U-series Isotopes Research Articles

U-series isotope and geodynamic constraints on mantle melting processes beneath the Newer Volcanic Province in South Australia

Young (< 5 kyr) olivine- and clinopyroxene-phyric ne-hawaiites from Mounts Gambier and Schank in the Newer Volcanic Province in South Australia have been analysed for major and trace elements as well as for Sr and Nd isotopes and 238U– 230Th disequilibria in order to constrain the mantle melting processes responsible for their origin. The rocks are relatively primitive (6.9–9.1% MgO), incompatible trace element-enriched alkali basalts with 87Sr/ 86Sr = 0.70398–0.70415 and 143Nd/ 144Nd = 0.51280–0.51271. Trace element modelling suggests that they reflect 3–6% partial melting in the presence of 2–8% residual garnet. Trends towards low K/K ⁎ are accompanied by decreasing 87Sr/ 86Sr and provide evidence for the involvement of hydrous phases during melting. 230Th excesses of 12–57% cannot be simulated by batch melting of the lithosphere and instead require dynamic melting models. It is argued that the distinction between continental basalts bearing significant U–Th disequilibria and those in secular equilibrium reflects dynamic melting in upwelling asthenosphere, rather than static batch melting within the lithosphere or the presence or absence of residual garnet. Upwelling rates are estimated at ∼ 1.5 cm/yr. A subdued, localised topographic uplift associated with the magmatism suggests that any upwelling is more likely associated with a secondary mode localised to the upper mantle, rather than a broad zone of deeply-sourced (plume) upwelling. Upper mantle, ‘edge-driven’ convection is consistent with seismic tomographic and anisotropy studies that imply rapid differential motion of variable thickness Australian lithosphere and the underlying asthenosphere. In this scenario, melting is linked to a significant contribution from hydrous mantle that is envisaged as resulting either from convective entrainment of lithosphere along the trailing edge of a lithospheric keel, or inherited variability in the asthenosphere.

Mantle plumes and geochemistry

There is considerable interest in the extent to which mantle plumes exist, how many there may be, and how best they can be recognized. It has proved unexpectedly difficult to image them consistently from seismology, and it has been suggested that they may be recognized from the geochemistry of rocks erupted at the Earth's surface. The difficulty is that plumes are physical features, and they can only be identified geochemically if they are either derived from, or entrain material that has a distinctive geochemical fingerprint. Too often plumes have been invoked because the erupted basalts have a within plate signature, irrespective of whether the volumes of magma or the rates of eruption can be estimated. U-series isotopes in basalts are sensitive to variations in the upwelling rates of their mantle source rocks, and those can be used to invoke upward movements that may be associated with mantle plumes. A contribution from the Earth's core might provide a possible geochemical tracer of deep-seated mantle plumes, although it is difficult to assess whether core material may be readily entrained in the mantle. Given the marked compositional differences between the core and the mantle, it should be possible to identify material that contains a contribution from the core and hence may be inferred to be from the core–mantle boundary. The core should have high Os and W abundances, and distinctive isotope ratios. Arguments for the recognition of core material using Os and W isotopes are therefore reviewed, as are those relying on rare gases to identify contributions from the lower mantle.

Arc dacite genesis pathways: Evidence from mafic enclaves and their hosts in Aegean lavas

Mafic enclaves are commonly found in intermediate arc magmas, and their occurrence has been linked to eruption triggering by pre-eruptive magma mixing processes. New major, trace, Sr–Nd and U–Th isotope data of rocks from Nisyros in the Aegean volcanic arc are presented here. Pre-caldera samples display major and trace element trends that are consistent with fractionation of magnetite and apatite within intermediate compositions, and zircon within felsic compositions, and preclude extensive hybridization between mafic and felsic magmas. In contrast, post-caldera dacites form a mixing trend towards their mafic enclaves. In terms of U-series isotopes, most samples show small 238U excesses of up to ∼ 10%. Mafic enclaves have significantly higher U/Th ratios than their dacitic host lavas, precluding simple models that relate the mafic and felsic magmas by fractionation or aging alone. A more complicated petrogenetic scenario is required. The post-caldera dacites are interpreted to represent material remobilized from a young igneous protolith following influx of fresh mafic magma, consistent with the U–Th data and with Sr–Nd isotope constraints that point to very limited (< 10%) assimilation of old crust at Nisyros. When these results are compared to data from Santorini in the same arc, there are many geochemical similarities between the two volcanic centers during the petrogenesis of the pre-caldera samples. However, striking differences are apparent for the post-caldera lavas: in Nisyros, dacites show geochemical and textural evidence for magma mixing and remobilization by influx of mafic melts, and they erupt as viscous lava domes; in Santorini, evidence for geochemical hybridization of dacites and mafic enclaves is weak, dacite petrogenesis does not involve protolith remobilization, and lavas erupt as less viscous flows. Despite these differences, it appears that mafic enclaves in intermediate Aegean arc magmas consistently yield timescales of at least 100 kyrs between U enrichment of the mantle wedge and eruption, on the upper end of those estimated for the eruptive products of mafic arc volcanoes. Finally, the data presented here provide constraints on the rates of differentiation from primitive arc basalts to dacites (less than ∼ 140 kyrs), and on the crustal residence time of evolved igneous protoliths prior to their remobilization by mafic arc magmas (greater than ∼ 350 kyrs).

The relationship between riverine U-series disequilibria and erosion rates in a basaltic terrain

U-series isotopes have been measured in the dissolved phase, suspended load and bedload of the main rivers draining basaltic catchments in Iceland. For the dissolved phase, ( 234U/ 238U) and ( 238U/ 230Th) range between 1.08 and 2.2, and 7.4 and 516, respectively. For the suspended load and bedload, ( 234U/ 238U) and ( 238U/ 230Th) range from 0.97 to 1.09 and from 0.93 to 1.05, respectively. Chemical erosion rates, calculated from dissolved major elements, range between 13 and 333 t km − 2 yr − 1 . Physical erosion rates have also been estimated, from existing data, and range between 21 and 4864 t/km 2/yr, with an average of 519 t km − 2 yr − 1 . U-series disequilibria indicate that weathering in Iceland operates at close to steady-state conditions. A model of continuous weathering indicates a maximum weathering timescale of 10 kyr, with an average rate of uranium release into water of 1.6 · 10 − 4 yr − 1 , which is significant when compared to granitic terrains located at similar latitudes and to tropical basaltic terrains. All river waters display ( 234U/ 238U) greater than secular equilibrium, consistent with the effects of alpha-recoil. The same dissolved phase ( 234U/ 238U) exhibit a negative trend with physical erosion rates, explained by the dominant effect of close-to-congruent chemical weathering of hyaloclastites in the younger basaltic terrains. Therefore, chemical erosion rate and mineral weathering susceptibility play a major role in determining 234U– 238U disequilibria in basaltic river waters. Comparison of global data for river basins in which weathering was recently strongly limited indicates a negative correlation between silicate weathering rates estimated with major elements and the age of weathering estimated with U-series disequilibria. This strongly suggests a key role of time and soil thickness on the chemical erosion of silicates.

Direct U-series dating of fossil human bone by laser ablation MC-ICPMS

The recent application of laser ablation ICP-MS to U-series dating allows the measurement of U and Th isotopes in tiny samples and at a high spatial resolution. This technique is ideal for Useries dating of valuable human fossils because it can be made virtually non-destructive, and because the spatial distribution of Useries isotopes is required to model uranium uptake in bone using the diffusion-adsorption model. We present results of recent dating studies on human fossils, including direct dating of the early anatomically modern human Omo Kibish 1.

238U– 230Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift

Trace element and U-series isotope analyses are presented for a suite of recent (< 10 ka) basalts from the axial portion of the Kenya rift. Samples from throughout the rift have LREE-enriched patterns with HREE > 10 × chondrite and the LREE between 60 and 200 × chondrite. REE fractionation is consistent with melting a garnet lherzolite source region with between 2% and 6% modal garnet. Other trace element ratios are distinct from OIB, notably Zr/Hf which ranges from 43 to 48, whilst at a given Zr content the Zr/Hf ratio is significantly greater than that found in OIB. ( 238U/ 232Th) range from 0.362 to 1.036, ( 230Th/ 232Th) from 0.503 to 1.109, with ( 230Th/ 238U) ranging from 0.783 to 2.966. All but two samples are in 230Th excess or in secular equilibrium. Samples with elevated ( 238U/ 232Th), also have Rb/Cs > 120, but unexceptional 208Pb ⁎/ 206Pb ⁎ and hence κ Pb values. These samples have experienced U and Cs loss and are excluded from further consideration. Of the unaltered samples, all have ( 238U/ 232Th) generally lower than OIB, with maximum values of < 0.8, and some < 0.6. The maximum ( 230Th/ 238U) is 1.39, similar to OIB. Although none of the basalts has a primary composition, ( 230Th/ 238U) does not vary systematically with indices of fractionation, and comparison with evolved rocks from Kenya indicates that 238U– 230Th disequilibrium in the basalts is not the product of fractionation and crustal residence, but a product of melt generation. The maximum ( 230Th/ 238U) that can be generated by batch melting, assuming a source mineralogy consistent with the REE variation is 1.05 and so the variation in ( 230Th/ 238U) is attributed to more complex models of melt generation and/or transport. Both dynamic melting and equilibrium porous flow suggest mantle upwelling rates of ≤ 2 cm year − 1 . It is suggested that the Kenya basalts represent melts derived from lithospheric mantle that has been thermally reactivated by and incorporated into the underlying (East African) mantle plume.

Evidence of a Stable Uranyl Site in Ancient Organic-Rich Calcite

The mechanism of uranium (U) incorporation into calcite (calcium carbonate) is of fundamental importance to the fate and transport of U at the surface and in the shallow subsurface and has implications for (a) the accuracy of U-Pb and U-series isotope ratio methods used to determine the ages of ancient deposits and (b) potential remediation strategies based on sequestration of U in the subsurface. Extended X-ray absorption fine structure (EXAFS) spectroscopy is uniquely suited to the study of U-calcite systems. The sensitivity of the EXAFS spectrum to the local atomic Ca coordination about U(VI) in the calcite structure results in an increase in the number and amplitude of Ca signals as the U(VI) becomes more ordered within the crystal structure. Our X-ray microprobe (10-microm) measurements of an ancient 298 million-year-old organic-rich calcite (calcrete) clearly revealed three coordination shells of Ca atoms, defining a well-ordered calcite structure about uranyl to a distance of approximately 6.5 angstroms. These results indicate that uranyl is incorporated at the Ca2+ site in calcite and that the uranyl environment may evolve over long time scales, becoming more calcite-like and more stable for long-term sequestration of uranium. These results therefore validate U-related dating methods and show that calcite can be effective at sequestering U in vadose zone sediments.

Remelting of recently depleted mantle within the Hawaiian plume inferred from the 226Ra– 230Th– 238U disequilibria of Puʻu ʻŌʻō eruption lavas

Lavas from the Puʻu ʻŌʻō eruption of Kīlauea Volcano are thought to partially bypass the shallow magma reservoir beneath the volcano's summit, and thus, provide a relatively direct “window” to the mantle. Here we use high-precision U-series isotope measurements of Puʻu ʻŌʻō lavas (1985–2001) to investigate the timing and mechanism of melt transport within the Hawaiian mantle plume. The lavas display small, but significant, temporal decreases in their 230Th– 238U (∼2.5% to 1.4% excess 230Th) and 226Ra– 230Th (∼14% to 12% excess 226Ra) disequilibria. These trends correlate systematically with larger decreases in abundance ratios of trace elements that are highly versus moderately incompatible during partial melting of the mantle. These ratios vary from a maximum of ∼23% for Ba / Yb to a minimum of ∼4% for Nd / Sm. Modeling of these geochemical signatures suggests that Puʻu ʻŌʻō lavas are increasingly derived from a mantle source that was recently depleted by prior melt extraction within the Hawaiian plume. The timing of this depletion must be longer than the ∼20-year duration of the eruption but less than several half-lives of 226Ra (< 8 kyr ago). A single magmatic process – the transfer of melt from pores in steady-state equilibrium with the residual mantle into chemically isolated channels – ultimately seems to control the rapid fluctuation in lava chemistry at Puʻu ʻŌʻō. Specifically, this mode of melt extraction (if it occurs frequently) would create numerous patches of recently depleted mantle within Kīlauea's source region that may subsequently remelt. When melt is extracted into channels to supply a given eruption, such as Puʻu ʻŌʻō, it must be drained from an ever increasing volume in order to sustain the flow of melt to the surface. As the eruption continues, the volcano might tap melt from more distal areas and tend to encounter a greater number of patches of melt from recently depleted mantle. The source region of Puʻu ʻŌʻō lavas within the Hawaiian plume is also thought to contain long-lived, small-scale compositional heterogeneities based on temporal variations of ratios of highly incompatible trace elements (Ba / Th or Th / U) and 87Sr / 86Sr. This melt-transport mechanism would allow Kīlauea to sample chemically and isotopically heterogeneous pockets of melt from “fresh” mantle that was never melted beneath Hawaiʻi (in addition to melt from recently depleted mantle), and thus, efficiently transmit the geochemical signatures of these compositional heterogeneities from the volcano's source region to the surface.

Direct dating of human fossils

The methods that can be used for the direct dating of human remains comprise of radiocarbon, U-series, electron spin resonance (ESR), and amino acid racemization (AAR). This review gives an introduction to these methods in the context of dating human bones and teeth. Recent advances in ultrafiltration techniques have expanded the dating range of radiocarbon. It now seems feasible to reliably date bones up to 55,000 years. New developments in laser ablation mass spectrometry permit the in situ analysis of U-series isotopes, thus providing a rapid and virtually non-destructive dating method back to about 300,000 years. This is of particular importance when used in conjunction with non-destructive ESR analysis. New approaches in AAR analysis may lead to a renaissance of this method. The potential and present limitations of these direct dating techniques are discussed for sites relevant to the reconstruction of modern human evolution, including Florisbad, Border Cave, Tabun, Skhul, Qafzeh, Vindija, Banyoles, and Lake Mungo.

In situ U-series dating by laser-ablation multi-collector ICPMS: new prospects for Quaternary geochronology

The capabilities and potential applications of in situ dating of Quaternary materials using laser ablation-MC-ICPMS are explored. 234U/ 238U and 230Th/ 234U can be measured with precision sufficient for dating at a spatial resolution of 100 μm or better in samples that contain as a little as 1 ppm uranium. Moreover, U and Th concentrations and U-series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth (e.g. in speleothems). These capabilities additionally permit the dating of bones, teeth and possibly molluscs, which are subject to post-mortem open-system behaviour of U-series isotopes, and can be employed to elucidate processes of U-series migration during weathering and diagenesis. A drawback of laser ablation-MC-ICPMS is that it cannot in general provide U-series age estimates with the high precision and accuracy of conventional TIMS or solution MC-ICPMS methods. However, sample preparation is straightforward, the amount of sample consumed negligible, and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods. Given further instrumental developments and the establishment of suitable matrix-matched standards for carbonates and other materials, we foresee that laser ablation-MC-ICPMS will play an increasingly important role in Quaternary dating research.

Partial melting and upwelling rates beneath the Azores from a U-series isotope perspective

We report U-series systematics (238U-230Th-231Pa-226Ra) for basalts from the Azores islands and the nearby mid-Atlantic ridge with the aim of constraining melting processes in this region of plume–ridge interaction. Both 230Th and 231Pa excess show rough negative trends with indices of source enrichment (Sr, Nd and Pb isotopes) which cannot be explained by melting of a source variably enriched by mafic components (e.g. pyroxenite). However, there is evidence for a distinct source component, possibly sediment, beneath Sao Miguel which may give rise to these trends. 231Pa excess and Sm-Nd fractionation are positively and negatively correlated (respectively) with distance from the centre of the plume, while 230Th excesses show no clear trend. This suggests that mantle upwelling velocity exerts a strong control on U-Pa fractionation and that melting is initiated deeper in the centre of the plume. These observations are used to infer melting conditions beneath the mid-Atlantic ridge and Azores islands. The mantle upwelling velocity is constrained to be 3–4 cm/a which is smaller than that inferred for Hawaii or Iceland. Predicted 226Ra-excesses are similar to, or lower than those measured, suggesting high velocity melt ascent in channels and rapid differentiation prior to eruption. The effect of the presence of volatiles in the source is also investigated and we show that the presence of water could explain the 230Th and 231Pa trends as a function of distance across the plume. A 3-D model for the Azores plume can reproduce the U-series isotope observations.

U/Th dating by SHRIMP RG ion-microprobe mass spectrometry using single ion-exchange beads

We present a new analytical method for U-series isotopes using the SHRIMP RG (Sensitive High mass Resolution Ion MicroProbe) mass spectrometer that utilizes the preconcentration of the U-series isotopes from a sample onto a single ion-exchange bead. Ion-microprobe mass spectrometry is capable of producing Th ionization efficiencies in excess of 2%. Analytical precision is typically better than alpha spectroscopy, but not as good as thermal ionization mass spectroscopy (TIMS) and inductively coupled plasma multicollector mass spectrometry (ICP-MS). Like TIMS and ICP-MS the method allows analysis of small samples sizes, but also adds the advantage of rapidity of analysis. A major advantage of ion-microprobe analysis is that U and Th isotopes are analyzed in the same bead, simplifying the process of chemical separation. Analytical time on the instrument is ∼60 min per sample, and a single instrument-loading can accommodate 15–20 samples to be analyzed in a 24-h day. An additional advantage is that the method allows multiple reanalyses of the same bead and that samples can be archived for reanalysis at a later time. Because the ion beam excavates a pit only a few μm deep, the mount can later be repolished and reanalyzed numerous times. The method described of preconcentrating a low concentration sample onto a small conductive substrate to allow ion-microprobe mass spectrometry is potentially applicable to many other systems.

Accurate in situ238U–234U–232Th–230Th analysis of silicate glasses and iron oxides by laser-ablation MC-ICP-MS

U-series microanalysis and dating is providing new insights into the timing and rates of formation of different minerals, including opals, authigenic iron oxides, zircons, and pedogenic carbonates, during the late Pleistocene. However, the low natural abundance of U and its decay chain isotopes, makes in situ U-series isotope analysis difficult. Using a state-of-the art excimer laser-ablation system coupled to a second generation MC-ICP-MS (Finnigan Neptune), we demonstrate the ability to measure [234U/238U] and [230Th/238U] activity ratios with in-run precision of 1‰ and 3‰, respectively, at a spatial resolution of 90 μm in materials containing 100–500 μg g−1 U. To verify the accuracy and precision of our approach we have prepared and thoroughly assessed two new ‘in-house’ reference materials (Al–Ca–Si glass and α-Fe2O3) for U-series isotopic composition and micro-scale homogeneity. Together with NIST 610, these reference materials allow us to monitor and correct for Th/U elemental fractionation and to accurately determine 230Th/U isotopic ratios. We show that calibration using silicate glasses imparts a small but significant (∼3%) matrix bias for U-series analysis of iron oxides, and highlights the need for matrix matched materials to obtain accurate U-series chronologies by laser-ablation MC-ICP-MS.

238U, 232Th profiling and U-series isotope analysis of fossil teeth by laser ablation-ICPMS

U and Th concentration profiles in fossil hominid and faunal teeth have been measured by laser ablation ICPMS. These profiles record diverse modes of U and Th uptake, particularly within enamel, that can be broadly related to the state of sample preservation. Observed U profiles are in general inconsistent with existing diffusion–adsorption models developed for U-uptake in bone and teeth. Where the models appear applicable, calculated diffusion rates are several orders of magnitude smaller than previous estimates. Laser ablation ICPMS offers a means of rapidly characterizing U and Th distributions in the enamel and dentine components of teeth as a precursor to ESR and U-series dating. In particular, it should allow the identification of teeth (and also bone) samples that have simple U-uptake histories and are amenable to precise dating by time-consuming and expensive Th–U and Pa–U TIMS techniques. We also demonstrated the use of laser ablation ICPMS to measure U-series isotopes in dentine and enamel samples with relatively high U concentrations (>20 ppm). These results, obtained using a quadrupole ICPMS, illustrate significant promise for in situ U-series isotope analysis, particularly when combined with the greater sensitivity and multi-collection capabilities of new sector ICPMS instrumentation. The latter may permit precise isotope ratio measurements on samples containing only a few ppm of U.

An open-system model for U-series age determinations of fossil corals

The source of excess 234U in fossil corals and its relationship to U-series age determinations has been an outstanding problem in geochronology for more than 20 years. With increasing numbers of U-series isotope measurements in corals, and significant improvements in analytical precision through mass spectrometry, it is increasingly apparent that a substantial fraction of observed isotope ratios cannot be reasonably explained by closed-system decay. Moreover, observations of a positive correlation between 234U/238U and 230Th/238U ratios in corals from the same terrace are difficult to explain. However, the decay of dissolved uranium and α-recoil mobilization of uranium daughters produce particle-reactive 234Th and 230Th, and the coupled addition of these Th isotopes could simultaneously increase coral 234U/238U and 230Th/238U. Here we present a quantitative model, based on decay-dependent redistribution of 234Th and 230Th, permitting calculation of open-system coral ages. These equations provide a general solution to the α-recoil redistribution problem, applicable to any alpha decay series. While measured isotope ratios of corals from the three youngest stratigraphically defined Barbados terraces are inconsistent with closed-system decay, they fall in broadly linear arrays agreeing with model predictions. Isotopic arrays of older Barbados corals, and corals from terraces around the world, are also consistent with model predictions suggesting the open-system model is generally applicable. Corals with extreme isotopic compositions that are impossible to produce by closed-system decay are consistent with the limited range of isotopic compositions predicted by the model at ages older than 600 ka. For corals from a single terrace, 234Th and 230Th redistribution appears to be a source of systematic conventional age error, even for corals with slightly elevated 234U. However, open-system ages are consistent, even for corals with extremely elevated 234U. For the youngest three Barbados terraces, mean open-system terrace ages are consistent with mean conventional terrace ages calculated from pristine samples. If the most accurate conventional ages are from corals with an initial 234U/238U identical to modern seawater, then the open-system model will improve the accuracy of coral U-series age determinations and dramatically increase the number of reliable ages.

Dating open systems by the 238U–234U–230Th method: application to Quaternary reef terraces

The sedimentation conditions and diagenetic processes of biogenic and inorganic carbonates are such that the assumptions of a closed system evolution and the absence of initial Th invoked in classical 238U–234U–230Th dating methods are very often not fulfilled for such material. This problem is of interest for all studies requiring accurate dating of corals or aragonite-rich sediments as in palaeoclimatology or neotectonics. Extending the approach of Henderson and Slowey [Nature 404 (2000) 61–66], we propose a consistent model that takes into account possible initial 230Th excess and where continuous selective redistribution (gain or loss) of 234U, 234Th and 230Th is controlled by recoil processes. This model applied to Quaternary marine terraces in Barbados, Marie Galante (Lesser Antilles arc) and Pacific islands, is able to explain the large scatter in isotopic compositions observed in the series of cogenetic corals or sediments. We show that recoil processes generally tend to increase the (234U/238U) and (230Th/234U) ratios with time, leading to an overestimation of the apparent ages calculated using classical methods. On the basis of simple assumptions (identical ‘marine’ initial (234U/238U)0 and (230Th/232Th)0 ratios), we propose a method for calculating a best estimate of the age of cogenetic series. Our model offers an alternative way to date old reef terraces where corals are almost systematically altered and where classical dating methods do not apply. It may also explain the apparent variations of the (234U/238U) seawater initial ratio estimated from corals formed in the last 300 ka and the apparent uncoupling between sea-level highstands and high-latitude Northern Hemisphere summer insolation. Our set of equations can be applied to all weathering processes where the mobility of U-series isotopes is controlled by recoil.

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 …

Introduction to U-series Geochemistry

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230Th-238U disequilibrium in East Scotia backarc basalts: Implications for slab contributions

Research Article| August 01, 2003 230Th-238U disequilibrium in East Scotia backarc basalts: Implications for slab contributions S. Fretzdorff; S. Fretzdorff 1Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany Search for other works by this author on: GSW Google Scholar K.M. Haase; K.M. Haase 1Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany Search for other works by this author on: GSW Google Scholar P.T. Leat; P.T. Leat 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar R.A. Livermore; R.A. Livermore 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar C.-D. Garbe-Schönberg; C.-D. Garbe-Schönberg 3Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany Search for other works by this author on: GSW Google Scholar J. Fietzke; J. Fietzke 3Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany Search for other works by this author on: GSW Google Scholar P. Stoffers P. Stoffers 3Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information S. Fretzdorff 1Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany K.M. Haase 1Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany P.T. Leat 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK R.A. Livermore 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK C.-D. Garbe-Schönberg 3Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany J. Fietzke 3Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany P. Stoffers 3Institut für Geowissenschaften, Universität Kiel, Olshausenstrasse 40, 24089 Kiel, Germany Publisher: Geological Society of America Received: 03 Jan 2003 Revision Received: 25 Apr 2003 Accepted: 28 Apr 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2003) 31 (8): 693–696. https://doi.org/10.1130/G19469.1 Article history Received: 03 Jan 2003 Revision Received: 25 Apr 2003 Accepted: 28 Apr 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S. Fretzdorff, K.M. Haase, P.T. Leat, R.A. Livermore, C.-D. Garbe-Schönberg, J. Fietzke, P. Stoffers; 230Th-238U disequilibrium in East Scotia backarc basalts: Implications for slab contributions. Geology 2003;; 31 (8): 693–696. doi: https://doi.org/10.1130/G19469.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract New U-series isotope data for lavas from the East Scotia backarc spreading center span a large range in (230Th/232Th) and (238U/232Th). Most of the backarc lavas have (238U/230Th) < 1, similar to the composition of mid-oceanic-ridge basalts (MORB). Lavas from two segments have (238U/230Th) > 1 and are enriched in fluid-mobile elements, implying a recent addition of a U-rich slab-derived component. The data from one segment suggest an influence from an aqueous fluid from altered MORB, whereas samples from the other slab-influenced segment show addition of a sediment melt. The slab-influenced samples form a distinct trend in the equiline diagram between aqueous fluid and sediment melt that is suggested to be a mixing line rather than an isochron. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Consequences of diffuse and channelled porous melt migration on uranium series disequilibria

Magmas erupted at mid-ocean ridges (MORB) result from decompression melting of upwelling mantle. However, the mechanism of melt transport from the source region to the surface is poorly understood. It is debated whether melt is transported through melt-filled conduits or cracks on short time scales (<∼ 10 3 yrs), or whether there is a significant component of slow, equilibrium porous flow on much longer time scales (>∼ 10 3–10 4 yrs). Radiogenic excess 226Ra in MORB indicates that melt is transported from the melting region on time scales less than the half life of 226Ra (∼1600 yrs), and has been used to argue for fast melt transport from the base of the melting column. However, excess 226Ra can be generated at the bottom of the melt column, during the onset of melting, and at the top of the melt column by reactive porous flow. Determining the depth at which 226Ra is generated is critical to interpreting the rate and mechanism of magma migration. A recent compilation of high quality U-series isotope data show that in many young basalts, 226Ra excess in MORB is negatively correlated with 230Th excess. The data suggest that 226Ra excess is generated independently of 230Th excess, and cannot be explained by “dynamic” or fractional melting, where observed radiogenic excesses are all generated at the base of the melt column. One explanation is that the negative correlation of activity ratios is a result of mixing of slow moving melt that has travelled through reactive, low-porosity pathways and relatively fast moving melt that has been transported in unreactive high-porosity channels. We investigate this possibility by calculating U-series disequilibria in a melting column in which high-porosity, unreactive channels form within a low-porosity matrix that is undergoing melting. The results show that the negative correlation of 226Ra and 230Th excesses observed in MORB can be produced if ∼60% of the total melt flux travels through the low-porosity matrix. This melt maintains 226Ra excesses via chromatographic fractionation of Ra and Th during equilibrium transport. Melt that travels through the unreactive, high-porosity channels is not able to maintain significant 226Ra excesses because Ra and Th are not fractionated from each other during transport and the transport time for melt in the channels to reach the top of the melt column is longer than the time scale for 226Ra excesses to decay. Mixing of melt from the high porosity channels with melt from the low-porosity matrix at the top of the melting column can produce a negative correlation of 226Ra and 230Th excesses with the slope and magnitude observed in MORB. This transport process can also account for other aspects of the geochemistry of MORB, such as correlations between La/Yb, α Sm/Nd, and Th/U and 226Ra and 230Th excess.