Lead isotopes behavior in the fumarolic environment of the Piton de la Fournaise volcano (Reunion Island)

International audienceThe recent activity of the Piton de la Fournaise volcano offers a rare opportunity to address the issue of Pb isotope behavior in volcanic fumaroles, as the composition of the degassing source is accurately and precisely known. Gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca. 100 degrees C include Na-K sulfate (aphthitalite), Ca-Cu sulfate (e.g., gypsum), Na sulfate (thenardite), Ca-Mg-Al-Fe fluoride (e.g., ralstonite) and native sulfur. The high-temperature deposits show trace element patterns typical of volcanic gas (with Pb concentration up to 836 ppm) while the low-temperature deposits are depleted in most volatile elements (Pb <1 ppm) with the exception of Pd and Tl (in fluorides) and Se (in native sulfur). Only for low-temperature fluoride samples do Pb isotope compositions plot significantly outside the field of lavas. The isotopic shift is ascribed to leaching ubiquitous unradiogenic phases (e.g., sulfides) by acidic gas condensates. The similarity in Pb isotope signature between lavas and sublimate samples more representative of the gas phase (sulfates) indicates that the net fractionation of Pb isotopes resulting from volatilization and condensation processes is smaller than the precision of Pb isotope measurements (better than 60 ppm/a.m.u.). The absence of net fractionation could result from negligible isotope fractionation during Pb volatilization followed by extensive condensation of gaseous Pb, with possibly significant isotopic fractionation at this stage. Although this scenario has to be refined by more direct measurement of the gas phase, and its general applicability tested, it suggests that a small fraction (<10\%) of initially volatilized Pb ultimately escapes to the atmosphere, while the remaining dominant fraction is trapped in sublimates. As sublimates are rapidly dissolved and entrained by runoff, the fumarolic environment appears as a factory efficiently transferring isotopically unfractionated Pb from magmas towards the hydrological system and seawater. Resolving very small isotopic differences between magmas and their gaseous products remains an analytical challenge. High-precision Pb isotope measurements rest not only on instrumental performance but also on high-yield chemistry, as Pb isotopes drastically fractionate (800 ppm/a.m.u.) upon elution on anionic resin. For 50\% Pb recovery, the estimated isotopic bias is plus or minus 60-80 ppm/a.m.u., depending on which of the early (isotopically light) or late (isotopically heavy) Pb fraction is lost. (c) 2012 Elsevier Ltd. All rights reserved

Field thermal monitoring during the August 2003 eruption at Piton de la Fournaise (La Réunion )

International audience[1] A detailed set of thermal images collected during the last day of the August 2003 eruption of Piton de la Fournaise (La Réunion), clearly revealed several dynamic processes associated with a spatter cone containing a lava pond and feeding a channelized lava flow. Periods of steady effusion were interrupted by brief pulses of lava effusion that closely coincide with peaks in seismic tremor amplitude. The thermal measurements show that roofing of a lava channel during steady effusion and cooling of surface flows decrease thermal radiance in two different ways. Here we show that the decrease in thermal radiance because of channel roofing is not related to a decrease in volcanic activity, as might be interpreted from satellite data. In addition, we introduce a new method of representing thermal data (hereby named ''Radiative Thermogramme'') that successfully describes thermal patterns produced by distinct eruptive processes within the same span of time. This graphic solution can be directly correlated with volcanic field processes and provides a useful tool for interpreting a high number of thermal data in a wide range of volcanic activities

A Nd Isotopic Study of Southern Sourced Waters and Indonesian Throughflow at Intermediate Depths in the Cenozoic Indian Ocean

We present Nd isotopic data for fossil fish teeth recovered from the past 40 m.y. at ODP Site 757, currentlylocated at 1650m water depth on the Ninetyeast Ridge in the Indian Ocean. Although Site 757 sits in a regionstrongly influenced by weathering inputs from the Himalayas and volcanic inputs from the Indonesian arc, the pattern of Nd isotopic variations does not appear to respond to these potential sources of Nd. Instead, secular variations correlate to changes in the composition of intermediate to deep water masses bathing thesite and circulation patterns in the Indian Ocean. From ~40 to 10 Ma, eNd values and the pattern of change at Site 757 closely match those of ODP Site 1090, a deep water site in the Atlantic sector of the Southern Ocean. Comparison to data from several Fe-Mn crusts in the Indian Ocean suggests that intermediate to deep waterflow paths were similar to the modern distribution of Circumpolar DeepWater. At approximately 10 Ma, Ndisotopic values increase in a step function by 2 eNd units, suggesting that plate motions had carried Site 757into a region influenced by Indonesian Throughflow. Estimates of the vertical and horizontal position of thissite at 10 Ma imply that Indonesian Throughflow extended as far south as ~20S and to a depth of ~1500 m. From 10 to 0 Ma, Nd isotopic variations at Site 757 appear to record variations in Indonesian Throughflow. From 10 to 5.5 Ma, values at Site 757 overlap with those from crusts located in the southwest Pacific,indicating extensive flow through the Indonesian Seaway. From 5.5 to 3.4 Ma, eNd values become lessradiogenic at Site 757 and more radiogenic in the southwest Pacific, suggesting increasing closure of theseaway and concomitant rerouting of equatorial Pacific waters. Beginning at 3.4 Ma, eNd values becomemore radiogenic again at Site 757, which may be attributed to enhanced opening of the seaway or to a changein the source of Throughflow waters from a southern to a northern Pacific region

Deep and bottom water export from the Southern Ocean to the Pacific Ocean over the past 38 million years

The application of radiogenic isotopes to the study of Cenozoic circulation patterns in the South Pacific Ocean has been hampered by the fact that records from only equatorial Pacific deep water have been available. We present new Pb and Nd isotope time series for two ferromanganese crusts that grew from equatorial Pacific bottom water (D137-01, “Nova,” 7219 m water depth) and southwest Pacific deep water (63KD, “Tasman,” 1700 m water depth). The crusts were dated using 10Be/9Be ratios combined with constant Co-flux dating and yield time series for the past 38 and 23 Myr, respectively. The surface Nd and Pb isotope distributions are consistent with the present-day circulation pattern, and therefore the new records are considered suitable to reconstruct Eocene through Miocene paleoceanography for the South Pacific. The isotope time series of crusts Nova and Tasman suggest that equatorial Pacific deep water and waters from the Southern Ocean supplied the dissolved trace metals to both sites over the past 38 Myr. Changes in the isotopic composition of crust Nova are interpreted to reflect development of the Antarctic Circumpolar Current and changes in Pacific deep water circulation caused by the build up of the East Antarctic Ice Sheet. The Nd isotopic composition of the shallower water site in the southwest Pacific appears to have been more sensitive to circulation changes resulting from closure of the Indonesian seaway

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N : implications for ridge crest plumbing and decadal changes in magma chamber compositions

Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 11 (2010): Q05T09, doi:10.1029/2009GC002977.Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short-term thermochemical changes in a mid-ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid-oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off-axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.This work has been supported by NSF grants OCE‐0525863 and OCE‐0732366 (D. J. Fornari and S. A. Soule), OCE‐0636469 (K. H. Rubin), and OCE‐ 0138088 (M. R. Perfit), as well as postdoctoral fellowship funds from the University of Florida

Radiogenic isotopes: Tracers of past ocean circulation and erosional input

The radiogenic isotope composition of dissolved trace metals in the ocean represents a set of relatively new and not yet fully exploited tracers with a large potential for oceanographic and paleoceanographic research on timescales from the present back to at least 60 Ma. The main topic of this review are those trace metals with oceanic residence times on the order of or shorter than the global mixing time of the ocean (Nd, Pb, Hf, and, in addition, Be). Their isotopic composition in the ocean has varied as a function of changes in paleocirculation, source provenances, style and intensity of weathering on the continents, as well as orogenic processes. The relative importance of these processes for each trace metal is evaluated, which is a prerequisite for reliable interpretation of their time series in terms of changes in paleocirculation or weathering inputs. This analysis of processes includes a discussion of the long-term isotopic evolution of Sr and Os, which are well mixed in the ocean and have thus not been influenced by circulation changes. The radiogenic isotope evolution of those trace metals with intermediate oceanic residence times can be used as paleoceanographic proxies to reconstruct paleocirculation and weathering inputs into the ocean. This is demonstrated by studies from different ocean basins, mainly carried out on ferromanganese crusts, which show that radiogenic trace metal isotopes provide important new insights and can complement results obtained by other well-established paleoceanographic tracers such as carbon isotopes

Pre- and syn-eruptive degassing and crystallisation processes of the 2010 and 2006 eruptions of Merapi volcano, Indonesia

The 2010 eruption of Merapi (VEI 4) was the volcano’s largest since 1872. In contrast to the prolonged and effusive dome-forming eruptions typical of Merapi’s recent activity, the 2010 eruption began explosively, before a new dome was rapidly emplaced. This new dome was subsequently destroyed by explosions, generating pyroclastic density currents (PDCs), predominantly consisting of dark coloured, dense blocks of basaltic andesite dome lava. A shift towards open-vent conditions in the later stages of the eruption culminated in multiple explosions and the generation of PDCs with conspicuous grey scoria and white pumice clasts resulting from sub-plinian convective column collapse. This paper presents geochemical data for melt inclusions and their clinopyroxene hosts extracted from dense dome lava, grey scoria and white pumice generated during the peak of the 2010 eruption. These are compared with clinopyroxene-hosted melt inclusions from scoriaceous dome fragments from the prolonged dome-forming 2006 eruption, to elucidate any relationship between pre-eruptive degassing and crystallisation processes and eruptive style. Secondary ion mass spectrometry analysis of volatiles (H2O, CO2) and light lithophile elements (Li, B, Be) is augmented by electron microprobe analysis of major elements and volatiles (Cl, S, F) in melt inclusions and groundmass glass. Geobarometric analysis shows that the clinopyroxene phenocrysts crystallised at depths of up to 20 km, with the greatest calculated depths associated with phenocrysts from the white pumice. Based on their volatile contents, melt inclusions have re-equilibrated during shallower storage and/or ascent, at depths of ~0.6–9.7 km, where the Merapi magma system is interpreted to be highly interconnected and not formed of discrete magma reservoirs. Melt inclusions enriched in Li show uniform “buffered” Cl concentrations, indicating the presence of an exsolved brine phase. Boron-enriched inclusions also support the presence of a brine phase, which helped to stabilise B in the melt. Calculations based on S concentrations in melt inclusions and groundmass glass require a degassing melt volume of 0.36 km3 in order to produce the mass of SO2 emitted during the 2010 eruption. This volume is approximately an order of magnitude higher than the erupted magma (DRE) volume. The transition between the contrasting eruptive styles in 2010 and 2006 is linked to changes in magmatic flux and changes in degassing style, with the explosive activity in 2010 driven by an influx of deep magma, which overwhelmed the shallower magma system and ascended rapidly, accompanied by closed-system degassing

Application des traceurs isotopiques à l’étude des sources des magmas

International audienceThis paper discusses the history of isotopic geochemistry at the Magmas et Volcans Laboratory (LMV) since the early 2000s. It focuses on the data acquired following the installation of new, solid- and plasma-source mass-spectrometer instruments. Covered topics concern both the analytical development of isotopic tools and the evolution of the discipline during this period, in terms of targets and subjects.Ce papier discute l’histoire de la géochimie isotopique au Laboratoire Magmas et Volcans (LMV) depuis le début des années 2000. Il se focalise plus particulièrement sur les données acquises suite à l’installation de nouveaux spectromètres de masse, de dernière génération, à source solide et à source plasma. Les thèmes abordés concernent aussi bien le développement analytique des outils isotopiques que l’évolution de la discipline au cours de cette période, en termes de cibles et de sujets

Initiation of a plume-ridge interaction in the South Pacific recorded by high-precision Pb isotopes along Hollister Ridge

The southern Pacific Ocean offers the rare possibility to study a situation where a spreading ridge (the Pacific-Antarctic Ridge (PAR)) migrates toward a fixed hot spot (the Louisville hot spot) (Small, 1995). Hollister Ridge is a 450 km long linear structure whose position, between the PAR axis and the most recent edifices of the Louisville hot spot trail, led some authors to suggest that the ridge is genetically related to the hot spot (Small, 1995; Wessel and Kroenke, 1997). Mapping and sampling of the ridge in 1996 revealed, however, that the contribution of the Louisville plume material to its mantle source is minor and suggested that it might be the result of intraplate deformation (Géli et al., 1998; Vlastélic et al., 1998). We report new, highly precise Pb isotopic data from Hollister Ridge, which (1) confirm that the maximal contribution of the Louisville plume, in the centrally, volcanic active part of the ridge, probably does not exceed 20% (15 and 35% for lower and upper limits) and (2) reveal through time an increasing plume influence. The initiation of the Louisville plume involvement in the source of Hollister Ridge is estimated to have occurred between 1.04 and 0.77 Myr ago. It thus followed closely the most recent volcanic activity reported along the Louisville trail (1.11 Ma (Koppers et al., 2004)). This suggests that Hollister Ridge has recorded the dispersion of the Louisville plume as the spreading ridge approached the hot spot. Assuming that the Louisville hot spot is located near the youngest seamount dredged along the Louisville seamount chain, Hollister Ridge lies along the shortest path of pressure release connecting the hot spot to the spreading axis. This path involves, first, an abrupt upwelling across the Eltanin fault system and, subsequently, a more progressive migration toward the spreading axis. Because Hollister Ridge is older than 2.5 Ma, the structure might not be the consequence of the plume-ridge flow. Instead, Hollister Ridge most likely emplaced through a lithospheric crack (Géli et al., 1998), which, subsequently, may have captured the plume-ridge flow

Miocene climate change recorded in the chemical and isotopic (Pb, Nd, Hf) signature of Southern Ocean sediments.

International audienceThe Middle Miocene transition from carbonate to biosilica sedimentation at DSDP site 266 (Australian-Antarctic basin) reflects a global transition toward a colder climate. The 143Nd/144Nd, 176Hf/177Hf, and Al/Ti of bulk sediments display systematic, coupled variations through time, which have been attributed to a change of the detrital source. This change could correspond to a reduction of input from the Antarctic continent, an increase of input from the Kerguelen volcanic province, or both. Mixing models based on Nd isotopes and Al/Ti suggest a 30–40% reduction of Antarctic input and an equivalent increase of Kerguelen input during the Miocene. Reduction of Antarctic input may result from the formation of a stable East Antarctic ice sheet. Consistently, Pb isotopes and trace element systematics suggest a change of weathering style during the Miocene, with an increase in physical weathering, or a reduction of chemical weathering, after 15 Ma. Increase of Kerguelen input may reflect the initiation, or enhancement, of the Antarctic Circumpolar Current (ACC), thus raising the possibility of a simultaneous onset of North Atlantic Deep Water production and the ACC during the Middle Miocene. In addition, large geochemical oscillations occurred during the Pliocene, possibly reflecting fluctuation in strength of the ACC or, alternatively, periods of instability of the Antarctic ice sheet