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111.
The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U-Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. 207Pb/206Pb dating of oscillatory-zoned zircon using SHRIMP RG (n = 142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69 Ga.Analyses of 18O/16O compositions of olivine from a harzburgite/dunite (n = 8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (δ18OOl = 6.0 ± 0.4‰) was slightly enriched in 18O, but not significantly different from that of the modern mantle. Zircon δ18O measurements from the six felsic rocks (n = 93) record mean or weighted mean compositions ranging from 4.9 ± 0.7‰ to 5.1 ± 0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ18OZr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ18OWR compositions (6.7-6.9‰) resemble those of modern adakites.LA-MC-ICPMS zircon 176Hf/177Hf analyses were obtained from six samples (n = 122). Five samples record weighted mean initial εHf compositions ranging from to 0.5 ± 0.6 to −0.1 ± 0.7 (calculated using λ176Lu = 1.867 × 10−11 yr−1), while one sample records a composition of 1.3 ± 0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf.Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro (δ18OWR = 5.5‰) with ∼20% hydrothermally altered gabbro/basalt (δ18OWR = 4.0‰) would produce tonalite magmas within the average compositional range observed. 18O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high 18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab. 相似文献
112.
V. V. Akinin A. V. Prokopiev J. Toro E. L. Miller J. Wooden N. A. Goryachev A. V. Alshevsky A. G. Bakharev V. A. Trunilina 《Doklady Earth Sciences》2009,426(1):605-610
U-Pb SHRIMP-dating of zircons from twenty five intrusions representative of the Main granitoid batholith belt and associated
dike swarms (Yano-Kolyma gold bearing province, North East of Asia) are mostly ∼150 ± 3 Ma (Kimmeridgian-Tithonian). Two less
widespread impulses of magmatism dated at 160–155 Ma and 146–143 Ma representing the full range of ages present in the Main
belt. Paleoproterozoic (∼1.8 Ga) inherited zircons were found in three intrusions from the south-western part of the belt
where Precambrian crust of the North-Asia craton is inferred to underlie it.
Published in Russian in Doklady Akademii Nauk, 2009, Vol. 426, No. 2, pp. 216–221.
The article was translated by the authors. 相似文献
113.
Francisco Artigas Ji Meng Loh Jin Young Shin Joe Grzyb Ying Yao 《Environmental Earth Sciences》2017,76(7):293
The relatively low cost of lands along with a privileged location near an urban center attracted industry to the Meadowlands of New Jersey and the absence of regulations resulted in vast amounts of industrial waste emitted into the air and dumped to nearby estuaries and marshlands. Hurricane Sandy created an unprecedented sea surge that overtopped berms and tide gates and extensively flooded approximately 22.8 km2 of a low lying basin that includes Berry’s Creek, a tributary to the Hackensack River and well known for its legacy of high levels of contamination. The sea surge connected Berry’s Creek with eastern creeks that flow into the Hackensack River for several tidal cycles. The objectives of this study were to establish a baseline for organic pollutants and heavy metals post Superstorm Sandy, determine whether contaminants from highly contaminated areas moved to the eastern creeks during the surge and measure contaminant gradients around tide gates. Cadmium, mercury and chromium were the most abundant contaminants in sediments, and pollutants responsible for the highest ecological risk were Hg and polychlorinated biphenyls (PCBs). Concentrations of PCBs were higher in the western creeks and contrary to metals did not show concentration gradients from either side of tide gates. Massive export of contaminants from western to eastern creeks due to the surge was not apparent. The abundance of heavy metals in the vicinity of tide gates shows that they play a role in their distribution across the estuary. 相似文献
114.
115.
Nearly one thousand oysters (Crassostrea virginica) were collected at 15 sites on four sampling trips to Galveston Bay during 1992–1993. Iron, silver, arsenic, cadmium, copper, lead, and zinc were determined in the whole soft part of the individual oysters. Trace metal concentrations were found to vary among individuals within a site on a given trip, and spatially and temporally around the Galveston Bay estuarine system. Differences in metal concentrations from site to site were a factor of 5 or more for some metals, and similar differences were found from time to time at a given site. Zinc was found in anomalously high concentrations at several sites, mainly along the west side of Galveston Bay. Isolated high values of other metals were found scattered throughout the bay; however, only one site (GBSL, near Swan Lake in lower Galveston Bay) was found to have significantly higher than average concentrations for several metals (Ag, Cu, Pb, and Zn). One obvious trend found in this study was that metal concentrations, except for As, were highest in the summer, followed by the fall, and lowest in the winter and spring. The temporal changes in metal concentrations in oysters cannot be explained by changes in body weight in different seasons, and are apparently caused by changes in environmental conditions (e.g., metal input, salinity) or biological activity (e.g., metabolism, depuration). 相似文献
116.
J. L. Wooden G. K. Czamanske M. L. Zientek 《Contributions to Mineralogy and Petrology》1991,107(1):80-93
Analyses of the Pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the Stillwater
Complex indicate a narrow range of apparent initial isotopic compositions (206Pb/ 204Pb=13.95; 207Pb/204Pb=14.95–15.01; 208Pb/204Pb=33.6). The uniformity of our data is in contrast to, but not necessarily contradictory to, other recent investigations
which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that
were presumably derived from different source regions. Samples from the Basal series of the complex have consistently higher
207Pb/204Pb ratios, suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas.
Apparent initial Pb isotopic compositions of the complex are very radiogenic compared to Late Archean model-mantle values,
but are nearly identical to initial Pb isotopic compositions found for the the adjacent, slightly older (2.73–2.79 Ga), Late
Archean crustal suite in the Beartooth Mountains. Contamination of magmas parental to the Stillwater Complex by the Late Archean
crustal suite is rejected for two reasons: (1) Th and U concentrations in Stillwater rocks and plagioclase are very low (about
0.08 and 0.02 ppm respectively), yet Th/U ratios are uniform at about 4, in contrast to the highly variable (2–26) but often
high Th/U ratios found for the Late Archean crustal complex; (2) it seems improbable that any contamination process would
have adjusted the isotopic compositions of the diverse magmas entering the Stillwater chamber to near-identical values. The
preferred hypothesis to explain the Pb isotopic data for the Stillwater Complex and the associated Late Archean crustal suite
involves a major Late Archean crust-forming event that resulted in a compositionally complex crust/mantle system with relatively
homogeneous and unusual Pb isotopic compositions. The parental magmas of the Stillwater Complex were generated at different
levels within this crust/mantle system, before isotopic contrasts could develop by radioactive decay within compositionally
discrete reservoirs. This situation limits the utility of all isotopic tracer systems in discriminating among the various
mantle and crustal reservoirs that may have affected the final isotopic character of the Stillwater magmas. The late Archean
crustal complex and the Stillwater Complex melts were ultimately derived from the same distinct mantle without obvious direct
interaction with the Middle to Early Archean crust present in the region. 相似文献
117.
118.
水-岩-气相互作用引起的水化学动态变化研究--以桂林岩溶试验场为例 总被引:45,自引:2,他引:45
利用多参数自动记录仪对桂林岩溶试验场的降水量、水位、水温、pH值和电导率进行了监测,数据采集间隔根据参数变化的程度由2min到1h不等。结果发现,岩溶裂隙水在洪水期间pH值呈降低趋势,而电导率呈升高的不寻常变化。与此相反,对于岩溶管道水,同样是在洪水期间,它的pH值是升高的,而电导率呈正常的降低。考虑到Ca2 和HCO-3分别为地下水中主要的阴阳离子(>90%),及它们与电导率的线性关系,计算得到了洪水期间方解石的饱和指数(SIc)和水的CO2分压(Pco2)的变化情况。发现洪水时裂隙水的Pco2高于正常情况的Pco2,而它的SIc值比正常情况低。与此相对,对于管道水,尽管同一洪水期间其SIc降低,但Pco2也降低。从这些结果,可以推断,至少有两个关键的过程控制着洪水期间的水化学变化。一个是雨水的稀释作用,另一个是水-岩-气的相互作用。然而,对于裂隙水来说,后者的作用可能更重要,即在洪水期间,高浓度的土壤CO2溶解于水中,则更具侵蚀性的水能溶解更多的石灰岩,从而增强水的电导率。而对于管道水,雨水的稀释作用更重要,因为研究区较高的pH和低电导率的雨水能更快地通过管道流出,所以,要了解岩溶系统水化学的变化,仅考虑水-岩相互作用是不够的,我们还必须重视CO2气体对岩溶系统中水化学变化的影响。总之,水-岩-气相互作 相似文献
119.
The main objectives of this study were (1) to examine what has beenrecovered and what has not yet been recovered in Nishinomiya City five years after the GreatHanshin-Awaji Earthquake, and (2) to find current service needs for the earthquake survivors,especially for the disadvantaged populations. Disadvantaged populations are minorities in society, andtheir needs and opinions are often ignored and not reflected in the services or mitigation policy,despite the fact that they are the ones who are affected most by various hazards. At the same time, becauseof their unique characteristics and positions in society, their needs differ from the majorityof the society. Six hundred eighty-six opinion cards were collected in nine grassroots workshopsessions with various disadvantaged groups in Nishinomiya City, such as the physically challenged,the visually impaired, the elderly living in public housing, junior and senior high school students,mothers with small children, and those who lost their own homes because of the earthquake. Theseopinion cards were sorted out and analyzed by using a TQM(Total Quality Management)-basedassessment method. Since the TQM-based assessment method allows bottom-up integration ofopinions from each individual, it helps us identify unique needs of the disadvantaged groupsand reflect their opinions in mitigation plan and policies. The main findings of this study include (1)the city redevelopment cannot be achieved without integrating the viewpoints of thedisadvantaged; (2) the social support was a key resource for both mental health and community development;and (3) disaster experiences and adaptation produced civic mindedness among people in a community. 相似文献
120.
Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway 总被引:4,自引:0,他引:4
B. R. Hacker T. B. Andersen D. B. Root L. Mehl J. M. Mattinson J. L. Wooden 《Journal of Metamorphic Geology》2003,21(6):613-629
The Solund–Hyllestad–Lavik area affords an excellent opportunity to understand the ultrahigh‐pressure Scandian orogeny because it contains a near‐complete record of ophiolite emplacement, high‐pressure metamorphism and large‐scale extension. In this area, the Upper Allochthon was intruded by thec. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410–400 Ma. Within < 5–10 Myr, all these units were exhumed by the Nordfjord–Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near‐ultrahigh‐pressure rocks to within c. 3 km vertical distance from the low‐grade Solund Conglomerate. 相似文献