Search

The Ediacaran age (ca. 570 Ma) Shuram excursion, a ca. 12‰ depletion in δ¹³C_(carb), may record a dramatic oxidation of marine sediments associated with a reorganization of Earth’s carbon cycle closely preceding the rise of large metazoans. However, several geochemical indicators suggest it may instead record secondary processes affecting the sediments such as post-depositional alteration. The stable isotopic composition of iron incorporated within carbonates (δ⁵⁶Fe_(carb)) reveals an anomalous ⁵⁶Fe-depletion (down to -1.05‰) in strata containing the Shuram excursion, while the underlying and overlying strata have crustal ⁵⁶Fe-δ⁵⁶Fe_(carb) values. These depleted δ⁵⁶Fe_(carb) data during the Shuram excursion reflect incomplete reduction of iron oxides, limited by low ambient organic carbon contents. This elevated pulse of sedimentary iron oxides would consume the majority of the limited pool of organic carbon and therefore would give rise to very low net organic carbon burial during a time of enhanced detrital delivery of oxidized iron to the sediments. These results imply a syndepositional origin for the Shuram excursion, which represents a shift in the redox composition of Earth’s sedimentary shell toward more oxidizing conditions, perhaps removing a long-lived buffer on atmospheric oxygen.

International audience; The authors regret that an error occurred whilst converting the SO2 normalized mass ratios to trace gas fluxes. This implies that Lascar volcanic gas fluxes, shown in Table 6 and discussed in Section 5.1.2.2, are lower than previously estimated. The correct trace element emission rate estimates are now presented in the Table below. The average rates over the 2009-2012 period range from 0.1 g/day (In) to 324 g/day (As), suggesting that Lascar does not represent such a significant local source of pollutants for the environment.

International audience; This study focuses on the chemical compositions of volcanic gases and aerosols emitted by Lascar volcano (Northern Chile). The sustained volcanic plume was sampled in April 2009, April 2011 and November 2012 through filter packs and analyzed for major gaseous species (SO2, HCl, and HF) and trace elements. During fieldwork, SO2 flux measurements were also performed by UV spectrometry (DOAS). The Lascar volcano is a significant and sustained emitter of SO2 (between 150 and 940 t/d), HCl (between 170 t/d and 210 t/d) and HF (up to 100 t/d). Combining the SO2-normalized trace element concentrations and the SO2 emission rates, we evaluate that the quiescent degassing of Lascar is an important local source of trace metals to the atmosphere with contributions to global volcanic fluxes generally less than 2%.Our data were used to infer the origin of the gas emitted at Lascar. Two major degassing sources were identified: a deep magmatic reservoir and a shallow hydrothermal system. The variable interaction between these two sources is the most likely scenario for explaining the compositional ranges in acid gases but also in a volatile and fluid–mobile trace element such as B. These variations are related to changes in volcanic activity: an extensive interaction between the hot ascending magmatic gases and the shallow aquifer occurred in 2009, possibly due to a long period of quiescence, before magmatic activity renewed in 2011 and 2012. Our study highlights, therefore, that filter-pack measurements may be used to study changes in subsurface processes that probably play a key role in triggering volcanic eruptions.

International audience; This study reports the concentrations of trace elements, including fluid mobile and semi-volatile species (e.g., Li, B, Cu, Re, In, Sn, Cd, Tl, Bi) in lavas erupted before and during the April 2007 collapse of the Piton de la Fournaise summit. Lavas erupted just prior to the collapse (April 5) display anomalous abundances in semi-volatile elements, with both depletion in Li, Cu and Tl (mostly on April 2) and enrichment in Cd, Bi, In and Sn (on April 3 and 4). These transient anomalies are thought to record unusual degassing conditions. Between March 30 and April 2, static decompression caused by magma withdrawal from the shallow magma reservoir might have triggered massive exsolution of a H2O- and S-rich phase in which Li and Cu might have partitioned. Alternatively, the Li-Cu depletion could record the degassing of a magma body that intruded at shallow depth during the same period. The Bi-Cd-In-Sn enrichment observed in the April 3-4 magmas requires conditions that prevent magma outgassing. In the absence of evidence for the pressurisation of the reservoir or the onset of collapse before April 5, it is suggested that the occurrence of less degassed lavas on April 3 and 4 reflects a high rate of magma transfer from the shallow magma reservoir to the eruption site just prior to summit collapse. The kinetic (diffusive) fractionation of elements accounts for the observed anomalies. The short time-scales required to fractionate Li from Cd diffusively (minutes to hours) and Cd from Bi (few hours to two days) support the idea that the magmas underwent rapid pressure variations a few days before the summit collapse.

Article history:Received 22 October 2012Received in revised form 10 June 2013Accepted 17 June 2013Available online 26 June 2013Editor: L. ReisbergKeywords:BoronIsotope dilutionICP-MSSilicate rockPrimitive mantleMagmatic degassing We determine the bulk boron concentration of silicate rocks using a new method that does not requiresample evaporation. After spiking ( 10 B enriched NIST SRM 952) and HF attack, the rock solution is directlyaspirated into the ICP-MS, and B concentration is determined by isotopic dilution. This technique is appliedto samples with B contents in the range of 0.1–30 ppm. The very low blank (b0.5 ng B) makes this procedureparticularly suitable for samples with low B content. Repeated analysis of the basaltic reference sampleBHVO-2 yields a B concentration of 2.81 ± 0.09 ppm (2σ), giving a 3% relative error. This concentration,like those obtained for other basaltic and andesitic reference materials (BIR-1a, BCR-2, JB-1 and JA-1), agreeswell with previously published values.The method is applied to 1)peridotitexenoliths from Mongolia and Russia, and2) freshlavas from Piton de laFournaise, the active volcano on Reunion Island. We use the xenolith data to examine the behavior of boronduring mantle melting and enrichment processes (i.e. metasomatism), and to estimate the primitive mantleB content (0.26 ± 0.04 ppm), while thelava data allows us to quantifyB loss due to magmatic degassing (be-tween 10 and 30%), using the composition of a primitive melt inclusion as the reference for an initial magmacomposition. Our knowledge of the degassing history of the samples allows us to quantify loss during lavaemplacement and cooling (4–10%), continuous open-system degassing (10%) and shallow closed-systemdegassing (5–8%).Our results indicate that 1) B abundances of ocean island basalts are ca. 60–75% less than estimates based onelements with similar incompatibility (e.g., Nb and Ta), 2) this deficiency could reflect source depletion and/or magmatic degassing, and 3) magma degassing accounts for less than half of the inferred boron anomalyseen on primitive mantle-normalized patterns, implying that the mantle sources of ocean island basaltsmust be depleted in boron. The robust technique presented in this study could provide useful constraintson boron distribution within the Earth's mantle as well as on its behavior in magmatic systems.© 2013 The Authors. Published by Elsevier B.V. All rights reserved.