Sr and Nd isotope geochemistry of coexisting alkaline magma series,Cantal, Massif Central,France

Sr and Nd isotope analyses are presented for Tertiary continental alkaline volcanics from Cantal, Massif Central, France. The volcanics belong to two main magma series, silica-saturated and silica-undersaturated (with rare nephelinites). Trace element and isotopic data indicate a common source for the basic parental magmas of both major series; the nephelinites in contrast must have been derived from a mantle source which is isotopically and chemically distinct from that which gave rise to the basalts and basanites.⁸⁷Sr/⁸⁶Sr initial ratios range from 0.7034 to 0.7056 in the main magma series (excluding rhyolites) and¹⁴³Nd/¹⁴⁴Nd ratios vary between 0.512927 and 0.512669; both are correlated with increasing SiO₂ in the lavas. The data can be explained by a model of crustal contamination linked with fractional crystallisation. This indicates that crustal magma chambers are the sites of differentiation since only rarely do evolved magmas not show a crustal isotopic signature and conversely basic magmas have primitive isotopic ratios unless they contain obvious crustal-derived xenocrysts. Potential contaminants include lower crustal granulites or partial melts of upper crustal units. Equal amounts of contamination are required for both magma series, refuting hypotheses of selective contamination of the silica-saturated series.The isotopic characteristics of the apparently primary nephelinite lavas demonstrates widespread heterogeneity in the mantle beneath Cantal. Some rhyolites, previously thought to be extremely contaminated or to be crustally derived, are shown to have undergone post-emplacement hydrothermal alteration.     

The effects of organic removal treatment on the integrity of δ<Superscript>18</Superscript>O measurements from biogenic silica

Prior to oxygen isotope analysis of biogenic silica by fluorination techniques, sediments must be cleaned of all organic compounds. Despite the expanding volume of research utilising oxygen isotope ratios in biogenic silica in palaeoclimatology, very little is known about the effects of different standard preparation methods for the removal of organic matter on the isotope values. Here we compare a number of methods for the removal of organic matter in order to assess their effectiveness and influence on subsequent isotope measurements. Not all the methods described here effectively removed organic matter, the best treatments were H₂O₂ and ignition in air at 550°C. Ignition in air at 950°C, and heating in vacuum at 250 and 450°C results in marked alteration of isotope values, whereas use of hydrogen peroxide (H₂O₂), nitric acid (HNO₃) and plasma ashing have little or no effect, within the bounds of analytical error. Ignition in air at 550°C was the most effective technique, and induced no alteration of δ¹⁸O_silica, but practical constraints may limit the application of this method routinely. Our findings indicate that considerable caution is required when preparing sediment for isotope analysis of biogenic silica.     

Modification of the subcontinental mantle beneath East Serbia: Evidence from orthopyroxene-rich xenoliths

Orthopyroxene-rich olivine websterite xenoliths (OWB₂) in Palaeogene basanites in East Serbia are mostly composed of tabular low-Al₂O₃ orthopyroxene (> 70 vol.%, Mg# 85–87) containing tiny Cr spinel inclusions. Orthopyroxene shows a slightly U-shaped primitive mantle-normalized trace element pattern with strong peaks at U and Pb, similar to that of orthopyroxene from normal regional peridotitic mantle. In between the orthopyroxenes are interstitial spaces composed of partially altered olivine (Mg# 85–87), clinopyroxene, Ti-rich spinel, Mg-bearing calcite, K-feldspar, apatite, ilmenite and relicts of a hydrous mineral. Clinopyroxene appears as selvages around orthopyroxene and as coarser euhedral crystals. Trace element patterns of the clinopyroxene selvages resemble those of adjacent orthopyroxene, whereas the coarser ones have flatter and more LREE- and LILE-enriched patterns, similar to that of metasomatic clinopyroxene. The OWB₂ xenoliths are interpreted as having formed in two stages. During Stage I orthopyroxene crystallized, along with some spinel, olivine and probably hydrous phase(s). This original OWB₂ lithology was a hydrous olivine-bearing orthopyroxenite that crystallised from subduction-related SiO₂-saturated, boninite-like magmas. During Stage II the interstitial minerals formed due to infiltration of a low-SiO₂, high-CaO and CO₂-rich external melt, accompanied by decomposition of original H₂O-bearing minerals. The calculated composition of the infiltrating liquid corresponds to a mafic alkaline melt similar to the basanitic host but more enriched in CO₂, LREE and LILE. Metasomatism is interpreted in terms of small degree melts related to the Palaeogene mafic alkaline magmatism.     

Magmagenesis in a subduction-related post-collisional volcanic arc segment: the Ukrainian Carpathians

Calc-alkaline magmatism in the south-west Ukraine occurred between 13.8 and 9.1 Ma and formed an integral part of the Neogene subduction-related post-collisional Carpathian volcanic arc. Eruptions occurred contemporaneously in two parallel arcs (here termed Outer Arc and Inner Arc) in the Ukrainian part of the Carpathians. Outer Arc rocks, mainly andesites, are characterized by LILE enrichment (e.g. K and Pb), Nb depletion, low compatible trace element abundances, high ⁸⁷Sr/⁸⁶Sr, high δ¹⁸O and low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios (0.7085–0.7095, 7.01–8.53, 0.51230–0.51245, respectively). Inner Arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas. They also show low compatible element abundances but have lower ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and higher ¹⁴³Nd/¹⁴⁴Nd ratios (0.7060–0.7085, 6.15–6.64, 0.5125–0.5126, respectively) than Outer Arc rocks. Both high-Nb and low-Nb lithologies are present in the Inner Arc. Based on the LILE enrichment (especially Pb), a higher fluid flux is suggested for the Outer Arc magmas compared with those of the Inner Arc.

Combined trace element and Sr–Nd–O isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex. Compositional differences between the Inner and Outer Arcs were produced by introduction of variable proportions of slab-derived sediments and fluids into a heterogeneous mantle wedge, and by different extents of upper crustal contamination. Degrees of magmatic fractionation also differed between the two arcs. The most primitive magmas belong to the Inner Arc. Isotopic modelling shows that they can be produced by adding 3–8% subducted terrigenous flysch sediments to the local mantle wedge source. Up to 5% upper crustal contamination has been modelled for fractionated products of the Inner Arc. The geochemical features of Outer Arc rocks suggest that they were generated from mantle wedge melts similar to the Inner Arc primitive magmas, but were strongly affected by both source enrichment and upper crustal contamination. Assimilation of 10–20% bulk upper crust is required in the AFC modelling, assuming an Inner Arc parental magma. We suggest that magmagenesis is closely related to the complex geotectonic evolution of the Carpathian area. Several tectonic and kinematic factors are significant: (1) hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes; (2) thermal disturbance caused by ascent of hot asthenospheric mantle during the back-arc opening of the Pannonian Basin; (3) clockwise translational movements of the Intracarpathian terranes, which facilitated eruption of the magmas.     

Ecological Footprint dynamics of Yunnan, China

A fundamental element of sustainable development is that humans live within nature＇s biological capacity. Quantifying this, however, remains a significant challenge for which there are many emerging tools. The concept of the Ecological Footprint is one such accounting tool for comprehensive assessment of the status of sustainable development, based on integration of resource consumption and land capacity, reflecting the human impact on the environment. A region＇s development is defined as unsustainable when the Ecological Footprint surpasses the biological capacity. In this paper, the Ecological Footprint concept Was applied in assessing the development of Yunnan Province, China in a period between 1988 and 2006. The results showed that the Ecological Footprint per capita in Yunnan rose from 0.854 gha in 1988 to 2.11 gha in 2006. Ecological deficit, defined as when the human demand on the land surpasses the regions biological productive capacity, emerged in 1991 and quickly increased from 0.02 gha in 1991 to 1.05 gha in 2006. The increase in the ecological deficit is primarily a result of the rapid increase in population and consumption level. To achieve sustainable development in Yunnan, production and consumption rates need to be modified.     

Carbonaceous chondrite clasts in the howardites Bholghati and EET87513

Abstract— Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.