238U Disequilibria Research Articles

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.

234Th: 238U disequilibrium as an indicator of sediment resuspension in Thermaikos Gulf, northwestern Aegean Sea

234Th: 238U disequilibria have been used extensively as tracers of particle dynamics in marine environments. 234Th ( t 1/2=24.1 days) can be used as “proxy” for particle reactive pollutants, due to their similar rapid rate of scavenging onto particles and subsequent removal from the water column, to the sediments. Radioactive disequilibrium can be exploited to determine the rates and time-scales of processes occurring over days to months; in this instance the residence times of dissolved and particulate species with the benthic nepheloid layer (BNL). Three sampling cruises were undertaken in Thermaikos Gulf (NW Aegean Sea) during contrasting periods, to examine the impact of natural and anthropogenic activity on sediment resuspension. September and October 2001 represented background and trawling periods, respectively; January 2002 represented a mixed period, of trawling and storms. Dissolved 234Th is scavenged actively at the BNL, in the presence of suspended particulate material (SPM), with a mean residence time of 16 days. There is a weak inverse correlation between dissolved 234Th residence time and SPM concentration in the BNL, with the shortest residence times occurring during October 2001. No relationship was observed between particulate 234Th activities and SPM concentrations, indicating that particles are rapidly removed from the system, either by sinking or advection. The mean particulate 234Th residence time is 5 days.

Vertical flux of particulate Al, Fe, Pb, and Ba from the upper ocean estimated from 234Th/ 238U disequilibria

The vertical sinking flux of particulate Al, Fe, Pb, and Ba from the upper 250 m of the Labrador Sea has been estimated from measurements of 234Th/ 238U disequilibrium and the respective metal/ 234Th ratios in >53 μm size particles. 234Th-derived particulate metal fluxes include in situ scavenged metals, labile lithogenic metals, and metals derived from external input (e.g., atmospheric supply). In contrast to the POC/ 234Th ratio, particle size-fractionated (0.4–10 μm, 10–53 μm, and >53 μm) Al/ 234Th, Fe/ 234Th and Pb/ 234Th, and Ba/ 234Th ratios generally increase with depth and exhibit no systematic change with particle diameter. Sinking fluxes of particulate Al (2.47–22.3 μmol m −2 d −1), Fe (2.69–16.3 μmol m −2 d −1), Pb (2.85–70 nmol m −2 d −1), and Ba (0.13–2.1 μmol m −2 d −1) at 50 m (base of the euphotic zone) and 100 m (base of the mixed layer) are largely within the range of previous sediment trap results from other ocean basins. Estimates of the upper ocean residence time of Al (0.07–0.28 yr) and Pb (0.8–2.9 yr) are short compared to previously reported values. The settling rate of >53 μm particles calculated from the 234Th data ranges from 14 to 38 m d −1.

Mobility of U-series nuclides during basalt weathering: An example from the Deccan Traps (India)

U-series disequilibria have been measured by TIMS and MC-ICP-MS in dissolved phases and suspended sediments of the main basaltic rivers from the Deccan Trap region (India). For dissolved phases ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range between 1.11 and 1.28, between 0.03×10 −2 and 1.35×10 −2, and between 0.02 and 0.16 respectively. For suspended particles, ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range from 0.95 to 1.1, from 1.23 to 1.59 and from 0.1 to 0.47 respectively. 230Th– 238U and 234U– 238U systems show direct links with indices of weathering. The data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady-state because present-day physical erosion rates are higher than expected (100–400 mm/ka versus 40 mm/ka for a steady-state process) for the Narmada and the Tapti basins. Models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40–90 ka. In contrast with basins located in Northern latitudes, erosion in the Deccan basins does not appear to have been significantly disturbed during the last glaciation. 234U– 238U disequilibria in the dissolved phase of large rivers world-wide are also explored and two key factors are highlighted in this study: the dissolution of carbonates, releasing dissolved U at secular equilibrium and, the silicate weathering rate estimated at the scale of the watershed. In particular, a positive correlation is found between dissolved ( 234U/ 238U) and basaltic weathering rates for the Deccan that could reflect a key role of physical erosion since, for silicate monolithological basins, chemical erosion rates are primarily controlled by physical erosion rates. For basins with mixed lithology, dissolved organic matter also plays a key role in the preferential release of 234U from silicate minerals. ( 234U/ 238U) ratios in rivers therefore reflect a mixture between U released by carbonate dissolution and 234U preferentially released from silicate minerals located in high physical erosion and peat areas.

Export fluxes of particulate organic carbon estimated from 234Th/ 238U disequilibrium during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS 2001)

The Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS) was carried out in the western subarctic gyre of the North Pacific to verify the iron-limitation hypothesis. Activities of dissolved 234Th in the upper 200 m of the water column and of particulate 234Th collected in a drifting sediment trap were measured during SEEDS 2001. Dissolved 234Th activities decreased and suspended particulate 234Th increased in the upper 20 m of the water column with the formation of an iron-induced phytoplankton bloom. The calculated residence times of the dissolved and particulate 234Th in the upper 20 m of the water column ranged from 2 to 13 d and 13 to 29 d, respectively. The ratios of particulate organic carbon (POC)/ 234Th p (μmol dpm −1) in the settling particles at 200 m depth varied between 1.94 and 4.02, and these levels were 10% those for the suspended particles. No large increase of the POC export flux was observed for the drifting sediment trap deployments during the observation period. In addition, the POC export fluxes derived from 234Th with a non-steady-state model varied from 10.1 to 38.3 mmol m −2 d −1. The ratio of the estimated POC export flux to primary production, Th E, was nearly constant (0.23–0.24) from July 20 to 27 (D2 and D9 after iron-enrichment) and decreased to 0.18 on July 31 (D13 after iron-enrichment). Regardless of whether the POC export fluxes increased, there was no change in Th E. The increase of the POC export fluxes estimated from 234Th was much smaller than the increase of primary production. Thus, no large POC export fluxes are concluded to have occurred from the iron-enrichment patch, though the removal flux of 234Th in settling particles increased with particulate biogenic silica fluxes as the phytoplankton bloom progressed.

U-Th-Pb and 230Th/ 238U disequilibrium isotope systematics: Precise accessory mineral chronology and melt evolution tracing in the Alpine Bergell intrusion

Abstract In order to investigate the potential of combined Th-U-Pb isotope and 230Th/ 238U disequilibrium systematics for tracing magmatic crystallization and melt evolution, conventional high-resolution single-crystal TIMS techniques have been applied to zircon, titanite and fragments of geochemically characterized growth zones of allanite. These minerals were extracted from a single tonalite specimen collected from the feeder zone of the Tertiary Bergell pluton (Southern Steep Belt, S Switzerland/N Italy). The isotopic results document an extended history of crystallization and melt evolution of at least 5 Ma, with well-resolved zircon ages defining an early interval of 33.0 to 32.0 Ma, followed by crystallization of zoned allanite from 32.0 to 28.0 Ma and formation of magmatic epidote possibly as late as 26 Ma. Trace and major element patterns in zoned allanite closely mirror melt evolution, characterized by increase of U concentration and sharp decrease of Th and LREE, reflecting early crystallization of phases low in U and, in particular, the dominating control by allanite precipitation. Preservation of substantial quantities of excess 206Pb derived from initial excess 230Th in all analyzed allanite grains indicates that their isotopic systems have not been reset by loss of radiogenic Pb during prolonged residence at magmatic conditions and regional-metamorphic cooling, and that the measured sequence of 208Pb/ 232Th dates translates into a real age sequence. Major loss of radiogenic Pb from compositionally zoned allanite by volume diffusion would have resulted in a data pattern grossly different from the observed one, as demonstrated by numerical modeling of 232Th- 208Pb- 238U- 230Th- 206Pb isotopic evolution. The results therefore suggest closure temperatures ≥700°C for magmatic allanite. Quantification of 230Th/ 238U disequilibrium relationships reveals a smooth, initially steep decrease of Th/U in the magma from values of 2.9 at 32.0 Ma to < 0.1 at 28.0 Ma in equilibrium with sequential allanite zoning. Comparison of calculated (Th/U) magma with measured total-rock Th/U = 0.79 requires fractional crystallization of allanite at an early stage. This process of removal of allanite, and thus Th, from the melt was operative at least until ∼31.5 Ma, providing an upper limit for the time of emplacement of the studied magma batch. The fact that bulk zircon and a large part of allanite crystallization predate emplacement cautions against equating ages determined on refractory minerals from deep-seated plutons with intrusion ages. The persistence of magmatic conditions over a period of ∼5 Ma, concentrated in a narrow collision zone between the European and the African plate, has been a major controlling factor for the Oligocene evolution of the adjacent Insubric fault, the main tectonic lineament of the Alps being active from late Cretaceous to Miocene times.

Uranium colloidal transport and origin of the 234U– 238U fractionation in surface waters: new insights from Mount Cameroon

In order to evaluate the spatial variability of the 234U– 238U fractionation and its origin in surface waters, ( 234U– 238U) activity ratios (a.r.) and trace element concentrations were measured in 34 streams and springs draining Mount Cameroon volcano. In addition, tangential ultrafiltration techniques were applied to 10 of the samples to assess the role of colloids on the transport of the uranium isotopes. U concentration variations in the different ultrafiltered fractions indicate that inorganic colloids can significantly influence the transport of this chemical element, even in diluted and organic-poor waters. Such influence, which may be related to the chemical and physical nature of colloids, does induce fractionations between U and Sr along with filter size. ( 234U– 238U) a.r. variations among the different ultrafiltered fractions of a sample remain small or even negligible. Only two samples exhibit a significant isotopic difference between colloid-associated and solute U. Colloids of these two samples seem to correspond to a phase which is distinct from the dissolved and particulate loads of the waters. At the scale of the volcano, the ( 234U– 238U) a.r. in surface waters is large, ranging from 1.038 to 1.397. Small 234U– 238U fractionations, between 1.038 and about 1.10, are attributed to surface and meteoric weathering of the bedrock. By contrast, the highest 234U– 238U fractionations (up to 1.397) are associated with groundwater contributions. Thus, variations in the ( 234U– 238U) a.r. could correspond to the water source and not to the surface weathering processes. Consequently, 234U– 238U disequilibrium may represent a powerful tool for detecting and quantifying the influence of groundwater inputs on the chemical fluxes carried by surface waters.

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A 2-year time series sampling with moored sediment traps deployed in Saanich Inlet, British Columbia, provides a monthly record of particulate organic carbon (POC) flux at three depths (50 m, 115 m, 180 m), which is compared to concurrent POC flux estimates obtained from 234Th:238U disequilibrium. The results obtained by both methods are in general agreement, supporting the validity of both approaches for measuring POC flux in these coastal waters. Closer comparison indicates that the 234Th-derived fluxes at 50 m are lower than measured by sediment traps. This difference is attributed to particle focusing from the narrowing of the fjord with depth, which leads to overestimating fluxes with sediment traps while underestimating these same fluxes with 234Th. Higher fluxes measured at greater depths and better agreement between the two methods are consistent with lateral input of sediment winnowed from the sill by tidal currents.The very high scavenging intensity at our study site results in large 234Th deficits and low dissolved 234Th activities. This limits lateral 234Th transport and the system largely maintains steady state. Our results indicate that the sinking flux of POC could be reasonably estimated by simply measuring 234Th and organic carbon in suspended particles at our study site. Such a relatively simple method may be applicable to other coastal settings with similar high scavenging intensity and limited exchange with the open ocean and would facilitate the implementation of sampling programs to monitor carbon cycling or contaminants dispersion in this type of environment.

Measuring low concentrations of 234Th in water and sediment

234Th/ 238U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle-reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of 234Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of 234Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a 229Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low 234Th/ 228Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure 234Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity.

Precise and accurate measurement of 226Ra– 230Th– 238U disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry

We describe analytical techniques for the measurement of 226Ra– 230Th– 238U disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry (MC-ICP-MS). Our methods build upon previous MC-ICP-MS techniques for 232Th/ 230Th and 238U/ 234U measurements with the addition of new chemical and/or mass spectrometric procedures for the determination of Th, U and 226Ra abundances. Using sample sizes that are similar to the requirements of state-of-the-art thermal ionization mass spectrometry (TIMS), we are able to measure Th isotope ratios and 226Ra– 230Th– 238U disequilibria with a precision that is at least ∼2–3× better than TIMS and U isotope ratios with a precision that is comparable to TIMS. Replicate analyses of a 226Ra– 230Th– 234U– 238U radioactive equilibrium rock standard (Table Mountain Latite) demonstrate that our MC-ICP-MS techniques are also highly accurate. An additional bonus is that analyses of Ra, Th and U isotope ratios by MC-ICP-MS are much less time-consuming than by TIMS. These analytical improvements will increase the usefulness of U-series isotopes as high-resolution tracers of the nature and timing of magmatic processes at active volcanoes.

Strong seasonality in particle dynamics of north-western Mediterranean surface waters as revealed by 234Th/ 238U

234Th was used to quantify sinking fluxes and residence times of particles in surface waters of the north-western Mediterranean Sea. Measurements of dissolved and particulate 234Th were made at the DYFAMED station (43°25′N–7°51′E, JGOFS-France program). Sampling covered 1 year on four cruises in 1994 (February 9, April 29, June 3, October 1) and focused on a transition period in mid-spring with six repeated profiles collected during May 1995. 234Th was nearly in equilibrium with its parent 238U most of the year, except in spring. The intensive sampling in May shows a rapid evolution throughout the month from a moderate 234Th deficit to near-equilibrium values. The time-series of 234Th were treated with steady-state and non-steady-state models. 234Th particulate fluxes clearly indicate large variability in export, with the highest values observed in spring. Particle residence times in the upper 40 m range from <10 to >250 days, and could increase by a factor of 10 within 2 weeks. POC fluxes from the upper 40 m and export ratios (ThE: ratio of 234Th-derived POC export to primary production), derived from the 234Th/ 238U disequilibrium in the water column and POC/ 234Th ratio on trapped material, decrease from about 9.5 mmol C m −2 d −1 and >22% in early May to less than 5 mmol C m −2 d −1 and 15% after mid-May. The 234Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity.

Constraints on melt movement beneath the East Pacific Rise from 230Th-238U disequilibrium.

We report 230Th-238U disequilibrium data on mid-ocean ridge basalts recovered 5 to 40 kilometers off the ridge axis near 9 degrees 30'N of the East Pacific Rise. These data indicate near-symmetrical eruptions of normal mid-ocean ridge basalts (NMORBs) and incompatible element-enriched mid-ocean ridge basalts (EMORBs) as far as 20 kilometers off axis. Our results suggest large-scale subsurface lateral transport of NMORB melt at 19 to 21 centimeters per year and also provide constraints on the petrogenesis of EMORBs of off-axis origin.

226Ra/ 238U disequilibrium in an upland organic soil exhibiting elevated natural radioactivity

This paper presents the results of a study into the anomalous 226Ra/ 238U disequilibrium ( 226Ra/ 238U of 0.5–9) exhibited by an upland organic soil in Co. Donegal, Ireland. Radiochemical speciation of 226Ra, 238U and 228Ra indicates that in this organic soil the high 226Ra/ 238U ratio is due to loss of 238U relative to 226Ra via oxidation and mobilisation of 238U in the upper layers of the soil and subsequent loss in solution. At the lower, more reducing depths of the soil profile, 238U and 226Ra are essentially in equilibrium. Loss of 238U appears to occur primarily from the easily oxidised organic and iron oxide fractions of the soil, samples exhibiting high 226Ra/ 238U ratios displaying significantly lower 238U levels in these fractions than samples whose ratio is below the average value for the soil of the valley. Selective enrichment of 226Ra by plants or preferential leaching of 226Ra from the underlying rock is not supported by the results of this study.

New production based on 228Ra-derived nutrient budgets and thorium-estimated POC export at the intercalibration station in the South China Sea

228Ra, 228Th and 234Th in the upper 300 m of the water column were measured to quantify new production and export flux of POC at the intercalibration station in the South China Sea during November 1997. Surprisingly high 228Ra concentrations of about 210 dpm m −3 in the surface mixed layer were observed. The concentration decreased considerably, from >200 dpm m −3 at 25 m to ∼43 dpm m −3 at 200 m, then increased to 87 dpm m −3 at 300 m close to the bottom (460 m), coincident with the typical distribution pattern of 228Ra in open oceans. Concentrations of dissolved and particulate 228Th ranged from ∼45 to ∼27 dpm m −3 and from ∼40 to ∼10 dpm m −3, respectively, and showed a general decrease with increasing depth. A 234Th deficit relative to 238U was also observed in the upper 150 m, as quantified by β-counting. The upward flux of nitrate into the euphotic zone was calculated by the coupled 228Ra-nitrate approach and further converted into a new production of 4.4 mmol C m −2 d −1 based on a Redfield ratio of 6.6 for C : N. The 234Th– 238U disequilibrium and the measured ratio of POC to particulate 234Th yielded a POC export flux of 5.7 mmol C m −2 d −1, which is in general agreement with the new production derived from nutrient budgets. POC export based on the 228Th– 228Ra disequilibrium was estimated to be 1.7 mmol C m −2 d −1, significantly lower than the derived new production. DOC transport or accumulation, data uncertainties and the different time scales to which the various methods were applied could be reasons for the discrepancy. The vertical fluxes of nitrate and phosphate to the euphotic zone bear a N/P molar ratio of 35, which is significantly higher than the Redfield ratio and suggests P-limitation in the southern South China Sea.

Pitfalls in 230Th– 238U dating of young Quaternary volcanic rocks: : a case study from Fornicher Kopf (East Eifel Volcanic Field, Germany)

The eruption age of the Fornicher Kopf basanite lava flow (East Eifel Volcanic Field, Germany) is of interest because it can provide an age constraint for part of the Quaternary Rhine river terrace chronology. Previous dating efforts by K–Ar and α-counting 230Th– 238U methods produced a wide dispersion of ages. This paper presents a fresh attempt to date this lava flow by the potentially more precise mass spectrometric 230Th– 238U mineral isochron technique. Unfortunately, this study also proved unsuccessful because it did not produce a meaningful mineral isochron: the only age information is provided by the fact that the groundmass shows 230Th– 238U disequilibrium, which implies that the eruption age was <350 ka. However, the mineral data clearly highlight some of the problems that can plague efforts to date young lavas by the 230Th– 238U method: (i) inherited older crystals; magnetite and pyroxene plot on the equiline (i.e. 230Th and 238U in secular equilibrium) and tielines to the groundmass data produce impossible ages, indicating that these crystals are older than the host magma, and (ii) open system behaviour; phlogopite has experienced variable, recent U exchange, which has implications for the reliability of this phase for alpha-recoil dating.

A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on 234Th : 238U disequilibrium

Depth profiles of total 234Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous 234Th studies. Significant zones of particle export ( 234Th deficiency) and particle remineralization ( 234Th excess) were measured both temporally and with depth. 234Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/ 234Th and PN/ 234Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. 234Th based export estimates of 4.0±2.3 mmol C m −2 d −1 and 0.53±0.19 mmol N m −2 d −1, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m −2 d −1 and 0.32±0.08 mmol N m −2 d −1. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for 234Th by a factor of 2 to 4. 234Th export ( ThE) ratios based on 234Th derived PC flux/ 14C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.

238U– 230Th constraints on mantle upwelling and plume–ridge interaction along the Reykjanes Ridge

238U– 230Th– 226Ra disequilibria data are presented for dredged glasses from the slow-spreading, plume-influenced Reykjanes Ridge, and lavas from the Reykjanes Peninsula, Iceland. Southern Reykjanes Ridge samples (57.7°–58.8°N), identified as ‘zero-age’ (i.e. <8 ka) from measured 226Ra– 230Th disequilibrium, lie on a shallow trend on the ( 230Th/ 232Th) vs. ( 238U/ 232Th) equiline diagram. Within this 150 km ridge section, samples north of the weak segment boundary at 58.42°N have lower U/Th, higher Th contents and higher ( 230Th/ 238U) than those to the south. This local compositional control on the degree of 230Th– 238U disequilibrium is inferred to result from variable mixing between melts from ‘enriched’ and ‘depleted’ components in the upwelling mantle. This is supported by a northward increase in 206Pb/ 204Pb and He isotope ratios within this region. A sample from the northern, shallow part of the Reykjanes Ridge (61.93°N) also lies within this trend, with relatively high U/Th. In contrast, post-glacial lavas on the Reykjanes Peninsula show broadly similar U/Th to the Reykjanes Ridge samples, but at significantly lower ( 230Th/ 232Th) (∼1.15 vs. ∼1.30). This distinction is most likely due to a difference in mantle upwelling rate – a high upwelling rate associated with the mantle plume beneath Iceland will give rise to lower 230Th– 238U disequilibrium for a particular U/Th value due to the reduced opportunity for 230Th in-growth in the melt column – although differences in initial melt productivity might also play a role. Active upwelling associated with the Iceland plume probably does not extend far along the Reykjanes Ridge.

226Ra–230Th–238U disequilibria of historical Kilauea lavas (1790–1982) and the dynamics of mantle melting within the Hawaiian plume

The geochemical variations of Kilauea’s historical summit lavas (1790–1982) document a rapid fluctuation in the mantle source and melting history of this volcano. These lavas span nearly the entire known range of source composition for Kilauea in only 200 yr and record a factor of ∼2 change in the degree of partial melting. In this study, we use high-precision measurements of the U-series isotope abundances of Kilauea’s historical summit lavas and two ‘ingrowth’ models (dynamic and equilibrium percolation melting) to focus on the process of melt generation at this volcano. Our results show that the 226Ra–230Th–238U disequilibria of these lavas have remained relatively small and constant with ∼12±4% excess 226Ra and ∼2.5±1.6% excess 230Th (both are ±2σ). Model calculations based mostly on subtle variations in the 230Th–238U disequilibria suggest that lavas from the 19th to early 20th centuries formed at significantly higher rates of mantle melting and upwelling (up to a factor of ∼10) compared to lavas from 1790 and the late 20th century. The shift to higher values for these parameters correlates with a short-term decrease in the size of the melting region sampled by the volcano, which is consistent with fluid dynamical models that predict an exponential increase in the upwelling rate (and, thus, the melting rate) towards the core of the Hawaiian plume. The Pb, Sr, and Nd isotope ratios of lavas derived from the smallest source volumes correspond to the ‘Kilauea’ end member of Hawaiian volcanoes, whereas lavas derived from the largest source volumes overlap isotopically with recent Loihi tholeiitic basalts. This behavior probably arises from the more effective blending of small-scale source heterogeneities as the melting region sampled by Kilauea increases in size. The source that was preferentially tapped during the early 20th century (when the melt fractions were lowest) is more chemically and isotopically depleted than the source of the early 19th and late 20th century lavas (which formed by the highest melt fractions). This inverse relationship between the magnitude of source depletion and melt fraction suggests that source fertility (i.e. lithology) controls the degree of partial melting at Kilauea. Thus, rapid changes in the size of the melting region sampled by the volcano (in the presence of these small-scale heterogeneities) may regulate most of the source- and melting-related geochemical variations observed at Kilauea over time scales of decades to centuries.

Multi-proxy constraints on the climatic significance of trace element records from a New Zealand speleothem

Trace element concentrations and uranium isotope ratios are reported for a speleothem from the South Island of New Zealand. TIMS uranium-series dating of the speleothem indicates it to have grown from 31 000 years ago to the present, providing a continuous record of the last deglaciation. Trace element abundances measured in this speleothem using ICP-MS are found to exhibit strong temporal relationships with δ 13C and other measured variables, and with the environmental changes inferred from them. Strontium and barium concentrations are positively correlated with inferred changes in the productivity and extent of vegetation cover above the cave. Magnesium concentration appears to have responded to changes in groundwater residence time, assumed to have an inverse relationship with effective meteoric precipitation above the cave. The degree of 234U/ 238U disequilibrium also appears to have varied in response to hydrological changes, and together with the magnesium data implies a post-glacial increase in regional effective precipitation to have culminated ca. 13 000 cal yr B.P., some 2000 years after a dramatic post-glacial increase in forest extent previously inferred for the region. This increase in forest extent is thus unlikely to have been caused by a significant increase in precipitation, and is assumed to have been driven predominantly by a rapid increase in regional temperature, centred on 15 000 calendar years before present.

Melt generation beneath ocean islands: a U-Th-Ra isotope study from Lanzarote in the Canary Islands

New major and trace element data, and Sr, Nd, Pb, U, Th and Ra isotopes are presented for prehistoric and historic lavas from Lanzarote in the Canary Islands. These rocks are amongst the most primitive found on intraplate ocean islands ranging in composition from basanite to alkali basalt, with MgO contents >9.3% and Mg numbers >67. The youngest are from two of the three 1824 vents, the largest group of samples is from the best known eruption episode, the 1730–36 Timanfaya eruptions, a smaller group of samples are from the northeast Corona region (∼50 ky) and the oldest samples are from the Famara complex and basement massif. The rocks have some of the characteristics of HIMU OIB, including high Ce/Pb, Nb/Ce and low Nb/U and restricted 87Sr/ 86Sr (0.70209–0.70332). There is significant 230Th/ 238U disequilibrium ( 230Th excesses range from 6–76%) with some of the intermediate silica composition Corona samples showing the greatest disequilibrium. The historic samples exhibit 226Ra excess. The major and trace element data have undergone fractionation corrections to Mg numbers of 70, requiring <5% olivine fractionation, and these inferred primary compositions are used to evaluate a number of melt generation and mixing models. The fractionation-corrected compositions for the 1824 and the 1730–36 have been modelled as 1–4% melts of a source similar to primitive mantle. However, Yb is incompatible, and so the amounts of residual garnet were < ∼5%, suggesting that there was no significant contribution from garnet pyroxenite source rocks. Rather the REE and the FeO contents are both consistent with melting in the garnet-spinel transition, at depths of 60–70 km. ( 230Th/ 238U) increases slightly with increasing La/Yb in the younger rocks, and they require some form of dynamic melting model. In the preferred model the upwelling rate is kept constant, and the differences in the degrees of melting are attributed to different lengths of the melt column, with the smaller degree melts being extracted from greater depths. Strikingly, ( 226Ra/ 230Th) increases with increasing degrees of melting, and so it reflects the time since extraction from the melt column rather than variations in the melting processes. Intra-suite minor and trace element variations are due to magma mixing, and not to progressive changes in the degrees of partial melting, and it is envisaged that such magma mixing occurred during the dynamic melting processes. Dynamic melting at depths of 60–70 km suggests that the regional uplift around the Canary Islands is at least in part due to thermal erosion of the underlying lithosphere. Variations of average Ce/Yb, Tb/Yb, ( 230Th/ 238U), SiO 2 and lithospheric age for different OIB highlight how the smaller degree melts tend to be generated at greater depths, and the mean pressure of melting increases with the thickness of the lithospheric lid. However, there is no general link between ( 230Th/ 238U) and the lithospheric age or thickness, and hence the integrated degrees of melting. High buoyancy fluxes result in higher degrees of melting and low ( 230Th/ 238U) (Chabaux and Allegre, 1994), but for OIB generated within low buoyancy plumes, ( 230Th/ 238U) and the degrees of melting primarily depend on the depths of melt extraction. Differences in the average composition of low buoyancy OIB depend on the thickness of the overlying lithosphere (Ellam, 1992; Haase, 1996), and the differences within an OIB suite, such as between the 1730-36 and 1824 lavas on Lanzarote, depend on the depth of extraction from the melt column.