Core–mantle boundary topography as a possible constraint on lower mantle chemistry and dynamics

The origin of large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa is not well constrained. We explore numerical convection calculations for two proposed hypotheses for these anomalies, namely, thermal upwellings (e.g., plume clusters) and large intrinsically dense piles of mantle material (e.g., thermochemical piles), each of which uniquely affects the topography on Earth's core–mantle boundary (CMB). The thermochemical pile models predict a relatively flat but elevated CMB beneath piles (presumed LLSVPs), with strong upwarping along LLSVP margins. The plume cluster models predict CMB upwarping beneath upwellings that are less geographically organized. Both models display CMB depressions beneath subduction related downwelling. While each of the two models produces a unique, characteristic style of CMB topography, we find that seismic models will require shorter length scales than are currently being employed in order to distinguish between the end-member dynamic models presented here.     

Zircon chemistry and new laser ablation U–Pb ages for uraniferous granitoids in SW Cameroon

The Ekomedion two-mica granite,southwestern Cameroon,has potential for uranium and molybdenum mineralization.Here,we present LA-ICP-MS U-Pb ages,Lu-Hf isotope characteristics,trace element concentrations and Ti-geothermometry of zircon from this granite hosting U-Mo mineralization in pegmatitic pods.The majority of zircon are CL-dark though some CL-bright cores were also identified.U-Pb zircon age data range from 121±3 to743±11 Ma with only 5 of 34 ages being near concordant.The concordant mean age of 603±12 Ma is similar to ages of granitic intrusions along the Central African Shear Zone in Cameroon.Apparent ages with mean of261±6 Ma reveal open system behavior with respect to Pb and/or U.Zircon e Hfi values range from-20.3 to-0.3.This implies that U-Mo was remobilized during partial melting of the surrounding gneiss.Zircon Th/U>0.1 as well as an increasing Hf with decreasing Th/U indicates that fractional crystallization was the main factor that controlled U-Mo mineralization in pegmatitic pods.Y and Y/Ho ratios cluster from 29 to 33 close to the chondritic ratio of 28 and indicate fractionation of Y and Ho with low F contents during the earliest stages of crystallization.Late stage accumulation of F-rich magmatic-hydrothermal fluids impacted U-Mo mineralization as a ligand.Zircon contains a prominent negative Eu anomaly pointing to a fractionating system rich in plagioclase.Calculated Ti-in-zircon temperatures span 672℃to1232℃with the temperatures at the high end reflecting interference from mineral inclusions in the zircon grains while the lower temperature values are linked to crystallization.     

A study of the role of ion–molecule chemistry in the formation of sporadic sodium layers

Over two campaigns in 1998 and 1999, multiple sporadic sodium events were observed by the Arecibo Observatory sodium density lidar while simultaneously monitoring the plasma density using the incoherent scatter radar. In this paper, we test the theoretical explanation proposed by Cox and Plane (1998) where Na⁺ in a plasma layer is neutralized via an ion–molecule mechanism to form a sporadic sodium layer. A particular challenge is to interpret observations made in a Eulerian frame of observation where the spatial and temporal characteristics of events cannot easily be separated. The reaction scheme in the original mechanism is modified to include the reactions NaO⁺+N₂→Na⁺·N₂+O and NaO⁺+O₂→Na⁺+O₃, following the results of theoretical quantum calculations. Six unique case studies of sporadic sodium layers are presented here, and excellent agreement between simulation and observations was obtained for five of them.     

Major element chemistry of ocean island basalts — Conditions of mantle melting and heterogeneity of mantle source

We estimate average compositions of near-primary, ‘reference’ ocean island basalts (OIBs) for 120 volcanic centers from 31 major island groups and constrain the depth of lithosphere–asthenosphere boundary (LAB) at the time of volcanism and the possible depth of melt–mantle equilibration based on recently calibrated melt silica activity barometer. The LAB depth versus fractionation corrected OIB compositions (lava compositions, X, corrected to Mg# 73, X_OIB^#73, i.e., magmas in equilibrium with Fo₉₀, if olivine is present in the mantle source) show an increased major element compositional variability with increasing LAB depths. OIBs erupted on lithospheres < 40 km thick approach the compositions (e.g. SiO₂^#73, TiO₂^#73, [CaO/Al₂O₃]^#73) of primitive ridge basalts and are influenced strongly by depth and extent of shallow melting. However, X_OIB^#73 on thicker lithospheres cannot be explained by melt–mantle equilibration as shallow as LAB. Melt generation from a somewhat deeper (up to 50 km deeper than the LAB) peridotite source can explain the OIB major element chemistry on lithospheres ≤ 70 km. However, deeper melting of volatile-free, fertile peridotite is not sufficient to explain the end member primary OIBs on ≥ 70 km thick lithospheres. Comparison between X_OIB^#73 and experimental partial melts of fertile peridotite indicates that at least two additional melt components need to be derived from OIB source regions. The first component, similar to that identified in HIMU lavas, is characterized by low SiO₂^#73, Al₂O₃^#73, [Na₂O/TiO₂]^#73, and high FeO?^#73, CaO^#73, [CaO/Al₂O₃]^#73. The second component, similar to that found in Hawaiian Koolau lavas, is characterized by high SiO₂^#73, moderately high FeO?^#73, and low CaO^#73 and Al₂O₃^#73. These two components are not evenly sampled by all the islands, suggesting a heterogeneous distribution of mantle components that generate them. We suggest that carbonated eclogite and volatile-free, silica-excess eclogite are the two most likely candidates, which in conjunction with fertile mantle peridotite, give rise to the two primitive OIB end members.     

Changes in surface water chemistry of the Chul’man River and its tributaries in coal mining regions

The physicochemical properties and chemistry of surface water of rivers in South-Yakutia Fuel and Production Complex were examined in the summer-autumn dry period. Changes in the properties and chemistry of the Chul’man River and its tributaries were analyzed to assess the effect of coal mining industry on the character and degree of technogenic pollution of surface water in the examined northern region.     

The role of paleohydrochemical factors in groundwater chemistry formation in oil-and-gas-bearing deposits of the northeastern Bol’shekhetskaya Megasyneclise

Groundwater chemistry in the northwestern Bol’shekhetskaya Megasyneclise is considered, and the predominance of inversion hydrogeochemical zonality is identified. The analysis of the hydrogeochemistry and genetic coefficients enabled the tentative identification of infiltrogenic, sedimentogenic, lithogenic, and condensatogenic types of waters. The implemented paleohydrogeochemical reconstructions confirmed the predominance in the cross-section of ancient infiltrogenic and slightly saline sedimentogenic water at all development stages of the sedimentary basin.     

Stratigraphy, chemistry, and eruptive dynamics of the 12.4 ka plinian eruption of Apoyeque volcano, Managua, Nicaragua

Apoyeque volcano, located 9 km northwest of Managua city, erupted explosively at 12.4 ka. The Plinian eruption deposited a widespread pumice fall deposit known as the Upper Apoyeque Tephra (UAq). The UAq is massive, reversely graded, and consists of white juvenile pumice (~78 vol.%), a variety of cognate lithics and accidental altered lithics. The whole-rock pumice composition is rhyodacitic (SiO₂?=?66.9–68.5 wt.%) with a mineral paragenesis of plagioclase, orthopyroxene, clinopyroxene, amphibole, titanomagnetite, and ilmenite in a rhyolitic glass groundmass (SiO₂?=?74.4?±?0.6 wt.%). The deposit’s dispersal axis is to the south, with the deposit covering a minimum area of 877 km² within the 50 cm isopach and has a total volume of 3 km³ (dense rock equivalent, 1.15 km³). The eruption column reached a maximum height of ca.28 km. The eruption ejected a total mass of 3?×?10¹² kg at an average rate of 2?×?10⁸ kg/s, and based on available models, we infer duration of almost 4 h. Petrographic and geochemical characteristics suggest that the eruption was triggered by magma mixing.     

Mineral chemistry, geochemistry and U-Pb SHRIMP zircon data of the Yangxin monzonitic intrusive in the foreland of the Dabie orogen

The lower Yangtze River area, situated at the fore-land of the Qinling-Dabie orogen, is an important re-gion for high-grade mineral deposits in Cen-tral-Eastern China. Nearly 300 different types of Cu and Au polymetal, Fe, and S mineral deposits have been found and mined in this zone[1,2]. The overall distribution of these deposits follows the trend of the Mesozoic igneous rocks, suggesting their fundamental controls on the formation of these deposits. Geo-physical and geologic observatio…     

The role of bromine and chlorine chemistry for arctic ozone depletion events in Ny-Ålesund and comparison with model calculations

During the Arctic Tropospheric Ozone Chemistry (ARCTOC) campaigns at Ny-Ålesund, Spitsbergen, the role of halogens in the depletion of boundary layer ozone was investigated. In spring 1995 and 1996 up to 30 ppt bromine monoxide were found whenever ozone decreased from normal levels of about 40 ppb. Those main trace gases and others were specifically followed in the UV-VIS spectral region by differential optical absorption spectroscopy (DOAS) along light paths running between 20 and 475 m a.s.l. The daily variation of peroxy radicals closely followed the ozone photolysis rate J(O₃(O¹D)) in the absence of ozone depletion most of the time. However, during low ozone events this close correlation was no longer found because the measurement of radicals by chemical amplification (CA) turned out to be sensitive to peroxy radicals and ClO_x. Large CA signals at night can sometimes definitely be assigned to ClO_x and reached up to 2 ppt. Total bromine and iodine were both stripped quantitatively from air by active charcoal traps and measured after neutron activation of the samples. Total bromine increased from background levels of about 15 ppt to a maximum of 90 ppt during an event of complete ozone depletion. For the spring season a strong source of bromine is identified in the pack ice region according to back trajectories. Though biogenic emission sources cannot be completely ruled out, a primary activation of halogenides by various oxidants seems to initiate an efficient autocatalytic process, mainly driven by ozone and light, on ice and perhaps on aerosols. Halogenides residing on pack ice surfaces are continuously oxidised by hypohalogenous acids releasing bromine and chlorine into the air. During transport and especially above open water this air mixes with upper layer pristine air. As large quantities of bromine, often in the form of BrO, have been observed at polar sunrise also around Antarctica, its release seems to be a natural phenomenon. The source strength of bromine from halogen activation on the pack ice, as based on the measured inorganic bromine levels, averages about 10¹² Br-atoms m⁻² s⁻¹ during sunlit periods in Arctic spring. The total source strength of inorganic bromine from sunlit polar regions may therefore amount to 30 kt y⁻¹.     

Effects of multiple stressors on lakes in south-central Ontario: 15 years of change in lakewater chemistry and sedimentary diatom assemblages

Freshwater ecosystems are increasingly affected by human influences. Since the pre-industrial era, lakes of the Muskoka–Haliburton region of south-central Ontario have had increases in shoreline residential development and acid deposition. Previous research on 54 of these lakes, using sediment cores and diatom-based transfer functions, showed changes in lakewater pH and total phosphorus concentration between the preindustrial era and 1992. Since 1992, there has been further change, which we have documented for the same set of lakes, using similar methods. For example, dissolved organic carbon has increased and there have been significant increases in planktonic diatoms (e.g. Cyclotella stelligera) commonly associated with climate warming. More striking diatom changes have occurred in the past 15 years than between pre-industrial times and 1992. Significant changes observed in both chemical (e.g. pH, Ca, DOC) and biological data suggest that novel stressors, such as declines in lake calcium concentrations, acting in conjunction with climate and land-use change, have created ecosystems for which there are no historical analogs.     

The 27-day solar rotational cycle in the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB CMAM CHEM)

We simulate twenty, perpetual Januaries each under solar minimum and maximum conditions using the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB CMAM CHEM), including the 27-day solar rotational cycle. Cross-correlation functions (r) of tropical ozone and ultraviolet (UV) suggest peak values of +0.5 to +0.6 (with % changes in ozone of 0.4–0.6%) in the middle atmosphere. Temperature in the middle atmosphere varies with UV by up to 0.5 K, which appears to be mainly dynamical in origin. We calculate a signal associated with the rotational cycle in the troposphere, although this requires further investigations.     

Magnetic properties of high-Ti basaltic rocks from the Krušné hory/Erzgebirge MTS. (Bohemia/Saxony), and their relation to mineral chemistry

This study provides new thermomagnetic and petrographic data on specific basaltic rock association from the broader vicinity of the Lou?ná-Oberwiesenthal volcanic centre, western Bohemia/Saxony. Two types of volcanic rocks were recognized there: (i) high-Ti types (3.5–5.2 wt% TiO₂) represented by (mela)nephelinite s.s., and sporadically present (ii) medium-Ti types (2.5–3.5 wt% TiO₂) of olivine nephelinite, nepheline basanite and phonotephrite compositions. In order to examine the rock-magnetic behaviour, they were studied for their variations in the Curie temperature (T_C) and field-dependent susceptibility, spinel group minerals, chemistry and petrology. Magnetic susceptibility of ulvöspinel-rich titanomagnetite, as a dominant magnetic carrier, depends on the amplitude of measured magnetic field, whereas pure magnetite is field-independent. Field dependence parameter ^kHD of the studied basaltic rocks ranges from 0.8 to 18.7%, TiO₂ contents in titanomagnetite range from 12.7 to 20.1 wt.%. TiO₂ content in titanomagnetite does not correlate with whole-rock TiO₂ content (2.8 to 5.6 wt.%). The content of substituted titanium in the sublattice of magnetite is also sensitively reflected in the Curie temperature, ranging from 200 to 580°C. The spinel group minerals are designated as titanomagnetite with the dominance of ulvöspinel, magnetite and magnesioferrite components, or titanomagnetite with the magnetite, ulvöspinel and magnesioferrite components. Only two samples are characterized by a significant presence of Cr-spinel and magnesiochromite components forming cores of titanomagnetites. The titanomagnetite-bearing rocks in the studied area, likewise the low- to medium-Ti basaltic rocks from the ?eské st?edoho?í Mts., provide similar thermomagnetic curves.