Creat membership Creat membership
Sign in

Forgot password?

Confirm
  • Forgot password?
    Sign Up
  • Confirm
    Sign In
home > search

Now showing items 1 - 16 of 33

  • New Olivine Reference Material for In Situ Microanalysis

    Batanova, Valentina G.   Thompson, Jay M.   Danyushevsky, Leonid V.   Portnyagin, Maxim V.   Garbe-Schoenberg, Dieter   Hauri, Erik   Kimura, Jun-Ichi   Chang, Qing   Senda, Ryoko   Goemann, Karsten   Chauvel, Catherine   Campillo, Sylvain   Ionov, Dmitri A.   Sobolev, Alexander V.  

    A new olivine reference material - MongOL Sh11-2 - for in situ analysis has been prepared from the central portion of a large (20 x 20 x 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic breccia at Shavaryn-Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5-2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA-ICP-MS, SIMS and bulk analytical methods (ID-ICP-MS for Mg and Fe, XRF, ICP-MS) for major, minor and trace elements at six institutions world-wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1-2). The presence of some mineral and fluid-melt micro-inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty-seven major, minor and trace elements.
    Download Collect
  • Enigmatic origin of diamond-bearing rocks revealed

    Chauvel, Catherine  

    Download Collect
  • Magnesium isotope geochemistry in arc volcanism

    Hu, Yan   Chauvel, Catherine  

    Incorporation of subducted slab in arc volcanism plays an important role in producing the geochemical and isotopic variations in arc lavas. The mechanism and process by which the slab materials are incorporated, however, are still uncertain. Here, we report, to our knowledge, the first set of Mg isotopic data for a suite of arc lava samples from Martinique Island in the Lesser Antilles arc, which displays one of the most extreme geochemical and isotopic ranges, although the origin of this variability is still highly debated. We find the delta Mg-26 of the Martinique Island lavas varies from -0.25 to -0.10, in contrast to the narrow range that characterizes the mantle (-0.25 +/- 0.04, 2 SD). These high delta Mg-26 values suggest the incorporation of isotopically heavy Mg from the subducted slab. The large contrast in MgO content between peridotite, basalt, and sediment makes direct mixing between sediment and peridotite, or assimilation by arc crust sediment, unlikely to be the main mechanism to modify Mg isotopes. Instead, the heavy Mg isotopic signature of the Martinique arc lavas requires that the overall composition of the mantle wedge is buffered and modified by the preferential addition of heavy Mg isotopes from fluids released from the altered subducted slab during fluid-mantle interaction. This, in turn, suggests transfer of a large amount of fluid-mobile elements from the subducting slab to the mantle wedge and makes Mg isotopes an excellent tracer of deep fluid migration.
    Download Collect
  • The size of plume heterogeneities constrained by Marquesas isotopic stripes

    Chauvel, Catherine   Maury, René C.   Blais, Sylvain   Lewin, Eric   Guillou, Hervé   Guille, Gérard   Rossi, Philippe   Gutscher, Marc-André  

    The scale and geometry of chemical and isotopic heterogeneities in the source of plumes have important scientific implications on the nature, composition and origin of plumes and on the dynamics of mantle mixing over time. Here, we address these issues through the study of Marquesas Islands, one of the Archipelagoes in Polynesia. We present new Sr, Nd, Pb, Hf isotopes as well as trace element data on lavas from several Marquesas Islands and demonstrate that this archipelago consists of two adjacent and distinct rows of islands with significantly different isotopic compositions. For the entire 5.5 Ma construction period, the northern islands, hereafter called the Ua Huka group, has had systematically higher Sr-87/Sr-86 and lower Pb-206/Pb-204 ratios than the southern Fatu Hiva group at any given Nd-143/Nd-144 value. The shape and curvature of mixing arrays preclude the ambient depleted MORB mantle as one of the mixing end-members. We believe therefore that the entire isotopic heterogeneity originates in the plume itself. We suggest that the two Marquesas isotopic stripes originate from partial melting of two adjacent filaments contained in small plumes or "plumelets" that came from a large dome structure located deep in the mantle under Polynesia. Low-degree partial melting under Marquesas and other "weak" Polynesian hot spot chains (Pitcairn-Gambier, Austral-Cook, Society) sample small areas of the dome and preserve source heterogeneities. In contrast, more productive hot spots build up large islands such as Big Island in Hawaii or Reunion Island, and the higher degrees of melting blur the isotopic variability of the plume source.
    Download Collect
  • The size of plume heterogeneities constrained by Marquesas isotopic stripes

    Chauvel, Catherine   Maury, Rene C.   Blais, Sylvain   Lewin, Eric   Guillou, Herve   Guille, Gerard   Rossi, Philippe   Gutscher, Marc-Andre  

    The scale and geometry of chemical and isotopic heterogeneities in the source of plumes have important scientific implications on the nature, composition and origin of plumes and on the dynamics of mantle mixing over time. Here, we address these issues through the study of Marquesas Islands, one of the Archipelagoes in Polynesia. We present new Sr, Nd, Pb, Hf isotopes as well as trace element data on lavas from several Marquesas Islands and demonstrate that this archipelago consists of two adjacent and distinct rows of islands with significantly different isotopic compositions. For the entire 5.5 Ma construction period, the northern islands, hereafter called the Ua Huka group, has had systematically higher Sr-87/Sr-86 and lower Pb-206/Pb-204 ratios than the southern Fatu Hiva group at any given Nd-143/Nd-144 value. The shape and curvature of mixing arrays preclude the ambient depleted MORB mantle as one of the mixing end-members. We believe therefore that the entire isotopic heterogeneity originates in the plume itself. We suggest that the two Marquesas isotopic stripes originate from partial melting of two adjacent filaments contained in small plumes or "plumelets" that came from a large dome structure located deep in the mantle under Polynesia. Low-degree partial melting under Marquesas and other "weak" Polynesian hot spot chains (Pitcairn-Gambier, Austral-Cook, Society) sample small areas of the dome and preserve source heterogeneities. In contrast, more productive hot spots build up large islands such as Big Island in Hawaii or Reunion Island, and the higher degrees of melting blur the isotopic variability of the plume source.
    Download Collect
  • A New Model for Barberton Komatiites: Deep Critical Melting with High Melt Retention

    Robin-Popieul, Christophe C. M.   Arndt, Nicholas T.   Chauvel, Catherine   Byerly, Gary R.   Sobolev, Alexander V.   Wilson, Allan  

    The oldest well-preserved komatiites, and the type examples, are found in the Barberton Greenstone Belt in South Africa (3 center dot 5-3 center dot 3 Ga). All three komatiite types are present, commonly within the same stratigraphic unit. Al-depleted komatiites have low Al/Ti, relatively high concentrations of incompatible elements and depleted heavy rare earth elements (HREE); Al-undepleted komatiites have chondritic Al/Ti and flat HREE patterns; and Al-enriched komatiites have high Al/Ti, low concentrations of incompatible elements, enriched HREE and extremely depleted light rare earth elements. Based on a comprehensive petrological and geochemical study, we propose a new melting model for the formation of these magmas. The basis of the model is the observation, from published experimental studies, that at great depths (similar to 13 GPa) the density of komatiitic liquid is similar to that of solid peridotite. At such depths, melting in a rising mantle plume produces near-neutrally buoyant komatiite melt that does not escape from the residual peridotite. As the source ascends to shallower levels, however, the pressure decreases and the density difference increases, eventually making melt escape possible. Al-depleted komatiites form first at about 13 GPa by equilibrium melting under conditions in which a large proportion of melt (30-40%) was retained in the source and the residue contained a high proportion of garnet (15%). Al-undepleted and Al-enriched komatiites form by fractional melting at intermediate to shallow depths after the escape of a large proportion of melt and after exhaustion of residual garnet. This model reproduces the chemical characteristics of all komatiite types in the Barberton belt and can probably be applied to komatiites in other parts of the world.
    Download Collect
  • New Olivine Reference Material for In Situ Microanalysis

    Batanova, Valentina G.   Thompson, Jay M.   Danyushevsky, Leonid V.   Portnyagin, Maxim V.   Garbe‐Schönberg, Dieter   Hauri, Erik   Kimura, Jun‐Ichi   Chang, Qing   Senda, Ryoko   Goemann, Karsten   Chauvel, Catherine   Campillo, Sylvain   Ionov, Dmitri   Sobolev, Alexander V.  

    Download Collect
  • Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust

    Chauvel, Catherine   Gar?on, Marion   Bureau, Sarah   Besnault, Adeline   Jahn, Bor-ming   Ding, Zhongli  

    Knowledge of the average composition of the upper continental crust is crucial to establish not only how it formed but also when. While well constrained averages have been suggested for its major and trace element composition, no values exist for its Nd and Hf isotopic compositions even though radiogenic isotopic systems provide valuable information on its average model age. Here we present Nd and Hf isotopic data determined on a large number of loess deposits from several continents. We demonstrate that these deposits have very uniform Nd and Hf isotopic compositions. We obtain an average Nd isotopic composition that is similar to previous estimates for the upper continental crust (epsilon(Nd) = -10.3 +/- 1.2 (1 sigma)) and we establish a new Hf average value at epsilon(Hf) = -13.2 +/- 2 (1 sigma). This average falls on the "Terrestrial Array", demonstrating that the two parent-daughter ratios are not decoupled during crust formation. Trace element data acquired on the same set of samples allow us to calculate an average Sm-147/Nd-144 ratio for the upper continental crust: 0.1193 +/- 0.0026, a value slightly higher than previous estimates. Based on the relationship between Sm/Nd and Nd isotopes, we estimate the average Nd extraction age of upper continental crust from the depleted mantle at T-DM(Nd) = 1.82 +/- 0.07 Ga. This model age is entirely consistent with previous suggestions made for example by Goldstein et al. (1984). Assuming that for each individual sample, the Hf model age cannot be younger than the Nd model age, our new Nd-Hf isotopic data provide a value for the very poorly known Lu-176/Hf-177 ratio of the upper crust. Our estimate is Lu-176/Hf-177 = 0.0125 +/- 0.0018, a value significantly lower than commonly used values (0.0150-0.0159; Griffin et al., 2002; Goodge and Vervoort, 2006; Hawkesworth et al., 2010) but higher than Rudnick and Gao's (2003) estimate of 0.0083. The impact of our new Lu-176/Hf-177 ratio on crustal model ages of zircon populations is not simple to evaluate but the Hf model ages calculated with this new Lu/Hf ratio could be younger by up to 500 Ma. (C) 2013 Elsevier B.V. All rights reserved.
    Download Collect
  • Sulfur and lead isotopic evidence of relic Archean sediments in the Pitcairn mantle plume

    Chauvel, Catherine   Thomassot, Emilie   Devey, Colin W.   Dazas, Baptiste  

    The isotopic diversity of oceanic island basalts (OIB) is usually attributed to the influence, in their sources, of ancient material recycled into the mantle, although the nature, age, and quantities of this material remain controversial. The unradiogenic Pb isotope signature of the enriched mantle I (EM I) source of basalts from, for example, Pitcairn or Walvis Ridge has been variously attributed to recycled pelagic sediments, lower continental crust, or recycled subcontinental lithosphere. Our study helps resolve this debate by showing that Pitcairn lavas contain sulfides whose sulfur isotopic compositions are affected by mass-independent fractionation (S-MIF down to Delta S-33 =3D -0.8), something which is thought to have occurred on Earth only before 2.45 Ga, constraining the youngest possible age of the EM I source component. With this independent age constraint and a Monte Carlo refinement modeling of lead isotopes, we place the likely Pitcairn source age at 2.5 Ga to 2.6 Ga. The Pb, Sr, Nd, and Hf isotopic mixing arrays show that the Archean EM I material was poor in trace elements, resembling Archean sediment. After subduction, this Archean sediment apparently remained stored in the deep Earth for billions of years before returning to the surface as Pitcairn's characteristic EM I signature. The presence of negative S-MIF in the deep mantle may also help resolve the problem of an apparent deficit of negative Delta S-33 anomalies so far found in surface reservoirs.
    Download Collect
  • Role of recycled oceanic basalt and sediment in generating the Hf–Nd mantle?array

    Chauvel, Catherine   Lewin, Eric   Carpentier, Marion   Arndt, Nicholas T.   Marini, Jean-Christophe  

    Following its subduction, oceanic crust either contributes to the source of island-arc volcanic rocks or it is recycled into the mantle(1). Most(2,3), but not all authors(4) believe that recycled crust is incorporated into the plume source of oceanic basalts. The hafnium (Hf) and neodymium (Nd) isotopic compositions of basalts from oceanic islands and mid-ocean ridges exhibit a linear relationship - the mantle array - which is thought to result from mixing between material from the depleted mantle and an enriched recycled component. Here, we model the Hf - Nd isotopic composition of oceanic basalts as a mixture of recycled oceanic crust and depleted mantle and find that recycling of basalt alone is not sufficient to reproduce the mantle array. We conclude that oceanic sediments, which have a relatively high (176)Hf/(177)Hf ratio, must also be recycled. Combining oceanic sediments with recycled oceanic basalts and subsequent mixing with depleted mantle peridotite produces Hf and Nd isotopic compositions that coincide with the mantle array. The composition of bulk continental crust requires the existence of a complementary low (176)Hf/(177)Hf reservoir, which we suggest is zircon-rich sediment.
    Download Collect
  • Stable Drainage Pattern and Variable Exhumation in the Western Himalaya since the Middle Miocene

    Chirouze, Francois   Huyghe, Pascale   Chauvel, Catherine   van der Beek, Peter   Bernet, Matthias   Mugnier, Jean-Louis  

    Sedimentary records in peripheral basins of mountain belts record changes in erosion dynamics and drainage-network reorganization, but it is often difficult to discriminate between these different controls. Geochemical provenance data on paleo-Indus deposits from the western Himalayan foreland provide constraints on the possible variation of the position of the drainage divide between the Indus and Ganges river systems. Here we present geochemical (trace element and Hf-Nd isotopic) and thermochronological (detrital zircon fission-track [DZFT]) analyses of modern Indus and Miocene Siwalik sediments from northern Pakistan and compare these with published data on the Indus Fan. Available bedrock isotopic data are used to define three end-member sediment sources (Himalaya, Karakorum, and the Kohistan-Ladakh arc) and to calculate the contribution of each of these sources to the foreland basin and Indus Fan. Our results indicate that since the Miocene the contribution of the Himalayan rivers reaching the Indus in the foreland remained constant, whereas the contributions of sediment sources of the upper Indus catchment changed: those of the Kohistan-Ladakh arc diminished strongly in favor of Karakorum and Himalayan sources. Analysis of the DZFT data from the Miocene foreland basin and sediments of the modern upper Indus reach suggests that the exhumation pattern changed due to an increase in exhumation rate of the Karakorum and Himalayan units of the syntaxis since Miocene times, whereas that in the Kohistan-Ladakh arc remained relatively stable. These results imply that the Indus sediments record changing relative erosion rates in the different source regions rather than widespread drainage rearrangement, as suggested previously.
    Download Collect
  • Removing the "heavy mineral effect" to obtain a new Pb isotopic value for the upper crust

    Garcon, Marion   Chauvel, Catherine   France-Lanord, Christian   Limonta, Mara   Garzanti, Eduardo  

    Based on the concept that sedimentary processes average large areas of exposed crust, sediment data have been widely used to estimate the average Pb isotopic composition of the upper continental crust. However, the possible effects of mineral sorting processes on sediment Pb isotopes have never been fully investigated. Here, we report Pb isotopic compositions of Himalayan river sediments as well as those of several grain-size fractions and mineral separates. We demonstrate that Pb isotopes of both bed loads and suspended loads are biased toward more radiogenic values than their source rocks due to a heavy mineral effect caused by mineral sorting during fluvial transport on continents. The sparse zircons, monazites and allanites present in all samples (<1 wt%), including suspended loads, generate a Pb isotopic variability as large as that observed in the Earth's mantle. After correction of this effect, we propose an average value for the composition of the upper Himalayan crust together with a new Pb isotopic value for the Earth's upper continental crust. We conclude that mineralogical effects must be evaluated carefully before using Pb isotopes of sediments as provenance and anthropogenic tracers.
    Download Collect
  • Beach placer, a proxy for the average Nd and Hf isotopic composition of a continental area

    Garcon, Marion   Chauvel, Catherine   Bureau, Sarah  

    Beach placer deposits concentrate detrital heavy minerals which are the erosion products of large areas of continental crust Here, we report the first analyses of Nd-Hf isotopic ratios and trace element concentrations that we measured in a beach placer from Camargue, France and in its pure mineral separates. Both the bulk composition of the placer and those of its pure mineral separates were determined. We also report mineral proportions obtained using observations under a binocular microscope and micro X-ray fluorescence mapping. Our results indicate that monazite totally controls the placer's Nd isotopic composition (epsilon(Nd) = -9.3) while zircon dominates its Hf isotopes (epsilon(Hf) = -13.0) even though both mineral phases represent only a small proportion of the heavy mineral assemblage (3.5 and 10% respectively). We demonstrate that the Camargue placer provides a good estimate of the average Nd and Hf isotopic composition of the continental area drained by the Rhone River in Western Europe (epsilon(Nd) approximate to -9 and epsilon(Hf) approximate to -13). Using these values, we calculate two stage model ages and show that almost all the placer minerals are derived from Proterozoic crustal protoliths. This provides valuable information on the history of the continental crust drained by the Rhone River. In particular, it suggests that little juvenile crust was created during the recent geological events that formed the Alps and the Massif Central, the two main massifs from which the placer minerals originate. More generally, we propose that similar measurements made on other worldwide beach placer deposits could provide estimates of the present-day Nd and Hf isotopic composition of large continental areas, values that are difficult to obtain due to the well-known heterogeneity of continental material but are essential to model the growth of continental crust through Earth history or to model the impact of crustal material when recycled into the mantle. (C) 2011 Elsevier B.V. All rights reserved.
    Download Collect
  • Silver and lead in high-altitude lake sediments: Proxies for climate changes and human activities

    Garcon, Marion   Chauvel, Catherine   Chapron, Emmanuel   Fain, Xavier   Lin, Mingfang   Campillo, Sylvain   Bureau, Sarah   Desmet, Marc   Bailly-Maitre, Marie-Christine   Charlet, Laurent  

    High-altitude lake sediments are often used as archives for environmental changes and their chemical and isotopic compositions provide significant constraints on natural and anthropogenic long-term changes that have occurred in their catchment area. Here, trace-element concentrations and Pb isotopes are presented for two sedimentary cores from Lake Blanc Huez in the French Alps, to trace the impact of climate changes and human activities over the Holocene. Lead and Ag contents are very high and clearly dominated by input from a Pb-Ag vein located a few meters from the lakeshore, a vein that also buffers the Pb isotopes. Mining of this vein in medieval times is recorded in the corresponding lake sediments with high Ag content coupled with high Pb/U ratio. These chemical characteristics can be used to constrain the major Holocene climate changes. Significant advances of glaciers next to the lake produced sediments with Ag and Pb concentration peaks and high Pb/U ratios due to accelerated erosion of the Pb-Ag vein, similar to the effects of the medieval mining. In contrast, reduced glacier activity led to the formation of organic-rich sediments with high U and As contents and low Pb/U ratios. More generally, the observed combination of chemical changes could be used elsewhere to decipher environmental changes over long periods of time. (C) 2011 Elsevier Ltd. All rights reserved.
    Download Collect
  • A hafnium isotope and trace element perspective on melting of the depleted mantle

    Chauvel, Catherine   Blichert-Toft, Janne  

    New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform (176Hf/177Hf0.28313–0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109–123; Patchett, Lithos 16 (1983) 47–51]. Atlantic MORB have 176Hf/177Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077–1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low 176Hf/177Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf–Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB): ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions – the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] – can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges.
    Download Collect
  • Pb-Nd isotopic constraints on sedimentary input into the Lesser Antilles arc system RID A-1762-2011

    Carpentier, Marion   Chauvel, Catherine   Mattielli, Nadine  

    The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill Core located east of Dominica Island complement the data published by White et al. [White, W.M., Dupre, B. and Vidal, P., 1985. Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region, Atlantic Ocean. Geochimica et Cosmochimica Acta, 49: 1875-1886.] and confirm their relatively uniform isotopic compositions (i.e., (206)Pb/(204)Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable ((206)Pb/(204)Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta 8/4 Values that characterize the Lesser Antilles arc. (C) 2008 Elsevier B.V. All rights reserved.
    Download Collect
1 2 3

Contact

If you have any feedback, Please follow the official account to submit feedback.

Turn on your phone and scan

Submit Feedback