Pb and other ore metals in modern seafloor tectonic environments: Evidence from melt inclusions

Many modern seafloor tectonic environments are host to hydrothermal systems and associated polymetallic sulfide deposits. Metal transport and precipitation are controlled by magmatic processes such as pre-eruptive degassing and the hydrothermal cycle. The original availability of Pb and other ore metals in a given setting is dependent on concentrations in the original magmatic source or additional enrichment processes. We have examined the Pb budget of melt inclusions from nine modern seafloor settings representing back-arcs, mid-ocean ridges and seamounts. Melt inclusions provide information on the characteristics of parental magmas, including insights into metal budgets. Trace element data in melt inclusions hosted in plagioclase, olivine and pyroxene were obtained by laser-ablation inductively-coupled mass-spectrometry.Results from back-arcs emphasize the impact of slab-subduction and dehydration processes on the chemical characteristics of generated magmas. Volatile- and fluid-mobile element-rich melt inclusions at Manus basin and Okinawa trough reflect a robust contribution of elements from the subducting slab as evidenced by relatively low Ce/Pb ratios. At Bransfield strait, on the other hand, melt inclusions are volatile poor, and fluid-mobile element ratios are similar to mid-ocean ridge values indicating little or no contribution from the slab. High Cu concentrations at Manus basin and Okinawa trough can be explained by fluxing of ferric iron from the subducting slab benefiting the production of sulfate over sulfide.Metal budgets for seamounts located on and nearby the axis of mid-ocean ridge segments appear to be independent of any input of mantle plume material. Results from the southern Explorer ridge (strong lower mantle influence, transitional- and enriched-MORBs), Pito and Axial seamounts (moderate lower mantle influence, transitional-MORBs) and a Foundation near-ridge seamount (little to no mantle influence, normal-MORB) show that, despite similar tectonic environments and varying contributions of mantle plume material, Cu, Zn and Pb values do not vary significantly between the enriched and non-enriched magma components of a given setting.     

SHRIMP zircon dating and Nd isotopic systematics of Palaeoproterozoic migmatitic orthogneisses in the Epupa Metamorphic Complex of northwestern Namibia

The Epupa Metamorphic Complex constitutes the southwestern margin of the Congo Craton and is exposed in a hilly to mountainous terrain of northwestern Namibia, bordering the Kunene River and extending into southern Angola. It consists predominantly of granitoid gneisses which are migmatized over large areas. This migmatization locally led to anatexis and produced crustal-melt granites such as the Otjitanda Granite. We have undertaken reconnaissance geochemical studies and single zircon U–Pb SHRIMP and Pb–Pb evaporation dating of rocks of the Epupa Complex. The granitoid gneisses, migmatites and anatectic melts are similar in composition and constitute a suite of metaluminous to peraluminous, calc-alkaline granitoids, predominantly with volcanic arc geochemical signatures. The zircon protolith ages for the orthogneisses range from 1861 ± 3 to 1758 ± 3 Ma. Anatexis in the migmatitic Epupa gneisses was dated from a melt patch at 1762 ± 4 Ma, and the anatectic Otjitanda Granite has a zircon age of 1757 ± 4 Ma. Migmatization and anatexis therefore occurred almost immediately after granitoid emplacement and date a widespread high-temperature Palaeoproterozoic event at ∼1760 Ma which has not been recorded elswhere in northern Namibia. The Nd isotopic systematics of all dated samples are surprisingly similar and suggest formation of the protolith from a source region that probably separated from the depleted mantle about 2.4–2.0 Ga ago. A major Archaean component in the source area is unlikely.     

Stable carbon isotope variations in northwest Europe during the last glacial–interglacial transition

Stable carbon isotope data that span part of the last glacial–interglacial transition (ca. 14-9 ka ¹⁴C BP; ca. 15–11 ka cal. BP), and which derive from organ-specific plant macrofossils recovered from two lake sediment profiles in the UK and one in Norway, are compared. The recorded temporal variations show similar trends, which, over a millennial time-scale appear to parallel the main drift in δ¹⁸O as determined for the GRIP ice-core. It is postulated that some smaller scale variations in the δ¹³C profiles may reflect the shorter term oscillations in δ¹⁸O values evident in the GRIP record, although this is less certain. Overall, however, the results suggest that stable carbon isotope measurements based on organ-specific terrestrial plant macrofossils may provide (i) a means for establishing correlations between terrestrial successions and (ii) additional paleoenvironmental information, as the apparent ‘shadowing’ of the GRIP record indicates a common forcing mechanism for both Greenland δ¹⁸O and northwest European δ¹³C variations. From the evidence available we suggest that the recorded δ¹³C variations reflect fluctuations in air temperature and/or changes in water vapour pressure in the atmosphere. © 1997 John Wiley & Sons, Ltd.     

An Investigation of the Influence of Cyclonic Airstreams on Midwestern Snowfall

Although previous climatological studies have investigated the relationship between cyclone frequency and trajectory and regional weather, analyses of structural airflow features and precipitation have been confined to meteorological case studies. In the following paper, the influence of Colorado cyclone airstreams on Midwestern snowfall is investigated using isentropic streamline maps for five cold season months. Results indicate that Colorado cyclones account for a minor percentage of Midwestern snowfall events and amounts. When snowfall is induced by Colorado cyclogenesis, diffluence and instability in the cold sector portions of the cyclone are the dominant lifting mechanisms.     

The transport of heat and matter by fluids during metamorphism

Non-dimensional solutions to the equations for the combined advective and diffusive one-dimensional transport of heat and solute in a layer are derived for fixed temperature/concentration on the boundaries and initial conditions of a linear gradient across the layer or a step function at the lower boundary. The solutions allow distinction of regimes in which advective or diffusive transport of either heat or solute predominate as a function of fluid flux, time and a length scale. The much lower diffusive coefficients for solute than heat results in a significant range of length scales and fluid flux rates characterised by advection of matter and diffusion of heat. The advective velocity of a component is a function of its fluid:rock partition coefficient. The most rapidly transported tracers which partition largely into the fluid phase, such as He, will travel orders of magnitude faster than heat or compatible solutes such as oxygen. Geochemical profiles in boundary layer regions where both advective and diffusive transport are significant are shown to be particularly informative as to properties of the rocks related to fluid flow such as porosity, permeability, time scales and fluid flux rates. The importance of advection can be directly estimated from the asymmetry of the geochemical profiles across individual layers.     

A Multiple Flare Scenario where the Classic Long-Duration Flare Was Not the Source of a CME

A series of flares (GOES class M, M and C) and a CME were observed in close succession on 20 January 2004 in NOAA 10540. Radio observations, which took the form of types II, III and N bursts, were associated with these events. We use the combined observations from TRACE, EIT, Hα images from Kwasan, MDI magnetograms and GOES to understand the complex development of this event. Contrary to a standard interpretation, we conclude that the first two impulsive flares are part of the CME launch process while the following long-duration event flare represents simply the recovery phase. Observations show that the flare ribbons not only separate but also shift along the magnetic inversion line so that magnetic reconnection progresses stepwise to neighboring flux tubes. We conclude that “tether cutting” reconnection in the sheared arcade progressively transforms it to a twisted flux tube, which becomes unstable, leading to a CME. We interpret the third flare, a long-duration event, as a combination of the classical two-ribbon flare with the relaxation process following forced reconnection between the expanding CME structure and neighboring magnetic fields. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Spatial differences in persistent organochlorine pollutant concentrations between the Bering and Chukchi Seas (1993)

During August-September 1993, a joint Russian-United States expedition to the Bering and Chukchi Seas took place. Surface water samples were collected from 21 sites and separated into dissolved (duplicates) and suspended solids; 19 sediment and 6 air samples were also collected. These samples were analysed for 19 organochlorine pesticides, 11 chlorobenzenes and 113 PCB congeners. The report provides data on selected compounds which occured in > or = 75% of the water samples. Highest water concentrations were observed for HCH in open waters north and south of the Bering Strait, both regions being similar (alpha-HCH; 2.2 ng/L and lindane: 0.35 ng/L). Air levels observed were also constant (alpha-HCH; 0.041 ng/m3, lindane: 0.0093 ng/m3). Suspended solids and air particulares contributed little to the concentrations in their respective media, an observation common to all analytes except for the PCBs and the DDT residues. The sum of PCB concentrations in water were higher in the Bering Sea area compared to the Chukchi Sea (1.0 vrs 0.67 ng/L) and lower for air (0.46 vrs 0.23 ng/m3). Sum of DDT in water was higher in the Bering Sea than in the Chukchi Sea (0.23 vrs 0.15 ng/L) while in sediments and air, the Bering Sea concentrations were lower (0.95 vrs 1.6 ng/g and 36 vrs 56 pg/m3, respectively). Other organochlorine compounds for which data are presented include: pp'-DDE, pp'-DDT, dieldrin, HCB, 3 chlorobenzenes and 3 PCB congeners. Fluxes of all these chemicals through the Berin Strait are estimated; they ranged from 57 t/a (alpha-HCH) through 26 t/a (for sum of PCBs) to 0.2 t/a (pp'-DDE, dieldrin and 1,2,3-trichlorobenzene). Fugacity ratios for the HCHs and PCBs indicate the alpha-HCH is degassing in both the Bering and Chukchi Seas and that the gamma-isomer is degassing in the Bering Sea and is close to equilibrium (weakly absorbing) in the Chuchi Sea; the sum of PCBs are strongly absorbing in both areas.     

Petrology of the Easter microplate region in the South Pacific

Submersible investigations along the East Rift segments, the Pito Deep and the Terevaka transform fault of the Easter microplate eastern boundary, and on a thrust-fault area of the Nazca Plate collected a variety of basalts and dolerites. The volcanics consist essentially of depleted (N-MORB), transitional (T-MORB) and enriched (E-MORB) basalts with low (0.01−0.1, < 0.7), intermediate (0.12–0.25, 0.7–1.2) and high (> 0.25, > 1.2–2) K/Ti and(La/Sm)_N ratios, respectively. The Fe-Ti-rich ferrobasalt encountered among the N-MORBs are found on the Pito Deep Central volcano, on the Terevaka intra-transform ridge, on the ancient (< 2.5 Ma) Easter microplate (called EMP, comprising the East Rift Inner pseudofaults and Pito Deep west walls) and on thrust-fault crusts. The most enriched (T- and E-MORB) volcanics occur along the East Rift at 25 °50′–27 °S (called 26 °S East Rift) and on the Pito seamount located near the tip of the East Rift at 23 °00′–23 °40′S (called 23 °S East Rift). The diversity in incompatible element ratios of the basalts in relation to their structural setting suggests that the volcanics are derived from a similar heterogenous mantle which underwent variable degrees of partial melting and magma mixing. In addition the Pito seamount volcanics have undergone less crystal fractionation (< 20%) than the lavas from the other Easter microplate structures (up to 35–45%). The tectonic segmentation of the East Rift observed between 23 and 27 °S corresponds to petrological discontinuities related to Mg# variations and mantle melting conditions. The highest Mg# (> 61) are found on topographic highs (2000–2300 m) and lower values (Mg# < 56) at the extremities of the East Rift segments (2500–5600 m depths). The deepest area (5600 m) along the East Rift is located at 23 °S and coincides with a Central volcano constructed on the floor of the Pito Deep. Three major compositional variabilities of the volcanics are observed along the East Rift segments studied: (1) the 26 °S East Rift segment where the volcanics have intermediate Na₈ (2.5–2.8%) and Fe₈ (8.5–11%) contents; (2) the 23 °S East Rift segment (comprising Pito seamount and Pito Deep Central volcano) which shows the highest (2.9–3.4%) values of Na₈ and a low (8–9%) Fe₈ content; and (3) the 25 °S (at 24 °50′–26 °10′S) and the 24 °S (at 24 °10′–25 °S) East Rift segments where most of the volcanics have low to intermediate Na₈ (2.6–2.0%) and a high range of Fe₈ (9–13%) contents. When modeling mantle melting conditions, we observed a relative increase in the extent of partial melting and decreasing melting pressure. These localized trends are in agreement with a 3-D type diapiric upwelling in the sense postulated by Niu and Batiza (1993). Diapiric mantle upwelling and melting localized underneath the 26, 25 and 23 °S (Pito seamount and Central volcano) East Rift segments are responsable for the differences observed in the volcanics. The extent of partial melting varies from 14 to 19% in the lithosphere between 18 and 40 km deep as inferred from the calculated initial (P_o=16kbar) and final melting (P_f=7kbar) pressures along the various East Rift segments. The lowest range of partial melting (14–16%) is confined to the volcanics from 23 °S East Rift segment including the Pito seamount and the Central volcano. The Thrust-fault area, and the Terevaka intra-transform show comparable mantle melting regimes to the 25 and 26 °S East Rift segments. The older lithosphere of the EMP interior is believed to have been the site of high partial melting (17–20%) confined to the deeper melting area (29–50 km). This increase in melting with increasing pressure is similar to the conditions encountered underneath the South East Pacific Rise (13–20 °S).     

Anomalous Behavior of the Linear Polarization of the Herbig Ae Star WW Vul during the 1997 Minimum

In the course of photopolarimetric monitoring of the Herbig Ae star WW Vul, an unusual variation of linear polarization upon energence from a deep minimum was recorded, which cannot be explained by the standard mechanism for such objects (UX Ori stars): a variable contribution of radiation scattered by circumstellar dust. An analysis of possible causes of the anomalous behavior of polarization shows that it was most likely due to the presence of aligned, aspherical dust grains in the dust cloud crossing the line of sight at that time.     

Selection of regional historical rainfall time series as input to urban drainage simulations at ungauged locations

It has become established practice during the past 20 years to use high-resolution historical rainfall time series as input to hydrological model packages for detailed simulation of urban drainage systems. However, sufficiently long rain series are rarely available from the exact catchment in question and simulations are hence often based on available rain series from other locations. Extreme rainfall properties of importance to the performance of urban storm drainage systems vary significantly even in regions with only minor physiographic differences. Part of this variation can be explained by regional variations of the mean annual rainfall and the remaining statistical residue can be interpreted as statistical uncertainty.In Denmark, more than 75 high-resolution rain gauges are installed across a total area of 43,000 m. About 40 gauges had sufficiently long records to be included in a comprehensive national investigation where newly developed statistical regionalisation procedures were used to model the regional variation of extreme rainfalls. On this basis, a spreadsheet model was made available for estimation of extreme design rainfalls and the associated uncertainty at any location in the country. Statistics were furthermore computed to classify historical rainfall time series according to the developed regional model, and this makes it possible to assess the uncertainty related with using different historical rain series for simulations at ungauged locations.This research indicates that use of historical point rainfall data at ungauged locations introduces a significant uncertainty that is largely overlooked in today's practice. The engineering recommendation is to select historical rain series based on an evaluation of the local physiographic characteristics (e.g., the mean annual rainfall) and a (pre-defined) desired safety level of the simulations.