Régis Doucelance

Helium and lead isotope geochemistry of the Azores Archipelago

New helium and lead isotopic data for basalts from the Azores archipelago (North Atlantic) show that the Azores have 4He/3He ratios both higher and lower than MORB values. Good covariations of helium and lead isotopes are observed at the scale of the archipelago, and suggest the coexistence of two mantle components in the Azores which are identified by data from Sao Miguel and Terceira. The eastern part of Sao Miguel island displays radiogenic helium (4He/3He > 140,000, R/Ra 100,000) were observed at latitude higher than 40°N and may reflect the influence of the Sao Miguel component at the ridge.

Pb-Sr-He isotope and trace element geochemistry of the Cape Verde Archipelago

Abstract New lead, strontium and helium isotopic data, together with trace element concentrations, have been determined for basalts from the Cape Verde archipelago (Central Atlantic). Isotopic and chemical variations are observed at the scale of the archipelago and lead to the definition of two distinct groupings, in keeping with earlier studies. The Northern Islands (Santo Antao, Sao Vicente, Sao Nicolau and Sal) present Pb isotopic compositions below the Northern Hemisphere Reference Line (NHRL) (cf. Hart, 1984) , unradiogenic Sr and relatively primitive 4He/3He ratios. In contrast, the Southern Islands (Fogo and Santiago) display Pb isotopes above the NHRL, moderately radiogenic Sr and MORB-like helium signatures. We propose that the dichotomy between the Northern and Southern Islands results from the presence of three isotopically distinct components in the source of the Cape Verde basalts: (1) recycled ∼1.6-Ga oceanic crust (high 206Pb/204Pb, low 87Sr/86Sr and high 4He/3He); (2) lower mantle material (high 3He); and (3) subcontinental lithosphere (low 206Pb/204Pb, high 87Sr/86Sr and moderately radiogenic 4He/3He ratios). The signature of the Northern Islands reflects mixing between recycled oceanic crust and lower mantle, to which small proportions of entrained depleted material from the local upper mantle are added. Basalts from the Southern Islands, however, require the addition of an enriched component thought to be subcontinental lithospheric material instead of depleted mantle. The subcontinental lithosphere may stem from delamination and subsequent incorporation into the Cape Verde plume, or may be remnant from delamination just before the opening of the Central Atlantic. Basalts from Sao Nicolau reflect the interaction with an additional component, which is identified as oceanic crustal material.

Reevaluation of precise lead isotope measurements by thermal ionization mass spectrometry: comparison with determinations by plasma source mass spectrometry

In this study we focus on precise lead isotope analysis by thermal ionization mass spectrometry (TIMS) with silica gel technique. We present extensive measurements of SRM981 and SRM982 standards, of chemically pure Pb and of an equal-atom mixture of the four stable Pb isotopes. Measurements at temperatures lower than 1175°C display reproducible values and are coherent with the usual mass discrimination laws at the level of precision achievable with multicollector mass spectrometers. In contrast, the evolution of mass fractionation at higher temperatures does not follow classical laws of mass discrimination. We observe systematic shifts, the most pronounced of which concerns 207Pb with a total deviation that can reach values up to 400 ppm. This confirms the recent observation by Thirlwall [Chem. Geol. 163 (2000) 299]. Similarly, we do not have at present a physical explanation for this abnormal behaviour. We propose new precise isotopic compositions for the SRM981 and SRM982 standards. Assuming a 208Pb/206Pb ratio equal to the nominal value (208Pb/206Pb=2.1681 and 1.00016, respectively, cf. J. Res. Natl. Bur. Stand. 72A (1968) 261), the values for 207Pb/206Pb and 204Pb/206Pb ratios are 0.914970±17 and 0.059019±5 (2σ) for SRM981, and 0.467080±10 and 0.027212±2 (2σ) for SRM982. These compositions are in agreement with the recent measurements achieved with multicollection mass spectrometers with an inductively coupled plasma-source (MC-ICP-MS).

Further investigations on optimized tail correction and high-precision measurement of uranium isotopic ratios using multi-collector ICP-MS

In the present paper, we further examine the optimum conditions for rapid, precise and accurate determination of 234U/238U ratios in geological materials by multiple collector (magnetic-sector) inductively coupled plasma mass spectrometry (MC-ICP-MS). In our experiments, isotopic measurements were performed on a Micromass IsoProbe™ instrument, using Faraday collectors in static mode only. Unlike the ion counting system coupled with an energy filter, this technique eliminates the difficulty of proper calibration of the Daly/Faraday gain ratio. On the other hand, since our Micromass instrument has a poor abundance sensitivity (the proportion of the 238U ion beam measured at mass 237 is ∼25 ppm), the major issue to be addressed is the tail correction. For this purpose, we have developed a tail correction method slightly modified from Thirlwall [J. Anal. At. Spectrom. 16 (2001) 1121]. This method is based on correction of the actual tail contribution under each peak, as assessed by the tail shape measurements on mono-isotopic ion beams, instead of the usual half-mass zeroes baseline estimation. Our approach enabled us to correct for the large offset that can be observed on isotopic data when tail correction is done by means of linear interpolation between half-mass zeroes, and showed that this latter tail correction method results in nearly 3% underestimation of 234U/238U ratios on the GEOTOP IsoProbe™ for material at secular equilibrium. A 236U–233U double spike was employed to correct for mass discrimination bias. Using an Aridus™ micro-concentric, desolvating nebulizer sample introducing system, a minimum of 200 ng of sample-U was consumed to carry out a precise 234U/238U analysis, thereby allowing a 234U signal of ∼4–5 mV to be monitored for 50 measurement cycles of 5 s each. This time-consuming, 10-min procedure allowed us to obtain an external reproducibility of 0.8‰ (2σ) for isotopic measurements of the NBL-112a standard solution. Replicate measurements of this reference material yielded a mean δ234U value of −36.42±0.80‰ (2σ, n=19), which is highly consistent with values reported by other laboratories. The total reproducibility, including both chemical separation and spectrometric measurement, was assessed using geological samples (a coral and a carbonate rock); the long-term reproducibility obtained was about 1.3‰ (2σ).

Diversity in Earth's early felsic crust: Paleoarchean peraluminous granites of the Barberton Greenstone Belt

Earth9s oldest preserved granitoid crust dates back to the Paleoarchean and consists predominantly of sodic tonalite-trondhjemite-granodiorite (TTG) granitoids that arose through the partial melting of hydrated metabasalts. In contrast, granites (sensu stricto) typically appear relatively late in the plutonic record of the old cratons. However, the existence of Hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust; although we have direct evidence, this earliest felsic crust is not preserved. Here we present evidence of an unusual variety of markedly low-CaO, K 2 O-rich, rutile-bearing, peraluminous granite and rhyolite that was produced concurrently with TTG magmas during three magmatic cycles in the Barberton Greenstone Belt (BGB), southern Africa. This material is not preserved as in situ rock units, but occurs as clasts within a younger conglomerate. Within these rocks, plagioclase feldspar is a rare inclusion in zircon, relative to alkali feldspar, and has low anorthite contents (An 2 O and Sr content, as well as the peraluminous character of the magmas, is a consequence of phengite melting in a metagraywacke source at pressures in excess of those of plagioclase stability. This process contributed to each episode of continental crustal growth through the Paleoarchean to Mesoarchean in the BGB, despite leaving no plutonic record at the typical mid-crustal level of exposure that the TTG plutons around the belt represent.

Geochemical insights into the internal dynamics of debris avalanches. A case study: The Socompa avalanche, Chile

One way to infer the internal dynamics of debris avalanches is to characterize the heterogeneity of their deposits. Here we present high-precision Sr-Nd isotope compositions, plus major and trace element concentrations, of matrix samples and rock fragments from the Socompa debris-avalanche deposit (Chile). The Socompa blocks are easily identifiable in the field, but distinguishing substrate debris from disaggregated material formed at the volcano is difficult to do with only field criteria. Combining isotope data with field observations can help with this. Measured Sr and Nd isotope ratios show significant variations, defining a binary mixing array where matrix and rock deposits overlap. This testifies to the mixing of crushed rocks during collapse and/or movement. Assimilation of Socompa basement appears to be variable; overall, it is far lower than was previously proposed. Comparison between matrix and block samples in contact, over the whole surface area of the deposit, shows that the isotopic heterogeneity increases from source to front. Close to the Socompa, matrices are resulting from simple crushing of adjacent rocks. At the front, rock samples with distinct compositions are found in a close relationship with matrices that result either from mixing of these (or some of these) rocks or from crushing of basement material. Between the source and the front, the efficient mixing of Socompa rocks (and basement rocks) generates matrices with isotopic compositions distinct from those of the blocks they are in contact with. We interpret these results as being due to more efficient vertical mixing during the avalanche emplacement.