Albert Galy

Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance

Abstract The mass-dependent fractionation laws that describe the partitioning of isotopes are different for kinetic and equilibrium reactions. These laws are characterized by the exponent relating the fractionation factors for two isotope ratios such that α2/1 = α3/1β. The exponent β for equilibrium exchange is (1/m1 − 1/m2)/(1/m1 − 1/m3), where mi are the atomic masses and m1

Higher erosion rates in the Himalaya: Geochemical constraints on riverine fluxes

The modern erosion rate of continental-scale mountains is difficult to estimate and is usually based on measurement of the suspended load flux of rivers combined with assumptions about river bedload transport and sedimentation in flood plains. These two parameters are very difficult to measure directly in continental-scale basins. In this paper we examine the chemical composition of the suspended load, bedload, and dissolved load of the Ganga and Brahmaputra Rivers and compare them with the average composition of Himalayan source rocks. A mass-balance equation of erosion fluxes shows that a Si-rich component is needed in addition to suspended and dissolved load fluxes to account for the composition of the source rock. It corresponds to bedload sediment and flood-plain deposits, which are enriched in quartz by mineral sorting during transport. The combined budget of Si, Al, and Fe in the river system allows us to estimate this Si-rich flux. By this method, the total Himalayan erosion is estimated to be twice the measured flux of suspended load. The comparison between the Brahmaputra and the Ganga shows that the eastern Himalaya has a higher erosion rate (2.9 mm/yr) than the western Himalaya (2.1 mm/yr). This is likely the result of the higher runoff in the Brahmaputra basin. The intensity of the monsoon acts as a first-order control of the erosion rate in the range.

Magnesium isotope heterogeneity of the isotopic standard SRM980 and new reference materials for magnesium-isotope-ratio measurements

Multicollector ICP-MS has been used for the precise measurement of variations in the isotopic composition of the isotopic standard of magnesium (SRM980) provided by the National Institute of Standards and Technology (Gaithersburg, MD, USA). The SRM980 consists of metal chips weighing between 1 and 50 mg and each unit delivered by the National Institute of Standards and Technology corresponds to a bottle containing about 0.3 g. Height units were analysed. Variations in sample 25Mg/24Mg, and 26Mg/24Mg ratios are expressed as δ25Mg and δ26Mg units, respectively, which are deviations in parts per 103 from the same ratio in a standard solution. The differences in δ25Mg and δ26Mg of the SRM980 are up to 4.20 and 8.19‰, respectively, while the long-term repeatability of δ25Mg and δ26Mg are 0.09 and 0.16‰, respectively, at 95% confidence. However, when plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. Overall limits of error of the SRM980 reported here fall within the previously reported overall limits of error. The isotopic heterogeneity not only corresponds to differences among units but has been found at the chip-size level. This result, due to the precision of the MC-ICP-MS technique, makes the SRM980 inappropriate for the international isotopic standard of magnesium. The SRM980 can still be used to report the excess of 26Mg, which is defined by the deviation from the mass-dependent relationship between 25Mg/24Mg, and 26Mg/24Mg ratios. Two large batches (around 10 g of Mg in each) of pure Mg solutions (in 0.3 M HNO3) have been prepared and characterised. These 2 solutions (DSM3 and Cambridge 1) are suitable reference material because they are immune to heterogeneity. DSM3 and Cambridge 1 are isotopically different (by 1.3‰ per u) and are available upon request from the first author. In addition, DSM3 has an isotopic composition very similar to the Mg-isotopic composition of carbonaceous chondrites (Orgueil and Allende). Because of the lack of heterogeneity and the cosmochemical and geochemical significance of DSM3, we urge the use of DSM3 as the primary isotopic reference material to report Mg-isotopic variations.

Mg isotopic composition of carbonate: insight from speleothem formation

Simultaneous high-precision measurement of 24 Mg, 25 Mg and 26 Mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry (MC-ICP-MS) relative to the international standard SRM980. Data are presented on low-Mg calcite speleothems and their associated host rocks and waters from four caves, one in the French Alps and three in Israel, covering various climate conditions. In addition, data are presented on three dolostones and three limestones from the Himalaya. The overall variation is 4.13x and 2.14x in N 26 Mg and N 25 Mg, respectively. This is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of Mg is not unique. Each speleothem shows a characteristic range of N 26 Mg values that are attributed to the isotopic composition of the local water. Differences between the isotopic composition of Mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to Mg isotopic fractionation during carbonate precipitation in the temperature range of 4^18‡C. The low-Mg calcite is enriched in light isotopes by 1.35x/AMU and the dependence on temperature has been found to be less than 0.02x/AMU/‡C. Despite various geological settings, the N 26 Mg of the studied dolostones is 2.0 @ 1.2x higher than the N 26 Mg of the limestones. All together, these results suggest a strong mineralogical control and a weak temperature effect on the Mg isotopic composition of carbonate. A 2002 Elsevier Science B.V. All rights reserved.

High-precision measurement of magnesium isotopes by multiple-collector inductively coupled plasma mass spectrometry

Multiple-collector inductively coupled plasma mass spectrometry has been used for the precise measurement of variations in the isotopic composition of Mg in a range of materials. The contributions of CC, CN, and MgH molecular species to the mass spectrum in the Mg mass region are minimised. Variations in sample 26Mg/24Mg and 25Mg/24Mg ratios are expressed as δ26Mg and δ25Mg units, which are deviations in parts per 103 from the same ratio in the SRM 980 Mg standard. The long-term repeatability of the 26Mg/24Mg and 25Mg/24Mg ratios of a sample Mg solution relative to the SRM 980 Mg isotope standard are 0.12‰ and 0.06‰, respectively, at 95% confidence. The addition of Na, Al, and Ca in a solution of Mg having a known isotopic composition induces 0.2‰–1‰ increase of δ26Mg. This chemical bias is a result of a mass-dependent process and is observed to be greater with Ca than Na. Isobaric interference from doubly charged 48Ca ions on mass 24 is observed to be significant when [Ca]/[Mg] ≥ 0.5. The results obtained on nine terrestrial material show a variation of Mg-isotopes of 4‰ in δ26Mg. When plotted in a three-isotope diagram, all the data fall on a single mass fractionation line. The excess of 26Mg has been determined by the deviation from that mass-dependent relationship, and its long-term repeatability is 0.06‰ at 95% confidence.

High-precision measurement of calcium isotopes in carbonates and related materials by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS)

Multi-collector ICP-MS has been used for the precise measurement of natural variations in the isotopic composition of Ca. The interference of Ar in the Ca mass region is assessed and the repeatability of the 44Ca/42Ca ratio of a sample calcium solution relative to the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 915a Calcium Carbonate (Clinical) standard is better than 0.1‰ at 95% confidence. Variations in sample 44Ca/42Ca ratio are expressed as δ44Ca units, which are deviations in parts per 103 from the same ratio in the NIST SRM 915a Ca standard. Measurements of δ44Ca are presented for terrestrial and marine carbonates, which show a variation of up to 0.7‰, in agreement with previous studies by thermal ionisation mass spectrometry (TIMS).

Efficient transport of fossil organic carbon to the ocean by steep mountain rivers: An orogenic carbon sequestration mechanism

Mountain building exposes fossil organic carbon (OC fossil ) in exhumed sedimentary rocks. Oxidation of this material releases carbon dioxide from long-term geological storage to the atmosphere. OC fossil is mobilized on hillslopes by mass wasting and transferred to the particu- late load of rivers. In large fl uvial systems, it is thought to be oxidised in transit, but in short, steep rivers that drain mountain islands, OC fossil may escape oxidation and re-enter geological storage due to rapid fl uvial transfer to the ocean. In these settings, the rates of OC fossil transfer and their controls remain poorly constrained. Here we quantify the erosion of OC fossil from the Taiwan mountain belt, combining discharge statistics with measurements of particulate organic carbon load and source in 11 rivers. Annual OC fossil yields in Taiwan vary from 12 ± 1 to 246 ± 22 tC km −2 yr −1 , controlled by the high physical erosion rates that accompany rapid crustal shortening and frequent typhoon impacts. Effi cient transfer of this material ensures that 1.3 ± 0.1 ◊ 10 6 tC yr −1 of OC fossil exhumed in Taiwan is delivered to the ocean, with <15% loss due to weathering in transit. Our fi ndings suggest that erosion of coastal mountain ranges can force effi cient transfer and long-term re-accumulation of OC fossil in marine sediments, further enhancing the role of mountain building in the long-term storage of carbon in the lithosphere.

Propagation of the thrust system and erosion in the Lesser Himalaya: Geochemical and sedimentological evidence

Centre de Recherches Pe´trographiques et Ge´ochimiques, BP 20, 54501 Vandoeuvre-les-Nancy,FranceABSTRACTSedimentological and Nd isotope data of two sections of the sub-Himalaya of westernNepal are used as new constraints for understanding the erosion history of the Himalaya.Throughout the deposition of the middle and upper members of the Siwalik Group, theLesser Himalaya contribution to the total detrital input progressively increased from lessthan 20% to 40%. The increasing proportion of Lesser Himalaya sediments started at ca.10–8 Ma and is associated with a coarsening of the maximum grain size at both micro-scopic and macroscopic scales. Thin-skinned tectonics of the Lesser Himalaya thrust sys-tem would have controlled the exhumation of the Lesser Himalaya rocks and would havebegun at 12–10 Ma, taking into account the delay for denudation. Together with otherstudies, these data restrict the onset of movement on the Lesser Himalaya thrust systemto less than 3 m.y. along more than 1750 km. This short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust; thus we suggest instead the simultaneous initiation of several thrusts aheadof the Main Central thrust at ca. 12 Ma. We suggest that a rapid rise of the TibetanPlateau at this time has transformed the Himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state. This regional rising could be the primecause of the increase of sediment influx at ca. 11 Ma around the Himalaya.Keywords: Siwalik formations, neodymium, grain size, erosion, thrust sheets, Himalaya.

Mg isotope heterogeneity in the Allende meteorite measured by UV laser ablation-MC-ICPMS and comparisons with O isotopes

First results from a new UV laser ablation MC-ICPMS method for measuring Mg isotope ratios in situ in meteoritical materials show that there are mass-dependent variations in 25 Mg and 26 Mg up to 1.5 ‰ per amu in chondrules and 0.3‰ per amu in a CAI from the Allende meteorite. In both cases the mass-dependent fractionation is associated with alteration. Comparisons with laser ablation O isotope data indicate that incorporation of pre-existing grains of forsterite with distinct Mg and O isotopic compositions and post-formation alteration both contributed to the variability in Mg isotope ratios in the chondrules, resulting in a correlation between high 25 Mg and low 17 O. The laser ablation analyses of the CAI show that high-precision determinations of both 25 Mg and 26 Mg can be used to discriminate features of the 26 Al- 26 Mg isotope system that are relevant to chronology from those that result from element mobility. Copyright

Evaluation of the accuracy of the determination of lead isotope ratios in wine by ICP MS using quadrupole, multicollector magnetic sector and time-of-flight analyzers

Different mass analysers [(quadrupole (Q), time-of-flight (TOF) and multicollector (MC) sector-field (SF)] of ions produced in an inductively coupled plasma were evaluated for the determination of lead isotope ratios in wine samples. A population of 20 wines of different origin including two reference wines from the EC Standards, Measurement and Testing Programme with concentrations varying between 7-140 mug Pb l(-1) was investigated. Wines were analyzed directly by Q ICP MS and MC ICP MS. The poor sensitivity of the TOF instrument, further aggravated by matrix signal suppression, did not allow the acquisition of data for wine samples that contained less than 50 mug l(-1) in the direct sample introduction mode. The separation and preconcentration of lead were therefore required. The precision obtained for the (206)Pb/(207)Pb and (208)Pb/(206)Pb were similar and equal to 0.14-2.7% for Q ICP MS, 0.04-0.17% for TOF ICP MS and 0.01-0.12% for MC ICP MS. The precision for (206)Pb/(204)Pb was 0.44-5.29, 0.15-1.7, 0.08-1.6%, respectively. On the level of accuracy, the data from TOF ICP MS and MC ICP MS were in good agreement. The accuracy of Q ICP MS data was judged satisfactory in comparison with the other techniques but their poor precision was a significant obstacle on the way of using these data for the determination of the geographic origin of wine.