210Pb/226Ra and 210Po/210Pb disequilibria in seawater and suspended particulate matter

The distribution of²¹⁰Po and²¹⁰Po in dissolved (<0.4 μm) and particulate (>0.4 μm) phases has been measured at ten stations in the tropical and eastern North Atlantic and at two stations in the Pacific. Both radionuclides occur principally in the dissolved phase. Unsupported²¹⁰Pb activities, maintained by flux from the atmosphere, are present in the surface mixed layer and penetrate into the thermocline to depths of about 500 m. Dissolved²¹⁰Po is ordinarily present in the mixed layer at less than equilibrium concentrations, suggesting rapid biological removal of this nuclide. Particulate matter is enriched in²¹⁰Po, with²¹⁰Po/²¹⁰Pb activity ratios greater than 1.0, similar to those reported for phytoplankton. Box-model calculations yield a 2.5-year residence time for²¹⁰Pb and a 0.6-year residence time for²¹⁰Po in the mixed layer. These residence times are considerably longer than the time calculated for turnover of particles in the mixed layer (about 0.1 year). At depths of 100–300 m,²¹⁰Po maxima occur and unsupported²¹⁰Po is frequently present. Calculations indicate that at least 50% of the²¹⁰Po removed from the mixed layer is recycled within the thermocline. Similar calculations for²¹⁰Pb suggest much lower recycling efficiencies.Comparison of the²¹⁰Pb distribution with the reported distribution of²²⁶Ra at nearby GEOSECS stations has confirmed the widespread existence of a²¹⁰Pb/²²⁶Ra disequilibrium in the deep sea. Vertical profiles of particulate²¹⁰Pb were used to test the hypothesis that²¹⁰Pb is removed from deep water by in-situ scavenging. With the exception of one profile taken near the Mid-Atlantic Ridge, significant vertical gradients in particulate²¹⁰Pb concentration were not observed, and it is necessary to invoke exceptionally high particle sinking velocities to account for the inferred²¹⁰Pb flux. It is proposed instead that an additional sink for²¹⁰Pb in the deep sea must be sought. Estimates of the dissolved²¹⁰Pb/²²⁶Ra activity ratio at depths greater than 1000 m range from 0.2 to 0.8 and reveal a systematic increase, in both vertical and horizontal directions, with increasing distance from the sea floor. This observation implies rapid scavenging of²¹⁰Pb at the sediment-water interface and is consistent with a horizontal eddy diffusivity of 3?6 × 10⁷ cm²/sec. The more reactive element Po, on the other hand, shows evidence of rapid in-situ scavenging. In filtered seawater,²¹⁰Po is deficient, on the average, by ca. 10% relative to²¹⁰Pb; a corresponding enrichment is found in the particulate phase. Total inventories of²¹⁰Pb and²¹⁰Po over the entire water column, however, show no significant departure from secular equilibrium.     

87Rb/87Sr study of diogenites

The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the⁸⁷Rb/⁸⁶Sr,⁸⁷Sr/⁸⁶Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10^?11a^?1), the initial⁸⁷Sr/⁸⁶Sr ratio being BABI.     

Identification of excess 40Ar by the 40Ar/39Ar age spectrum technique

⁴⁰Ar/³⁹Ar incremental heating experiments on igneous plagioclase, biotite, and pyroxene that contain known amounts of excess⁴⁰Ar indicate that saddle-shaped age spectra are diagnostic of excess⁴⁰Ar in igneous minerals as well as in igneous rocks. The minima in the age spectra approach but do not reach the crystallization age. Neither the age spectrum diagram nor the⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar isochron diagram reliably reveal the crystallization age in such samples.     

226Ra, 210Pb and 210Po disequilibria in the Western North Pacific

²²⁶Ra,²¹⁰Pb and²¹⁰Po were measured in oceanic profiles at two stations near the Bonin and Kurile trenches.²¹⁰Po is depleted by 50% on average relative to²¹⁰Pb in the surface water. In the deep water,²¹⁰Pb is about 25% deficient relative to²²⁶Ra. Based on the deficiency,²¹⁰Pb residence time with respect to removal by particulate matter was estimated to be less than 96 years in the deep water.²¹⁰Pb deficiency in the bottom water was significantly greater than that of the adjacent deep water, indicating more effective removal near or at the bottom interface.²¹⁰Pb,²¹⁰Po and Th appear to have similar overall rate constants of particulate removal throughout the water column.     

143Nd/144Nd,87Sr/86Sr and trace element constraints on the petrogenesis of Aleutian island arc magmas

¹⁴³Nd/¹⁴⁴Nd,⁸⁷Sr/⁸⁶Sr and trace element results are reported for volcanic and plutonic rocks of the Aleutian island arc. The Nd and Sr isotopic compositions plot within the mantle array with ε_Nd values of from 6.5 to 9.1 and⁸⁷Sr/⁸⁶Sr ratios of from 0.70289 to 0.70342. Basalts have mildly enriched light REE abundances but essentially unfractionated heavy REE abundances, while andesites exhibit a greater degree of light to heavy REE fractionation. Both the basalts and andesites have significant large ion lithophile element to light rare earth element (LILE/LREE) enrichments. Variations in the isotopic compositions of Nd and Sr are not related to the spatial distribution of volcanoes in the arc, nor are they related to temporal differences. ε_Nd and⁸⁷Sr/⁸⁶Sr do not correlate with major element compositions but do, however, correlate with certain LILE/LREE ratios (e.g. Ba_N/La_N). Plutonic rocks have isotropic and trace element characteristics identical to some of the volcanic rocks. Rocks that make up the tholeiitic, calc-alkaline and alkaline series in the Aleutians do not come from isotopically distinct sources, but do exhibit some differing LILE characteristics.Given these elemental and isotopic constraints it is shown that the Aleutian arc magmas could not have been derived directly from homogeneous MORB-type mantle, or fresh or altered MORB subducted beneath the arc. Mixtures of partially altered MORB with deep-sea sediment can in principle account for the isotopic characteristics and most of the observed LILE/LREE enrichments. However, some samples have exceedingly high LILE/LREE enrichments which cannot be accounted for by sediment contamination alone. For these samples a more complex scenario is considered whereby dehydration and partial melting of the subducted slab, containing less than 8% sediment, produces a LILE-enriched (relative to REE) metasomatic fluid which interacts with the overlying depleted mantle wedge. The isotopic and LILE characteristics of the mantle are extremely sensitive to metasomatism by small percentages of added fluid, whereas major elements are not substantially effected, Major element compositions of Aleutian magmas are dominantly controlled by the partial melting of this mantle and subsequent crystal fractionation; whereas isotopic and LILE characteristics are determined by localized mantle heterogeneities.     

表生明矾石族矿物40Ar/39Ar年代学概述

近年来,激光40Ar/39Ar法成功测定了硫化物矿床氧化带中表生明矾石族矿物(明矾石和黄钾铁矾)的年龄,表明这类矿物适用于 40Ar/39Ar法定年。激光熔样方法引入40Ar/39Ar定年,不仅减少了样品的用量,同时也降低了系统的空白本底水平; 利用阶段加热法得到的年龄谱图,不仅可以有效识别原生矿物的混染和多世代矿物的存在,还可以检测样品中的过剩、继承Ar。虽然风化矿物定年还存在一些潜在的问题,但是只要有科学的采样和合理的分析流程,并结合其他检测手段,就可以获得有意义的风化年龄。这为研究新生代以来化学风化、氧化带形成时间和古气候提供了新的方法。     

230Th,226Ra and222Rn in abyssal sediments

A model that predicts the flux of²²²Rn out of deep-sea sediment is presented. The radon is ultimately generated by²³⁰Th which is stripped from the overlying water into the sediment. Data from many authors are compared with the model predictions. It is shown that the continental contribution of ionium is not significant, and that at low sedimentation rates, biological mixing and erosional processes strongly affect the surface concentration of the ionium. Two cores from areas of slow sediment accumulation, one from a manganese nodule region of the central Pacific and one from the Rio Grande Rise in the Atlantic were analyzed at closely spaced intervals for²³⁰Th,²²⁶Ra, and²¹⁰Pb. The Pacific core displayed evidence of biological mixing down to 12 cm and had a sedimentation rate of only 0.04 cm/kyr. The Atlantic core seemed to be mixed to 8 cm and had a sedimentation rate of 0.07 cm/kyr. Both cores had less total excess²³⁰Th than predicted.Radium sediment profiles are generated from the²³⁰Th model. Adsorbed, dissolved, and solid-phase radium is considered. According to the model, diffusional losses of radium are especially important at low sedimentation rates. Any particulate, or excess radium input is ignored in this model. The model fits the two analyzed cores if the fraction of total radium available for adsorption-desorption is about 0.5–0.7, and ifK, the distribution coefficient, is about 1000.Finally, the flux of radon out of the sediments is derived from the model-generated radium profiles. It is shown that the resulting standing crop of²²²Rn in the overlying water may be considered as an added constraint in budgeting²³⁰Th and²²⁶Ra in deep-sea sediments.     

The effects of assimilation of country rocks by magmas on 18O/16O and 87Sr/86Sr systematics in igneous rocks

Examples of positive correlations between initial ⁸⁷Sr/⁸⁶Sr and δ¹⁸O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high-¹⁸O, high-⁸⁷Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope “mixing” curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ¹⁸O-⁸⁷Sr/⁸⁶Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ¹⁸O-⁸⁷Sr/⁸⁶Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.     

241Pu/239+240Pu ratios in polar glaciers

The analyses of plutonium isotopes in dated strata of polar ice sheets indicate that the²⁴¹Pu/^239+240Pu activity ratio produced in the U.S.-dominated nuclear tests during the 1950s was about 26, while that in the U.S.S.R.-dominated weapons tests in the early 1960s was between 12 and 14. This difference provides time horizons for sedimentary deposits. Further, the²³⁹Pu/²⁴⁰Pu ratio may show a similar difference in fallout values from these two periods of weapons testing and may provide an additional, and perhaps more sensitive, chronology for sediments.     

210Pb/226Ra disequilibrium in the Santa Barbara Basin

The vertical distributions of²¹⁰Pb and²²⁶Ra in the Santa Barbara Basin have been measured. The²¹⁰Pb/²²⁶Ra activity ratio is close to unity in surface water, but ranges from 0.2 to 0.6 in deep water with a mean value of 0.3 (d > 250m), suggesting rapid removal of²¹⁰Pb from the water column. The²¹⁰Pb concentrations in the particulate phase at different water depths indicate that the removal of²¹⁰Pb is due to adsorption on settling particles.It is estimated that the particulate²¹⁰Pb contributes about 50–70% of the total²¹⁰Pb measured on unfiltered water samples of the Santa Barbara Basin. The fate of²¹⁰Pb (and Pb) in the water column is thus strongly controlled by the settling particles, which have a mean residence time of one year or less in the basin. Material balance calculation for²¹⁰Pb in the basin suggests that there is an external source supplying about 70–80% of the²¹⁰Pb observed in particulate material or sediments. This excess²¹⁰Pb is most likely provided by particles entering the basin loaded already with²¹⁰Pb.     

18O/16O ratios in sulfate from fossil shells

The measurement of the oxygen isotopic composition of the sulfate separated from fossil shells supplies no reliable information on the isotopic values of dissolved ocean water sulfate in the past. The results obtained suggest that fossils behave as open systems from the point of view of the possible influence of post-depostional processes on the isotopic composition of shell sulfate.     

226Ra,210Pb and210Po in the Red Sea

Profiles of²²⁶Ra and dissolved²¹⁰Pb have been measured at several stations in the Red Sea. At one station in the central Red Sea an expanded profile was measured including²²⁶Ra and dissolved and particulate²¹⁰Pb and²¹⁰Po. These profiles show several distinct features: (1)²²⁶Ra displays a mid-depth maximum of about 13 dpm/100 kg at about 500 m; (2) dissolved²¹⁰Pb concentrations are uniformly low at about 2 dpm/100 kg with little lateral or vertical variation; (3) the surface-water²¹⁰Pb excess which is commonly observed in low-latitude open ocean regions is entirely lacking; (4)²¹⁰Pb and²¹⁰Po activities are essentially identical to each other in both particulate and dissolved phases although²¹⁰Po activities appear somewhat lower; (5) about 20% of the²¹⁰Pb and²¹⁰Po in the water column residues on particulate matter.Assuming the atmospheric²¹⁰Pb flux to be in the dissolved form and at the lower level of the normal range i.e. 0.5 dpm/cm² yr, the residence time of the dissolved Pb is about 1.5 years. However, if the same atmospheric flux is entirely in particulate form, then the residence time of the dissolved Pb is about 5 years. The residence time of Pb in the particulate phase is less than 0.4 years if all the Pb is removed only by sinking particles.     

18O/16O ratios in lunar fines and rocks

¹⁸O/¹⁶O ratios have been measured for Luna 20 and Apollo 15 fines and Apollo 15 rocks.Isotopic composition and fractionation between minerals are compared with previous results.Partial fluorination experiments on Luna 20 soil and Apollo 15021 extreme fines show large¹⁸O enrichments in grain surfaces. These results are discussed.     

Removal of 230Th and 231Pa at ocean margins

Uranium, thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the Panama Basin and by in-situ filtration of large volumes of seawater in the Panama and Guatemala Basins. Concentrations of dissolved Th and Pa isotopes were determined by extraction onto MnO₂ adsorbers placed in line behind the filters in the in-situ pumping systems.Concentrations of dissolved ²³⁰Th and ²³¹Pa in the Panama and Guatemala Basins are lower than in the open ocean, whereas dissolved ²³⁰Th/²³¹Pa ratios are equal to, or slightly greater than, ratios in the open ocean. Particulate ²³⁰Th/²³¹Pa ratios in the sediment trap samples ranged from 4 to 8, in contrast to ratios of 30 or more at the open ocean sites previously studied. Particles collected by filtration in the Panama Basin and nearest to the continental margin in the Guatemala Basin contained ²³⁰Th/²³¹Pa ratios similar to the ratios in the sediment trap samples. The ratios increased with distance away from the continent.Suspended particles near the margin show no preference for adsorption of Th or Pa and therefore must be chemically different from particles in the open ocean, which show a strong preference for adsorption of Th. Ocean margins, as typified by the Panama and Guatemala Basins, are preferential sinks for ²³¹Pa relative to ²³⁰Th. Furthermore, the margins are sinks for ²³⁰Th and, to a greater extent, ²³¹Pa transported by horizontal mixing from the open ocean.     

234Th/238U activity ratios in Pacific Ocean bottom waters

The²³⁴Th/²³⁸U activity ratios in the near-bottom waters at a station in the South Pacific have been measured. The activity ratios are close to the secular equilibrium value, ranging between 0.9 and 1.13 (± 8%), suggesting that the rate of removal of²³⁴Th by bottom-water scavenging processes at this station is slow compared to its rate of radioactive decay. The mean²³⁴U/²³⁸U activity ratios in these waters is 1.14 ± 0.02, the same as the reported values for the world oceans.     

Intercalibration of international and domestic 40Ar/39Ar dating standards

Four international standards, Ga1550, MMhb-1, Lp-6, Bem 4M, and one domestic standard BT-1 have been intercalibrated. The repeated measurements on MMhb-1 with different mass demonstrate that MMhb-1 is inhomogeneous in age and its average age is 519.8 Ma. The results of Bern 4M and Lp-6 reflect that they have an invariable value of ⁴⁰Ar*/³⁹Ar_k (F) and the ages we obtained are consensus with their K-Ar age: Lp-6=127.7Ma; Bern 4M=18.2 Ma. Analyses of BT-1 age spectra, Ca/K and Cl/K spectra as well as inverse isochrons indicate that the sample is homogeneous and invariable and keeps close chemically, with its trapped argon isotope composition close to the atmosphere. The dating results show that age values are reproducible and steady, total fusion age, step-heating age, plateau age and isochron age are in accord with each other within the error range (2σ). Therefore, we recommend 28.7 Ma as the calibrated age of BT-1. We also discuss the variation in neutron flux gradients of Beijing 49-2 reactor. It was found that the neutron flux gradient varies considerably, and more monitors (standard samples) are needed to fix the trend of variation. The coefficient of the 49-2 reactor that transfers the ratio of production rate of ³⁷Ar_Ca/³⁹Ar_K into Ca/K ratio is 1.78. This is different from that reported earlier, 2.0, which may be caused by the reconstruction of the reactor.     

87Sr/86Sr ratios for basalt from Loihi Seamount,Hawaii

⁸⁷Sr/⁸⁶Sr ratios of 15 samples of basalt dredged from Loihi Seamount range from 0.70334 to 0.70368. The basalt types range from tholeiite to basanite in composition and can be divided into six groups on the basis of abundances of K₂O, Na₂O, Rb and Sr and ⁸⁷Sr/⁸⁶Sr ratio. The isotopic data require that the various basalt types be derived from source regions differing in Sr isotopic composition. The Loihi basalts may be produced by mixing of isotopically distinct sources, but the tholeiites and alkalic basalts from Loihi do not show a well-developed inverse trend between Rb/Sr and ⁸⁷Sr/⁸⁶Sr that is characteristic of the later stages of Hawaiian volcanoes such as Haleakala and Koolau.     

40Ar/39Ar dating of the long range dikes,Newfoundland

Stepwise outgassing experiments were performed on eleven whole rock samples from a sequence of diabase dikes from the Great Northern Peninsula. Gas released at high temperatures (> ? 925–950°C) was consistently characterized by anomalously high apparent ages. These are attributed to the release of excess radiogenic argon, gas which probably resides in the mineral augite. At temperatures below ? 700°C, a wide range in apparent age was observed. On the other hand, the medium temperature region (? 700–950°C) is characterized by a relatively small range in apparent age. Data points corresponding to this region are rather well correlated in an isochron-type diagram; the slope of the best-fitting straight line corresponds to an aget = 605 ± 10m.y. We suggest that the outgassing of unaltered plagioclase is dominating the age spectra in this temperature interval. Consequently, we take 605 ± 10 m.y. as the time of emplacement of the Long Range dike swarm. This event may have marked the opening of a proto-Atlantic Ocean. The Acadian Orogeny (perhaps the ocean-closing event ? 380 m.y. ago) appears to be recorded in the low-medium temperature regions in the cases of two rather extensively altered samples.     

40Ar/39Ar age and thermal history of the Kirin chondrite

The Kirin meteorite, a large (2800kg) H5 chondrite, fell in Kirin Province, China in 1976. A sample from each of the two largest fragments (K-1, K-2) yield⁴⁰Ar/³⁹Ar total fusion ages of 3.63 ± 0.02b.y. and 2.78 ± 0.02b.y. respectively.⁴⁰Ar/³⁹Ar age spectra show typical diffusional argon loss profiles. Maximum apparent ages of 4.36 b.y. (K-1) and ～4.0 b.y. (K-2) are interpreted as possible minimum estimates for the age of crystallization of the parent body.The⁴⁰Ar/³⁹Ar ages found for gas released at low temperature are about 2.2 b.y. for K-1 and about 0.5 b.y. for K-2, suggesting that this meteorite may have suffered two discrete collisional events that caused degassing of radiogenic argon. Modelling of possible thermal events in the parent body indicates that samples K-1 and K-2 were at a depth of less than 3 m from the base of an impact melt of a thickness less than 7 m and separated by no more than ～2 m from one another at the time of the heating event about 0.5 b.y. ago. Further, the duration of heating was probably less than a few years.Calculations from³⁸Ar data yield exposure ages for samples K-1 and K-2 of about 5 m.y., similar to that found for many other H chondrites.