻¹ to 0.64 μg g⁻¹ at FMQ + 4, and 0.54 μg g⁻¹ to 0.1 μg g⁻¹ at FMQ. The measured solubility of Au in olivine and clinopyroxene was consistently below the LA-ICP-MS limit of detection (i.e., 0.1 μg g⁻¹). These melt solubility data place important limitations on the dissolved Au content of water-saturated, Cl- and S-bearing basaltic liquids at geologically relevant fO₂ values. The new data are compared to published, experimentally-determined values for Au solubility in dry and hydrous silicate liquids spanning the compositional range from basalt to rhyolite, and the effects of melt composition, oxygen fugacity, pressure and temperature are discussed.     

Northward transport of pacific summer water along the Northwind Ridge in the Western Arctic Ocean

The recent sea-ice reduction in the Arctic Ocean is not spatially uniform, but is disproportionally large around the Northwind Ridge and Chukchi Plateau compared to elsewhere in the Canada Basin. In the Northwind Ridge region, Pacific Summer Water (PSW) delivered from the Bering Sea occupies the subsurface layer. The spatial distribution of warm PSW shows a quite similar pattern to the recent ice retreat, suggesting the influence of PSW on the sea-ice reduction. To understand the regionality of the recent ice retreat, we examine the dynamics and timing of the delivery of the PSW into this region. Here, we adopt a two-layer linearized potential vorticity equation to investigate the behavior of Rossby waves in the presence of a topographic discontinuity in the high latitude ocean. The analytical results show a quite different structure from those of mid-latitude basins due to the small value of β. Incident barotropic waves excited by the sea-ice motion with large annual variation can be scattered into both barotropic and baroclinic modes at the discontinuity. Since the scattered baroclinic Rossby wave with annual frequency cannot propagate freely, a strong baroclinic current near the topographic discontinuity is established. The seasonal variation of current near the topographic discontinuity would cause a kind of selective switching system for shelf water transport into the basin. In our simple analytical model, the enhanced northward transport of summer water and reduced northward transport of winter water are well demonstrated. The present study indicates that these basic dynamics imply that a strengthening of the surface forcing during winter in the Canada Basin could cause sea-ice reduction in the Western Arctic through the changes of underlying Pacific Summer Water.     

Polar lakes of the World: Current data and status of investigations

Current bathymetric and limnologic data on natural polar lakes of the World is discussed. The principal limnologic data on the largest polar lakes and also on the saline, cold amictic, and meromictic polar lakes are generalized. The geographic, “national”, and “administrative” distributions of polar lakes and also their distribution by the genesis of lake basins are analyzed. The distribution functions of polar lake areas and the regressions relating the water areas of lakes and the storages of lakes are calculated. The total water area and storage of polar lakes are estimated. The study is based on published data for 1432 Arctic and 174 Antarctic lakes, which are collected in the authors’ WORLDLAKE database. Attention is called to the inadequate knowledge of polar lakes.     

Fairweather atmospheric electricity at Antarctica during local summer as observed from Indian station,Maitri

Surface measurements of the atmospheric electrical parameters like Maxwell current, electric field and conductivity studied at the Indian station, Maitri (70.75° S, 11.75° E, 117 m above mean sea level), Antarctica, during austral summer have been analyzed for the years 2001 to 2004. A total of 69 days were selected which satisfied the ‘fairweather’ conditions, i.e., days with absence of high winds, drifting or falling snow, clouds, and fog effects. The diurnal variation curve of electric field and vertical current averaged for 69 fairweather days is a single periodic with a minimum at 03:00 UT and a maximum near 19:00 UT, which is very similar to the Carnegie curve. The correlation coefficient between these measured parameters has a high value (more than 0.9) for all the days. During fairweather days the measured current and field variations are similar and hence it is clear that the conductivity is more or less stable. During magnetically disturbed days, the dawn-dusk potential drop has clear influences on the diurnal variation and it modifies the conductivity. Apart from the day-to-day variation in low latitude thunderstorm activity, there are diurnal, seasonal, inter-annual variations in the electric potential and the currents, as well as solar influences on the measured parameters. This study will help us to examine the impact of solar and geophysical phenomena like solar flares, geomagnetic storms and substorms on the global electric circuit.     

Brightness power spectral distribution and waves in Jupiter's upper cloud and hazes

In this work we analyze the spatial structure of Jupiter's cloud reflectivity field in order to determine brightness periodicities and power spectra characteristics together with their relationship with Jupiter's dynamics and turbulence. The research is based on images obtained in the near-infrared (∼950 nm), blue (∼430 nm) and near-ultraviolet (∼260 nm) wavelengths with the Hubble Space Telescope in 1995 and the Cassini spacecraft Imaging Science Subsystem in 2000. Zonal reflectivity scans were analyzed by means of spatial periodograms and power spectra. The periodograms have been used to search for waves as a function of latitude. We present the values of the dominant wavenumbers for latitude bands between 32° N and 42° S. The brightness power spectra analysis has been performed in the meridional and zonal directions. The meridional analysis of albedo profiles are close to a k⁻⁵ law similarly to the wind profiles at blue and infrared wavelengths, although results differ from that in the ultraviolet. The zonal albedo analysis results in two distributions characterized by different slopes. In the near infrared and blue wavelengths, average spectral slopes are n₁=−1.3±0.4 for shorter wavenumbers (k<80), and n₂=−2.5±0.7 for greater wavenumbers, whereas for the ultraviolet n₁=−1.9±0.4 and n₂=−0.7±0.4, possibly showing a different dynamical regime. We find a turning point in the spectra between both regimes at wavenumber k∼80 (corresponding to L∼1000 km) for all wavelengths.     

Seasonal changes in submarine groundwater discharge to coastal salt ponds estimated using Ra and Ra as tracers

Submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes ²²⁶Ra (t_1/2 = 1600 y) and ²²⁸Ra (t_1/2 = 5.75 y). Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith–Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, from January 2002 to August 2003; additional porewater samples were collected in August 2005. Surface water activities ranged from 12–83 dpm 100 L^− 1 (60 dpm = 1 Bq) and 21–256 dpm 100 L^− 1 for ²²⁶Ra and ²²⁸Ra, respectively. Porewater ²²⁶Ra activities ranged from 16–736 dpm 100 L^− 1 (2002–2003) and 95–815 dpm 100 L^− 1 (2005), while porewater ²²⁸Ra activities ranged from 23–1265 dpm 100 L^− 1. Combining these data with a simple box model provided average ²²⁶Ra-based submarine groundwater fluxes ranging from 11–159 L m^− 2 d^− 1 and average ²²⁸Ra-derived fluxes of 15–259 L m^− 2 d^− 1. Seasonal changes in Ra-derived SGD were apparent in all ponds as well as between ponds, with SGD values of 30–472 L m^− 2 d^− 1 (Winnapaug Pond), 6–20 L m^− 2 d^− 1 (Quonochontaug Pond), 36–273 L m^− 2 d^− 1 (Ninigret Pond), 29–76 L m^− 2 d^− 1 (Green Hill Pond), and 19–83 L m^− 2 d^− 1 (Pt. Judith–Potter Pond). These Ra-derived fluxes are up to two orders of magnitude higher than results predicted by a numerical model of groundwater flow, estimates of aquifer recharge for the study period, and values published in previous Ra-based SGD studies in Rhode Island. This disparity may result from differences in the type of flow (recirculated seawater versus fresh groundwater) determined using each technique, as well as variability in porewater Ra activity.     

Analysis of microseismicity associated with the 2017 seismic swarm near the Aegean coast of NW Turkey

A thorough spatiotemporal analysis of the intense seismic activity that took place near the Aegean coast of NW Turkey during January–March 2017 was conducted, aiming to identify its causative relation to the regional seismotectonic properties. In this respect, absolute and relative locations are paired and a catalog consisting of 2485 events was compiled. Relative locations are determined with high accuracy using the double-difference technique and differential times both from phase pick data and from cross-correlation measurements. The spatial distribution of the relocated events revealed a south-dipping causative fault along with secondary and smaller antithetic segments. Spatially, the seismicity started at the westernmost part and migrated with time to the easternmost part of the activated area. Temporally, two distinctive periods are observed, namely an early period lasting 1 month and a second period which includes the largest events in the sequence. The investigation of the interevent time distribution revealed a triggering mechanism, whereas the ETAS parameters show a strong external force (μ?>?1), which might be attributed to the existence of the Tuzla geothermal field.     

Gold-bearing rocks of Mangalur greenstone belt,Gulbarga District,Karnataka

The Mangalur greenstone belt of Dharwar Craton, South India, is an Archaean schist belt dominated by metavolcanic rocks. The gold mineralization occurs within the metavolcanics and the fabric, mineralogy and geochemistry of these host rocks indicate that they were tholeiitic basalts regionally metamorphosed under medium to low-grade greenschist facies. The basic metavolcanic rocks occur as tholeiitic metabasalts and amphibolites. The rocks have undergone some fractionation and appear to be derived from melts generated by 10 to 25% melting of the mantle at depths 30 to 35 km around temperature 1200°C and pressure 12 kb. The source of gold is mainly in the basalts and not in the surrounding granites.