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International audience; To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate = 1.8 cm/yr; 67 degrees 05'-70 degrees 26'N) display ((230)Th/(238)U) < 1 and ((230)Th/(238)U) > 1 with ((230)Th/(238)U) from 0.95 to 1.30 and have low U (11.3-65.6 ppb) and Th (33.0 ppb-2.40 ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e. g. (87)Sr/(86)Sr = 0.70272-0.70301, epsilon(Nd) = 8.4-10.5, epsilon(Hf) = 15.4-19.6 (La/Yb)(N) = 0.28-0.84) encompass a narrow range of ((230)Th/(232)Th) (1.20-1.32) over a large range in ((238)U/(232)Th) (0.94-1.32), producing a horizontal array on a ((230)Th/(232)Th) vs. ((238)U/(232)Th) diagram and a large variation in ((230)Th/(238)U). However, the ((230)Th/(238)U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with ((234)U/(238)U) (1.001-1.031). Samples with low ((230)Th/(238)U) and elevated ((234)U/(238)U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium (234)U/(238)U have high ((230)Th/(238)U) values (>= 1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the ((230)Th/(238)U) and ((231)Pa/(235)U) ratios in uncontaminated Kolbeinsey lavas, but low ((231)Pa/(235)U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment

Current models using U- and Th-series disequilibria to study radioisotope transport in groundwater systems mostly consider a steady-state situation. These models have limited applicability to the vadose zone (UZ) where the concentration and migratory behavior of radioisotopes in fluid are often transitory. We present here, as a first attempt of its kind, a model simulating the non-steady state, intermittent fluid transport in vadose layers. It provides quantitative constraints on in-situ migration of dissolved and colloidal radioisotopes in terms of retardation factor and rock–water interaction (or water transit) time. For uranium, the simulation predicts that intermittent flushing in the UZ leads to a linear relationship between reciprocal U concentration and 234U/238U ratio in percolating waters, with the intercept and slope bearing information on the rates of dissolution and α-recoil of U isotopes, respectively. The general validity of the model appears to be borne out by the measurement of uranium isotopes in UZ waters collected at various times over a period during 1995–2006 from a site in the Pena Blanca mining district, Mexico, where the Nopal I uranium deposit is located. Enhanced 234U/238U ratios in vadose-zone waters resulting from lengthened non-flushing time as prescribed by the model provide an interpretative basis for using 234U/238U in cave calcites to reconstruct the regional changes in hydrology and climate. We also provide a theoretical account of the model’s potential applications using radium isotopes.

We present new uranium-series and cosmogenic nuclide age data obtained by sensitive thermal ionization and accelerator mass spectrometric techniques for samples of dusty polar ice from Allan Hills, Antarctica. These data are used to further evaluate the applicability of these nuclides for direct radiometric dating of polar ice. In order to minimize artifacts during sample processing we developed an improved method utilizing ethylenediaminetetraacetic acid (EDTA) to stabilize uranium-series nuclides dissolved in polar ice during ice melting and filtration. Our measurements revealed low uranium-series daughter abundances and large uranium-series ( 234 U/ 238 U, 230 Th/ 234 U, 226 Ra/ 230 Th, 231 Pa/ 235 U) disequilibria in the filtered 10 Be/ 36 Cl ratios are most likely recently inherited from precipitation. The short-lived daughter 226 Ra is the exception and is enhanced in the 238 U– 234 U– 230 Th– 226 Ra disequilibria in the volcanic dust bands also suggests that ice ages are greater than 10 ka, consistent with recoil-based ages. Our relatively young ice ages can be compared with previously published relatively old terrestrial ages of meteorites from this area and suggests that meteorites at Allan Hills have experienced a multi-stage history of ice accumulation and/or ice flow. Our results also suggest that large uranium-series disequilibria can be inherited from precipitation, which may provide a method for dating clear ice samples.

We present new uranium-series plagioclase and glass data for the 1996 eruption of the North Gorda Ridge. The glass data provide a more accurate estimate of ‘zero-age’ disequilibria for use in external isochron dating than was previously available. Furthermore, plagioclase–glass 226Ra–230Th disequilibria delimit the degree of initial fractionation of radium from barium during crystal growth, with effective DRa/DBa ∼0.25–0.5. These data are inconsistent with the common assumption that DRa=DBa but are qualitatively consistent with theoretical model predictions that radium and barium should be fractionated during crystallization, with DRa/DBa∼0.2. In more detail, differences between model predictions and data could be explained by an extreme combination of model and data uncertainties, but more likely suggest suppression of efficient fractionation during rapid crystallization. We also assess the extent to which use of barium as an analog for radium would result in underestimating 226Ra–230Th disequilibria produced during melting. Effects of a lower value of DRa on most melting models are small, with a slight increase in the porosity or melt fraction for a given value of 226Ra/230Th. The new plagioclase data also indicate that plagioclase accumulation and assimilation in the crustal reservoir would have only a negligible effect on mantle-derived 226Ra–230Th disequilibria. External isochron ages from U–Th, U–Pa, and Th–Ra data calculated using initial disequilibria from the 1996 sample are concordant, with one exception. Anomalously young ages for off-axis samples most likely reflect volcanism up to 1 km off-axis. Radium ages for near-axis dredge samples for other areas of the North Gorda Ridge generally range from 2000 to 4000 yr, similar to calculated steady-state eruptive periodicity of 3000 yr for this ridge segment. However, comparison of radium excesses for the 1996 eruption and a nearby older lava suggests that recent volcanism at this site occurs with relatively short-lived (

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (238U-230Th-226Ra and 235U-231Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, 230Th excesses and 226Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226Ra, this inverse correlation between 230Th and 226Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226Ra and 230Th excesses also vary with lava composition: 226Ra excesses are negatively correlated with Na8 and La/Yb and positively correlated with Mg#. Conversely, 230Th excesses are positively correlated with Na8 and La/Yb and negatively correlated with Mg#. Th/U, 230Th/232Th, and 230Th excesses are also variable and correlated to one another. 231Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na8. The isotope ratios 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 208Pb/206Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, 230Th, and 226Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.

We present new uranium-series and radiocarbon measurements for deep-sea corals from the Southern Ocean. These data are used to reconstruct ventilation ages, both at present and at the end of the last glacial period approximately 16 500 years ago. We apply an improved two-component mixing approach to correct uranium-series dates for contaminant thorium and protactinium present in oxide coatings. Calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area. This indicates that deep-sea corals can accurately record seawater radiocarbon distributions. Two of three glacial samples experienced open-system uranium-series systematics, however, a third sample from the Drake Passage yields concordant thorium and protactinium dates as well as seawater values for initial 234U/238U. This coral yields a ventilation age that is approximately 20–40% greater than modern values for its location. This increase is consistent with published deep-sea coral and calibrated planktonic–benthic foraminifera radiocarbon data, suggesting that the glacial oceans as a whole may have been substantially less ventilated, presumably due to decreased formation of North Atlantic Deep Water. An overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pCO2 during the past glacial period.

Previous estimates of crustal storage time of magmas at Kilauea Volcano, Hawaii, range from a few years to a few thousand years, leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation. We present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226Ra–230Th disequilibria, and apply this approach to the early phase of the 1955 east rift eruption at Kilauea. When fractionation of Ra from Ba (a proxy for initial Ra in the crystals) during crystal growth is considered along with the effects of inclusions in the minerals, the data are consistent with co-precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000+300−400 a. Unless a significant fraction (>30%) of the crystals are remnants from an earlier batch of evolved magma in the system, these data constrain the minimum magmatic residence time to be ∼550 yr, considerably longer than most previous estimates of storage time at Kilauea as well as those for some other basaltic systems. For the temperature interval of augite+plagioclase growth in the early 1955 magma, a maximum constant cooling rate of 0.1°C/yr (1×10−5°C/h) is derived from the minimum magmatic residence time of 550 yr. The total magma storage time would be >2500 yr if this cooling rate applied to the entire thermal history of the magma, although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr. The disparate estimates of magma residence at Kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma. More work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and/or of Kilauean lavas in general.

Mass spectrometric measurements of 230 Th– 226 Ra, 235 U– 231 Pa and 238 U– 230 Th disequilibria are used to determine eruption ages for four mid-ocean ridge basalts from the median valley of the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK area). Three samples were collected across-axis on the Axial Volcanic Ridge (i.e. the Neovolcanic Ridge) near the Snake Pit hydrothermal mound, and one sample was collected near the crest of Serocki Volcano ∼50 km south of Snake Pit. ( 226 Ra)/( 230 Th) and ( 230 Th)/( 238 U) activity ratios are low and uniform for all four samples, while ( 231 Pa)/( 235 U) activity ratios are elevated and somewhat more variable. Age constraints suggest that these lavas, from the most robust volcanic edifices in the MARK area, are 10 000–20 000 yr old. The age data are used to evaluate the efficacy of commonly used age estimate scales based on qualitative indicators (e.g. sediment cover, glass quality) and to begin to quantify the temporal and spatial dependence of volcanic, tectonic and hydrothermal processes at slow-spreading oceanic ridges.

Uranium and thorium-decay series disequilibria in groundwater occur as a result of water-rock interactions, and they provide site-specific, natural analog information for assessment of in-situ, long-term migration of radionuclides in the far field of a nuclear waste disposal site. In this study, a mass balance model was used to relate the decay-series radionuclide distributions among solution, sorbed and solid phases in an aquifer system to processes of water transport, sorption-desorption, dissolution-precipitation, radioactive ingrowth-decay, and α recoil. Isotopes of U (238U, 234U), Th (232Th, 230Th, 228Th, 234Th), Ra (226Ra, 228Ra, 224Ra), and Rn (222Rn) were measured in 23 groundwater samples collected from a basaltic aquifer at the Idaho National Engineering and Environmental Laboratory (INEEL), Idaho. The results show that groundwater activities of Th and Ra isotopes are 2–4 orders lower than those of their U progenitors which average 1.35 ± 0.40 dpm 238U/L, with 234U/238U ratios of ∼1.6–3.0. 222Rn activities range from 20 to 500 dpm/L. Modeling of the observed disequilibria places the following constraints on the time scale of radionuclide migration and water-rock interaction at INEEL: (1) Time for sorption is minutes for Ra and Th; time for desorption is days for Ra and years for Th; and time for precipitation is days for Th, years for Ra, and centuries for U. (2) Retardation factors due to sorption average >106 for 232Th, ∼104 for 226Ra, and ∼103 for 238U. (3) Dissolution rates of rocks are ∼70 to 800 mg/L/y. (4) Ages of groundwater range from

Measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (230Th/238U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr−1 for tholeiites and from 1 to 3 cm yr−1 for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr−1 for tholeiites and from 0.1 to 1 cm yr−1 for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.

With precise 234U, 230Th, and 231Pa data available, 230Th and 231Pa ages can now be tested rigorously for concordancy. If the material is not concordant, the isotopic characteristics of this material may be examined in some detail. Here, models similar to those used to describe the U-Pb system are evaluated for use in U-Th-Pa studies, for the case in which initial 230Th and 231Pa concentrations are effectively zero. The systematics of concordia plots in relation to models of variation in δ234U, episodic U loss or gain, continuous U loss or gain, and continuous 234U, 230Th and 231Pa gain or loss are considered for the case in which initial U concentration is significant (for example, in many carbonate deposits). We also examine linear U uptake models for the case in which initial U concentration is effectively zero (for example, in teeth and bones). Such models should prove useful in interpreting data from materials that have behaved as open-systems. In particular, these models may help constrain the nature of diagenetic processes, and in some situations it may be possible to determine or constrain true ages with materials that have behaved as open-systems.

We present here the first survey of (231Pa/235U) ratios in volcanic rocks; such measurements are made possible by new mass spectrometric techniques. The data place new constraints on the timing and extent of magma source and evolutionary processes, particularly due to the sensitivity of the231Pa-235U pair and its intermediate time scale (231Pat1/2 = 33 ky). (231Pa/235U) is found to vary widely, from 0.2 in carbonatites to 1.1–2.9 in basalts and 0.9–2.2 in arcs. Substantial Pa enrichment is nearly ubiquitous, suggestive of the relative incompatibility of Pa, qualitatively consistent with available partitioning data. The level of231Pa-235U disequilibrium typically far exceeds that of230Th-238U and is comparable to226Ra-230Th. The high (231Pa/235U) ratios in MORB and other basalts reflect a large degree of discrimination between two incompatible elements, posing challenges for modelling of melt generation and migration. Fundamental differences in (231Pa/235U) among different basaltic environments are likely related to contrasts in melting zone conditions (e.g., melting rate). Strong (231Pa/235U) disequilibria in continental basalts, for which (230Th/238U) disequilibria are small or absent, demonstrate that Pa-U fractionation is possible in both garnet and spinel mantle stability fields. In arcs, correlation of (231Pa/235U) and (230Th/238U) is consistent with U enrichment via slab-derived fluids, a process which is additional to the still dominant Pa enrichment. An important new constraint is provided by the observation that the near-equilibrium (230Th/238U) common to arcs and continental basalts is not typically accompanied by near-equilibrium (231Pa/235U), arguing against the influence of long magma history, crustal material, or equilibrium mantle sources in affecting decay-series ratios. Small sample sets from two silicic centers illustrate: (1) recent, rapid U enrichment in the magma chamber (El Chichon); and (2) the failure of substantial magma H2O-CO2 degassing to effect U-Th-Pa fractionation (Mono Craters).

Liquid carbonate/ liquid silicate partition coefficients (D) for a suite of incompatible elements (Nb, Mo, Ba, Ce, Pb, Th, U) have been determined experimentally. For one experiment, D's for Ra and Pa were also determined. The partitioning behavior is shown to be a regular function of the ionic field strength (zr) of the partitioned elements. At constant temperature, increasing pressure from 7 to 20 kbar caused little change in the measured D. At constant pressure, increasing temperature causes all D's to approach unity, as the temperature nears the critical temperature and the two liquids become compositionally similar. Addition of P or F to the system enlarges the stability field of carbonate but does not affect the partitioning behavior of our suite of elements. However, both P and F are strongly partitioned into the carbonate liquid. Omphacite/carbonate liquid D's at 50–60 kbar have also been determined. Our results for DU and DTH are similar to those found previously for augite/silicate liquid. Thorium is more compatible than U in pyroxene, indicating that excesses of 230Th/238U found in MORB cannot be explained by fractionation between pyroxene and melt. The Soret effect in carbonate liquids is quite small, in contrast to the effect in silicate liquids. In general, we see no evidence for Soret separation in our experiments, implying that endmember Ca-, Na-, and Mg-carbonate liquids have simple speciations and mix nearly ideally. This observation suggests that the results of our liquid/liquid partitioning studies do not strongly depend on the exact composition of the carbonate. Our experimentally determined liquid-liquid partition coefficients can explain recent fractionations of 210T/238U and 226Ra/238U in carbonatitec lavas from the Oldoinyo Lengai volcano, but cannot explain similar fractionations in 230Th/238U. 210Pb has the shortest half-life of these nuclides, and is, therefore, the most sensitive to the exact timing and mechanism of fractionation. Because we can explain 210Pb/238U in terms of recent, instantaneous liquid immiscibility, we favor this model for the origin of the deviations of 210Pb/238U and 226Ra/238U from those expected under secular equilibrium. If our interpretation is correct, some other fractionation process(es) must have fractionated Th (and perhaps Pa) from U, Ra, and Pb in the Oldoinyo Lengai carbonatites.

U-series nuclides, beryllium, and lead isotopes have been measured on historic lavas from eight volcanoes in Nicaragua. Low-Ti samples from northern Nicaragua have ( 230 Th) ( 232 Th) ratios from 2.23 to 2.56 and are enriched in 238U and 234U over 230Th by 1–16%. Those from southern Nicaragua have ( 230 Th) ( 232 Th) ≈ 2.1 and have ( 238 U) ( 230 Th) = 1.0–0.9. High-Ti samples have intermediate ( 230 Th) ( 232 Th) ratios and are strongly enriched in 230Th. Lead isotopic data for all samples plot in the mantle array with 206 Pb 204 Pb = 18.50–18.63 . Th isotopic ratios for all samples imply source U/Th ratios that are significantly higher than those implied by lead isotopic ratios. All historic samples have ( 210 Po) ( 230 Th) and thus ( 226 Ra) ( 230 Th) > 1 , suggesting that lavas erupt less than 8,000 y after generation. Thorium isotopic ratios for all volcanic samples correlate well with 10 Be 9 Be and B/Be as well as with 87 Sr 86 Sr and Ba/La ratios from the literature. ( 238 U) ( 230 Th) ratios do not correlate well with any of these ratios but rather anti-correlate with Th concentrations and published La Yb ratios. These observations, and inferences therefrom, lead to the following preferred, although not completely unique, interpretations. Comparison of thorium, beryllium, and strontium isotopic ratios of the Cocos plate sediments at DSDP 495 with the low-Ti volcanic regression trends shows that the subducted component transferred to the mantle has isotopic compositions similar to the bulk sediments. B/Be and Ba/La ratios of the volcanic samples are well correlated with the isotopes, but the inferred ratios in the subduction component are at least 10 and 5 times higher, respectively, than values in the bulk sediment. These relationships suggest that the subduction component combines the element-transport properties of both a melt and a hydrous fluid. The ( 230 Th) ( 232 Th) and 87 Sr 86 Sr ratios corresponding to a 10 Be 9 Be ratio of zero are about 2.0 and 0.704, significantly higher than expected in the postulated MORB source for the lavas. We attribute these characteristics to subduction related addition of strontium, uranium, plus or minus thorium, and related tracers to the mantle prior to about 4.5 Ma, such that the 10Be signal of this event has decayed away. The observed good correlations are generated primarily by addition of 10Be, U >Th, B and Ba between 4.5 Ma. and ca. 200–300 ka. The most recent event occurred since 200–300 ka and perhaps more recently than 8 ka. It transported only a fraction of the total slab-derived flux of 10Be, B, Ba, and probably thorium stored within the mantle, but added U >Th and may have raised the partition coefficient of uranium over thorium. The magnitude of the uranium excess thus relates to the extent of partial melting of the mantle source during this event. The U-series and beryllium isotope systematics of the high-Ti lavas are consistent with their derivation from the mantle residue after extraction of low-Ti melts.

Mass spectrometric measurements of235U231Pa and238U230Th disequilibria are used to further constrain processes producing U-series disequilibria in young mid-ocean ridge basalts (MORB) and to determine eruption ages for samples from 9–10°N East Pacific Rise and Juan de Fuca-Gorda ridges.231Pa/235U activity ratios for axial basalts are large and relatively uniform within ridge segments. The systematically large231Pa/235U,230Th/238U, and226Ra/230Th activity ratios in MORB are inconsistent with simple equilibrium melting models and crustal contamination or seawater alteration processes. More complicated melting processes involving disequilibrium or heterogeneous melting of mantle sources and disequilibrium transport of melts to the crust could produce the large U-series disequilibria in MORB. 231Pa and230Th model ages were determined by estimating initial231Pa/235U and230Th/232Th ratios from axial samples and by assuming constancy of these ratios over time. All231Pa and230Th ages agree to ±0–7 ka for samples with ages of 0–130 ka. These concordant ages validate the model assumptions of the two dating methods and suggest that mantle sources and melt generation processes have been uniform for these areas during the past 130 ka. Consequently,231Pa and230Th age-dates can be used to quantify the temporal and spatial dependence of volcanism at oceanic spreading centers during the past 375 ka.

High-precision mass spectrometer and alpha scintillation measurements of 238U-series nuclides were obtained for whole-rock pumice and constituent mineral and glass separates to help interpret the timing and nature of magmatic processes that led up to the 1985 eruption of Nevado del Ruiz volcano. Internal isochron diagrams for (226Ra)Ba vs. (230Th)Ba and (230Th232Th) vs. (238U232Th) show that data define linear arrays, indicating an average crystallization age of 6.1 ± 0.5 ka (Ra-Th isochron) and 7 ± 6 ka (Th-U isochron). Stratigraphic, petrographic, geochemical, repose time, and eruption volume data for the Holocene eruptive sequence of Nevado del Ruiz indicate that significant changes occurred in the subvolcanic magma chamber between eruptions R9 (≈8.6 ka) and R8 (≈3.0 ka). This coincides with the average crystallization age derived from the U-series data and may represent the injection of a new batch (or batches) of mantle-derived magma into the subvolcanic magma chamber.

A highly precise mass spectrometric method of analysis was used to determine238U—234U—230Th—232Th in axial and off-axis basalt glasses from Juan de Fuca (JDF) and Gorda ridges. Initial230Th activity excesses in the axial samples range from 3 to 38%, but generally lie within a narrow range of 12 to 15%. Secondary alteration effects were evaluated usingδ234U and appear to be negligible; hence the230Th excesses are magmatic in origin. Direct dating of MORB was accomplished by measuring the decrease in excess230Th in off-axis samples.238U—230Th ages progressively increase with distance from axis. Uncertainties in age range from 10 to 25 ka for U—Th ages of 50 to 200 ka. The full spreading rate based on U—Th ages for Endeavour segment of JDF is 5.9 ± 1/2 cm/yr, with asymmetry in spreading between the Pacific (4.0 ± 0.6 cm/yr) and JDF (1.9 ± 0.6 cm/yr) plates. For northern Gorda ridge, the half spreading rate for the JDF plate is found to be 3.0 ± 0.4 cm/yr. These rates are in agreement with paleomagnetic spreading rates and topographic constraints. This suggests that assumptions used to determine ages, including constancy of initial 230Th/232Th ratio over time, are generally valid for the areas studied. Samples located near the axis of spreading are typically younger than predicted by these spreading rates, which most likely reflects recent volcanism within a 1–3 km wide zone of crustal accretion. Initial230Th/232Th ratios and230Th activity were also used to examine the recent Th/U evolution and extent of melting of mantle sources beneath these ridges. A negative anomaly in 230Th/232Th for Axial seamount lavas provides the first geochemical evidence of a mantle plume source for Axial seamount and the Cobb-Eickelberg seamount chain and indicates recent depletion of other JDF segment sources. Large230Th activity excesses for lavas from northern Gorda ridge and Endeavour segment indicate formation from a lower degree of partial melting than other segments. An inverse correlation between230Th excess and 230Th/232Th for each ridge indicates that these lower degree melts formed from slightly less depleted sources than higher degree melts. Uniformity in230Th excess for other segments suggests similarity in processes of melt formation and mixing beneath most of the JDF-Gorda ridge area. The average initial230Th/232Th activity ratio of 1.31 for the JDF-Gorda ridge area is in agreement with the predicted value of 1.32 from the Th—Sr isotope mantle array.

We have developed a mass spectrometric technique for the measurement of 230Th/232Th ratios in young volcanic rocks. We show that we can measure 230Th/232Th ratios on MORB samples of ∼- 1 gram with an accuracy and reproducibility of 0.5–1.0%. This represents an improvement of at least a factor of 5–10 in sample size and precision over conventional alpha spectrometry methods. Using this technique, we have measured distinct excesses of230Th activity relative to238U activity for axial samples from the Juan de Fuca (JDF) and Gorda Ridges. These enrichments are 13–15% but range up to 40% in one sample. Low boron concentrations and 234U/238U ratios corresponding to secular equilibrium verify the absence of detectable seawater contamination. From this we infer that primary magmatic processes are the source of the measured230Th activity excesses, and that Th is indeed more incompatible than U during partial melting of MORB sources. 230Th/232Th activity ratios for the axial samples from JDF appear to be relatively high for uncontaminated MORB, ranging from 1.35 to 1.41. From this we infer that the JDF basalts formed from a uniform source highly depleted in Th relative to U (Th/Uwt= 2.15–2.25). One Gorda axial sample has a somewhat lower 230Th/232Th activity ratio of 1.31, suggesting formation from a source only slightly less depleted (Th/U= 2.32 ± 0.02). Based on the small range in 30Th/232Th ratios for axial basalts from JDF, dating off-axis JDF basalts may be feasible.