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41.
LOWENSTERN J. B.; CHARLIER B. L. A.; CLYNNE M. A.; WOODEN J. L. 《Journal of Petrology》2006,47(11):2105-2122
Rhyolite pumices and co-erupted granophyric (granite) xenolithsyield evidence for rapid magma generation and crystallizationprior to their eruption at 15·2 ± 2·9 kaat the Alid volcanic center in the Danikil Depression, Eritrea.Whole-rock U and Th isotopic analyses show 230Th excesses upto 50% in basalts <10 000 years old from the surroundingOss lava fields. The 15 ka rhyolites also have 30–40%230Th excesses. Similarity in U–Th disequilibrium, andin Sr, Nd, and Pb isotopic values, implies that the rhyolitesare mostly differentiated from the local basaltic magma. Giventhe (230Th/232Th) ratio of the young basalts, and presumablythe underlying mantle, the (230Th/232Th) ratio of the rhyolitesupon eruption could be generated by in situ decay in about 50000 years. Limited (5%) assimilation of old crust would hastenthe lowering of (230Th/232Th) and allow the process to takeplace in as little as 30 000 years. Final crystallization ofthe Alid granophyre occurred rapidly and at shallow depths at20–25 ka, as confirmed by analyses of mineral separatesand ion microprobe data on individual zircons. Evidently, 30000–50 000 years were required for extraction of basaltfrom its mantle source region, subsequent crystallization andmelt extraction to form silicic magmas, and final crystallizationof the shallow intrusion. The granophyre was then ejected duringeruption of the comagmatic rhyolites. KEY WORDS: U-series; zircon; ion microprobe; volcano; geochronology 相似文献
42.
The Rabigh area, a coastal region north of Jeddah city, Saudi Arabia contains raised Quaternary coral reefal terraces and reworked coral fragments mixed with sand and gravel. This area has a thin exposure Lower Miocene shallow marine carbonate rocks that laterally pass into evaporites. The Miocene carbonate and evaporite rocks conformably overly the Lower Miocene siliciclastic sequence, are in turn capped by the Harrat basaltic boulders. The Miocene carbonates are made up of dolomitic packstone, wackestone and mudstone, whereas the overlying Quaternary reefal terraces are composed of coral boundstone and grainstones.The Quaternary reefal terraces of Rabigh area have been dated using the uranium-series dating method to obtain precise dates for these corals. The calculated ages (128, 212 and 235 ka) indicate that deposition took place during high sea level stands associated with interglacial times during Oxygen Isotope Stages (OIS) 5 and 7. The youngest age (128 ka) clearly corresponds to stage 5e of the last interglacial period. The obtained ages correlate well with those of the emerged reefs on the Sudanese and Egyptian coasts at the western side of the Red Sea. The broad distribution of wet climate, pluvial deposits on the continents and high sea level stands indicate a wide geographical range of the interglacial events of the Oxygen Isotope Stages (OIS) 5 and 7.The oxygen and carbon isotopic composition of the Miocene and Quaternary carbonate rocks in Rabigh area show a broad range of δ13C and δ18O. The Quaternary carbonate rocks have significantly higher δ13C than the Miocene ones, but low δ13C values of the Miocene samples likely indicate a high contribution of carbon from organic sources at the time of deposition. Linear trends are evident in both groups of samples supporting the likelihood of secondary alteration. 相似文献
43.
EDWARDS R. Lawrence 《中国科学:地球科学(英文版)》2010,(5)
Based on 33 U/Th dates and 1020 oxygen isotopic data from stalagmite Y1 from Yamen Cave, Guizhou Province, China, a record of the Asian Summer Monsoon (ASM) was established. The record covers the last deglaciation and the early Holocene (from 16.2 to 7.3 ka BP) with an average oxygen isotope resolution of 9 years. The main millennial-scale deglacial events first identified in Greenland (Greenland Interstadial Events: GIS 1e through GIS 1a) and later in China are clearly present in the Y1 record. By analogy to earlier work, we refer to these as Chinese Interstadials (CIS): CIS A.1e to CIS A.1a. The onset of these events in Y1 δ18O records are nominally dated at: 14750±50, 14100±60, 13870±80, 13370±80, and 12990±80 a BP. The end of CIS A.1a or the beginning of the Younger Dryas (YD) event is nominally at 12850±50 a BP and the end of the YD dates to 11500±40 a BP. The δ18O values shift by close to 3‰ during the transition into the Bφlling-Allerφd (BA, the onset of CIS A.1e) and at the end of the YD. Comparisons of Y1 to previously published early Holocene records show no significant phase differences. Thus, the East Asia Monsoon and the Indian Monsoon do not appear to have been out of phase during this interval. The Y1 record confirms earlier work that suggested that solar insolation and North Atlantic climate both affect the Asian Monsoon. 相似文献
44.
A detailed geomorphologic and morphostratigraphic investigation of raised marine terraces at Cape Cuvier, Western Australia, reveals two morphologically distinct units. A lower, well-developed accretional reef terrace between 3 and 5.5 m above MLWS (mean low-water springs; hereafter denoted as “+”) represents an extended interval of stable sea level. An upper erosional terrace and incipient coralgal rim between + 8.5 to 10.5 m represents a brief sea-level stillstand at this higher elevation. These features suggest the lower and upper terraces developed during discrete sea-level events. In an attempt to better define the timing of emplacement of each marine unit, 20 coral samples collected along vertical and lateral reef growth axis from both terraces were analysed with U-series dating. Unfortunately, all coral samples exhibited elevated δ234Uinitial values, suggesting that pervasive uptake of 234U-enriched uranium and 230Th thorium had occurred. Despite the shortcomings of absolute dating, a succession of events can be resolved though morphostratigraphic relationships. Comparison of the facies relationships, coral growth, and morphostratigraphic features between the lower and upper terraces indicates that an early to mid MIS 5e stillstand at + 3 to 5 m was followed by a late rise to + 8.5 to 10.5 m. This agrees with an emerging global view of MIS 5e sea-level history derived from stable carbonate platforms, rejecting the hypothesis that these higher sea-level benchmarks are an artefact of localized tectonic processes. 相似文献
45.
阳原县虎头梁的泥河湾层夹有两层第四纪的叠层石,即上、下叠层石。文章分析了从3个地点采集的叠 层石样品,分别用热电离质谱铀系法和α能谱铀系法测量了它们的年龄。同一样品致密部分和非致密部分的年龄 及铀含量明显不一致,同时致密部分的铀系年龄出现倒序,此结果表明虎头梁的叠层石不适合用铀系法测年。根 据沉积物光谱光度测量,文章发现亮度值随剖面深度的变化曲线与北太平洋的风尘通量曲线形态相似,通过对比, 可以尝试将后者的时间标尺转移到前者上来,从而可建立虎头梁的泥河湾层的亮度时间标尺。它指示阳原县虎头 梁杏沟剖面上、下叠层石的年龄分别为73kaB.P.和274kaB.P.,分别是间冰期晚期和冰期的产物,前人所报道的孢 粉组合所代表的古气候环境支持这个年龄。 相似文献
46.
Discovered accidently during factory building construction works, the Middle Palaeolithic site of Biache-Saint-Vaast (Pas-de-Calais, France) was excavated from 1976 to 1982 by a team of Lille University directed by Alain Tuffreau. An abundant archaeological and paleontological material, including two human skulls, was there recovered from fossil alluvial deposits of the Scarpe River. In order to determine the ages of these remains, the ESR/U-series method was applied on bone and teeth. As the U-series data obtained of the main part of the analyzed tissues do not allow the use of the classical US-ESR model, the recently proposed AU-ESR model, taking into account if necessary U-leaching from some of the tissues, was used to calculate combined ESR/U-series ages for these samples. The obtained ages suggest a MIS7 attribution to the faunal remains and permit an age of ca 240 ka to be assigned for the human remains and associated archaeological material, in accordance with the stratigraphic data and the large mammal associations. 相似文献
47.
Microatoll record for large century-scale sea-level fluctuations in the mid-Holocene 总被引：2，自引：0，他引：2
Coral microatolls have been long used as precise indicators of past sea level, but their use for precise definition of detailed sea-level fluctuations is still rare. Here we report twelve high-precision thermal ionization mass spectrometric 230Th ages for twelve rims of five mid-Holocene microatolls from an emerged reef terrace at Leizhou Peninsula, northern South China Sea. This is a tectonically stable area, enabling us to reconstruct both the timing and trajectory of local sea-level fluctuations accurately. The elevations of these microatoll rims and cores were accurately determined relative to the surface of modern living microatolls at the same site. The results indicate that the sea level during the period of 7050–6600 yr bp (years before AD 1950) was about 171 to 219 cm above the present, with at least four cycles of fluctuations. Over this 450 yr interval, sea level fluctuated by 20–40 cm on century scales. 相似文献
48.
This paper reports U-series dates on speleothem samples from Locality 15 at Zhoukoudian, one of the richest Paleolithic sites in northern China. The age of the lower part of Layer 2 is securely bracketed between 155,000 and 284,000 yr. The underlying Layer 3 dates back at least 284,000 yr. Layer 4, further below, should be older still, possibly by a cycle on the SPECMAP time scale before 284,000 yr ago. These ages, much greater than the previous estimates of 110,000-140,000 yr from U-series and electron spin resonance dating of fossil teeth, suggest that Locality 15 was broadly contemporaneous with Locality 4 (New Cave) and with the uppermost strata of Locality 1 (Peking Man site). The physical evolution and cultural development evidenced by the timing of the Zhoukoudian localities are in line with the opinion of Chinese anthropologists for a regional transition from Homo erectus to archaic Homo sapiens. 相似文献
49.
长江上源布曲盆地晚更新世湖相沉积的发现及其古植被与古气候 总被引：4，自引：1，他引：4
在青海省西南部青藏公路东南侧的通天河支流——布曲北岸 (33°4 4′4 5 .2″N,92°11′2 8.1″E) ,发现一条拔河 0～ 18m的湖相沉积剖面。湖相沉积物的 U系测年结果为 88.9～ 5 6 .2 ka,确定其沉积时代为晚更新世早中期。结合孢粉分析结果 ,本文探讨了青海省西部地区通天河及布曲流域晚更新世早中期的古植被与古气候。初步研究表明 ,在该剖面的下部和上部沉积期间 ,当地生长松、桦、冷杉等乔木和蒿等草本植物 ,气候比较温暖湿润。剖面中部沉积期间 ,蒿、藜、禾本科和麻黄等草原植被明显增多 ,气候凉爽干燥。剖面沉积期间气候曾经历过温暖湿润和凉爽干燥的波动。植被与气候的变化具有从末次间冰期晚期经末次冰期早冰段到末次冰期间冰段中期的特征 相似文献
50.
Immature and mature calcretes from an alluvial terrace sequence in the Sorbas basin, southeast Spain, were dated by the U-series isochron technique. The immature horizons consistently produced statistically reliable ages of high precision. The mature horizons typically produced statistically unreliable ages but, because of linear trends in the dataset and low errors associated with each data point, it was still possible to place a best-fit isochron through the dataset to produce an age with low associated uncertainties. It is, however, only possible to prove that these statistically unreliable ages have geochronological significance if multiple isochron ages are produced for a single site, and if these multiple ages are stratigraphically consistent. The geochronological significance of such ages can be further proven if at least one of the multiple ages is statistically reliable. By using this technique to date calcretes that have formed during terrace aggradation and at the terrace surface after terrace abandonment it is possible not only to date the timing of terrace aggradation but also to constrain the age at which the river switched from aggradation to incision. This approach, therefore, constrains the timing of changes in fluvial processes more reliably than any currently used geochronological procedure and is appropriate for dating terrace sequences in dryland regions worldwide, wherever calcrete horizons are present. 相似文献