Thorium and radium isotopic relationships in manganese nodules and sediments at MANOP Site S     

Willard S Moore 《Geochimica et cosmochimica acta》1984,48(5):987-992

Relationships among Th and Ra isotopes in nodule, sediment and water phases at MANOP Site S establish the most likely source for Th in the nodules, the frequency of nodule turning, and the similarity of micro and macro nodules. Manganese nodules and bottom waters have ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios considerably higher than other phases at this site suggesting that sea water is the likely source of Th for the nodules. Similar ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios in nodule tops and bottoms and in certain cases departure from expected ${}^{226}\text{Ra}{}^{230}\text{Th}$ activity ratios in nodule tops and bottoms indicate that the nodules rotate every one to ten thousand years. The micro nodules have diffusion coefficients of Ra similar to macro nodule bottoms. I suggest that they may act as a carrier phase for transporting metals through oxic sediments to nodules.  相似文献   

Application of the ${}^{227}\text{Th}{}^{230}\text{Th}$ method to dating Pleistocene carbonates and comparison with other dating methods     

M. Gascoyne 《Geochimica et cosmochimica acta》1985,49(5):1165-1171

The ${}^{227}\text{Th}{}^{230}\text{Th}$ dating method is described in detail and its usefulness investigated by comparing ages of sixteen Pleistocene carbonates (mainly cave deposits) with those determined by the ${}^{231}\text{Pa}{}^{235}\text{U}$ and ${}^{230}\text{Th}{}^{234}\text{U}$ methods. The ${}^{227}\text{Th}{}^{230}\text{Th}$ ages are found to be critically dependent on corrections for decay of ²²⁷Th prior to alpha counting and ingrowth of daughter isotopes of ²³²Th derived from clastic detritus. Of nineteen sets of ages determined for the sixteen samples, good agreement is found for only seven sets. Differences are attributed to low U content of some samples and the possibility of excess ²²⁷Th in the calcite of samples younger than ~50 ky, possibly due to the coprecipitation of ²³¹Pa during formation. Calculated “negative” ${}^{227}\text{Th}{}^{230}\text{Th}$ ages may be a direct result of this process and the fact that, unlike the other methods, the activity ratio is non-zero at zero age. Nevertheless, the ${}^{227}\text{Th}{}^{230}\text{Th}$ is found to be a useful alternative dating technique for carbonates which are between ~50 and 300 ky, because no spiking is required. It also serves as a check for partial concordancy with ages dated by the other methods.  相似文献   

Comparison of 230Th-238U disequilibrium systematics in lavas from three hot spot regions: Hawaii,Prince Edward and Samoa     

S Newman  R.C Finkel  J.D Macdougall 《Geochimica et cosmochimica acta》1984,48(2):315-324

²³⁰Th-²³⁸U disequilibrium systematics reveal several important characteristics of the mantle source regions and petrogenesis of volcanic rocks in the presumed hot spots of Hawaii, Marion Island (Prince Edward hot spot), and Samoa. The (${}^{230}\text{Th}{}^{232}\text{Th}$) activity ratios of lavas from these three hot spots (1.06 ± 0.07, 1.04 ± 0.08, and 0.81 ± 0.06, respectively) imply that the source regions are each nearly homogeneous with $\text{Th}\text{U}$ weight ratios of 2.9, 3.0, and 3.8. For Marion Island and Mauna Kea, Hawaii, negligible secular variation occurs in the (${}^{230}\text{Th}{}^{232}\text{Th}$) initial ratios. This supports other evidence for very short transfer time between source and surface. Significant residence time at depth prior to eruption cannot be ruled out for the Samoan lavas we have studied; however, the data for one of these flows deviate from the proposed (${}^{230}\text{Th}{}^{232}\text{Th}$)-${}^{87}\text{Sr}{}^{86}\text{Sr}$ correlation (Condomineset al., 1981a) in the opposite sense from that expected for such residence. If it is assumed that the measured (${}^{230}\text{Th}{}^{232}\text{Th}$) ratios of the young lavas reflect $\text{Th}\text{U}$ in their mantle sources, then the observed variations among these three hot spots, combined with those reported by other workers for Iceland, the Azores and Tristan de Cunha, suggest that these sources are characterized by $\text{Th}\text{U}$ ratios ranging from values similar to that of MORB source (~2.5) to values similar to those of bulk earth (~3.8). Mixing of different proportions of depleted and enriched mantle may be responsible for the observed range.  相似文献   

Radiochemical observations on manganese nodules from three sedimentary environments in the north Pacific     

Hun Chih-An  Ku Teh-Lung 《Geochimica et cosmochimica acta》1984,48(5):951-963

Decay-series isotopes were measured on manganese nodules from three sedimentary environments, characterized by substrates of red clay (R), siliceous ooze (S) and hemipelagic clay (H). Growth rates of nodules are shown to be site dependent: 1–2 mm/Myr at site R, 3–8 mm/Myr at sites S, and 20–50 mm/ Myr at site H. Correlation between growth rate and the parameter Mn/Fe² suggests that regional diagenetic variations control the compositions and growth rates of the nodules. The frequency of nodule turnover and the period of their growth are assessed from the “top” vs. “bottom” distributions of several nuclides. Based on the ²³⁰Th, ²³¹Pa and ²²⁶Ra data, turnover times of 10³ to 10⁵ years are estimated and they vary with relative size and shape of the specimens at a given site. The presence of unsupported ²¹⁰Pb and ²²⁸Th in the top surfaces but not in the bottom surfaces of the surface nodules suggests active growth during the last decades or years, despite their old ages. The data also indicate that turnover rates are not more frequent than once every several years or decades.The ²³⁰Th-²²⁶Ra, ²²⁶Ra-²¹⁰Pb and ²³²Th-²²⁸Th disequilibrium relationships in the nodules allow the migratory behavior of ²²⁶Ra, ²²²Rn and ²²⁸Ra and their fluxes to be deduced: thus radium leaves the top sides but enters the bottom sides of the surface nodules at sites R and S. At site R there is a net loss of radium to the sea; the opposite is true at site S. Surface nodules at site H trap radium from both sides, probably due to more intense diagenetic input of radium from sediment pore water. The effective diffusivities of ²²⁶Ra in nodules vary from $\text{3 × 10}{}^{\mathrm{?12}}$ to $\text{8 × 10}{}^{\mathrm{?14}}$ cm²/s, dependent on the textural variation of the nodule material, which crudely reflects the growth rate and hence sedimentary environment. ²²²Rn diffuses out of nodules from all sides, with an effective diffusivity of ca. $\text{(2–6) × 10}{}^{\mathrm{?8}}\text{cm}{}^2\text{/s}$. At all sites nodules serve as a more effective source of ²²²Rn to sea water than their adjacent sediments. The outward flux of ²²²Rn from nodules relative to that from adjacent sediments tends to be higher in more reduced environments, an effect caused by the fact that Mn-rich nodules from more reduced environment act as a more efficient trap for ²²⁶Ra.  相似文献   

Redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite     

Jacqueline Michel 《Geochimica et cosmochimica acta》1984,48(6):1249-1255

Previous studies of the distribution of U and Th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering. In this study, the distribution of U and Th decay series isotopes was determined in a weathering profile of a granitic saprolite, which showed textural preservation indicating isovolumetric weathering. Two types of dissolution methods were used: a whole-rock dissolution and a sodium-citrate dithionite leach to preferentially attack noncrystalline phases of weathering products. Using volume-based activities, 45–70 percent of the total ²³²Th was gradually removed during weathering. Although the whole-rock ${}^{228}\text{Th}{}^{232}\text{Th}$ activity ratios were in equilibrium, there were large excesses of ²²⁸Th in the leachable fraction of both parent rock (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 2.06}$) and partially weathered saprolite (${}^{228}\text{Th}{}^{232}\text{Th}\text{= 3–6.5}$), due to alpha recoil and release of daughter ²²⁸Th to the weathering rind of the mineral grain. For the most weathered sample, 81 percent of the thorium was in the teachable fraction and ${}^{228}\text{Th}{}^{232}\text{Th}\text{= 1}$, indicating that even the more resistant minerals were attacked.The total U activities showed as much variation in the six parent rock samples as in the weathered profile, and ${}^{234}\text{U}{}^{238}\text{U}$ were in equilibrium in both the whole-rock and leachable fractions. ²³⁰Th was deficient relative to ²³⁴U and ²²⁶Ra in both fractions, suggesting recent addition of U and Ra to the entire profile. The large variation in U was not from absorption onto the intermediate weathering products, because only 11–23 percent of the U was in the leachable fraction.  相似文献   

Incorporation of Be isotopes and other trace metals into marine ferromanganese deposits     

Masashi Kusakabe  Teh-Lung Ku 《Geochimica et cosmochimica acta》1984,48(11):2187-2193

Two-to threefold inward increase of ¹⁰Be, ⁹Be, Cu and Ni have been found in surface layers of marine ferromanganese deposits. We interpret this phenomenon as reflecting the manner in which the Be isotopes and probably other trace metals (e.g. Cu and Ni) are incorporated. On reaching the sea floor exchangeable Be is released from its carrier phase to the pore water of the ferromanganese deposits, diffuses inward and gets fixed into the deposits. The fixation process may be related to the mineralogical reorganizations suggested by previous studies. We have modeled this concept and applied it to a Mn crust on which detailed analyses of Cu, Ni and natural decay-series nuclides, as well as Be isotopes are made. Post-depositional fixation of Th isotopes may also occur. The redistributions of Be and Th isotopes are confined to regions very close to the surface of the deposits. For all practical purposes, they will not affect growth rate determinations conventionally done using these isotopes. The “linearly” extrapolated ${}^{10}\text{Be}{}^9\text{Be}$ ratio to the surface from deep layers of a Mn deposit does not necessarily represent that ratio in sea water.  相似文献   

Uranium-series dating of lacustrine limestones from pan deposits with final Acheulian assemblage at Rooidam,Kimberley district,South Africa     

B.J. Szabo  K.W. Butzer 《Quaternary Research》1979,11(2):257-260

Lacustrine limestone samples from sedimentary pan deposits at Rooidam, near Kimberley, South Africa, that contain late Acheulian (Fauresmith) artifacts have been dated by ${}^{230}\text{Th}{}^{234}\text{U}$ and ${}^{231}\text{Pa}{}^{235}\text{U}$ methods. Results indicate a minimum age of about 200,000 yr B.P. for the terminal Acheulian in the interior of South Africa.  相似文献   

Anomalously high concentrations of uranium,radium and radon in water from drilled wells in the Helsinki region     

M Asikainen  H Kahlos 《Geochimica et cosmochimica acta》1979,43(10):1681-1686

The concentrations of uranium. ²²⁶Ra and ²²²Rn were determined in 308 drilled and 58 dug wells in the Helsinki region. The study area was about 400 km² and geologically highly variable, with granites, amphibolites and migmatites the dominant rocks. The radioactivity of water in the dug wells was on a “normal” level, but in numerous drilled wells it was anomalously high. In 14 drilled wells the concentration of uranium exceeded 1000 μg/l, the highest concentration being 14,870 μg/l. For ²²²Rn the maximum concentration was 880,000 pCi/l. The ${}^{226}\text{Ra}{}^{228}\text{Ra}$ and ${}^{230}\text{Th}{}^{232}\text{Th}$ activity ratios showed the isotopes of the uranium series to be dominant in the study area. A state of disequilibrium between ²³⁸U and ²³⁴U was very common in the samples. The ${}^{234}\text{U}{}^{238}\text{U}$ activity ratios varied in the range 1.0–4.0 regardless of the amount of uranium in the water. The conclusion can be drawn from the isotopic data that the high radioactivity of water is in some cases caused by primary uranium mineralizations, but mostly by uranium deposited in fissures of the bedrock. The paper includes a summary of the results of two studies carried out between 1967 and 1977.  相似文献   

Accumulation rates of Th-230, Pa-231, and some transition metals on the Bermuda Rise     

Michael P Bacon  John N Rosholt 《Geochimica et cosmochimica acta》1982,46(4):651-666

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 ${}^{231}\text{Pa}{}_{\mathrm{ex}}{}^{230}\text{Th}{}_{\mathrm{ex}}$ 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, ²³⁰Th_ex, and ²³¹Pa_ex 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 ²³⁰Th_ex 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   

The effect of bioturbation and adsorption gradients on solid and dissolved radium profiles in sediments from the eastern equatorial Pacific     

《Geochimica et cosmochimica acta》1987,51(6):1613-1623

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Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from 230Th measurements     

Michael P. Bacon 《Chemical Geology》1984,46(2):97-111

The cause of the climatically controlled fluctuations in the carbonate content of deep-sea sediments remains the subject of uncertainty and debate. Three variables are involved: supply of biogenic carbonate, loss by dissolution, and dilution by non-carbonate phases. It is suggested that ²³⁰Th, which is produced in the ocean at a constant rate provides a reliable reference for measuring variations in rate of sedimentation on a regional scale. Results of a preliminary analysis based on published data indicate that, for depths at and above the lysocline, the carbonate fluctuations observed in cores from the North Atlantic Ocean are due primarily to variations in the terrigenous clay input, which was 2–5 times higher during glacials than during interglacials. Carbonate deposition appears to have been somewhat reduced during glacials, but probably not by more than a factor of 2. From published ${}^{230}\text{Th}{}^{232}\text{Th}$ profiles it appears that the South Atlantic Ocean also received increased inputs of terrigenous clay during glacial periods.  相似文献   

The RbSr isotope system as an index of origin and diagenetic evolution of southern Pacific red clays     

Norbert Clauer  Michel Hoffert  Anne-Marie Karpoff 《Geochimica et cosmochimica acta》1982,46(12):2659-2664

Morphological, mineralogical, chemical and RbSr isotopic studies have been made on Fesmectites (nontronites) from southern Pacific red clays cored near the Marquisas Islands. These minerals have at the top of the core, an ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of 0.70917 ± 0.00007, which indicates an authigenic origin in isotopic equilibrium with seawater. Weak leaching experiments with 1N HCl show that the nontronites also contain a volcanic component with a lower ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio which, combined with the morphology of the particles, suggests a transportation by bottom currents of clay formed elsewhere.During burial, the nontronites experience diagenetic modifications resulting in an increase in Fe, K and Rb contents, a concomitant decrease of Mg, Ca, Ti, Na and Sr, and a preferential migration of radiogenic ⁸⁷Sr from the clays into the surrounding pore waters.The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Sr adsorbed on the outermost surfaces of the nontronites does not change with depth in the core, and is, therefore, independent of diagenetic influence, which is rather characterized by the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the interstitial waters. The isotopic composition of both the adsorbed Sr and that of the pore fluids may yield useful information on the crystallization environment and subsequent history of deep sea red clays.  相似文献   

Magmatic evolution of a volcano studied by ²³⁰Th-²³⁸U disequilibrium and trace elements systematics: The Etna case     

M Condomines  J.C Tanguy  G Kieffer  C.J Allègre 《Geochimica et cosmochimica acta》1982,46(8):1397-1416

Age determinations of several lava flows from Etna through ²³⁰Th-²³⁸U disequilibrium (internal isochrons) yield a precise chronology of the volcano's history for the last 200,000 years, and emphasize the main episodes in the formation of this huge complex strato-volcano. Study of $\text{(}{}^{230}\text{Th}{}^{232}\text{Th}\text{)}{}_0$ initial ratios of lavas together with their trace-element compositions yields a consistent model of magmatic evolution implying the existence, for 200,000 years, of a deep reservoir of alkalic magma periodically mixed with magmas of tholeiitic affinity. These short periods of mixing appear to be related to the formation of the large calderas of Etna. In addition to these processes affecting the deep reservoir, fractional crystallization also occurred in more superficial levels of the volcanic edifice, thereby yielding several series of differentiation of relatively short duration. As for its geochemistry, Etna's volcanism is of oceanic type but with its own characteristics and in a peculiar geodynamic context, at the edge of the African Plate.  相似文献   

$\text{U}{}^{234}\text{U}{}^{238}$ Ratios in limestone cave seepage waters and speleothem from West Virginia     

Peter Thompson  Derek C Ford  Henry P Schwarcz 《Geochimica et cosmochimica acta》1975,39(5):661-669

Speleothem from West Virginia, ranging in age from 2000 to 200,000 yr B.P. contains uranium with $\text{U}{}^{234}\text{U}{}^{238}$ ratios significantly greater than unity. This ratio varies from one speleothem to another, as does average U content. Initial ratios, corrected for age, are remarkably constant for a given speleothem. By contrast, $\text{U}{}^{234}\text{U}{}^{238}$ ratios in seepage waters vary significantly from month to month at a given drip site, and their average values differ from that of the speleothem which they are depositing. This discrepancy is attributed either to long-term averaging-out of fluctuations, or fractional precipitation on the speleothem of a chemical species of uranium with a more constant ratio. Constancy of initial $\text{U}{}^{234}\text{U}{}^{238}$ ratios from $\text{Th}{}^{230}\text{U}{}^{234}$. datable portions of speleothems should permit $\text{U}{}^{234}\text{U}{}^{238}$-dating of older portions of the same speleothem, back to about 10⁶ yr B.P., with estimated precision of ±5 per cent.  相似文献   

Paleoclimatic implications of U-series dates for lake sediments and travertines in the Arava Rift Valley,Israel     

A. Livnat  J. Kronfeld 《Quaternary Research》1985,24(2):164-172

The Sayif travertines and lacustrine limestones cropping out in the northern Arava Valley, a region of extreme aridity, have been dated by the ${}^{230}\text{Th}{}^{234}\text{U}$ disequilibrium method. The consistency between the stratigraphic position and the apparent ages at each of the sampling sites strongly supports the reliability of the ages. Most of the ages fall within two clusters that are coincident with ages representative of oceanic oxygen-isotope stages 5 and 7. The colder intervening stage 6 was not a period of substantial travertine formation. During the warmer global episodes of isotopic stages 5 and 7 the arid south of Israel apparently was considerably wetter than at present.  相似文献   

Holocene sea-level changes in Palau,West Caroline Islands     

William H. Easton  Teh-Lung Ku 《Quaternary Research》1980,14(2):199-209

¹⁴C ages supplemented by ${}^{230}\text{Th}{}^{234}\text{U}$ determinations have been obtained for calcareous deposits on Koror, Babelthuap, Auluptagel, and Adorius Islands in the Palau Group, West Caroline Islands. Test borings for a bridge between Koror and Babelthuap reveal shoreface terraces consisting largely of bioclastic sand, but resembling fringing reefs. The base of the shoreface terraces dates to between 7000 and 8000 ¹⁴C yr B.P. and the upper surfaces are slightly younger than 4000 yr B.P. Spacing of the subsurface isochrons indicates that the rate of sedimentation increased up-section on Koror terrace and decreased up-section on Babelthuap terrace. The average rate of deposition in the terraces was 0.5 cm/yr. Isochrons in the Babelthuap shoreface terrace are 7 m higher than those in the Koror terrace. If deposition was sufficient to keep the surfaces of the terraces at low tide level, then the Babelthuap side was essentially stable and the Koror side was uplifted between about 8000 and 6300 yr ago, and then subsided until 4000 yr ago; since then there has been about 2 m of uplift. “Top hat” microatolls on Koror terrace indicate that relative sea level has dropped about 30 cm in the past 75 yr. In the absence of good evidence for changes of level in water ponded in a moat, it is likely that the microatolls indicate uplift of the terrace. On the other hand, if tectonic activity was minimal, then differences in the two terraces are due to differences in sedimentation with the Koror side of the channel being substantially subtidal between about 7000 and 5000 yr B.P. Taking the composite eustatic sea-level curve of Hawaii and elsewhere as a reference standard, it is deduced that Auluptagel Island has risen 0.8 m in the last 2900 yr, and Adorius Island has risen approximately 8 m in the last 5000–6000 yr.  相似文献   

Ground water geochemistry of ²²⁸Ra, ²²⁶Ra and ²²²Rn     

Philip T. King  Jacqueline Michel  Willard S. Moore 《Geochimica et cosmochimica acta》1982,46(7):1173-1182

²²⁸Ra, ²²⁶Ra, and ²²²Rn activities were determined on over 150 ground water samples collected from drilled, public water supply wells throughout South Carolina. A wide range of aquifer lithologies were sampled including the crystalline rocks of the Piedmont and sedimentary deposits of the Coastal Plain. A significant linear relationship between log ²²⁸Ra and log ²²⁶Ra (n = 182, r = 0.83) was indistinguishable between Piedmont and Coastal Plain ground water. Median ${}^{228}\text{Ra}{}^{226}\text{Ra}$ activity ratios for the Piedmont, 1.2, and Coastal Plain, 1.3, ground water are close to estimated average crustal ${}^{232}\text{Th}{}^{238}\text{U}$ activity ratios of 1.2 to 1.5 corresponding to Th/U weight ratios of 3.5 to 4.5. A linear correlation was also found between log ²²²Rn and log ²²⁶Ra for Piedmont (n = 68, r = 0.62) and Coastal Plain (n = 89, r = 0.64) ground water. However, the median ${}^{222}\text{Rn}{}^{226}\text{Ra}$ activity ratio for Piedmont ground water, 6100, was much higher than for Coastal Plain ground water, 230. Higher excess ²²²Rn activities may be due to greater retention of ²²⁶Ra by the chemically active Piedmont aquifers compared to the more inert sand aquifers sampled in the Coastal Plain. The relationship between log ²²⁸Ra and log ²²⁶Ra was used to predict total Ra (${}^{228}\text{Ra +}{}^{226}\text{Ra}$) distributions in Appalachian and Atlantic and Gulf Coastal Plain ground water. Predictions estimate that 2.4% of Appalachian and 5.3% of Atlantic and Gulf Coastal Plain ground water supplies contain total Ra activities in excess of the 5 pCi/l limit established by the U.S. Environmental Protection Agency. These predictions also indicate that 40–50% of these ground water wells may be overlooked using the presently suggested screening activity of 3.0 pCi/l of ²²⁶Ra for ²²⁸Ra analysis.  相似文献   

Uplift rates of the Loyalty Islands as determined by ${}^{230}\text{Th}{}^{234}\text{U}$ dating of raised coral terraces     

John F. Marshall  Jean Launay 《Quaternary Research》1978,9(2):186-192

Uplift rates of the Loyalty Islands (S.W. Pacific) have been determined from ${}^{230}\text{Th}{}^{234}\text{U}$ dating of raised coral terraces standing 2 to 7.5 m above sea level. The ages of the terraces on Ouvéa and Lifou correspond closely to previously documented periods of high sea levels at 120,000 and 180,000 yr BP. A +2-m terrace on Beautemps-Beaupré is considered to be beyond the range of the dating technique. The uplift rates of the various islands show a decrease toward the west. This lends support to the hypothesis of a lithospheric bulge of the leading edge of the Australian plate prior to its subduction at the level of the New Hebrides trench.  相似文献   

Helium-uranium dating of corals     

Michael L. Bender 《Geochimica et cosmochimica acta》1973,37(5):1229-1247

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UThPb systematics of the eucrite “Juvinas”: Precise age determination and evidence for exotic lead     

G Manhes  C.J Allegre  A Provost 《Geochimica et cosmochimica acta》1984,48(11):2247-2264

The U-Th-Pb isotope systematics of the eucrite “Juvinas” have been studied in whole rock fragments as well as in plagioclases and pyroxenes. The results show that this monomict breccia crystallized with a very high $\text{U}\text{Pb}$ initial ratio at T = 4.539 ± 0.004 AE ago. There is evidence for a less radiogenic Pb component (${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 13.0}$; ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 13.5}$; ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 32.71}$) interpreted as “exotic lead” induced by a meteoritical impact at the surface of the Juvinas parent body, 1.92 ± 0.06 AE ago.  相似文献