Optically stimulated luminescence dating of Ocean Drilling Program core 658B: Complications arising from authigenic uranium uptake and lateral sediment movement     

《Quaternary Geochronology》2015

Ocean Drilling Program Site 658 lies under the North African summer dust plume, and ought to be an ideal target for optically stimulated luminescence (OSL) dating, since the main clastic input is far-travelled Saharan dust. However, OSL ages for coarse silt-sized quartz (40–63 μm) are systematically lower than independent age estimates when dose rates are calculated using a model which assumes detrital ²³⁸U, ²³²Th and ⁴⁰K and excess ²³⁰Th and ²³¹Pa. Ages which are in good agreement with independent age control are obtained from the coarse silt samples when a correction for authigenic uranium uptake is incorporated into the dose rate model. Authigenic uranium uptake occurs under reducing conditions, which are common at the sediment–water interface, and some degree of authigenic uranium correction may be required for most marine sediments. Using this revised dose rate model, ages produced using fine silt-sized quartz (4–11 μm) are up to 100% older than both independent and coarse silt ages. In addition, the fine silt ages show a consistent pattern of age decrease with depth over 1.5 m of core. ²³⁰Th data from Site 658 indicate that this site receives 3 times more sediment laterally than vertically. It is concluded that the fine silt at Site 658 contains a substantial reworked component, making it unsuitable for dating. Conversely the coarse silt fraction, which settles through water at ∼40 times the rate of fine silt, appears to be derived from dust input over the site at the time of deposition. Since prominent nepheloid (cloudy) layers occur in various deep ocean basins, and the material suspended in these layers often consists of reworked fine silt-sized sediments, coarser material should be dated where possible.  相似文献   

An approach for optimizing in situ cosmogenic 10Be sample preparation     

《Quaternary Geochronology》2016

Optimizing sample preparation for the isotopic measurement of ¹⁰Be extracted from quartz mineral separates has a direct positive effect on the accuracy and precision of isotopic analysis. Here, we demonstrate the value of tracing Be throughout the extraction process (both after dissolution and after processing), producing pure Be (by optimizing ion exchange chromatography methods and quantifying quartz mineral separate and final Be fraction purity), and minimizing backgrounds (through reducing both laboratory process blanks and ¹⁰B isobaric interference). These optimization strategies increase the amount of ¹⁰Be available for analysis during accelerator mass spectrometry (AMS), while simultaneously decreasing interference and contamination, and ensuring that sample performance matches standard performance during analysis. After optimization of our laboratory's extraction methodology, ⁹Be³⁺ ion beam currents measured during AMS analysis, a metric for sample purity and Be yield through the extraction process, matched the ⁹Be³⁺ beam currents of AMS standards analyzed at the same time considering nearly 800 samples. Optimization of laboratory procedures leads to purer samples that perform better, more consistently, and more similarly to standards during AMS analysis, allowing for improved precision and accuracy of measurements used for dating and quantification of Earth surface processes.  相似文献   

Retrospective analysis of the impacts of major hurricanes on sediments in the lower Everglades and Florida Bay     

W.-J.?Kang  J.?H.?TrefryEmail author 《Environmental Geology》2003,44(7):771-780

A record of the impacts of major hurricanes on sediment stratigraphy and composition in subtropical ecosystems has been preserved in the lower Everglades and Florida Bay. These impacts were observed in discontinuous layers of sediment that were identified from high-resolution, vertical profiles of excess ²¹⁰Pb and ¹³⁷Cs. Discontinuities were found at different geographic locations and at two to three different depths in the sediment column; however, the layers were each deposited within time periods that corresponded with the passing of category 3–5 hurricanes during 1960, 1948 and 1935. A simple mass balance model for excess ²¹⁰Pb was used to show net changes of ±20–100% in excess ²¹⁰Pb inventory that resulted from sediment disturbances of <1 to >22 cm. Abrupt shifts in sediment composition were often observed in hurricane-impacted layers. Ratios of organic (C/P) were four- to fivefold higher than normal in post-hurricane layers of sediment at open bay sites. These layers are phosphorus poor and seem to reflect preferential decomposition of organic P relative to organic C in association with hurricanes. The net effect is for major hurricanes to redistribute sediment, organic matter and nutrients.This revised version was published online in July 2003.  相似文献   

Volcanic ash layers in the Upper Cretaceous of the Central Apennines and a numerical age for the early Campanian     

Daniel?BernoulliEmail author  Urs?Schaltegger  Willem?B.?Stern  Martin?Frey?  Michèle?Caron  Simonetta?Monechi 《International Journal of Earth Sciences》2004,93(3):384-399

At Montagna della Maiella and at Gola del Furlo (central Apennines) two discrete layers of bentonic clay are intercalated within the pelagic (Furlo) and turbiditic/pelagic limestones (Maiella) of the Upper Cretaceous basinal succession of the Umbrian basin (Scaglia facies). The bentonite layers are dated by planktonic foraminifera to the Globotruncanita elevata zone, early Campanian, and by calcareous nannofossils to the Aspidolithus parcus zone (CC 18); they fall into the reversed interval of chron 33. Detailed correlation shows the layers to be of exactly the same age. The upper layer is dated by U/Pb on magmatic zircons to 81.67±0.21 Ma, an age compatible with the Cretaceous time-scale of Obradovich. The mineralogy of the bentonitic clays is almost pure montmorillonite and contrasts sharply with the clay mineral assemblage of the enclosing pelagic and turbiditic limestones, which is dominated by soil-derived smectite and illite in different proportions. The bentonite seams are interpreted as the submarine alteration products of wind-borne volcanic ashes. They can be followed with only minor changes in thickness over 200 km and must be derived from distant volcanic sources and related to extreme volcanic events. A possible source area is present in the Dinarides where Upper Cretaceous subduction-related magmatic rocks are widespread.  相似文献   

Geochronological framework of the Quadrilátero Ferrífero, with emphasis on the age of gold mineralization hosted in Archean greenstone belts     

Carlos Mauricio Noce  Colombo Tassinari  Lydia Maria Lobato 《Ore Geology Reviews》2007,32(3-4):500

Archean terrains of the Quadrilátero Ferrífero comprise a greenstone belt association surrounded by granitoid–gneiss complexes, mainly composed of banded TTG gneisses whose igneous protoliths are older than 2900 Ma. This early continental crust was affected by three granitic magmatic episodes during the Neoarchean: ca. 2780 to 2760 Ma; 2720 to 2700 Ma; and 2600 Ma. Dating of felsic volcanic and volcaniclastic rocks defines a felsic magmatic event within the greenstone belt association around 2772 Ma, contemporaneous with emplacement of several of the granitic plutons and constrains a major magmatic and tectonic event in the Quadrilátero Ferrífero. Lead isotopic studies of lode–gold deposits indicate that the main mineralization episode occurred at about 2800 to 2700 Ma.Proterozoic evolution of the Quadrilátero Ferrífero comprises deposition of a continental-margin succession hosting thick, Lake Superior-type banded iron formations, at ca. 2500 to 2400 Ma, followed by deposition of syn-orogenic successions after 2120 Ma. The latter is related to the Transamazonian Orogeny. The western part of the Quadrilátero Ferrífero was also affected by the Brasiliano Orogeny (600 to 560 Ma).  相似文献   

Geochronology of layered mafic intrusions from the Pan–Xi area in the Emeishan large igneous province, SW China     

Hong Zhong  Wei-Guang Zhu 《Mineralium Deposita》2006,41(6):599-606

The Panzhihua–Xichang (Pan-Xi) area hosts mafic/ultramafic intrusions, which are part of the Permian Emeishan large igneous province. Some of these intrusions host giant Fe–Ti–V deposits and minor Ni–Cu–PGE mineralization. In the present study, zircon U–Pb ages of 259.3±1.3 and 260.7±0.8 Ma have been obtained from the giant Fe–Ti–V ore-bearing Hongge and the unmineralized Binggu intrusions, respectively, by isotope dilution thermal ionization mass spectrometry method. In combination with the ages of other ore-bearing intrusions, this age shows that these mafic/ultramafic intrusions were emplaced at ca. 260 Ma. The Hongge and Binggu intrusions cut the lower part of the rapidly deposited Emeishan flood basalt sequence but no further into the upper volcanic sequence in the Pan-Xi area. Thus, emplacement and mineralization of the mafic/ultramafic intrusions were almost contemporaneous with the eruption of the Emeishan flood basalts during a relatively short time span.  相似文献   

Kinematic link between episodic trapdoor collapse of the Negra Muerta Caldera and motion on the Olacapato-El Toro Fault Zone, southern central Andes     

Juliane Ramelow  Ulrich Riller  Rolf L. Romer  Onno Oncken 《International Journal of Earth Sciences》2006,95(3):529-541

A combined geochronological and structural analysis of the Miocene Negra Muerta Caldera was designed to better understand caldera formation associated with prominent faults on the central Andean plateau. Rb–Sr ages of the caldera outflow facies indicate that caldera formation occurred in two volcano-tectonic episodes. The first episode commenced with explosive eruption of the 9.0±0.1 Ma andesitic Acay Ignimbrite followed by a period of volcanic quiescence and moderate tectonic activity. Dominant volcanic and tectonic activity occurred during the second episode, which is bracketed by eruption of the 7.6±0.1 Ma rhyolitic Toba 1 Ignimbrite and effusive discharge of the 7.3±0.1 Ma rhyodacitic to andesitic lava flows. Structural relationships between rocks of the Negra Muerta Volcanic Complex and collapse-induced normal faults, notably NE-striking normal faults, agree with simultaneous volcanic activity and floor subsidence of the caldera during the second episode. Floor subsidence was achieved by tilting on an outward dipping reverse fault to the northwest of the caldera floor around a hinge zone located south of the caldera floor. This induced horizontal extension of the caldera floor and was accomplished by fragmentation of, and intrusion of dikes into, the floor. Collapse-induced and post-collapse fault populations of the caldera do not differ significantly in the directions of their axes of maximum extension and are in this respect kinematically compatible with left-lateral slip on the nearby Olacapato-El Toro Fault Zone. This furnishes evidence for a kinematic control by prominent faults on the formation of collapse calderas in the central Andes. The structural analysis of the Negra Muerta Caldera shows that collapse calderas can serve as deformation markers that contribute in elucidating the regional kinematic regime and the time of activity of prominent dislocations genetically related to collapse calderas.  相似文献   

Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France   总被引：4，自引：0，他引：4  

E. Be Mezeme  A. Cocherie  M. Faure  O. Legendre  Ph. Rossi 《Lithos》2006,87(3-4):276-288

U–Th–Pb dating of monazite with the electron probe microanalyser (EPMA) is increasingly documented as a reliable geochronological method offering high spatial resolution. This method has been applied on monazite from the Cévennes migmatites and granitoids from the southeast of the French Massif Central. Measurements were performed on separated grains after systematic back-scattered electron (BSE) imaging. Monazites from migmatites record two main ages: (i) a protolith age of about 550–543 Ma obtained on inherited cores, and (ii) a migmatization event between 329 ± 5 and 323 ± 3 Ma recorded by monazite rims and all other monogenetic grains. Monazite from the peraluminous Rocles pluton yields a 318 ± 3 Ma age. Finally, three granite dykes are dated at 333 ± 6, 318 ± 5 and 311 ± 5 Ma; the older dyke is the most deformed of them and is interpreted as linked to the migmatization event; the two other dykes are geochronologically, petrologically and structurally coeval with the Rocles pluton. The data constrain the timing of crustal melting following Variscan thickening in the northern Cévennes area. Migmatization of Ordovician protoliths took place at 329–323 Ma and was shortly followed by intrusion of leucogranite at 318–311 Ma. The study shows that EPMA dating of monazite can be successfully used to resolve a close succession of regional melting events.  相似文献   

Provenance implications of Th–U–Pb electron microprobe ages from detrital monazite in the Carboniferous Upper Silesia Coal Basin, Poland     

Monika Agnieszka Kusiak  Artur K&#x;dzior  Mariusz Paszkowski  Kazuhiro Suzuki  Ignacio Gonzlez-lvarez  Boles&#x;aw Wajsprych  Marek Doktor 《Lithos》2006,88(1-4):56-71

This paper reports the results of CHIME (chemical Th–U–Pb isochron method) dating of detrital monazites from Carboniferous sandstones in the Upper Silesia Coal Basin (USCB). A total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence. Age distributions were identified in detrital monazites from the USCB sequence and correlated with specific dated domains in potential source areas. Most monazites in all samples yielded ca. 300–320 Ma (Variscan) ages; however, eo-Variscan, Caledonian and Cadomian ages were also obtained. The predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic Bohemian Massif (BM), which suggests that various crystalline lithologies in the BM were the dominant sources of USCB sediments.  相似文献   

Zircon Ce4+/Ce3+ ratios and ages for Yulong ore-bearing porphyries in eastern Tibet     

Hua-Ying Liang  Ian H. Campbell  Charlotte Allen  Wei-Dong Sun  Cong-Qiang Liu  Heng-Xiang Yu  Ying-Wen Xie  Yu-Qiang Zhang 《Mineralium Deposita》2006,41(2):152-159

Yulong ore-bearing porphyries, along the northwestern extension of the Red River–Ailao Shan fault system in eastern Tibet, consist of five porphyry deposits, containing a total of more than 8 million tons of copper resources. U–Th–Pb laser inductively coupled plasma mass spectrometry dating of zircon shows that the porphyries were emplaced in Early Tertiary (41.2–36.9 Ma), covering a period of ∼4.3 Ma, with formation ages decreasing systematically from northwest to southeast. The start of porphyry magmatism coincided with the onset of transpressional movement along the Red River–Ailao Shan fault system, implying a close link between these two events. Age sequence in intrusions can be plausibly explained by assuming that a region of melting in the lower northwestern plate moved southeasternward along the Tuoba–Mangkang fault relative to the upper plate. Zircon grains from the Yulong ore-bearing porphyries have higher Ce⁴⁺/Ce³⁺ than those from barren porphyries in the region. This suggests that the ore-bearing porphyries crystallized from a relatively oxidized magma, which has important implications for future ore exploration in the region and other Cu deposits in convergent margin environments in general.  相似文献