Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

A geochemical method for removing the effect of tephra on lake diatom oxygen isotope records

The analysis of oxygen isotope ratios (δ¹⁸O) in biogenic silica (e.g. diatoms, sponge spicules, phytoliths) is an increasingly utilised palaeoclimatic proxy for lakes and oceans, providing an alternative to carbonate δ¹⁸O. Currently, the problems associated with the cleaning and extraction of pure diatom silica, especially from lake sediments, may compromise the precision associated with δ¹⁸O_diatom records. This is because, despite improvements in cleaning techniques, it is still difficult to remove silica-containing contaminants (e.g. tephra, silts and clays) from biogenic silica. Where it has proved difficult to remove all the silica bearing contaminants from a sample, mass balance calculations involving point counting have been used, but these are inherently inaccurate as they only consider the surface area of the contaminant, rather than volume. The new method described here assesses the volume of contamination in each sample of biogenic silica by using major and trace element geochemistry, allowing the δ¹⁸O of the contamination to be removed from the biogenic silica δ¹⁸O value by linear mass balance. In this case, diatom silica was contaminated with tephra, however the method should also be applicable to other contaminants such as silts and clays.     

Post-collisional Tertiary–Quaternary mafic alkalic magmatism in the Carpathian–Pannonian region: a review

Mafic alkalic volcanism was widespread in the Carpathian–Pannonian region (CPR) between 11 and 0.2 Ma. It followed the Miocene continental collision of the Alcapa and Tisia blocks with the European plate, as subduction-related calc-alkaline magmatism was waning. Several groups of mafic alkalic rocks from different regions within the CPR have been distinguished on the basis of ages and/or trace-element compositions. Their trace element and Sr–Nd–Pb isotope systematics are consistent with derivation from complex mantle-source regions, which included both depleted asthenosphere and metasomatized lithosphere. The mixing of DMM-HIMU-EMII mantle components within asthenosphere-derived magmas indicates variable contamination of the shallow asthenosphere and/or thermal boundary layer of the lithosphere by a HIMU-like component prior to and following the introduction of subduction components.Various mantle sources have been identified: Lower lithospheric mantle modified by several ancient asthenospheric enrichments (source A); Young asthenospheric plumes with OIB-like trace element signatures that are either isotopically enriched (source B) or variably depleted (source C); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII-EMI components and slightly influenced by Miocene subduction-related enrichment (source D); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII components and significantly influenced by Miocene subduction-related enrichment (source E). Melt generation was initiated either by: (i) finger-like young asthenospheric plumes rising to and heating up the base of the lithosphere (below the Alcapa block), or (ii) decompressional melting of old asthenosphere upwelling to replace any lower lithosphere or heating and melting former subducted slabs (the Tisia block).     

Laboratory Kinetic Studies of the Reactions of Cl Atoms with Species of Biogenic Origin: δ3-Carene, Isoprene, Methacrolein and Methyl Vinyl Ketone

The rate coefficients for the reaction between atomic chlorine and a number of naturally occurring species have been measured at ambient temperature and atmospheric pressure using the relative rate technique. The values obtained were (4.0 ± 0.8) × 10^-10, (2.1 ± 0.5) × 10^-10, (3.2 ± 0.5) × 10^-10, and (4.9 ± 0.5) × 10^-10 cm³ molecule^-1 s^-1, for reactions with isoprene, methyl vinyl ketone, methacrolein and ³-carene, respectively. The value obtained for isoprene compares favourably with previously reported values. No values have been reported to date for the rate constants of the other reactions.     

Interviews and Questionnaires as Mixed Methods in Population Geography: TheCase of Lone Fathers in Newcastle,Australia

A mixed method approach was adopted to study the experiences of lonefathers, using a classic triangulation approach of interview and questionnaire data. This study utilised an empirical realist framework of scientific enquiry, with the ‘soft’ individual interview data seen as an adjunct to the ‘hard’ aggregate quantitative methods. A review of this study found that the interviews worked well as a pilot study in a classic mixed methods framework. The questionnaires provided a range of information about thecharacteristics of this group of lone fathers, but it was the interviews which provided astonishing depth on the causes of marital breakdown and post-marital conflict, and on the discourses and other structures which sustain social processes. In this study, the interview techniques could have been used differently, in a different framework of analysis (that of critical rather than empirical realism) without the support of other mixed methods.     

Determination of Incompatible Trace Elements in Mantle Clinopyroxenes by LA-ICP-MS: A Comparison of Analytical Performance with Established Techniques

Incompatible trace element abundances have been determined in mantle-derived clinopyroxenes by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in a comparative study with established microanalytical and bulk techniques. Individual clinopyroxene grains were sampled for the laser ablation study from sieved mineral separate fractions, from which similar mineral grains had been previously extracted for microprobe (SIMS) and bulk (INAA and ID-TIMS) analysis. Mineral grains were ablated with variable degrees of surface spatial resolution (50-200 μm) in order to maximise ICP-MS analyte count rates and to improve detection limits. A comparison of results from the different techniques reveals that for the most homogeneous samples LA-ICP-MS can achieve excellent levels of agreement with other techniques (10%) and good precision for most of the studied elements (1-5% RSD). Variations in calculated concentrations by LA-ICP-MS confirm inter- and intra-mineral heterogeneity determined by SIMS, reflecting changes in sample composition and texture. The long-term reproducibility of the technique is shown by the consistency of results for one sample analysed on thirteen occasions over a period of nine months.     

Decomposition of mangrove roots: Effects of location,nutrients, species identity and mix in a Kenyan forest

Mangrove trees may allocate >50% of their biomass to roots. Dead roots often form peat, which can make mangroves significant carbon sinks and allow them to raise the soil surface and thus survive rising sea levels. Understanding mangrove root production and decomposition is hence of theoretical and applied importance. The current work explored the effects of species, site, and root size and root nutrients on decomposition. Decomposition of fine (≤3 mm diameter) and coarse (>3 mm diameter, up to a maximum of ∼9 mm) roots from three mangrove species, Avicennia marina, Bruguiera gymnorrhiza and Ceriops tagal was measured over 12 months at 6 sites along a tidal gradient in Gazi Bay, Kenya. C:N and P:N ratios in fresh and decomposed roots were measured, and the effects on decomposition of root size and age, of mixing roots from A. marina and C. tagal, of enriching B. gymnorrhiza roots with N and P and of artefacts caused by bagging roots were recorded. There were significant differences between species, with 76, 47 and 44 % mean dry weight lost after one year for A. marina, B. gymnorrhiza and C. tagal respectively, and between sites, with generally slower decomposition at dryer, high tidal areas. N enriched B. gymnorrhiza roots decomposed significantly faster than un-enriched controls; there was no effect of P enrichment. Mixing A. marina and C. tagal roots caused significantly enhanced decomposition in C. tagal. These results suggest that N availability was an important determinant of decomposition, since differences between species reflected the initial C: N ratios. The relatively slow decomposition rates recorded concur with other studies, and may overestimate natural rates, since larger (10–20 mm diameter), more mature and un-bagged roots all showed significantly slower rates.     

Phreatomagmatic to Strombolian eruptive activity of basaltic cinder cones: Montaña Los Erales, Tenerife, Canary Islands

Phreatomagmatic activity results from the interaction of magma and external water during a volcanic eruption and is a frequent eruptive phenomenon worldwide. Such ‘fuel-coolant’ reactions change the eruptive dynamics, thus generating particles that reflect the degree of explosivity. Different eruptive phases may thus be identified from deposits, allowing us to reconstruct conditions that prevailed in the past and use these to predict the level of explosivity in a given geological setting in the future.A detailed study of the deposits from Montaña Los Erales, a 70 m-high Quaternary cinder cone belonging to a rift-related chain of vents in the Bandas del Sur region, in Southeast Tenerife, was undertaken. Field observations on excavated sections and SEM analysis of tephra suggest that the eruption style changed progressively from an initial phreatomagmatic phase, through a transitional stage, to one that was entirely Strombolian. To investigate the causes and the nature of these changes in eruptive style, products from each major unit were analysed for their morphology using hand specimen observations, secondary electron microscopy, backscatter electron microscopy, and reflected light microscopy to examine fragment size variation, fragment morphologies, vesicularity, and the level of secondary hydrous alteration (e.g. palagonitisation and zeolitisation). Study results demonstrate that the initial phase of activity was largely driven by magma–water interaction, where magma may have interacted with a lens of fresh ground- or surface water. With proceeding eruptive activity the water became exhausted, giving rise to an entirely Strombolian eruptive style.Examples of phreatomagmatic activity that occur on typical rift-related basaltic vent alignments are not infrequent in the Canary Islands. These vent systems usually erupt in Strombolian fashion, producing scoria and lava flows that do not generally extend far beyond the vent area. However, aligned feeders may intersect different strata, structural features, and different hydrological situations, thus giving rise to activity that is less predictable in setting, intensity, and duration. The occurrence of phreatomagmatism in an otherwise low-explosivity basaltic eruptive environment increases the need for awareness of the geo-hydrological situation in volcanically active areas. Studying the past eruptive history is therefore essential to derive realistic scenarios for future vulnerability evaluation and risk assessment, especially in densely populated areas like the Canary Islands.     

Potential Impacts of Climate Change on Tropical Asian Forests Through an Influence on Phenology

Changes in plant phenology will be one of the earliest responses to rapid global climate change and could potentially have serious consequences both for plants and for animals that depend on periodically available plant resources. Phenological patterns are most diverse and least understood in the tropics. In those parts of tropical Asia where low temperature or drought impose a seasonal rest period, regular annual cycles of growth and reproduction predominate at the individual, population, and community level. In aseasonal areas, individuals and populations show a range of sub- to supra-annual periodicities, with an overall supra-annual reproductive periodicity at the community level. There is no evidence for photoperiod control of phenology in the Asian tropics, and seasonal changes in temperature are a likely factor only near the northern margins. An opportunistic response to water availability is the simplest explanation for most observed patterns where water is seasonally limiting, while the great diversity of phenological patterns in the aseasonal tropics suggests an equal diversity of controls. The robustness of current phenological patterns to high interannual and spatial variability suggests that most plant species will not be seriously affected by the phenological consequences alone of climate change. However, some individual plant species may suffer, and the consequences of changes in plant phenology for flower- and fruit-dependent animals in fragmented forests could be serious.