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1.
The 230Th content of subsurface Bahamas seawater has been estimated from incrementally deposited aragonite skeletons of sclerosponges using U-series age models which imply syndepositional incorporation of 230Th. First order age models, assuming constant growth rate and zero initial 230Th activity, display an average surface age of 63 ± 18 y despite the fact that the surfaces of the organisms were known to be alive at the time collection in the 1990s. A knowledge of the initial 230Th activity is necessary to incorporate the surface age constraint into an age model. In the Bahamas, this approach estimates an initial 230Th/232Th activity ratio of 4.3 ± 1.1 when applied to several individuals of the Caribbean sclerosponge Ceratoporella nicholsoni. These results differ from the same approach applied to previous work on Jamaican sclerosponges accreting aragonite in an environment closer to crustal sources and with less carbonate substrate. We report new measurements of Jamaican sclerosponge suggesting initial activity ratios of 230Th/232Th of ∼1, albeit with a high amount of uncertainty, indicating a regional difference in initial thorium activities. The estimates of initial 230Th/232Th activity ratios in the Bahamas are similar to the dissolved ratio rather than the particulate ratio and can be explained by mixing of open ocean water and bank-top surface water, however no information regarding temporal changes in the amount of initial 230Th/232Th activity available for incorporation into biogenic aragonite can be constrained using this approach. Growth rates do not change after correction for initial thorium and they display an inverse relationship with depth. Although this paper deals uniquely with sclerosponge aragonite, it is likely that the approach can be used on any incrementally deposited biogenic aragonite, provided information is known a priori about the first order growth model and there is evidence that initial thorium incorporation is representative of ambient environment rather than individual physiology.  相似文献   

2.
We present U, Th, and Pa isotope data for young lavas from Costa Rica and Nicaragua in the Central American arc. Thorium isotopic ratios for Costa Rica and Nicaragua differ dramatically: Costa Rican lavas are characterized by low (230Th/232Th) (1 to 1.2) and, for four out of five lavas, (230Th/238U) greater than unity. Nicaraguan lavas have high (230Th/232Th) (2.2 to 2.7) and, for five of six samples, (230Th/238U) less than unity. All lavas have (231Pa/235U) greater than unity, with initial values ranging from 1.27 to 1.77, but those from Costa Rica have larger 231Pa excesses. There is a broad positive correlation between (231Pa/235U) and (230Th/238U) similar to the worldwide trend for arcs outlined by Pickett and Murrell (1997), although many of the Nicaraguan lavas skirt the high end of that trend. In greater detail, the Central American data appear to divide into separate high-(231Pa/235U) and low-(231Pa/235U) tiers. These tiers may be different because of either different residence times in the crust or different proportions of sedimentary components from the slab.Substantial (231Pa/235U) excesses (>1.5) in both Costa Rica and Nicaragua require a melting process that allows for enhanced daughter (231Pa) ingrowth. With increasing U addition, (231Pa/230Th) increases in a manner that cannot be explained adequately by aging of fluid components before partial melting and eruption. Thus, either some 231Pa is added from the slab, or melting-enhanced 231Pa ingrowth is greater in sources that have experienced a larger amount of slab-derived flux and a higher extent of melting. These observations can be explained if regions that have undergone greater extents of fluxing and melting have experienced these processes over a longer time interval than those that have had little flux added and little melt extracted. We propose a flux-ingrowth melting model in which corner flow in the mantle wedge supplies fresh hot mantle into a zone of slab fluid addition. Partial melting occurs in response to this fluxing. We assume critical melting at low porosity (∼10−3), rapid fluid flux to the melting region, and rapid melt transport. Solid mantle traverses the melting region over 105 to 106 yr, thereby allowing 231Pa and 230Th ingrowth from U retained in the residues of melt extraction. Magmas are aggregated from all parts of the melting regime, mixing melts from incipiently fluxed regions with those from sources that have experienced more extensive fluid addition, partial melting, and daughter nuclide ingrowth. With suitable assumptions about component addition from the slab, this flux-ingrowth model matches a wide range of U-series and trace element data from Costa Rican and Nicaraguan lavas, with required average extents of melting of ∼1 to 3% and 7 to 15%, respectively. Upwelling and/or extensive melt-rock reaction are not required to explain large (231Pa/235U) excesses in Central America or other arcs. On Th isotope equiline plots, the model produces linear arrays that resemble isochrons but that have no age significance. Instead, these arrays are generated by mixing of melts from sources that have experienced fluid addition and partial melting over a range of time intervals, as seems likely in arc source regions. Finally, the flux-ingrowth model predicts considerable 226Ra excesses for integrated magmas. If we assume that 226Ra is added continuously with the slab-derived fluid, the model predicts large and increasing (226Ra/230Th) with increasing melting and slab-component addition, without requiring the addition of a distinct late fluid.  相似文献   

3.
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°05′-70°26′N) display (230Th/238U) < 1 and (230Th/238U) > 1 with (230Th/238U) 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. 87Sr/86Sr = 0.70272-0.70301, εNd = 8.4-10.5, εHf = 15.4-19.6 (La/Yb)N = 0.28-0.84) encompass a narrow range of (230Th/232Th) (1.20-1.32) over a large range in (238U/232Th) (0.94-1.32), producing a horizontal array on a (230Th/232Th) vs. (238U/232Th) diagram and a large variation in (230Th/238U). However, the (230Th/238U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with (234U/238U) (1.001-1.031). Samples with low (230Th/238U) and elevated (234U/238U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium 234U/238U have high (230Th/238U) 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 (230Th/238U) and (231Pa/235U) ratios in uncontaminated Kolbeinsey lavas, but low (231Pa/235U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment.  相似文献   

4.
The 230Th/234U/238U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system 230Th/234U/238U ages is elevated 234U/238Uinitial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have 234U/238Uinitial values close to seawater, there is a need for age validation. Redundant 230Th/234U/238U and 231Pa/235U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both 230Th/234U/238U and 231Pa/235U ages in fragments as small as 500 mg. We have obtained excellent agreement between 230Th/234U/238U and 231Pa/235U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the 230Th/234U/238U age accuracy.Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby 238U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of 230Th/234U/238U and 231Pa/235U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for 231Pa and from undetermined sources currently limit the 231Pa/235U age uncertainty to about ±2.5%. U isotope data and 230Th/234U/238U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.  相似文献   

5.
Hydrogenetic ferromanganese crusts (Fe-Mn crusts) provide a secular record of the variations of seawater composition responding to changes in ocean circulation and erosion processes. In this respect, the acquisition of an absolute and reliable chronology in Fe-Mn crusts is a prerequisite. Here we combine four different and complementary chronometers (10Be, 230Thex, 230Thex/232Th, 234U/238U) in a Fe-Mn crust dredged at ∼2000 m depth in the east Atlantic to first establish a reliable chronology over the Quaternary period. Then, we use EDS chemical analysis to look for correlation between major element chemistry and climate changes. (230Thex), (230Thex/232Th), and Be data give very consistent growth rates. In particular, the good match between (230Thex) and (230Thex/232Th) data indicates that at the location of crust 121DK, 230Th and 232Th fluxes in the water column change simultaneously and suggests that the normalization of 230Thex to 232Th makes (230Thex/232Th) a better chronometer. Our best-fit model suggests that crust 121DK experienced changes in growth rates at ∼122 and 312 ka and a growth with a constant 230Th initial flux. This chronology returns an age of 680 ka for the uppermost 1.5 mm. The (234U/238U) depth profile, however, was clearly affected by diffusion of 234U in the porous crust and can therefore not be used to derive a reliable chronology. One part of the crust seems isolated from pore water diffusion and can be physically recognized as a zone of very small porosity. On the basis of the (230Thex/232Th) chronology, major element chemistry is shown to be linked to climate change. Mn/Fe variations compare well with those in a Fe-Mn crust from the Pacific, showing systematic maxima during glacial stages 2 and 4. High Mn/Fe are tentatively interpreted to reflect expansion of the oxygen minimum zone during glacial periods, resulting from higher bioproductivity. In addition we note that the surface (230Th/232Th) activity ratio of crust 121DK is entirely consistent with advection of deep water from the western toward the eastern Atlantic basin.  相似文献   

6.
We present major and trace element and Sr-Nd-Pb and U-Th-Pa-Ra isotope data for a small sample suite of primarily post-glacial, mildly alkalic volcanic rocks from the Snaefellsjökull central volcano situated off the main rift systems in western Iceland. The volcanic rocks are crystal-poor and range from olivine alkali basalt to trachyte and show tight correlations of major and trace elements that are explained by fractional crystallization involving removal of olivine, plagioclase, clinopyroxene, Fe-Ti oxide and apatite. Sr-Nd-Pb isotopes are practically invariant, consistent with derivation from the same source region. During fractionation from primitive basalt to evolved trachyte, (230Th/232Th), (230Th/238U) and (231Pa/235U) decrease progressively at broadly constant (238U/232Th). A continuous closed-system fractionation model that assumes constant initial (230Th/232Th) in the basaltic precursor melt indicates that hawaiite was derived from olivine basalt by ∼50% fractional crystallization within and trachyte by ∼80% fractionation within . An overrepresentation of evolved basalts and hawaiites with young inferred magma ages in the dataset is consistent with the parental precursor to these magmas intruded into the sub-volcanic magma plumbing system as a consequence of lithospheric rebound caused by deglaciation. Lavas affected by apatite removal have higher (231Pa/235U) than predicted for simple radioactive decay, suggesting apatite significantly fractionates U from Pa. The proposed fractionation model consistently explains our U-series data assuming and and . If applicable, these D values would indicate that the effect of apatite fractionation must be adequately considered when assessing differentiation time scales using (231Pa/235U) disequilibria data.  相似文献   

7.
We present data for U and its decay series nuclides 230Th, 226Ra, 231Pa, and 210Po for 14 lavas from Kick’em Jenny (KEJ) submarine volcano to constrain the time-scales and processes of magmatism in the Southern Lesser Antilles, the arc having the globally lowest plate convergence rate. Although these samples are thought to have been erupted in the last century, most have (226Ra)/(210Po) within ±15% of unity. Ten out of 14 samples have significant 226Ra excesses over 230Th, with (226Ra)/(230Th) up to 2.97, while four samples are in 226Ra-230Th equilibrium within error. All KEJ samples have high (231Pa)/(235U), ranging from 1.56 to 2.64 and high 238U excesses (up to 43%), providing a global end-member of high 238U and high 231Pa excesses. Negative correlations between Sr, sensitive to plagioclase fractionation, and Ho/Sm, sensitive to amphibole fractionation, or K/Rb, sensitive to open system behavior, indicate that differentiation at KEJ lavas was dominated by amphibole fractionation and open-system assimilation. While (231Pa)/(235U) does not correlate with differentiation indices such as Ho/Sm, (230Th)/(238U) shows a slight negative correlation, likely due to assimilation of materials with slightly higher (230Th)/(238U). Samples with 226Ra excess have higher Sr/Th and Ba/Th than those in 226Ra-230Th equilibrium, forming rough positive correlations of (226Ra)/(230Th) with Sr/Th and Ba/Th similar to those observed in many arc settings. We interpret these correlations to reflect a time-dependent magma differentiation process at shallow crustal levels and not the process of recent fluid addition at the slab-wedge interface.The high 231Pa excesses require an in-growth melting process operating at low melting rates and small residual porosity; such a model will also produce significant 238U-230Th and 226Ra-230Th disequilibrium in erupted lavas, meaning that signatures of recent fluid addition from the slab are unlikely to be preserved in KEJ lavas. We instead propose that most of the 238U-230Th, 226Ra-230Th, and 235U-231Pa disequilibria in erupted KEJ lavas reflect the in-growth melting process in the mantle wedge (reflecting variations in U/Th, daughter-parent ratios, fO2, and thermal structure), followed by modification by magma differentiation at crustal depths. Such a conclusion reconciles the different temporal implications from different U-series parent-daughter pairs and relaxes the time constraint on mass transfer from slab to eruption occurring in less than a few thousand years imposed by models whereby 226Ra excess is derived from the slab.  相似文献   

8.
Abundances of 238U, 234U, 232Th, 226Ra, 228Ra, 224Ra, and 222Rn were measured in groundwaters of the Ojo Alamo aquifer in northwest New Mexico. This is an arid area with annual precipitation of ∼22 cm. The purpose was to investigate the transport of U-Th series nuclides and their daughter products in an old, slow-moving groundwater mass as a means of understanding water-rock interactions and to compare the results with a temperate zone aquifer. It was found that 232Th is approximately at saturation and supports the view of Tricca et al. (2001) that Th is precipitated irreversibly upon weathering, leaving surface coatings of 232Th and 230Th on aquifer grains. Uranium in the aquifer waters has very high [234U/238U] ∼ 9 and low 238U concentrations. These levels can be explained by low weathering rates in the aquifer (w238U ∼ 2 × 10−18 to 2 × 10−17s−1) using a continuous flow, water-rock interaction model. The Ra isotopes are roughly in secular equilibrium despite their very different mean lifetimes. The 222Rn and 228Ra isotopes in the aquifer correspond to ∼10% of the net production rate of the bulk rock. This is interpreted to reflect an earlier formed irreversible surface coating of Th that provides Ra and Rn to the aquifer waters. The surface waters that appear to be feeding the aquifer have low [234U/238U] and high 238U concentrations. The flow model shows that it is not possible to obtain the high [234U/238U] and low [238U] values in the aquifer from a source like the present vadose zone input. It follows that the old aquifer waters studied cannot be fed by the present vadose zone input unless they are greatly diluted with waters with very low U concentrations. If the present sampling of vadose zone sources is representative of the present input, then this requires that there was a major change in water input with much larger rainfall some several thousand years ago. This may represent a climatic change in the Southwest.  相似文献   

9.
The processes involved in the formation and transport of partial melts above subducting plates remain poorly constrained relative to those at mid-ocean ridges. In particular, 238U-230Th-226Ra disequilibria, that might normally be used to constrain melting dynamics, tend to be swamped by the effects of fluid addition from the down-going plate. The 231Pa-235U system provides an exciting exception to this because the highly incompatible nature of Pa means that fractionation and in-growth during partial melting overwrite the effects of fluid U addition. We present 231Pa-235U data on 50 well-characterised lavas from seven subduction zones in order to examine partial melting processes. Measured (231Pa/235U) ratios are all >1 and 15% are >2. Overall (231Pa/235U) shows broad positive correlations with (230Th/238U) and La/Yb and negative trends against Ba/Th and (226Ra/230Th). These systematics can differ from arc to arc but suggest that (231Pa/235U) tends to be higher in sediment-rich arc lavas where the effects of fluid addition are muted and there is less of a 231Pa deficit for melting to overprint. We have explored the effects of decompression melting, frictional drag dynamic melting with and without ageing subsequent to fluid U addition to the wedge as well as flux melting models. Globally, average (231Pa/235U) appears to correlate negatively with convergence rate and so in the numerical models we use the local subduction rate for the rate of matrix flow through the melting zone. Using this assumption and reasonable values for other parameters, the melting models can simulate the overall range of (231Pa/235U) and some of the data trends. However, it is clear that local variations in some parameters, especially source composition and extent of melting, exert a major influence on 231Pa-235U disequilibria. Some data, which lie at a high angle to the modelled trends, may be explained by mixing between small degree hydrous melts formed near the slab and larger degree, decompression melts produced at shallow depth.  相似文献   

10.
U-series disequilibria are presented for Holocene samples from the Canary Islands and interpreted with special emphasis on the separate roles of plume vs. lithospheric melting processes. We report Th and U concentrations and (238U)/(232Th), (230Th)/(232Th), (230Th)/(238U) and (234U)/(238U) for 43 samples, most of which are minimally differentiated, along with (226Ra)/(230Th) and (231Pa)/(235U) for a subset of these samples, measured by thermal ionization mass spectrometry (TIMS). Th and U concentrations range between 2 and 20 ppm and 0.5 and 6 ppm, respectively. Initial (230Th)/(238U) ranges from 1.1 to 1.6. (226Ra)/(230Th)o ranges between 0.9 and 1.8 while (231Pa)/(235U)o ranges between 1.0 and 2.0.Our interpretation of results is based on a three-fold division of samples as a function of incompatible element ratio, such as Nb/U. The majority of samples have Nb/U = 47 ± 10, similar to most MORB and OIB. Higher ratios are found exclusively in alkali basalts and tholeiites from the eastern Canary Islands whereas lower ratios are exclusively found in differentiated rocks from the western Canary Islands. Those with ordinary Nb/U ratios are attributed to melting within the slowly ascending HIMU-dominated Canary plume.Higher Nb/U, generally found in more silica rich basalts from the eastern islands, is attributed to lithospheric contamination. Based on their trace element characteristics, two possible contaminants are amphibole veins (± other minerals) crystallized in the mantle from previous plume-derived basanite or re-melted plume-derived intrusive rocks. The high Nb/U signature of these materials is imparted on a melt of the lithosphere created either by the diffusive infiltration of alkalis or by direct reaction between basanites and peridotite. Mixing between plume-derived basanite and lithospheric melt accounts for the U-series systematics of most eastern island magmas including the well-known Timanfaya eruption. Lower Nb/U ratios in differentiated rocks from the western islands are attributed to fractional crystallization of amphibole ± phlogopite ± sphene from basanite during its ascent through the lithosphere. Based on changes in disequilibria, phonolites and tephrites are interpreted to result from rapid differentiation of primitive parents within millennia.  相似文献   

11.
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.  相似文献   

12.
A deep-sea Mn nodule handpicked from top of a 20 cm diameter North Pacific tripod core was directly counted by an α-spectrometer with a silicon surface barrier detector. A distinct 210Po peak was recorded for the nodule upper side in contact with bottom seawater. Based on the spectrum, the 210Po (supported by 210Pb with 22 year half life) is thought to be confined in the top few microns of the nodule. Since the 210Po peak diminishes considerably for the bottom side resting on sediment, nondestructive α-spectrometry is useful for determining the sea floor orientation of nodules. This method was tested for a western North Pacific dredged Mn nodule, and was successful in identifying the orientation (and also surface loss of the nodule by dredging).Radiochemical analysis was also carried out for carefully scraped samples from the top and bottom of the two nodules. The growth rates of the hand-picked nodule based on excess 230Th and 231Pa profiles were 1 to 4 mm/106 y and apparently varied with time. The dredged sample has grown at 2 to 6 mm/ 106 y and the growth has likely been episodic. The radionuclide composition was significantly different between the top and bottom. Evidence was found that the radionuclide fluxes of 230Th and 231Pa into the nodules considerably changed during the growing histories. The 230Th/232Th activity ratios in the nodule tops are comparable with those in bottom seawater suggesting that the top is growing with metals of seawater origin. However it remains unresolved whether the nodule bottoms are growing by utilizing metals in the sediment pore water.  相似文献   

13.
U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for 238U, 234U, 232Th and 230Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas (234U/238U) and (230Th/232Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in 234U relative to 238U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between (230Th/232Th) activity ratios and 87Sr/86Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by (234U/238U) > 1 resulting from α-recoil effect of 234Th. Groundwaters are characterized by a more radiogenic signature in 87Sr/86Sr than the rocks. Moreover, (230Th/232Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10-35 and from 1 × 104 to 2 × 105 were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km2), a characteristic migration time from recharge to riverine discharge of 200-600 yr for U and 0.2-3.7 Myr for Th was obtained.  相似文献   

14.
This study centers on the question: How sensitive are 231Pa/230Th and 10Be/230Th to sediment composition and redistribution? The natural radionuclides 231Pa, 230Th and 10Be recorded in deep sea sediments are tracers for water mass advection and particle fluxes. We investigate the influence of oceanic particle composition on the element adsorption in order to improve our understanding of sedimentary isotope records. We present new data on particle size specific 231Pa and 10Be concentrations. An additional separation step, based on settling velocities, led to the isolation of a very opal-rich phase. We find that opal-rich particles contain the highest 231Pa and 10Be concentrations, and higher 231Pa/230Th and 10Be/230Th isotope ratios than opal-poor particles. The fractionation relative to 230Th induced by the adsorption to opal-rich particles is more pronounced for 231Pa than for 10Be. We conclude that bulk 231Pa/230Th in Southern Ocean sediments is most suitable as a proxy for past opal fluxes. The comparison between two neighboring cores with rapid and slow accumulation rates reveals that these isotope ratios are not influenced significantly by the intensity of sediment focusing at these two study sites. However, a simulation shows that particle sorting by selective removal of sediment (winnowing) could change the isotope ratios. Consequently, 231Pa/230Th should not be used as paleocirculation proxy in cases where a strong loss of opal-rich material due to bottom currents occurred.  相似文献   

15.
During recent humid episodes, stromatolites were built along paleolake margins, some 60 m above the modern water level of Lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently covering pebbles and boulders eroded from the older ones. The youngest one yielded 14C ages ranging from approximately 12,000 to 10,000 yr B.P. Their δ13C values (≥2.6%) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO2, thereby lending credence to the reliability of the 14C. An initial 230Th/232Th ratio in the detrital component was determined by Th/U measurements on the 14C dated stromatolites. Using this value a 230Th/234U chronology for the older stromatolites was calculated. Ages of ≥240,000 and 135,000 ± 10,000 yr were obtained for the first and second generations, respectively. A humid episode apparently characterized eastern Africa during each glacial-interglacial transition. 18O and 13C measurements on stromatolites, when compared to values on modern waters and carbonates, provide paleohydrological information. Long residence time of the paleolake waters and less seasonally contrasted regimes are inferred.  相似文献   

16.
The miliolite deposits of Saurashtra have been dated by234U,230Th,231Pa and14C methods. Concordant ages of 105 years using the U decay series isotopes are obtained which agree with the ages of the coral reefs of Okha-Dwaraka coast suggesting a contemporaneous origin for both. The lower14C ages (≤40,000 years) may be due to a recent influx of seawater or ground water. Quartz and clay minerals together constitute only ≤10% by weight, as such the aeolian characteristics of quartz grains may not be relevant to the origin of the miliolites.  相似文献   

17.
The 238U-230Th-226Ra and 235U-231Pa disequilibria have been measured by mass spectrometry in historic lavas from the Kamchatka arc. The samples come from three closely located volcanoes in the Central Kamchatka Depression (CKD), the most active region of subducted-related volcanism in the world. The large excesses of 226Ra over 230Th found in the CKD lavas are believed to be linked to slab dehydration. Moreover, the samples show the uncommon feature of (230Th/238U) activity ratios both lower and higher than 1. The U-series disequilibria are characterized by binary trends between activity ratios, with (231Pa/235U) ratios all >1. It is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting. We suggest that they are more likely to reflect mixing between two end-members: a high-magnesia basalt (HMB) end-member with a clear slab fluid signature and a high-alumina andesite (HAA) end-member reflecting the contribution of a slab-derived melt. The U-Th-Ra characteristics of the HMB end-member can be explained either by a two-step fluid addition with a time lag of 150 ka between each event or by continuous dehydration. The inferred composition for the dehydrating slab is a phengite-bearing eclogite. Equilibrium transport or dynamic melting can both account for 231Pa excess over 235U in HMB end-member. Nevertheless, dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid-derived 226Ra/230Th. A continuous flux melting model is also tested. In this model, 231Pa-235U is quickly dominated by fluid addition and, for realistic extents of melting, this process cannot account for (231Pa/235U) ratios as high as 1.6, as observed in the HMB end-member.The involvement of a melt derived from the subducted oceanic crust is more likely for explaining the HAA end-member compositions than crustal assimilation. Melting of the oceanic crust is believed to occur in presence of residual phengite and rutile, resulting in no 226Ra-230Th disequilibrium and low 231Pa excess over 235U in the high-alumina andesites. Consequently, it appears that high-alumina andesites and high-magnesia basalts have distinct origins: the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle. It seems that there is no genetic link between these two magma types, in contrast with what was previously believed.  相似文献   

18.
Uranium (U) concentrations and activity ratios (δ234U) of authigenic carbonates are sensitive recorders of different fluid compositions at submarine seeps of hydrocarbon-rich fluids (“cold seeps”) at Hydrate Ridge, off the coast of Oregon, USA. The low U concentrations (mean: 1.3 ± 0.4 μg/g) and high δ234U values (165-317‰) of gas hydrate carbonates reflect the influence of sedimentary pore water indicating that these carbonates were formed under reducing conditions below or at the seafloor. Their 230Th/234U ages span a time interval from 0.8 to 6.4 ka and cluster around 1.2 and 4.7 ka. In contrast, chemoherm carbonates precipitate from marine bottom water marked by relatively high U concentrations (mean: 5.2 ± 0.8 μg/g) and a mean δ234U ratio of 166 ± 3‰. Their U isotopes reflect the δ234U ratios of the bottom water being enriched in 234U relative to normal seawater. Simple mass balance calculations based on U concentrations and their corresponding δ234U ratios reveal a contribution of about 11% of sedimentary pore water to the bottom water. From the U pore water flux and the reconstructed U pore water concentration a mean flow rate of about 147 ± 68 cm/a can be estimated. 230Th/234U ages of chemoherm carbonates range from 7.3 to 267.6 ka. 230Th/234U ages of two chemoherms (Alvin and SE-Knoll chemoherm) correspond to time intervals of low sealevel stands in marine isotope stages (MIS) 2, 4, 5, 6, 7 and 8. This observation indicates that fluid flow at cold seep sites sensitively reflects pressure changes of the hydraulic head in the sediments. The δ18OPDB ratios of the chemoherm carbonates support the hypothesis of precipitation during glacial times. Deviations of the chemoherm δ18O values from the marine δ18O record can be interpreted as to reflect temporally and spatially varying bottom water and/or vent fluid temperatures during carbonate precipitation between 2.6 and 8.6°C.  相似文献   

19.
Measurements of 238U, 234U, 230Th, 232Th, 231Pa, Mn, Fe, Co, Ni, Cu, and Zn were made on 23 samples from core GPC-5, a 29-m giant piston core from a water depth of 4583 m on the northeastern Bermuda Rise (33°41.2′N, 57°36.9′W). This area is characterized by rapid deposition of sediment transported by abyssal currents. Unsupported 230Th and 231Pa are present throughout the core but, because of large variations in the sedimentation rate, show marked departures from exponential decay with depth. The trend with depth of the 231Paex230Thex ratio is consistent with the average accumulation rate of 36 cm/1000 y reported earlier on the basis of radiocarbon dating and CaCO3 stratigraphy. When expressed on a carbonate-free basis, concentrations of Mn, Co, Ni, Cu, Zn, 230Thex, and 231Paex all show cyclic variations positively correlated with those of CaCO3. The correlations can be explained by a model in which all of these constituents, including CaCO3, are supplied to the sediments from the water column at a constant rate. Concentration variations are controlled mainly by varying inputs of terrigenous detritus, with low inputs occurring during interglacials and high inputs during glacials. Relationships between the metal and 230Thex concentrations permit estimates of the rates at which the metals are removed to the sediment by scavenging from the water column. The results, in μg/cm2-1000 y, are: 4300 ± 1100 for Mn, 46 ± 16 for Ni and 76 ± 26 for Cu. These rates are somewhat larger than ocean-wide averages estimated by other methods, and the absolute rate of 230Th accumulation in GPC-5 averages about nine times higher than production in the overlying water column. This part of the Bermuda Rise and similar bottom-current deposits may act as important accumulators of elements scavenged from seawater.  相似文献   

20.
The Lost City Hydrothermal Field (LCHF) is a serpentinite-hosted vent field located 15 km west of the spreading axis of the Mid-Atlantic Ridge. In this study, uranium-thorium (U-Th) geochronological techniques have been used to examine the U-Th systematics of hydrothermal fluids and the 230Th ages of hydrothermally-precipitated carbonate chimneys at the LCHF. Fluid sample analyses indicate that endmember fluids likely contain only 0.0073 ng/g U or less compared to 3.28 ± 0.03 ng/g of U in ambient seawater. For fluid samples containing only 2-21% ambient seawater (1.1-11 mmol/kg Mg), Th concentration is 0.11-0.13 pg/g and surrounding seawater concentrations average 0.133 ± 0.016 pg/g. The 230Th/232Th atomic ratios of the vent fluids range from 1 (±10) × 10−6 to 11 (±5) × 10−6, are less than those of seawater, and indicate that the vent fluids may contribute a minor amount of non-radiogenic 230Th to the LCHF carbonate chimney deposits. Chimney 238U concentrations range from 1 to 10 μg/g and the average chimney corrected initial δ234U is 147.2 ± 0.8, which is not significantly different from the ambient seawater value of 146.5 ± 0.6. Carbonate 232Th concentrations range broadly from 0.0038 ± 0.0003 to 125 ± 16 ng/g and 230Th/232Th atomic ratios vary from near seawater values of 43 (±8) × 10−6 up to 530 (±25) × 10−3. Chimney ages, corrected for initial 230Th, range from 17 ± 6 yrs to 120 ± 13 kyrs. The youngest chimneys are at the intersection of two active, steeply-dipping normal faults that cut the Atlantis Massif; the oldest chimneys are located in the southwest portion of the field. Vent deposits on a steep, fault-bounded wall on the east side of the field are all <4 kyrs old, indicating that mass wasting in this region is relatively recent. Comparison of results to prior age-dating investigations of submarine hydrothermal systems shows that the LCHF is the most long-lived hydrothermal system known to date. It is likely that seismic activity and active faulting within the Atlantis Massif and the Atlantis Fracture Zone, coupled with volumetric expansion of the underlying serpentinized host rocks play major roles in sustaining hydrothermal activity at this site. The longevity of venting at the LCHF may have implications for ecological succession of microorganisms within serpentinite-hosted vent environments.  相似文献   

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