234Th/238U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle-reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of 234Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of 234Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a 229Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low 234Th/228Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure 234Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity. 相似文献
Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S‐type granitoids; (ii) a small I‐type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I‐types; and (iii) a suite of dyke rocks ranging in composition from calc‐alkaline lamprophyre through hornblende and biotite porphyrite to aplite. Ion‐microprobe U‐Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 ±3 Ma (weighted mean 206Pb/238U age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S‐type granitoids from a relatively juvenile protolith. Deformation of the Rockvale Adamellite occurred after complete crystallization of the pluton and prior to emplacement of dykes and I‐type intrusives. K‐Ar biotite and hornblende ages show broadly synchronous intrusion of I‐type magmas and lamprophyre dykes at ca 255 Ma, indicating that mantle magmatism associated with lamprophyres was contemporaneous with the crustal production of I‐type melts. Chemical similarities between the most mafic Moonbi Suite members and calc‐alkaline lamprophyres may also indicate a direct mantle contribution to some I‐type magmas. 相似文献
The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics
which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could
be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from
the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity
force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the
fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system
and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13
AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation
of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini,
for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which
would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect
to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the
Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and
fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition
to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend
the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in
the outer Solar System [(4) in the above list].
相似文献
Broadly the science of geology has passed through a number of distinct phases. In the early days attention was focussed on establishing a stratigraphic framework, concentrating on fossils and lithologies—the days of mapping and systematizing of sedimentary successions and the uncovering of the succession of life. Later, in the early twentieth century, geologists became much more interested in igneous rocks. By the 1960s attention turned to the ocean basins, culminating in the acceptance of the paradigm of plate tectonics. At the end of the 1960s, one area of geology that remained relatively little understood was the huge span of time represented by the Precambrian, about 80 per cent of Earth's history. By the 1960s this was changing. Radiometric dating was beginning to show the relative ages of such terranes and new methods of mapping were beginning to be used. 相似文献
The Hillgrove mineral field, in the southern part of the New England Orogen of northeastern New South Wales, Australia, contains numerous mesothermal Au---Sb vein systems. Calc-alkaline (shoshonitic) lamprophyre (CAL) dykes are also associated with mineralisation with dilational lode structures acting as conduits for dyke intrusion, which has occurred before and after major quartz-stibnite veining. Dykes include minette and vogesite compositions and were emplaced in the late Permian (247–255 Ma), at the same time as regionally extensive I-type magmatism in the New England Orogen. Least-altered dykes are enriched in Mg, K, Ba, Rb, Sr, Zr, Th, Cr and Ni relative to I-type intrusives although chemical affinities are evident between lamprophyres and the more mafic members of the high-K Moonbi Plutonic Suite.
Hillgrove lamprophyres are commonly enriched in Sb, As, Hg, Au, W and Bi with respect to average CAL compositions. Evidence indicates this is most likely due to contamination of magma during intrusion through mineralised structures, rather than a primary magmatic feature. Partially resorbed xenocrystic stibnite occurs in dykes which have intruded lode structures, probably facilitated by the low melting point of stibnite (550°C) and its incorporation into the magma. Carbon and oxygen isotopic data from carbonates in least-altered, post-lode lamprophyres are indistinguishable from carbonate in altered dykes and veins, implying that hydrothermal interaction continued after dyke intrusion. Although it is unlikely that lamprophyre dykes have been a direct source for mineralisation at Hillgrove, the close temporal and spatial relation of dykes, mesothermal Au---Sb veins and I-type intrusions are interpreted to be manifestations of the post-collisional setting and influx of mantle-derived heat and partial melts into the New England Orogen during the late Permian. 相似文献
Kent, P.E., 1976. Major synchronous events in continental shelves. In: M.H.P. Bott (editor), Sedimentary Basins of Continental Margins and Cratons. Tectonophysics, 36 (1–3): 87–91.The modern Atlantic-type continental shelves mostly originated around the Permian, and showed a history of dominantly fault-controlled subsidence through the Trias, Jurassic and Early Cretaceous. In Albian—Aptian times there was a widespread nearly simultaneous change to subsidence by intercratonic downwarping, which has continued through Late Cretaceous and Tertiary into Recent times. 相似文献
On the sea bed of the West Florida continental shelf about 45 km SSW of Ft. Myers, Florida, an 85-km2 area has been discovered in which six thermal springs discharge warm, chemically altered seawater from vents and seepage zones. The spring water apparently originates in the subsurface ocean around the Florida Platform and penetrates the highly porous strata of the platform about 500–1000 meters below sea level. It percolates toward the interior of the platform and is geothermally heated to about 40°C en route. Then it rises along more vertical flow channels and is discharged in warm submarine springs.Beneath the platform, several chemical processes alter the percolating seawater. One process seems to be a secondary dolomitization of the limestone of the platform because, in the discharging seawater, magnesium is lower by 2.7 mmole/kg and calcium higher by 3.6 mmole/kg than in normal seawater with the same chlorinity. Other reactions within the sediments of the platform enrich the spring effluents 1000-fold in226Ra, 10,000-fold in222Rn, and 90-fold in228Ra compared to the seawater surrounding the platform. Thus, the springs may be important sources of radionuclides for the Gulf of Mexico. The percolating seawater also loses all of its oxygen and nitrate to reduction processes, loses most of its phosphate and 40% of its238U, and roughly quadruples its silica content.Coastal carbonate platforms are fairly common geological features. Thus, processes like those beneath the West Florida Shelf may function on a world-wide basis to play an important role in the diagenesis of carbonate sediments. 相似文献