Osmium isotope of the Co-rich crust from seamount Allison,central Pacific and its use for determination of growth hiatus and growth age     

XianWei Meng  YanGuang Liu  WenJun Qu  XueFa Shi 《中国科学D辑(英文版)》2008,51(10):1446-1451

Based on its microstructure, Co-rich crust A1-1 from seamount Allison, central Pacific, was scraped at averaged interval of 1.3 mm to measure osmium isotopic composition, and subsequently to establish the 187Os/188Os profile of scraping section of the crust. By observing the variation of 187Os/188Os under 10Be chronology and matching it to the well-known seawater Os isotope evolution of the past 40 Ma, two growth hiatuses (H1 and H2) occurring in the periods respectively between 13.6 and 29.6 Ma and between 8 and 9.8 Ma in the crust were recognized. According to the two hiatuses, the dating scheme for each scraped layer of the crust was suggested. For the upper layers younger than 6.8Ma, their growth ages were calibrated under 10Be chronology; for the lower layers older than 6.8Ma, their growth ages were obtained from 187Os/188Os evaluation curve by linear interpolation. Hereby, the age for the most inner layer of the crust was determined to be 39.5 Ma. H1 and H2 exactly correspond to the boundary between phosphatization and non-phosphatization and volcanic ash layer in the crust, respectively.  相似文献   

The relationship between the growth process of the ferromanganese crusts in the pacific seamount and Cenozoic ocean evolvement     

Xuan Ding  LianFeng Gao  NianQiao Fang  WenJun Qu  Jian Liu  JiangShan Li 《中国科学D辑(英文版)》2009,52(8):1091-1103

Base on the Os isotope stratigraphy together with the empirical growth rate models using Co concentrations, the growth ages of the ferromanganese crusts MHD79 and MP3D10 distributed in the seamount of Pacific are confirmed. Through the contrast and research on the previous achievements including ODP Leg 144 and the crusts CD29-2, N5E-06 and N1–15 of the seamount of the Central Pacific, the uniform five growth and growth hiatus periods of them are found, and closely related to the Cenozoic ocean evolvement process. In the Paleocene Carbon Isotope Maximum (PCIM), the rise of the global ocean productivity promoted the growth of the seamount crust; the first growth hiatus (I) of the ferromanganese crust finished. In the Paleocene-Eocene Thermal Maximum (PETM), though the vertical exchange of seawater was weakened, the strong terrestrial chemical weathering led to the input of a great amount of the terrigenous nutrients, which made the bioproductivity rise, so there were no crust hiatuses. During 52–50 Ma, the Early Eocene Optimum Climate (EECO), the two poles were warm, the latitudinal temperature gradient was small, the wind-driven sea circulation and upwelling activity were weak, the terrestrial weathering was also weakened, the open ocean bioproductivity decreased, and the ferromanganese crust had growth hiatus again (II). From early Middle Eocene-Late Eocene, Oligocene, it was a long-term gradually cooling process, the strengthening of the sea circulation and upwelling led to a rise of bioproductivity, and increase of the content of the hydrogenous element Fe, Mn and Co and the biogenous element Cu, Zn, so that was the most favorable stage for the growth of ferromanganese crust (growth periods III and IV) in the studied area. The hiatus III corresponded with the Eocene- Oligocene boundary, is inferred to relate with the global climate transformation, celestial body impact event in the Eocene-Oligocene transition. From the early to the middle Miocene, a large-scale growth hiatus (hiatus period IV) of the ferromanganese crust in the studied area is inferred to relate with temporary warm up climate and ephemeral withdrawal of Antarctic bottom water in the early Miocene. After that, the Antarctic ice sheets extended, the bottom water circumfluence strengthened, the ocean fertility increased, and the once interrupted crust continued to grow in the late Miocene (growth period V). Supported by China Ocean Mineral Resources Research and Development Association “10th Five Year” Topic (Grant No. DY105-01-04-14)  相似文献   

Constraining the timing of the India-Asia continental collision by the sedimentary record   总被引：5，自引：0，他引：5  

XiuMian Hu  JianGang Wang  Wei An  Eduardo Garzanti  Juan Li 《中国科学:地球科学(英文版)》2017,60(4):603-625

Placing precise constraints on the timing of the India-Asia continental collision is essential to understand the successive geological and geomorphological evolution of the orogenic belt as well as the uplift mechanism of the Tibetan Plateau and their effects on climate,environment and life.Based on the extensive study of the sedimentary record on both sides of the Yarlung-Zangbo suture zone in Tibet,we review here the present state of knowledge on the timing of collision onset,discuss its possible diachroneity along strike,and reconstruct the early structural and topographic evolution of the Himalayan collided range.We define continent-continent collision as the moment when the oceanic crust is completely consumed at one point where the two continental margins come into contact.We use two methods to constrain the timing of collision onset:(1) dating the provenance change from Indian to Asian recorded by deep-water turbidites near the suture zone,and(2) dating the age of unconformities on both sides of the suture zone.The first method allowed us to constrain precisely collision onset as middle Palaeocene(59±l Ma).Marine sedimentation persisted in the collisional zone for another 20-25 Ma locally in southern Tibet,and molassic-type deposition in the Indian foreland basin did not begin until another 10-15 Ma later.Available sedimentary evidence failed to firmly document any significant diachroneity of collision onset from the central Himalaya to the western Himalaya and Pakistan so far.Based on the Cenozoic stratigraphic record of the Tibetan Himalaya,four distinct stages can be identified in the early evolution of the Himalayan orogen:(1) middle Palaeocene-early Eocene earliest Eohimalayan stage(from 59 to 52 Ma):collision onset and filling of the deep-water trough along the suture zone while carbonate platform sedimentation persisted on the inner Indian margin;(2) early-middle Eocene early Eohimalayan stage(from 52 to 41 or 35 Ma):filling of intervening seaways and cessation of marine sedimentation;(3) late Eocene-Oligocene late Eohimalayan stage(from 41 to 25 Ma):huge gap in the sedimentary record both in the collision zone and in the Indian foreland;and(4) late Oligocene-early Miocene early Neohimalayan stage(from 26 to 17 Ma):rapid Himalayan growth and onset of molasse-type sedimentation in the Indian foreland basin.  相似文献   

Age and isotope geochemistry of the Archaean Pongola and Usushwana suites in Swaziland, southern Africa: a case for crustal contamination of mantle-derived magma     

E. Hegner  A. Krner  A.W. Hofmann 《Earth and Planetary Science Letters》1984,70(2):267-279

The igneous rocks of the Pongola Supergroup (PS) and Usushwana Intrusive Suite (UIS) represent a case of late Archaean continental magmatism in the southeastern part of the Kaapvaal craton of South Africa and Swaziland.

U-Pb dating on zircons from felsic volcanic rocks of the PS yields a concordia intercept age of 2940 ± 22Ma that is consistent with a Sm-Nd whole rock age of 2934 ± 114Ma determined on the PS basalt-rhyolite suite. The initial ε_Nd of−2.6 ± 0.9 is the lowest value so far reported for Archaean mantle-derived rocks. Rb-Sr whole rock dating of the PS yields a younger isochron age of 2883 ± 69Ma, which is not significantly different form the accepted U-Pb zircon age.

An internal (cpx-opx-plag-whole rock) isochron for a pyroxenite from the younger UIS yields an age of 2871 ± 30 Ma and initial ¹⁴³Nd/¹⁴⁴Nd that lies off the CHUR growth curve by ε_Nd −2.9 ± 0.2. However, Sm-Nd whole-rock data for the UIS yield an excessively high age of 3.1 Ga that conflicts with firm geological evidence showing the UIS to be intrusive into the PS.

The negative deviations of initialε_Nd from the chondritic Nd evolution curve suggest significant contamination of the PS and UIS melts by older continental crust. A mixing process with continental crust after magma segregation is supported by a high initial ⁸⁷Sr/⁸⁶Sr ratio of0.703024 ± 24 for a clinopyroxene sample from a UIS pyroxenite, compared with an expected value of 0.701 for the 2.9 Ga mantle. We therefore interpret the linear array of data points for the UIS gabbros as a mixing line between 2.87 Ga old magma and older continental crust.

Parallel LREE-enriched REE patterns, negative Nb-Ti anomalies, a distinctive and uniform ratio of Ti/Zr 46 and a narrow span of initial Nd indicate a common source for both the PS and UIS suites which is different from primitive mantle.  相似文献   

Paleoenvironmental implications of high-density records in Co-rich seamount crusts from the Pacific Ocean   总被引：1，自引：0，他引：1  

XiaoHong Wang  LiPing Zhou  YiMin Wang  XueHua Zhang  XiaoMing Liu  XingTao Fan  KeXin Liu  JianXiong Zhou 《中国科学D辑(英文版)》2008,51(10):1460-1469

Co-rich seamount crusts have been shown to possess great potential for providing information on paleoceanographic and paleoclimatic changes. High resolution data are essential to decipher and correctly understand such high-density records. With the development of modern microprobe techniques, detailed sampling of crusts can be performed and it is possible to retrieve detailed information about envi- ronmental changes recorded in the seamount crusts. We report here geochemical results of more than 40 elements (including all rare earth elements) of four Co-rich seamount crust samples, which were collected from seamounts in the central and western Pacific Ocean. These data were obtained with two micro-probe techniques: Electron Probe Micro Analyzer and Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The chronological framework of the seamount crust samples was determined using the cos- mogenic 10Be and the Co-chronometer. Records of elemental composition, P, and Al/(Fe Mn) and Y/Ho ratios across the sections of the four samples are used to identify paleoceanographic and paleoclimatic events over the past ~30 Ma. These data show that: (1) Al/(Fe Mn) in the western Pacific seamount crust is a useful proxy for the assessment of changes of source materials related to the variability of the Asian monsoon; (2) P and Y/Ho can be used as proxies to infer biogenic episodes. Finally we discuss the methodology related to dating and micro-probe analysis used in crust study.  相似文献   

Evolution of the tertiary volcanic rocks in the Izu-Mariana arc     

K. Shiraki  N. Kuroda  S. Maruyama  H. Urano 《Bulletin of Volcanology》1978,41(4):548-562

Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite.  相似文献   

大陆岩石圈地幔形成与演化研究的新进展   总被引：7，自引：0，他引：7  

任向文  吴福元 《地球物理学进展》2002,17(3):514-524

研究大陆岩石圈地幔形成与演化的主要途径是利用岩石－地球化学探针研究幔源橄榄岩的年龄、组成及其演化，目前多将岩石圈地幔划分为大洋型和克拉通型，并认识到，岩石圈地幔的矿和相、成分、年龄在垂向上具有分层性；根据研究资料，目前已提出了以熔体抽取为岩石圈地幔形成的主要方式，并通过交代、拆沉等过程使岩石圈地幔遭受改造，中国东部岩石圈地幔的成分主体上是饱满的，与其上的太古代地壳并不协调，一些研究者提出了岩石圈拆沉和地幔置换两种具有代有性的模型来解释岩石圈地幔成分的这种特点，人们对中国东部岩石圈地幔作了大量的研究工作，然而，若要形成较为合理的解释，还需应用新技术（如，Re/Os同位素技术）做进一步的工作。  相似文献   

Discovery of Paleogene marine stratum along the southern side of Yarlung-Zangbo suture zone and its implications in tectonics   总被引：5，自引：0，他引：5  

Hyesu Yun 《中国科学D辑(英文版)》2005,48(5):647-661

The final withering of Tibetan Tethys predicated the absolute retreat of seawater from Tibet, one of the most direct and valid marks of which is the age of the latest marine deposits in the area. Therefore, the dis-cussion on the closing age of Tibetan Tethys actuallyis the dating of the latest marine sediment in the area. In the study of late evolution of Tethys, the study on the latest marine sediments in southern Tibet is the major object of geologists. In this field, a lot of works have…  相似文献   

The Late Eocene 187Os / 188Os excursion: Chemostratigraphy,cosmic dust flux and the Early Oligocene glaciation     

《Earth and Planetary Science Letters》2006,241(3-4):477-492

High resolution records (ca. 100 kyr) of Os isotope composition (¹⁸⁷Os / ¹⁸⁸Os) in bulk sediments from two tropical Pacific sites (ODP Sites 1218 and 1219) capture the complete Late Eocene ¹⁸⁷Os / ¹⁸⁸Os excursion and confirm that the Late Eocene ¹⁸⁷Os / ¹⁸⁸Os minimum, earlier reported by Ravizza and Peucker-Ehrenbrink [Earth Planet. Sci. Lett. 210 (2003) 151–165], is a global feature. Using the astronomically tuned age models available for these sites, it is suggested that the Late Eocene ¹⁸⁷Os / ¹⁸⁸Os minimum can be placed at 34.5 ± 0.1 Ma in the marine records. In addition, two other distinct features of the ¹⁸⁷Os / ¹⁸⁸Os excursion that are correlatable among sections are proposed as chemostratigraphic markers which can serve as age control points with a precision of ca. ± 0.1 Myr. We propose a speculative hypothesis that higher cosmic dust flux in the Late Eocene may have contributed to global cooling and Early Oligocene glaciation (Oi-1) by supplying bio-essential trace elements to the oceans and thereby resulting in higher ocean productivity, enhanced burial of organic carbon and draw down of atmospheric CO₂. To determine if the hypothesis that enhanced cosmic dust flux in the Late Eocene was a cause for the ¹⁸⁷Os / ¹⁸⁸Os excursion can be tested by using the paired bulk sediment and leachate Os isotope composition; ¹⁸⁷Os / ¹⁸⁸Os were also measured in sediment leachates. Results of analyses of leachates are inconsistent between the south Atlantic and the Pacific sites, and therefore do not yield a robust test of this hypothesis. Comparison of ¹⁸⁷Os / ¹⁸⁸Os records with high resolution benthic foraminiferal δ¹⁸O records across the Eocene–Oligocene transition suggests that ¹⁸⁷Os flux to the oceans decreased during cooling and ice growth leading to the Oi-1 glaciation, whereas subsequent decay of ice-sheets and deglacial weathering drove seawater ¹⁸⁷Os / ¹⁸⁸Os to higher values. Although the precise timing and magnitude of these changes in weathering fluxes and their effects on the marine ¹⁸⁷Os / ¹⁸⁸Os records are obscured by recovery from the Late Eocene ¹⁸⁷Os / ¹⁸⁸Os excursion, evidence of the global influence of glaciation on supply of Os to the ocean is robust as it has now been documented in both Pacific and Atlantic records.  相似文献   

青藏高原腹地中新世从造山阶段向造高原阶段的转变及其动力学机制   总被引：2，自引：1，他引：1       下载免费PDF全文

王刚  伊海生  刘顺  沈军辉 《地球物理学报》2010,53(6):1384-1398

通过对前人研究的综述,发现青藏高原新生代地质演化与高原东南缘构造演化密切相关.俯冲下插的印度地壳在藏南发生部分熔融并注入青藏高原中部地壳,这些塑性流变的地壳物质在高原东南缘先后沿两个通道流出高原内部：早期为印支通道,开放时间为35 Ma以前并持续到12 Ma;后期为川滇通道,开放时间为12 Ma至今.由于喜马拉雅东构造结与四川盆地之间强烈的挤压,印支通道不断变窄,并在12 Ma被关闭.两个通道的差异,通道的打开和关闭,造成高原中地壳物质流出速率在中新世发生明显变化,在23 Ma以来流出速率小于注入速率,在12 Ma流出速率最小,部分熔融的印度地壳物质不断滞留于高原地壳内部,使得地势相对平坦、面积巨大的青藏高原逐渐形成并分别向南和向北扩展.通过简单的力学分析,本文将高原腹地变形划分为两个阶段：大于35~23 Ma的造山阶段,受控于造山机制;23 Ma至今的造高原阶段,受控于造高原机制.  相似文献   

Osmium isotopes as petrogenetic and geological tracers     

Claude J. Allègre  Jean-Marc Luck 《Earth and Planetary Science Letters》1980,48(1):148-154

Using terrestrial osmium-enriched samples of known ages, we have shown that¹⁸⁷Os/¹⁸⁶Os varies with time in result of the¹⁸⁷Re β^? radioactivity. Such a variation in the earth's mantle can be fitted by a straight line corresponding to¹⁸⁷Re/¹⁸⁶Os = 3.15 for the mantle, comparable to C1 carbonaceous chondrites. Using this result and the Re and Os contents of various rocks, several theoretical considerations and predictions can be deduced for the chemical evolution of the earth, such as a method for distinguishing between different processes of development of the continental crust. The special result of¹⁸⁷Os/¹⁸⁶Os from Bushveld is discussed with respect to the possible existence of an “enriched” subcontinental mantle.  相似文献   

Rhenium-osmium isotope constraints on the age and source of the platinum mineralization in the Lower Cambrian black rock series of Hunan-Guizhou provinces,China   总被引：2，自引：0，他引：2  

李胜荣  肖启云  申俊峰  孙丽  刘波  阎柏琨  江永宏 《中国科学D辑(英文版)》2003,46(9):919-927

The bottom of the Lower Cambrian series is an important bed typical of boundary event. The bed had been enriched with many useful elements such as Ni, Mo, Cu, Pb, Zn, Au, Ag, Ru, Rh, Pd, Os, Ir and Pt, many rare and dispersed elements such as Cd, Se, Tl, …  相似文献   

PROGRESS OF DETRITAL ZIRCON CHRONOLOGY IN SEDIMENT PROVENANCE STUDIES IN THE YANGTZE RIVER BASIN     

LI Ya-wei  LI Chang-an  ZHANG Yu-fen  LIN Xu  WANG Jie-tao  SUN Xi-lin  WEI Chuan-yi  GUO Ru-jun  LENG Yong-hui 《地震地质》2019,41(2):521-544

Evolution of the Yangtze River in East Asia is closely linked to the evolving topography following India-Eurasia collision and plays an important role in connecting the Tibetan plateau and the marginal sea, which is of great significance for understanding the evolution of modern Asian landform pattern and exploring the response of river development to tectonic uplift and monsoon evolution. Thus, many methods have been performed to reconstruct the evolution history of the Yangtze River, but there are still some disputes about the age of the Yangtze River, which has been strongly debated for over a century with estimates ranging from late Cretaceous to late Pleistocene. At present, sediment provenance tracing is one of the most important methods for studying the Yangtze River drainage evolution, for the provenance tracers could effectively represent the source area information and the various dating methods would provide reliable chronology framework. Previous studies showed that the zircon high closure temperature, wide distribution in fluvial sediment, and convenient sampling and analyzing made the zircon U-Pb dating a unique indicator recording the source area information. However, the Yangtze River drains a large basin and runs through different geological blocks with complicated lithology, as well as the abundant thermal historical events, leading to the zircon U-Pb dating a challenge work in tracing the sediment source within the Yangtze River Basin. In this study, based on the combination of previous research data and the "source to sink" system, the limitations and disadvantages of the detrital zircon U-Pb dating in the studies of sediment provenance tracing of the Yangtze River Basin were re-analyzed and re-discussed. Considering the evolving process of the large river system, some key areas and diagnostic information carrier, including bedrock and fluvial sediments deposited in present day or geo-history, would provide significant constraints on the evolution process. The former records the original information of the source region, and the latter reserves the practical information preserved in the downstream sink. As for the Yangtze River Basin, the limitation and disadvantages of the detrital zircon U-Pb dating in tracing sediment provenance are showed as follows:Firstly, six major tectonic units in the source region shows four similar age peaks, which closely corresponds to the previously identified synchronous major granitoid magmatic episodes. Five similar age peaks obviously exist in the sediment of the downstream sink both in the modern fluvial sediment and the geo-historical deposits such as outcrops and basin sediments. Thus, detrital zircon U-Pb chronology is indistinguishable from source to sink, making it unreliable in provenance tracing of the Yangtze River. Secondly, comparing with the detrital zircon spectra of tributary downstream and the upper reaches, all the tributaries below the Three Gores, the running-through of which is regarded as the symbol of the establishment of the modern Yangtze River system, could make up the similar spectra with the modern river sediments. Moreover, Sichuan Basin and Jianghan Basin, which is the last basin and first basin in western and eastern of the Three Gorges, are crucial basins for recording the incision information. However, sediment in these two basins show the similar spectra with five major age peaks from early Jurassic to late Cretaceous, which means the detrital zircon U-Pb chronology could not efficiently record the capture information no matter in spatial scale or time scale. In addition, the same results are also shown in Neogene gravel layer both in Jianghan Basin and Nanjing area. In summary, we propose that the similarity of the detrital zircon age spectra exists widely in Yangtze River system, and this greatly restricts the application of detrital zircon chronology in provenance tracing in the Yangtze River Basin, and the combination of multi-index and multi-method will shed new light in the future studies of provenance tracing within Yangtze River drainage system.  相似文献   

华北沧东断裂的构造特征   总被引：28，自引：3，他引：25       下载免费PDF全文

高战武  徐杰  宋长青 《地震地质》2000,22(4):395-404

地质和地球物理资料表明 ,沧东断裂为发育于上地壳的上陡下缓的铲状断裂 ,断面于 10km深处的上地壳底部变为近水平的拆离滑脱面。沧东断裂经历了中、新生代两个裂陷旋回 ,开始形成于晚侏罗世 ,此时为正断裂性质 ,早白垩世末期的燕山运动使断裂反转为逆断裂 ,并有一定的左旋走滑 ,形成正花状构造。早第三纪始新世时在中生代断裂的基础上改造成为正断裂 ,控制了黄骅坳陷下第三系的分布。早第三纪为断裂主要活动时期 ,塑造了现今沧东断裂的主要形态 ,断裂不同区段的产状、发育历史和活动强度表现出鲜明的分段性。晚第三纪以来断裂活动微弱。从浅层地震勘探结果看 ,沧东断裂晚更新世以来基本不活动 ,不是活动断裂  相似文献   

The longevity of subcontinental lithospheric mantle beneath Jiangsu-Anhui Region ——The Os isotope model age of mantle-derived peridotite xenoliths     

ZHI Xiachen  PENG Zicheng  CHEN Daogong  YU Chuniiang  Sun Weidong  Laurie Reisberg 《中国科学D辑(英文版)》2001,44(12)

The basalt-borne peridotite xenoliths from Jiangsu-Anhui provinces were analyzed for whole rock Os isotopic compositions in two laboratories of USTC, China and CRPG, France, respectively. The 187Os/188Os ratio of the sample set ranges from 0.119 to 0.129 (25 samples, USTC)and from 0.117 to 0.131 (17 samples, CRPG). The Os isotopic compositions of most samples are less than 0.129 and depleted relatively to the primitive mantle, showing a good correlation with the major element compositions. With the 187Os/188Os-Al2O3 alumichron, the samples yield a model age of 2.5 ± 0.1 Ga (data of USTC) and 1.9 ± 0.1 Ga (data of CRPG), late Archean to early Proterozoic, The two samples with the lowest 187Os/188Os ratio (0.119 and 0.117) have the TRD (Re depleted age) of 1.1 Ga (USTC) and 1.4 Ga (CRPG), mid-Proterozoic. The Os isotope model age shows that the peridotite xenoliths from Cenozoic alkali basalt in Jiangsu-Anhui provinces have an old formation age (early- to mid- Proterozoic). They are not newly produced mantle after the Phanerozoic replacement of the lithosphere mantle, but residual fractions of Proterozoic mantle.  相似文献   

Osmium-strontium-neodymium-lead isotopic covariations in mid-ocean ridge basalt glasses and the heterogeneity of the upper mantle   总被引：4，自引：0，他引：4  

Pierre Schiano  Jean-Louis Birck  Claude J. Allgre 《Earth and Planetary Science Letters》1997,150(3-4):363-379

Osmium, strontium, neodymium, and lead isotopic data have been obtained for 30 hand picked samples of basaltic glass from the Pacific, Atlantic and Indian mid-oceanic ridges. Large variations in Os isotopic ratios exist in the glasses, from abyssal peridotite-like values to radiogenic compositions similar to oceanic island basalts (¹⁸⁷Os/¹⁸⁶Os and ¹⁸⁷Os/¹⁸⁸Os ratios range from 1.06 to 1.36 and from 0.128 to 0.163, respectively). Os isotopic and elemental data suggest the existence of mixing correlations. This relationship might be ascribed to secondary contamination processes; however, such a hypothesis cannot account for the negative correlation observed between Os and Nd isotopes and the existence of complementary covariations between Os and SrPb isotopes. In this case, OsSrNdPb isotopic variations are unrelated to late post-eruption or shallow level contamination. These relationships provide strong evidence that the Os isotopic composition of the samples are derived from the mantle and thus implies a global chemical heterogeneity of the oceanic upper mantle. The results are consistent with the presence of recycled oceanic crust in the mantle sources of mid-ocean ridge basalts, and indicate that the unique composition of the upper mantle below the Indian ocean results from its contamination by a mantle component characterized by radiogenic Os and particularly unradiogenic Nd and Pb isotopic compositions.  相似文献   

High-resolution dating of Co-rich crusts: A comparative study using the methods of orbital pacing and <Superscript>230</Superscript>Th<Subscript>ex</Subscript>/<Superscript>232</Superscript>Th dating     

XiQiu Han  ZhongYan Qiu  WeiLin Ma  JiangChu Qian 《中国科学D辑(英文版)》2009,52(4):484-488

Due to their slow growth rates, seamount Co-rich crusts are very difficult to date with high resolution and precision. This paper is to test the use of orbital pacing on the growth profile of crusts to determine high-resolution age and growth rate. Crust CB14 from the central Pacific Ocean was selected for this study. We first examined the growth pattern in detail under a reflected-light microscope and ascertained that the growth environment was stable for the sub-layer 1 (0–3 mm). We then used electron microprobe line-scanning to obtain elemental profiles. The pattern of the power spectrum analysis of the Al-profile revealed that there are significant cycles of 113.9, 87.8, 51.5, 42.2 and 25.8 μm. These cycles correspond to the Milankovitch cycles of 53.1, 41, 24, 19.7 and 12 ka, respectively, and yield the growth rate of about 2.14 mm/Ma and an age of about 1.40 Ma for the boundary between the sub-layer 1 and sub-layer 2. We also used a drilling machine with a numerically controlled drive to obtain high-resolution samples at 0.1mm intervals, and used the ²³⁰Th_ex/²³²Th method to date the samples. For the uppermost 1.3 mm, the growth rate was about 2.15 mm/Ma, and the age for the layer at the depth of 3 mm was about 1.40 Ma, which coincides perfectly with the results obtained from orbital pacing. Thus, it is considered that orbital pacing is a new and effective method to determine the growth rate of the seamount Co-rich crust. This method is applicable for establishing a high-resolution age frame for the crusts of the world’s oceans. Supported by China Ocean Mineral Resources R & P Association (Grant No. DY105-01-01-08) and National Natural Science Foundation of China (Grant Nos. 40106005, 40476050)  相似文献   

Geochemistry and zircon SHRIMP U-Pb dating of granitoids from the west segment of the North Qaidam   总被引：5，自引：0，他引：5  

CaiLai Wu  Joseph L. Wooden  Paul T. Robinson  YunHong Gao  SuoPing Wu  QiLong Chen  Frank K. Mazdab  Chris Mattinson 《中国科学D辑(英文版)》2009,52(11):1771-1790

Granitoid intrusives such as Saishitenshan, Tuanyushan, Aolaohe and Sanchagou occur widely in the western segment of North Qaidam. All these bodies trend NW, roughly parallel to the regional structure. Zircon SHRIMP dating for these granites show that they range in age from Ordovician to Permian; 465.4±3.5 Ma for Saishitenshan, 469.7±4.6 Ma and 443.5±3.6 Ma for Tuanyushan, 372.1±2.6 Ma for Aolaohe, and 271.2±1.5 Ma and 259.9±1.2 Ma for Sanchagou. Both the Tuanyshan and Aolaohe plutons record two distinct intrusive events. Geochemically, the early Paleozoic granites have an island arc or active continental margin affinity, and their protolith may have been Mesoproterozoic oceanic crust derived from depleted mantle. The protolith of the late Paleozoic granites may have been Mesoproterozoic lower crust from the root of an island arc with the magmas reflecting a mixture of mantle and crustal material.  相似文献   

Thermal evolution of the northeastern Siberian platform in the light of apatite fission-track dating of the deep drill core     

O. M. Rosen  A. V. Soloviev  D. Z. Zhuravlev 《Izvestiya Physics of the Solid Earth》2009,45(10):914-931

Thermal evolution of the continental crust beneath the northeastern Siberian craton was studied based on the interpretation of apatite fission-track ages. The samples selected for AFT dating were collected from depths between 2 and 3 km along a 1000-km-long profile, from the crystalline basement of the Siberian platform. The AFT ages range from 185 to 222 Ma, indicating that in the late Triassic-early Cretaceous, the top of the crystalline basement was cooled below ∼100°C. Once the apatite cooled below this temperature, it began to accumulate and preserve tracks produced by spontaneous fissioning of ²³⁸U, and the number of tracks preserved is effective in determining the ages of events using the apatite fission-track method (AFT). The study showed that the apatite from Archean rocks was largely formed at 1.8–1.9 Ga as a result of a Paleoproterozoic metamorphic overprinting during the terrane collision and the subsequent accretion of the Siberian craton. The last thermal event, the self-heating of the collision prism, was terminated by cooling at ∼1.3 Ga. At that time, the Rb-Sr isotopic system became closed and the upper crust passed the ~300°C isograd. The calculation results showed that on further cooling, the ∼100°C isograd was passed at 1143 Ma. This age estimate could be obtained using AFT dating if the above event had been the last one in the thermal history of the Siberian craton. The obtained track ages indicate the existence of a repeated, significantly younger, heating of the crystalline crust due to some local reason.  相似文献   

Rhenium–osmium isotope and elemental behaviour during subduction of oceanic crust and the implications for mantle recycling     

《Earth and Planetary Science Letters》2007,253(1-2):211-225

This study presents major-, trace-element, and rhenium–osmium (Re–Os) isotope and elemental data for basalts and gabbros from the Zermatt-Saas ophiolite, metamorphosed to eclogite-facies conditions during the Alpine orogeny. Igneous crystallisation of the gabbros occurred at 163.5 ± 1.8 Ma and both gabbro and basalt were subject to ‘peak’ pressure–temperature (P–T) conditions of > 2.0 GPa and ~ 600 °C at about 40.6 ± 2.6 Ma.Despite such extreme P–T conditions, Re–Os isotope and abundance data for gabbroic rocks suggest that there has been no significant loss of either of these elements during eclogite-facies metamorphism. Indeed, ¹⁸⁷Re–¹⁸⁷Os isotope data for both unaltered gabbros and gabbroic eclogites lie on the same best-fit line corresponding to an errorchron age of 160 ± 6 Ma, indistinguishable from the age of igneous crystallisation. In contrast, metamorphosed basalts do not yield age information; rather most possess ¹⁸⁷Re/¹⁸⁸Os ratios that cannot account for the measured ¹⁸⁷Os/¹⁸⁸Os ratios, given the time since igneous crystallisation. Taken with their low Re contents these data indicate that the basalts have experienced significant Re loss (∼ 50–60%), probably during high-pressure metamorphism. Barium, Rb and K are depleted in both gabbroic and basaltic eclogites. In contrast, there is no evident depletion of U in either lithology.Many ocean-island basalts (OIB) possess radiogenic Os and Pb isotope compositions that have been attributed to the presence of recycled oceanic crust in the mantle source. Published Re–Os data for high-P metabasaltic rocks alone (consistent with this study) have been taken to suggest that excessive amounts of oceanic crust are required to generate such signatures. However, this study shows that gabbro may exert a strong influence on the composition of recycled oceanic crust. Using both gabbro and basalt (i.e. a complete section of oceanic crust) calculations suggest that the presence of ≥ 40% of 2 Ga oceanic crust can generate the radiogenic Os compositions seen in some OIB. Furthermore, lower U/Pb ratios in gabbro (compared to basalt) serve to limit the ²⁰⁶Pb/²⁰⁴Pb ratios generated, while having a minimal effect on Os ratios. These results suggest that the incorporation of gabbro into recycling models provides a means of producing a range of OIB compositions having lower (and variable) ²⁰⁶Pb/²⁰⁴Pb ratios, but still preserving ¹⁸⁷Os/¹⁸⁸Os compositions comparable to HIMU-type OIB.  相似文献