Bernard Bourdon

146Sm-142Nd evidence from Isua metamorphosed sediments for early differentiation of the Earth's mantle

Application of the 147Sm–143Nd chronometer (half-life of 106 Gyr) suggests that large-scale differentiation of the Earths mantle may have occurred during the first few hundred million years of its history. However, the signature of mantle depletion found in early Archaean rocks is often obscured by uncertainties resulting from open-system behaviour of the rocks during later high-grade metamorphic events. Hence, although strong hints exist regarding the presence of differentiated silicate reservoirs before 4.0 Gyr ago, both the nature and age of early mantle differentiation processes remain largely speculative. Here we apply short-lived 146Sm–142Nd chronometry (half-life of 103 Myr) to early Archaean rocks using ultraprecise measurement of Nd isotope ratios. The analysed samples are well-preserved metamorphosed sedimentary rocks from the 3.7–3.8-Gyr Isua greenstone belt of West Greenland. Our coupled isotopic calculations, combined with an initial ɛ143Nd value from ref. 6, constrain the mean age of mantle differentiation to 4,460 ± 115 Myr. This early Sm/Nd fractionation probably reflects differentiation of the Earths mantle during the final stage of terrestrial accretion.

Introduction to U-series Geochemistry

During the last century, the Earth Sciences underwent two major revolutions in understanding. The first was the recognition of the great antiquity of the Earth and the second was the development of plate tectonic theory. These leaps in knowledge moved geology from its largely descriptive origins and established the modern, quantitative, Earth Sciences. For any science, and particularly for the Earth Sciences, time scales are of central importance. Until recently, however, the study of time scales in the Earth Sciences was largely restricted to the unraveling of the ancient history of our planet. For several decades, Earth scientists have used a variety of isotope chronometers to unravel the long-term evolution of the planet. A fuller understanding of the physical and chemical processes driving this evolution often remained elusive because such processes occur on time scales (1–105 years) which are simply not resolvable by most conventional chronometers. The U-series isotopes, however, do provide tools with sufficient time resolution to study these Earth processes. During the last decade, the Earth Sciences have become increasingly focused on fundamental processes and U-series geochemistry has witnessed a renaissance, with widespread application in disciplines as diverse as modern oceanography and igneous petrology. The uranium and thorium decay-series contain radioactive isotopes of many elements (in particular, U, Th, Pa, Ra and Rn). The varied geochemical properties of these elements cause nuclides within the chain to be fractionated in different geological environments. while the varied half-lives of the nuclides allows investigation of processes occurring on time scales from days to 105 years. U-series measurements have therefore revolutionized the Earth Sciences by offering some of the only quantitative constraints on time scales applicable to the physical processes that take place on the Earth. The application of U-series geochemistry to the Earth Sciences was thoroughly summarized in 1982 …

226Ra-230Th evidence for multiple dehydration events, rapid melt ascent and the time scales of differentiation beneath the Tonga-Kermadec island arc

Abstract U-series disequilibria can be used to constrain the time scales and processes of fluid transfer, partial melting, magma ascent and magma differentiation beneath island arcs. However, maximum information is provided by studies involving more than one parent–daughter pair. Here we present mass spectrometric 226Ra measurements of Tonga–Kermadec arc lavas to complete the first detailed U–Th–Pa–Ra study of island arc lavas. (226Ra/230Th) activity ratios in Tonga–Kermadec lavas range from ∼1 up to 6.2, and they are negatively correlated with SiO2 constraining the time scale for differentiation from basalt to dacite and rhyolite to be

Insights into Magma Genesis at Convergent Margins from U-series Isotopes

Convergent margins (oceanic and continental arcs) form one of the Earth’s key mass transfer locations, being sites where melting and transfer of new material to the Earth’s crust occurs and also where crustal materials, including water, are recycled back into the mantle. Volcanism in this tectonic setting constitutes ~15% (0.4–0.6 km3/yr) of the total global output (Crisp 1984) and the composition of the erupted magmas is, on average, similar to that of the continental crust (Taylor and McLennan 1981). Moreover, many arc volcanoes have been responsible for the most hazardous, historic volcanic eruptions. Yet, despite their importance, many fundamental aspects of convergent margin magmatism remain poorly understood. Key among these are the rates of processes of fluid addition from the subducting plate. Furthermore, in stark contrast to the ocean ridges, where adiabatic decompression provides a simple and robust physical model for partial melting, no consensus has yet been reached about the physics of the partial melting process and the mechanism of melt extraction beneath arcs. Preceding chapters concerned with partial melting in this volume (Lundstrom 2003; Bourdon and Sims 2003) have discussed how the differing half-lives and distribution coefficients of the various U-series nuclides result in disequilibria through in-growth. This provides important information on the nature and timing of mantle partial melting processes. In convergent margin settings the differential fluid mobility of U and Ra relative to Th and Pa provides an additional source of fractionation leading to in-growth and this is crucial to understanding the timing and mechanisms of fluid addition. Here we review the role that the proliferation of high quality U-series isotope data, over the last decade, have had in obtaining precise information on time scales and the development of quantitative physical models for convergent margin magmatism. Our approach is to use trace element …

Erosion timescales derived from U-decay series measurements in rivers

The relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades. The principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates. In this study, (234U/238U), (230Th/238U), and (226Ra/238U) were measured by TIMS and by MC–ICP–MS in the dissolved and suspended loads of rivers from the Mackenzie Basin (Northwest Territories, Canada). The data show a complementary nature between (234U/238U), (230Th/238U) in the dissolved and suspended loads while 226Ra is characterized by a more complex behavior. Modeling of fractionation of U-series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed (9–28±10 ka), as well as the rates of release of U-series nuclides. The results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the Mackenzie River.

Equilibrium Mercury Isotope Fractionation between Dissolved Hg(II) Species and Thiol-Bound Hg

Stable Hg isotope ratios provide a new tool to trace environmental Hg cycling. Thiols (-SH) are the dominant Hg-binding groups in natural organic matter. Here, we report experimental and computational results on equilibrium Hg isotope fractionation between dissolved Hg(II) species and thiol-bound Hg. Hg(II) chloride and nitrate solutions were equilibrated in parallel batches with varying amounts of thiol resin resulting in different fractions of thiol-bound and free Hg. Mercury isotope ratios in both fractions were analyzed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). Theoretical equilibrium Hg isotope effects by mass-dependent fractionation (MDF) and nuclear volume fractionation (NVF) were calculated for 14 relevant Hg(II) species. The experimental data revealed that thiol-bound Hg was enriched in light Hg isotopes by 0.53 per thousand and 0.62 per thousand (delta(202)Hg) relative to HgCl(2) and Hg(OH)(2), respectively. The computational results were in excellent agreement with the experimental data indicating that a combination of MDF and NVF was responsible for the observed Hg isotope fractionation. Small mass-independent fractionation (MIF) effects (<0.1 per thousand) were observed representing one of the first experimental evidences for MIF of Hg isotopes by NVF. Our results indicate that significant equilibrium Hg isotope fractionation can occur without redox transition, and that NVF must be considered in addition to MDF to explain Hg isotope variations.

Silicon Isotope Evidence Against an Enstatite Chondrite Earth

Building Blocks of Earth Earth formed from an explosive and energetic series of collisions that accreted material over millions of years. Comparisons between rocks from Earths interior and more primitive extraterrestrial samples can help tease apart the composition of Earths starting material; however, discrepancies between the abundance of certain elements or their isotope ratios often obscure their origin. Fitoussi and Bourdon (p. 1477, published online 1 March) analyzed the silicon isotopes of a suite of rocks from chondritic meteorites and the Moon to reconcile some of the previous models. By tuning Earth accretion models to account for these Si isotope signatures, enstatite chondrites could be ruled out as the sole end-member composition for bulk Earth. Instead, a heterogeneous mixture of several types of chondritic meteorites is more likely. Earth accreted from materials with a heterogeneous mix of chondritic meteorite compositions. The compositions of Earth materials are strikingly similar to those of enstatite chondrite meteorites in many isotope systems. Although this suggests that Earth largely accreted from enstatite chondrites, definitive proof of this model has been lacking. By comparing the silicon (Si) isotope signatures of several extraterrestrial materials with terrestrial samples, we show that they cannot be explained by core-formation scenarios involving a bulk Earth of enstatite chondrite composition. Si isotope similarities between the bulk silicate Earth and the Moon preclude the existence of a hidden reservoir in the lower mantle, a necessary condition of the enstatite chondrite model, and require an equilibrium process after the Moon-forming impact. A three–end-member chondritic mixing model for Earth reconciles the Si isotope similarities between enstatite chondrites and Earth.

Osmium isotope variations in the oceans recorded by Fe-Mn crusts

This study presents osmium (Os) isotope data for recent growth surfaces of hydrogenetic ferromanganese (Fe‐Mn) crusts from the Pacific, Atlantic and Indian Oceans. In general, these data indicate a relatively uniform Os isotopic composition for modern seawater, but suggest that North Atlantic seawater is slightly more radiogenic than that of the Pacific and Indian Oceans. The systematic difference in the Os isotopic composition between the major oceans probably reflects a greater input of old continental material with a high Re =Os ratio in the North Atlantic Ocean, consistent with the distribution of Nd and Pb isotopes. This spatial variation in the Os isotope composition in seawater is consistent with a residence time for Os of between 2 and 60 kyr. Indian Ocean samples show no evidence of a local source of radiogenic Os, which suggests that the present-day riverine input from the Himalaya‐Tibet region is not a major source for Os. Recently formed Fe‐Mn crusts from the TAG hydrothermal field in the North Atlantic yield an Os isotopic composition close to that of modern seawater, which indicates that, in this area, the input of unradiogenic Os from the hydrothermal alteration of oceanic crust is small. However, some samples from the deep Pacific (4 km) possess a remarkably unradiogenic Os isotope composition ( 187 Os= 186 Os ratios as low as 4.3). The compositional control of Os incorporation into the crusts and mixing relationships suggest that this unradiogenic composition is most likely due to the direct incorporation of micrometeoritic or abyssal peridotite particles, rather than indicating the presence of an unradiogenic deep-water mass. Moreover, this unradiogenic signal appears to be temporary, and local, and has had little apparent effect on the overall evolution of seawater. These results confirm that input of continental material through erosion is the dominant source of Os in seawater, but it is not clear whether global Os variations are due to the input of mantle or meteoritic material, or simply indicate that the continental source itself is not uniform.

Ra–Th–Sr isotope systematics in Grande Comore Island: a case study of plume–lithosphere interaction

Here, we present new mass spectrometry measurements of U–Th–Ra disequilibria and Sr isotopes for historical volcanics from the Karthala and La Grille volcanoes in Grande Comore Island, Comores Archipelago. Alkali basalts from the Karthala are characterised by large 230Th (33–47%) and 226Ra excesses (21–53%), and radiogenic Sr compositions (0.7034–0.7041). In contrast, La Grille basanites have less radiogenic 87Sr/86Sr (∼0.7032) with less 226Ra excesses (21%) but similar 230Th–238U disequilibria (45%). The Karthala samples display much more scatter in the Ra–Th isochron diagram than in the Th–U isochron diagram, suggesting a decoupling of the two parent–daughter systems. Correlations between the Sr isotopes and U–Th–Ra disequilibria suggest a mixing relationship between La Grille and Karthala sources. La Grille basanites are shown to result from partial melting of the old metasomatised oceanic lithosphere beneath the archipelago whereas Karthala lavas are derived from the Comore plume. The presence of amphibole in the lithosphere is responsible for Ra–Ba, Ba–Th and Ra–Th fractionation during melting such that DBa > DTh > DRa. The partitioning of these elements clearly differs from partial melting of garnet–lherzolite where DTh > DRa > DBa. Part of the variations in the 226Ra excesses is attributed to radioactive decay between the source and the surface. Short residence times (<1000 year) preclude the presence of a large magma chamber and may be an argument for rapid segregation of the melts through cracks or veins during melt transport.

On the preservation of mantle information in ultramafic nodules, glass inclusions within minerals versus interstitial glasses

This study questions the assumption that silicate melts preserved as glass inclusions in minerals and as interstitial films or pockets in mantle xenoliths have identical chemical compositions to one another and are both suitable for inferring deep mantle melt compositions. Theoretical models of the elastic behavior of melt inclusions indicate that only limited decompression of the inclusion takes place during ascent of the host xenolith, whereas the pressure of an interstitial melt follows the external pressure. Consequences of such behavior are considered using simple model systems, such as the alkali-bearing system forsterite–nepheline–SiO2. With decreasing pressure, phase boundaries shift to silica normative compositions. Consequently, reequilibration of small-degree melts of peridotite, which are characterized by olivine–nepheline normative compositions at moderate pressure, yield quartz normative compositions at a lower pressure. Also, melt inclusions are simpler systems, i.e. the trapped melts are in contact with a single mineral phase, in contrast with interstitial glasses. We show here that experimental heating of the inclusions restores the original melt composition of melt inclusions prior to cooling, which is not possible for interstitial glasses. Data obtained for rehomogenized glass inclusions and interstitial glasses associated in the same xenoliths from intraplate ocean islands and subduction zones confirm the model predictions. In intraplate nodules, the highly silicic, alkali-rich melts preserved as glass inclusions inside minerals are olivine–nepheline normative and represent high-pressure (1 GPa) near-solidus melts in equilibrium with a peridotitic assemblage. In contrast, the silica-normative composition of the interstitial glasses records their last pressure of equilibration, i.e., shallow-level conditions. In subduction zone settings, exsolution of the oversaturated H2O-rich volatile phase and decompression cause conflicting effects on the composition of the melts trapped in xenoliths. The composition of glass inclusions is characterized by higher levels of volatile elements, mainly H2O, and by a higher quartz norm than interstitial glasses. This is consistent with the hypothesis that the glass inclusions represent quenched melts from H2O-saturated peridotite or amphibolite/eclogite systems, whereas the composition of the interstitial glasses indicates reequilibration at low pressure, probably induced by the H2O-rich volatile loss