Lithostratigraphy and petrology of Lachman Crags and Cape Lachman lava-fed deltas, Ulu Peninsula, James Ross Island, north-eastern Antarctic Peninsula: Preliminary results

This paper presents the preliminary results regarding the lithostratigraphy, petrography and petrology of James Ross Island Volcanic Group dominating the Lachman Crags and Cape Lachman lava-fed deltas in the Ulu Peninsula, James Ross Island north-eastern Antarctic Peninsula. Studied lava-fed deltas were produced via Late Miocene to Pleistocene sub-marine and sub-glacial volcanism and made up four main lithofacies: a- bottomset pillow lavas, peperites and associated volcanoclastic/siliciclastic deposits; b- foreset-bedded hyaloclastite breccias; c- intrusions (feeder dykes, sills, and plugs) and d- topset subaerial lavas. Collectively these lithofacies record the transition from an effusive subaqueous to an effusive subaerial eruption environment. All lava samples and dykes from bottomset, foreset and topset lava-fed delta associations are olivine-phyric alkali basalts and are mineralogically and geochemically homogeneous. These eruptive products display significant enrichments in alkali contents and have ocean island basalt (OIB)-type, intra-plate geochemical signatures characterized by enrichments in all highly to moderately incompatible trace elements relative to basaltic rocks from ocean ridge settings. Volcanic products from a number of different eruptive periods display limited variations in major and trace element relative abundances, indicating derivation from a relatively homogeneous mantle source. The results of quantitative modelling of geochemical data is consistent with the view that the primary melts from which these mafic alkaline rocks were originated are the products of relatively small degrees (~3-7%) of partial melting of a volatile-bearing, metasomatized mantle source. The magmatism is likely the result of extension-driven mantle upwelling

Causes and effects of geochemical variations in late Cenozoic volcanism of the Foça volcanic centre, NW Anatolia, Turkey

The Foça volcanic centre (FVC) occupies a NNE-SSW-oriented highland between two EW-trending structural grabens in western Anatolia, and includes early-middle Miocene mafic and felsic extrusive suites. Its evolutionary history consists of an older volcano stage (16.6-16.1Ma) and a younger volcano stage (15.2-14.1Ma), which are characterized by different eruption styles and compositional and geochemical features. The older units include high-K calc-alkaline basalt, andesite, trachyandesite, rhyolite, and associated pyroclastic rocks, which formed during ignimbrite eruptions and plinian-subplinian air-fall episodes. The younger sequences are composed of shoshonitic-alkaline basalt lavas and dikes, trachytes, phonolites, and phonolitic ignimbrites that formed strombolian cones. The Foça volcanic rocks display high initial 87Sr/86Sr ratios (0.7075-0.7082 for the calc-alkaline mafic lavas, 0.7073-0.7064 for calc-alkaline felsic lavas, and 0.7063-0.7075 for the alkaline series) and low 143Nd/144Nd (0.5123-0.5125 in both series with εNd values varying from -1.3 to -6.0). These FVC geochemical features are consistent with those of other volcanic centres in western Anatolia (i.e. Bodrum, Urla-Cumaovas) and on the Aegean islands (i.e. Samos, Patmos, Chios). The geochemical and Sr-Nd isotopic compositions of the Foça volcanic units suggest that both lithospheric and asthenospheric mantle melts were involved in their evolution; however, the mantle lithosphere fingerprint was diminished by the middle Miocene, as the asthenospheric mantle melt input became dominant. These findings, combined with the bimodal character of post-collisional volcanism in the study area, suggest that geochemical variations in the nature of volcanism from calc-alkaline to alkaline and the changes in tectonic regimes through time may have been caused by successive thermal relaxations associated with possible 'piecemeal' removal of the base of subcontinental lithospheric mantle beneath western Anatolia. This interpretation is more plausible than a catastrophic collapse or wholesale delamination of the entire lithospheric mantle. Asthenospheric upwelling caused by this inferred convective thinning provided underplating of mantle-derived magmas, which interacted with the previously metasomatized lithospheric mantle and the overlying crust, resulting in their partial melting and in production of high-K calc-alkaline to mildly alkaline, incompatible element enriched magmas in separate magma chambers in which fractional crystallization occurred

Source components and magmatic processes in the genesis of Miocene to Quaternary lavas in western Turkey: constraints from HSE distribution and Hf-Pb-Os isotopes

Hf-Pb-Os isotope compositions and highly siderophile element (HSE) abundance variations are used to evaluate the mantle source characteristics and possible effects of differentiation processes in lavas from western Turkey, where the eruption of Late Miocene to Quaternary OIB-type intraplate mafic alkaline lavas followed pre-Middle Miocene convergent margin-type volcanism. Concentrations of Os, Ir, and Ru (IPGE) in the OIB-type intraplate lavas decrease with fractionation for primitive melts (MgO > 10 wt%), suggesting that these elements reside predominantly in olivine and associated HSE retaining trace phases and behave compatibly during olivine-dominated fractionation. Fractional crystallization trends indicate distinctly lower bulk partition coefficients for IPGE in more evolved lavas, possibly reflecting a change in the fractionating assemblages. Pd and Re in the primitive melts display negative correlations with MgO, demonstrating moderately incompatible behavior of these elements during fractionation, while the significantly scattered variation in Pt against MgO may indicate the effects of micronuggets of a Pt-rich alloy. Os-rich alkaline primary lavas (> 50 ppt Os) exhibit a limited range of Os-187/Os-188 (0.1361-0.1404), with some xenolith-bearing lavas displaying depletions in Os-187/Os-188 (0.1131-0.1232), suggesting slight compositional modification of primitive melts through contamination with highly depleted, Os-rich mantle lithosphere. More radiogenic Os isotope ratios (Os-187/Os-188 > 0.1954) in the evolved lavas reflect contamination of the magmas by high(187)Os/Os-188 crustal material during shallow differentiation. The OIB-type lavas show limited variations in Hf and Pb isotopes with Hf-176/Hf-177 = 0.282941-0.283051, Pb-206/Pb-204 = 18.683-19.091, Pb-207/Pb-204 = 15.57915.646, Pb-208/Pb-204 = 38.550-38.993; Hf-176/Hf-177 ratios correlate negatively with Pb-208*/Pb-206*, suggesting the effects of similar mantle processes on the evolution of time-integrated Th/U and Lu/Hf. These lavas have distinctly higher Hf-176/Hf-177 and lower Pb-208*/Pb-206* than the Early-Middle Miocene lavas of the region, which are interpreted as melts of enriched mantle with an overprint by sediment-derived subduction component. The source region for the OIB-type alkaline melts is interpreted to be a sub-lithospheric reservoir enriched in Hf and Pb isotopes with respect to depleted MORB mantle. Combined evaluation of Hf, Pb, and Os isotopes suggests that the relative enrichment in this domain is related to mixing of ancient oceanic crust with the ambient mantle through long-term plate recycling processes

Lithological controls on lake water biogeochemistry in Maritime Antarctica

International audienceAlthough the Antarctic lakes are of great importance for the climate and the carbon cycle, the lithological influences on the input of elements that are necessary for phytoplankton in lakes have so far been insufficiently investigated. To address this issue, we analyzed phytoplankton cell concentrations and chemical compositions of water samples from lakes, ponds and a stream on Fildes and Ardley Islands of King George Island in the South Shetland Archipelago. Furthermore, lake sediments, as well as soil and rock samples collected from the littoral zone were analyzed for their mineralogical/petrographic composition and pollutant contents of polycyclic aromatic hydrocarbons (PAHs). In addition, leaching experiments were carried out to with the lithologic samples to investigate the possible changes in pH, alkalinity, macronutrients (N, P, Si), micronutrients (e.g. Fe, Zn, Cu, Mn), anions (S, F, Br), and other cations (e.g. Na, K, Mg, Ca, Al, Ti, V, Cr, Co, Ni, As, Se, Pb, Sb, Mo, Ag, Cd, Sn, Ba, Tl, B). Our results showed that phytoplankton levels varied between 15 and 206 cells/mL. Chlorophyll-a concentrations showed high correlations with NH4, NO3. The low levels of PO4 (<0.001 mg/L) indicated a possible Plimitation in the studied lakes. The composition of rock samples ranged from basalt to trachybasalt with variable major oxide (e.g. SiO2, Na2O and K2O) contents and consist mainly quartz, albite, calcite, dolomite and zeolite minerals. The concentrations of total PAHs were below the toxic threshold levels (9.55–131.25 ng g− 1 dw). Leaching experiments with lithologic samples indicated major increase in pH (up to 9.77 ± 0.02) and nutrients, especially PO4 (1.03 ± 0.04 mg/L), indicating a strong P-fertilization impact in increased melting scenarios. Whereas, toxic elements such as Pb, Cu, Cd, Al and As were also released from the lithology, which may reduce the phytoplankton growth