Nitrogen isotopes in the recent solar wind from the analysis of Genesis targets: Evidence for large scale isotope heterogeneity in the early solar system

We have analyzed nitrogen, neon and argon abundances and isotopic ratios in target material exposed in space for 27 months to solar wind (SW) irradiation during the Genesis mission. SW ions were extracted by sequential UV (193 nm) laser ablation of gold-plated material, purified separately in a dedicated line, and analyzed by gas source static mass spectrometry. We analyzed gold-covered stainless steel pieces from the Concentrator, a device that concentrated SW ions by a factor of up to 50. Despite extensive terrestrial N contamination, we could identify a non-terrestrial, ¹⁵N-depleted nitrogen end-member that points to a 40% depletion of ¹⁵N in solar-wind N relative to inner planets and meteorites, and define a composition for the present-day Sun (¹⁵N/¹⁴N = [2.26 ± 0.67] × 10⁻³, 2σ), which is indistinguishable from that of Jupiter’s atmosphere. These results indicate that the isotopic composition of nitrogen in the outer convective zone of the Sun has not changed through time, and is representative of the protosolar nebula. Large ¹⁵N enrichments due to e.g., irradiation, low temperature isotopic exchange, or contributions from ¹⁵N-rich presolar components, are therefore required to account for inner planet values.     

Mineral dissolution kinetics as a function of distance from equilibrium – New experimental results

We revisit a fundamental question in mineral dissolution kinetics, namely: is the function of dissolution rate versus the distance from equilibrium continuous, or does the “switch” between two different reaction mechanisms cause a discontinuity, i.e., a kinetic bifurcation? Based on new insight from experimental results, including direct observations of retreating crystal surfaces with vertical scanning interferometry (VSI), we present evidence that a discontinuity does indeed exist. Through a carefully designed near-equilibrium albite dissolution experiment, we show how a non-steady-state dissolution rate observed on a crystal surface reflects reactivity inherited from earlier episodes of undersaturation. This outcome forces us to re-think the common practice of extrapolating overall dissolution rates measured far-from-equilibrium to near-equilibrium conditions.     

Different styles of metasomatic veining in ultramafic xenoliths from the TUBAF Seamount (Bismarck Microplate,Papua New Guinea)

Petrologic, geochemical and isotopic investigations on two ultramafic xenoliths with metasomatic veins from the TUBAF Seamount in the Bismarck Archipelago NE of Papua New Guinea reveal different styles of metasomatic overprinting. The first xenolith, a clinopyroxene–poor spinel lherzolite, was part of the depleted upper mantle. It contains an orthopyroxene-rich vein that formed by hydrous metasomatism at ~ 980 °C and ~ 1.5 GPa. The second xenolith is a clinopyroxene-dominated spinel olivine websterite that formed as a magmatic cumulate at the transition of the upper mantle to the oceanic crust. The websterite contains a vein with orthopyroxenes and clinopyroxenes, which give evidence for high-temperature crystallization at ~ 1300 °C and < 0.36 GPa. Both xenoliths were transported to the seafloor by a Quaternary trachybasalt in a fore-arc position. The vein minerals show a strong affinity to a supra-subduction zone or island arc setting. The REE pattern of the vein in the clinopyroxene–poor lherzolite strongly resembles the one from the host trachybasalt, with a high enrichment of the LREE and a strong to moderate enrichment of the MREE and HREE. Although broadly similar in shape, the REE pattern of the vein in the websterite shows a much weaker enrichment. The same applies to the trace-element patterns, although there are significant differences in the Eu, Zr, Hf and Nb concentrations. The isotope signatures of both veins suggest a derivation from a subducted slab that had been hydrothermally altered by seawater (high ⁸⁷Sr/⁸⁶Sr values).The contrasting crystallization temperatures of the vein minerals as well as their overall geochemical differences indicate that the metasomatic agents responsible for the vein in the websterite were mobilized from a previously depleted source at a much deeper mantle level than those forming the vein of the clinopyroxene–poor lherzolite. The metasomatic agents may also have been mobilized at different times and from different plates, i.e., the deeply subducted Solomon Sea Microplate (for the veins in the websterite) and the shallow dehydrating Pacific Plate (for the veins in the clinopyroxene–poor lherzolite).Metasomatic agents responsible for similar petrologic phenomena, i.e., modal or cryptic metasomatism, may have distinctly different origins and show contrasting histories. A strongly depleted lherzolite may totally lose its initial geochemical signature by the influence of an enriched metasomatic agent, whereas a primarily enriched ultramafic rock, e.g., a websterite, may strongly obscure the trace-element pattern of a less enriched metasomatic vein. Furthermore, the geochemistry of the ultramafic xenoliths may reflect polyphase cryptic and modal metasomatism related to veining and later transport by the hosting melt to the seafloor.     

An iterative version of the adaptive Gaussian mixture filter

The adaptive Gaussian mixture filter (AGM) was introduced as a robust filter technique for large-scale applications and an alternative to the well-known ensemble Kalman filter (EnKF). It consists of two analysis steps, one linear update and one weighting/resampling step. The bias of AGM is determined by two parameters, one adaptive weight parameter (forcing the weights to be more uniform to avoid filter collapse) and one predetermined bandwidth parameter which decides the size of the linear update. It has been shown that if the adaptive parameter approaches one and the bandwidth parameter decreases, as an increasing function of the sample size, the filter can achieve asymptotic optimality. For large-scale applications with a limited sample size, the filter solution may be far from optimal as the adaptive parameter gets close to zero depending on how well the samples from the prior distribution match the data. The bandwidth parameter must often be selected significantly different from zero in order to make large enough linear updates to match the data, at the expense of bias in the estimates. In the iterative AGM we introduce here, we take advantage of the fact that the history matching problem is usually estimation of parameters and initial conditions. If the prior distribution of initial conditions and parameters is close to the posterior distribution, it is possible to match the historical data with a small bandwidth parameter and an adaptive weight parameter that gets close to one. Hence, the bias of the filter solution is small. In order to obtain this scenario, we iteratively run the AGM throughout the data history with a very small bandwidth to create a new prior distribution from the updated samples after each iteration. After a few iterations, nearly all samples from the previous iteration match the data, and the above scenario is achieved. A simple toy problem shows that it is possible to reconstruct the true posterior distribution using the iterative version of the AGM. Then a 2D synthetic reservoir is revisited to demonstrate the potential of the new method on large-scale problems.     

Origin and age of ultramafic rocks and gabbros in the southern Puna of Argentina: an alleged Ordovician suture revisited

Ultramafic rocks and gabbros are exposed in the southern Puna (NW Argentina) in tectonic association with continental arc-related Ordovician (volcano) sedimentary successions and granitoids. The origin of this mafic rock suite has been debated for three decades as either representing an Ordovician terrane suture, primitive Ordovician arc-related rocks or relics of the pre-Ordovician basement in tectonic contact with the Ordovician retro-arc basin successions. We present the first U–Pb ages of primary and inherited zircon from gabbros of this mafic–ultramafic assemblage. LA-ICP-MS analyses on cores and rims of these zircon grains yielded a concordia age of 543.4 ± 7.2 Ma for the gabbroic rocks. Other analysed zircons have Mesoproterozoic, and Early Ediacaran core and rim ages indicating that the magmas also assimilated Meso- and Neoproterozoic crustal material prior to final crystallization. The mafic rocks witnessed higher metamorphic grade than associated Ordovician rocks, which are unmetamorphosed or only affected by anchimetamorphism. The gabbros are mostly tholeiitic and enriched in Zr, Th, as well as other incompatible elements and have εNd _t=540Ma ranging from 1.3 to 7.4 with most of the values between 5 and 7. ¹⁴⁷Sm/¹⁴⁴Nd ratios show evidence of weak crustal contamination. The mafic rocks do not reveal any affinity to mid-ocean ridge basalts in their geochemistry but point instead to an emplacement in an active plate margin arc environment. Chromites from ultramafic rocks show typical Ti, Al, Cr#, Fe³⁺ abundances found in magmatic arc rocks. The formation of the gabbros and the associated ultramafic rocks in the southern Argentine Puna is related to the evolution of the margin of the Pampia terrane, including the Puncoviscana basin, during the Late Neoproterozoic and earliest Cambrian. In contrast to previous interpretations, the rocks predate the Ordovician evolution of the Central proto-Andean active margin. Consequently, interpretations assuming these rocks to represent an oceanic terrane suture of Ordovician age have to be dismissed as much as all palaeotectonic models that define Ordovician terranes in the Central Andes based on assumption that the ultramafic rocks and gabbros exposed in the southern Puna mark plate boundaries.     

Geochemical studies on urban soil from two sampling depths in Tampere Central Region,Finland

Preliminary geochemical mapping was carried out within urban areas in Tampere Central Region, Finland, to gain a better understanding of element concentrations in urban soil and to provide information on baselines in soil within urban centres for soil contamination assessment purposes. The soil samples were taken from central city parks, day-care centres and school yards, and residential areas. Various sampling depths have generally been used in urban geochemical surveys. The aim of this study was to compare the results from two commonly used sample types taken from the same sites in urban soil: single samples of minerogenic topsoil from the 0–25 cm layer and composite samples of minerogenic topsoil from a depth of 0–2 cm. The concentrations of most of the studied trace elements showed a significant correlation between samples from 0 to 2 and 0 to 25 cm, but element concentrations differed between the two studied sample depths. For most of the studied elements, the median concentrations were higher in the 0–25 cm samples, but anomalous concentrations were more often found in the 0–2 cm samples. Some elements had elevated concentrations when compared with the Finnish guideline values for soil contamination assessment. This study did not conclusively establish whether a sampling depth of 0–2 or 0–25 cm should be recommended for similar studies in the future. Selection of the sampling depth in geochemical studies greatly depends on the aim of the project. In order to determine the upper limits of geochemical baseline variation, the deeper sampling depth appears to be more feasible. However, for the preliminary health risk assessment of areas with sensitive land uses, e.g. children’s playgrounds, samples from 0–2 cm depth are considered informative. Such samples may also be used to indicate local sources of dusting creating site-specific hotspots of potentially harmful elements in urban topsoil.     

Genetic relationship between silver–lead–zinc mineralization in the Wutong deposit,Guangxi Province and Mesozoic granitic magmatism in the Nanling belt,southeast China

More than 50 % of the world's total reserves of tungsten are in China and most tungsten deposits are located in the Nanling range in southeast China. This study explores the potential genetic relationship between tungsten–tin (W–Sn) mineralization and shallower Ag–Pb–Zn deposits in the Nanling range based on data from the Wutong deposit, Guangxi Province. The lead, oxygen, carbon, sulfur, and strontium isotopic compositions of minerals at Wutong indicate that a single crustal-derived fluid was responsible for mineralization. Wutong likely formed at relatively low temperatures (～200–300 °C) and low pressures, as indicated by the similarity between homogenization temperatures of fluid inclusions and those estimated from S isotopic compositions of minerals. The hübnerite age (92.3–104.4 Ma) indicates that the Wutong mineralization is likely related to nearby Late Yanshanian (Cretaceous) S-type granites derived from Proterozoic crust. This mineralization event coincides with the last W–Sn mineralization event and the Cretaceous peak of mineralization in the Nanling range.     

‘PALEOVAN’, International Continental Scientific Drilling Program (ICDP): site survey results and perspectives

Lake Van is the fourth largest terminal lake in the world (volume 607 km³, area 3570 km², maximum depth 460 m), extending for 130 km WSW–ENE on the Eastern Anatolian High Plateau, Turkey. The sedimentary record of Lake Van, partly laminated, has the potential to obtain a long and continuous continental sequence that covers several glacial–interglacial cycles (ca 500 kyr). Therefore, Lake Van is a key site within the International Continental Scientific Drilling Program (ICDP) for the investigation of the Quaternary climate evolution in the Near East (‘PALEOVAN’). As preparation for an ICDP drilling campaign, a site survey was carried out during the past years. We collected 50 seismic profiles with a total length of ～850 km to identify continuous undisturbed sedimentary sequences for potential ICDP locations. Based on the seismic results, we cored 10 different locations to water depths of up to 420 m. Multidisciplinary scientific work at positions of a proposed ICDP drill site included measurements of magnetic susceptibility, physical properties, stable isotopes, XRF scans, and pollen and spores. This core extends back to the Last Glacial Maximum (LGM), a more extended record than all the other Lake Van cores obtained to date. Both coring and seismic data do not show any indication that the deepest part of the lake (Tatvan Basin, Ahlat Ridge) was dry or almost dry during past times. These results show potential for obtaining a continuous undisturbed, long continental palaeoclimate record. In addition, this paper discusses the potential of ‘PALEOVAN’ to establish new results on the dynamics of lake level fluctuations, noble gas concentration in pore water of the lake sediment, history of volcanism and volcanic activities based on tephrostratigraphy, and paleoseismic and earthquake activities.