Potential geodynamic relationships between the development of peripheral orogens along the northern margin of Gondwana and the amalgamation of West Gondwana

The Neoproterozoic-Early Cambrian evolution of peri-Gondwanan terranes (e.g. Avalonia, Carolinia, Cadomia) along the northern (Amazonia, West Africa) margin of Gondwana provides insights into the amalgamation of West Gondwana. The main phase of tectonothermal activity occurred between ca. 640–540 Ma and produced voluminous arc-related igneous and sedimentary successions related to subduction beneath the northern Gondwana margin. Subduction was not terminated by continental collision so that these terranes continued to face an open ocean into the Cambrian. Prior to the main phase of tectonothermal activity, Sm-Nd isotopic studies suggest that the basement of Avalonia, Carolinia and part of Cadomia was juvenile lithosphere generated between 0.8 and 1.1 Ga within the peri-Rodinian (Mirovoi) ocean. Vestiges of primitive 760–670 Ma arcs developed upon this lithosphere are preserved. Juvenile lithosphere generated between 0.8 and 1.1 Ga also underlies arcs formed in the Brazilide Ocean between the converging Congo/São Francisco and West Africa/Amazonia cratons (e.g. the Tocantins province of Brazil). Together, these juvenile arc assemblages with similar isotopic characteristics may reflect subduction in the Mirovoi and Brazilide oceans as a compensation for the ongoing breakup of Rodinia and the generation of the Paleopacific. Unlike the peri-Gondwanan terranes, however, arc magmatism in the Brazilide Ocean was terminated by continent-continent collisions and the resulting orogens became located within the interior of an amalgamated West Gondwana. Accretion of juvenile peri-Gondwanan terranes to the northern Gondwanan margin occurred in a piecemeal fashion between 650 and 600 Ma, after which subduction stepped outboard to produce the relatively mature and voluminous main arc phase along the periphery of West Gondwana. This accretionary event may be a far-field response to the breakup of Rodinia. The geodynamic relationship between the closure of the Brazilide Ocean, the collision between the Congo/São Francisco and Amazonia/West Africa cratons, and the tectonic evolution of the peri-Gondwanan terranes may be broadly analogous to the Mesozoic-Cenozoic closure of the Tethys Ocean, the collision between India and Asia beginning at ca. 50 Ma, and the tectonic evolution of the western Pacific Ocean.     

Experimental estimate of a relation between sea wave energies and the Earth’s crust microdeformations

The experimental data in the microseismic frequency range obtained using the seismo-acoustic-hydrophysical measurement complex are analyzed. The emphasis is put on estimating the ratio between the energy of surface sea wind waves in the area of the Japan Sea where the complex was located and the Earth’s crust microdeformations in this frequency range. The experimental evaluate obtained allow us to estimate the energy re-distribution at the hydrosphere-lithosphere boundary.     

Turning off low and high currents in a transmitter loop used in the transient electromagnetic method

In near-surface transient electromagnetic studies, it is desirable to measure the transient response starting from the earliest possible time. This requires the current in the transmitter loop to be switched off quickly, which necessitates working with a low transmitter current. As for deep-target transient electromagnetic studies, the transmitter current is as high as possible. The transmitter current's turn-off waveform and total duration affect the transient voltage response, especially at early times, which is to be accounted for when interpreting transient electromagnetic data. This article discusses the difference in switching off low and high current in a horizontal loop used as the source of the primary magnetic field in the transient electromagnetic method. Low and high currents are turned off in fundamentally different ways. When the current to be switched off is low, the loop can be represented as a symmetric combination of two transmission lines grounded at the middle of the loop perimeter. Such a representation of a loop allows calculating the current turn-off waveform at any point of the loop. The waveform and total duration of switching off a low current does not depend on its magnitude, but is determined by the period of natural oscillations of the current in the loop and the resistance of a shunting resistor. Switching off a low current in a loop can be represented as the sum of stepped current waves travelling along the loop wire. As a consequence, the current at different points of the loop perimeter is turned off at different times. In contrast to a low current, a high current is switched off linearly in time and synchronously at all points of the loop perimeter. The wave phenomena appear only at the very beginning of the current shutdown for a time interval that is much less than the total current turn-off duration. Presentation of the loop using a simple lumped-circuit model predicts the waveform and duration of the high current turn-off that coincide with the measured ones. There are two reasons why the article may be of interest to those engaged in the theory and/or practice of electromagnetic geophysical methods. First, it contributes to a general understanding of how the current in the transmitter loop is turned off. Second, the article shows how the parameters of a transmitter loop determine the current turn-off duration and thus the minimum depth of the transient electromagnetic sounding method.     

Kimberlitic sources of super-deep diamonds in the Juina area,Mato Grosso State,Brazil

The Juina diamond field, in the 1970–80s, was producing up to 5–6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006–2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2–1.8 ct/m³. Chrome spinel has 30–61 wt.% Cr₂O₃. Picroilmenite contains 6–14 wt.% MgO and 0.2–4 wt.% Cr₂O₃. Manganoan ilmenite has less than 3 wt.% MgO and 0.38–1.41 wt.% MnO. The ¹⁷⁶Hf/¹⁷⁷Hf ratio in kimberlitic zircons is 0.028288–0.28295 with ε_Hf = 5.9–8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79–80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean ²⁰⁶Pb/²³⁸U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma.     

Dynamical zodiacal cloud models constrained by high resolution spectroscopy of the zodiacal light

The simulated Doppler shifts of the solar Mg I Fraunhofer line produced by scattering on the solar light by asteroidal, cometary, and trans-neptunian dust particles are compared with the shifts obtained by Wisconsin H-Alpha Mapper (WHAM) spectrometer. The simulated spectra are based on the results of integrations of the orbital evolution of particles under the gravitational influence of planets, the Poynting-Robertson drag, radiation pressure, and solar wind drag. Our results demonstrate that the differences in the line centroid position in the solar elongation and in the line width averaged over the elongations for different sizes of particles are usually less than those for different sources of dust. The deviation of the derived spectral parameters for various sources of dust used in the model reached maximum at the elongation (measured eastward from the Sun) between 90° and 120°. For the future zodiacal light Doppler shifts measurements, it is important to pay a particular attention to observing at this elongation range. At the elongations of the fields observed by WHAM, the model-predicted Doppler shifts were close to each other for several scattering functions considered. Therefore the main conclusions of our paper do not depend on a scattering function and mass distribution of particles if they are reasonable. A comparison of the dependencies of the Doppler shifts on solar elongation and the mean width of the Mg I line modeled for different sources of dust with those obtained from the WHAM observations shows that the fraction of cometary particles in zodiacal dust is significant and can be dominant. Cometary particles originating inside Jupiter's orbit and particles originating beyond Jupiter's orbit (including trans-neptunian dust particles) can contribute to zodiacal dust about 1/3 each, with a possible deviation from 1/3 up to 0.1-0.2. The fraction of asteroidal dust is estimated to be ∼0.3-0.5. The mean eccentricities of zodiacal particles located at 1-2 AU from the Sun that better fit the WHAM observations are between 0.2 and 0.5, with a more probable value of about 0.3.     

Spectrum of hot water in the 4750–13 000 cm−1 wavenumber range (0.769–2.1 μm)

The high resolution laboratory spectrum of hot water vapour has been recorded in the 500–13 000 cm⁻¹ wavenumber range and we report on the analysis of the 4750–13 000 cm⁻¹ (0.769–2.1 μm) portion. The emission spectrum was recorded using an oxy-acetylene welding torch and a Fourier transform spectrometer. Line assignments in the laboratory spectrum as well as in an absorption spectrum of a sunspot umbra were made with the help of the BT2 line-list. Our torch spectrum is the first laboratory observation of the 9300 Å'steam bands' seen in M-stars and brown dwarfs.     

Spectroscopic and photometric observations of symbiotic nova PU Vul during 2009–2016

A new set of low-resolution spectral and UBVJHKL-photometric observations of the symbiotic nova PU Vul is presented. The binary has been evolving after its symbiotic nova outburst in 1977 and now it is in the nebular stage. It is found that the third orbital cycle(after 1977) was characterized by great changes in associated light curves. Now, PU Vul exhibits a sine-wave shape in all the light curves(with an amplitude in the U band of about 0.7 mag), which is typical for symbiotic stars in the quiescent state. Brightness variability due to pulsations of the cool component is now clearly visible in the VRI light curves. The amplitude of the pulsations increases from 0.5 mag in the V band to 0.8 mag in the I band. These two types of variability, as well as a very slow change in the physical parameters of the hot component due to evolution after the outburst of 1977, influence the spectral energy distribution(SED)of the system. The variability of emission lines is highly complex. Only hydrogen line fluxes vary with orbital phase. An important feature of the third orbital cycle is the first emergence of the OVI, 6828  Raman scattering line. We determine the temperature of the hot component by means of the Zanstra method applied to the He II, 4686  line. Our estimate is about 150 000 K for the spectrum obtained near orbital maximum in 2014. The VO spectral index derived near pulsation minimum corresponds to M6 spectral class for the cool component of PU Vul.     

Atmospheric teleconnection patterns and severity of winters in the Laurentian Great Lakes basin

Abstract

We analyzed the relationship between an index of Great Lakes winter severity (winters 1950–1998) and atmospheric circulation characteristics. Classification and Regression Tree analysis methods allowed us to develop a simple characterization of warm, normal and cold winters in terms of teleconnection indices and their combinations. Results are presented in the form of decision trees. The single most important classifier for warm winters was the Polar/Eurasian index (POL). A majority of warm winters (12 out of 15) occurred when this index was substantially positive (POL > 0.23). There were no cold winters when this condition was in place. Warm winters are associated with a positive phase of the Western Pacific pattern and El Niño events in the equatorial Pacific. The association between cold winters and La Niña events was much weaker. Thus, the effect of the El Niño/Southern Oscillation (ENSO) on severity of winters in the Great Lakes basin is not symmetric. The structure of the relationship between the index of winter severity and teleconnection indices is more complex for cold winters than for warm winters. It takes two or more indices to successfully classify cold winters. In general, warm winters are characterized by a predominantly zonal type of atmospheric circulation over the Northern Hemisphere (type W1). Within this type of circulation it is possible to distinguish two sub‐types, W2 and W3. Sub‐type W2 is characterized by a high‐pressure cell over North America, which is accompanied by enhanced cyclonic activity over the eastern North Pacific. Due to a broad southerly “anomalous” flow, surface air temperatures (SATs) are above normal almost everywhere over the continent. During the W3 sub‐type, the polar jet stream over North America, instead of forming a typical ridge‐trough pattern, is almost entirely zonal, thus effectively blocking an advection of cold Arctic air to the south. Cold winters tend to occur when the atmospheric circulation is more meridional (type C1). As with warm winters, there are two sub‐types of circulation, C2 and C3. In the case of C2, the jet stream loops southward over the western part of North America, but its northern excursion over the eastern part is suppressed. In this situation, the probability of a cold winter is higher for Lake Superior than for the lower Great Lakes. Sub‐type C3 is characterized by an amplification of the climatological ridge over the Rockies and the trough over the East Coast. The strongest negative SAT anomalies are located south of the Great Lakes basin, so that the probability of a cold winter is higher for the lower Great Lakes than for Lake Superior.     

Spectroscopic constraints on growth of Siberian mixed-habit diamonds

Notable within-crystal variability of mineralogical and geochemical properties of single natural diamonds are commonly attributed to changing chemistry of parental fluids, sources of carbon and redox conditions of diamond precipitation. A distinct type of compositional heterogeneity (mixed-habit structure) is well-known to occur in diamonds as well as in many other minerals due to purely “structural” reasons that are unequal crystal chemistry of crystallographically different faces and selective absorption and fractionation of impurities between adjacent growth pyramids. Based on the combined cathodoluminescence, Fourier-transformed infrared spectroscopy and photoluminescence spectroscopy, study of nine diamond crystals with different growth histories and external morphology, but all showing mixed-habit patterns at different growth stages, we show that mixed-diamonds may grow in closed system conditions or with a slowly decreasing growth rate from a media with a much lower impurity content than previously thought. Intracrystal nitrogen distribution seems to be a function of growth rate even in the cases of unusual impurity partitioning between growth sectors. Generally poor with IR-active hydrogen at moderate nitrogen aggregation parameters, studied diamonds likely resemble the low hydrogen content from the growth medium that, for cubic diamonds, was typically suggested hydrogen-rich and a crucial factor for growth of cubic and mixed-habit diamonds. We also show that mixed-habit diamond growth may occur not only in peridotitic suite but also in an extended field of geochemical affinities from high-Ni to low-Ni or maybe even Ni-free environments, such as pyroxenitic or eclogitic.