Página 1 dos resultados de 4315 itens digitais encontrados em 0.012 segundos

Chemical process to separate iron oxides particles in pottery sample for EPR dating

Watanabe, Shigueo; Farias, Thiago Michel de Brito; Gennari, Roseli Fernandes; Ferraz, Gilberto Marcon; KUNZLI, Ruth; Chubaci, Jose Fernando Diniz
Fonte: PERGAMON-ELSEVIER SCIENCE LTD Publicador: PERGAMON-ELSEVIER SCIENCE LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.97%
Ancient potteries usually are made of the local clay material, which contains relatively high concentration of iron. The powdered samples are usually quite black, due to magnetite, and, although they can be used for thermoluminescene (TL) dating, it is easiest to obtain better TL reading when clearest natural or pre-treated sample is used. For electron paramagnetic resonance (EPR) measurements, the huge signal due to iron spin-spin interaction, promotes an intense interference overlapping any other signal in this range. Sample dating is obtained by dividing the radiation dose, determined by the concentration of paramagnetic species generated by irradiation, by the natural dose so as a consequence, EPR dating cannot be used, since iron signal do not depend on radiation dose. In some cases, the density separation method using hydrated solution of sodium polytungstate [Na(G)(H(2)W(12)O(40))center dot H(2)O] becomes useful. However, the sodium polytungstate is very expensive in Brazil: hence an alternative method for eliminating this interference is proposed. A chemical process to eliminate about 90% of magnetite was developed. A sample of powdered ancient pottery was treated in a mixture (3:1:1) of HCI, HNO(3) and H(2)O(2) for 4 h. After that...

Fission-track dating of South American natural glasses: an overview

Bigazzi, G.; Hadler, J. C.; Iunes, P. J.; Araya, AMO
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 585-594
ENG
Relevância na Pesquisa
36.97%
Although many glass-bearing horizons can be found in South American volcanic complexes or sedimentary series, only a relatively few tephra and obsidian-bearing volcanic fields have been studied using the fission-track (FT) dating method. Among them, the volcanics located in the Sierra de Guamani (east of Quito, Ecuador) were studied by several authors. Based upon their ages, obsidians group into three clusters: (1) very young obsidians, similar to 0.2Ma old, (2) intermediate-age obsidians, similar to 0.4- similar to 0.8 Ma old, and (3) older obsidians, similar to 1.4- similar to 1.6 Ma old. The FT method is also an efficient alternative technique for identification of the sources of prehistoric obsidian artefacts. Provenance studies carried out in South America have shown that the Sierra de Guamani obsidian occurrences were important sources of raw material for toot making during pre-Columbian times. Glasses originated from these sources were identified in sites distributed over relatively wide areas of Ecuador and Colombia.Only a few systematic studies on obsidians in other sectors were carried out. Nevertheless, very singular glasses have been recognised in South America, such as Macusanite (Peru) and obsidian Quiron (Argentina)...

Geomorphological correlation of the tectonically displaced Tejo River terraces (Gavião–Chamusca area,central Portugal) supported by luminescence dating

Martins, A., Cunha, P., Huot, S., Murray, A., Buylaert, J.,
Fonte: Elsevier Ltd and INQUA Publicador: Elsevier Ltd and INQUA
Tipo: Artigo de Revista Científica
POR
Relevância na Pesquisa
37.02%
A suite of fluvial terraces (T1–T6,from top to bottom of the staircase) occur along a _70km stretch of the Tejo River in central Portugal, between the small towns of Gavião and Chamusca. Terrace correlation was based upon the following: (a)aerialphotograph analysis, geomorphological mapping and field topographic survey; (b) sedimentology of the terrace deposits, namely the maximum particle size, clast composition and lithofacies identification; and (c) K-feldspar infrared stimulated luminescence (IRSL) dating of the three lower terraces levels, as quartz optically stimulated (OSL) signal of the samples from these terraces was too close to saturation for all but two samples.The two upper terraces (T1andT2) lack suitable materials for luminescence dating (e.g.sands/silts),but also their probable ages are beyondthe upper range of the dating method. Fault saffecting terraces and older deposits have been reported. The luminescence dating results suggest that some assignments of local terrace remnants were in correct because of fault-related vertical displacements. The luminescence dating procedure also included a correction for anomalous fading in order to obtain more reliable estimates of the burial ages. The fading rate was identical for all samples...

K-feldspar IRSL dating of a Pleistocene river terrace staircase sequence of the Lower Tejo River (Portugal, western Iberia)

Martins, António; Cunha, Pedro; Buylaert, Jan Pieter; Huot, Sébastien; Murray, Andrew; Dinis, Pedro; Stokes, Martin
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.97%
We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs, geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced by faults (known from previous studies). Integration of these new ages with those obtained previously in the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley. However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces (with uncorrected De values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the natural signals close to luminescence saturation...

Optical dating of clastic deposits generated by an extreme marine coastal flood: The 1755 tsunami deposits in the Algarve (Portugal)

Cunha, P., Buylaert, J., Murray, A., Andrade, C., Freitas. M., Fatela, F., Munhá, J., Martins, A., Sugisaki, S.,
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
36.97%
Identification of past tsunamis is important for risk assessment and management of coastal areas. Obtaining accurate and precise ages of sediments originating from such extreme marine coastal floods is crucial for a reliable estimation of the recurrence interval of these often devastating events. We present here the results of quartz optical dating and 14C dating of two sites (Boca do Rio and Martinhal) on the Algarve coast (southern Portugal). These sites contain deposits of the great tsunami of November 1, 1755. The sections were described using sedimentological techniques; at both sites tsunami-laid sands and gravels were identified, intercalated between estuarine muds. Quartz luminescence ages from these sedimentary successions were derived using standard SAR-OSL dating using multi-grain sub-samples. A multiple sampling strategy was employed with several samples taken from the AD 1755 tsunami deposit and from the sediments bracketing the tsunami layer. Our SAR-OSL protocol was shown to be appropriate using dose recovery measurements (measured/given dose ratio of 1.004 0.007, n ¼ 165). The several OSL ages from the 1755 tsunamigenic deposits are internally reproducible but yield age overestimates of between 20 and 125% (60–310 years respectively); this is in agreement with values reported in the literature for similar deposits. The age overestimation of the tsunami-laid sands is presumably due to the rapid erosion and deposition of older sediments...

Online dating - aspectos comportamentais e emocionais: um estudo exploratório

Vaz, Carolina Lima
Fonte: Universidade da Beira Interior Publicador: Universidade da Beira Interior
Tipo: Dissertação de Mestrado
Publicado em //2010 POR
Relevância na Pesquisa
37.02%
Os sites e canais de online dating são plataformas online, através das quais os utilizadores efectuam um registo que lhes permite aceder a uma enorme rede de utilizadores, com diferentes perfis, através da qual escolhem perfis desejados, com a finalidade de estabelecer relações românticas. O principal objectivo desta investigação é explorar e compreender os processos associados aos encontros virtuais, nomeadamente de que forma a utilização de sites e canais de online dating influencia a conduta e vida pessoal dos seus utilizadores. Desta forma, foram estabelecidos três objectivos específicos, sendo que 1) explorar os comportamentos e emoções associados ao online dating; 2) avaliar diferenças relativamente às variáveis sócio-demográficas; e, 3) determinar as correlações entre comportamentos e emoções dos utilizadores de sites e canais de online dating. Os dados foram obtidos através da aplicação do Questionário de Avaliação dos Comportamentos e Emoções associados ao Online Dating a 240 utilizadores deste tipo de sites e canais. Através da análise do questionário foi possível verificar que a utilização de sites e canais de encontros virtuais não influencia de forma significativa a vida dos seus utilizadores...

A study of Pleistocene Lacustrine Sediments at the Southern Front of the Tibetan Plateau : Dating and Palaeoclimate record; A study of Pleistocene Lacustrine Sediments at the Southern Front of the Tibetan Plateau : Dating and Palaeoclimate record; Pleistozäne lakustrine Sedimente, eine Studie am südlichen Rand des Tibetischen Plateaus

Goddu, Srinivasa Rao
Fonte: Universidade de Tubinga Publicador: Universidade de Tubinga
Tipo: Dissertação
EN
Relevância na Pesquisa
36.97%
The rise of the Himalaya and the Tibetan plateau strongly controlled the monsoon system. Lacustrine sediments of long living lakes at the southern margin of the Tibetan plateau are useful archives to study long-term palaeoenvironmental changes with high time resolution. Three sites at the southern hinge of the Tibetan plateau, i.e. Kashmir basin in the west, Kathmandu basin in the central part, and Heqing Basin in the far eastern part were sampled. Detailed studies on Heqing and Kathmandu basins are reported in this thesis. High-resolution magnetostratigraphy, cyclostratigraphy, and AMS radiocarbon dating are integrated to establish an optimum age model for a 168 m long drill core of lacustrine sediments from Heqing Basin, Yunnan Province, southwestern China. A 14C age of 51.62 +2.42/-1.85 kyr BP is obtained at a depth of 7.3 m. Remanent magnetization resides in maghemite and partly magnetite, both showing the same direction. The polarity sequence clearly reveals the Brunhes/Matuyama (B/M) boundary at 141.5 m. The upper boundary of Jaramillo is indicated at 167 m, as well as Blake Event between 16.3 to 17.5 m. Fourier analysis was done on carbonate, magnetic susceptibility and saturation isothermal remanent magnetization (SIRM) in wavelength domain for sliding windows with different window lengths. Almost all spectra within the range of window centers (30-140 m) show a dominant long wavelength...

Testing the TT-OSL single-aliquot protocol for quartz sediment dating.

Moffatt., Jillian Elizabeth
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2014
Relevância na Pesquisa
37.09%
Thermally-transferred optically stimulated luminescence (TT-OSL) is a form of optically stimulated luminescence that saturates at much higher doses than conventional OSL (Wang et al, 2006b). Luminescence sediment dating is a technique whereby the natural radiation dose given to a sample is measured. This is divided by the environmental radiation rate of the sample site to give the sample's age. As TT-OSL is able to measure higher doses than conventional OSL, it has been considered a candidate for long range luminescence sediment dating, beyond one million years. In this thesis, TT-OSL single-aliquot sediment dating protocols were tested on selected samples from the south-east of South Australia (SESA) stranded dune sequence, a sequence of ancient dunes ranging from 0 to 900 thousand years of age that have previously been independently dated using luminescence and non-luminescence dating methods. A young sample with a high natural dose from Baldina Creek, Burra, South Australia was also dated. Measurements of the thermal depletion of the TT-OSL signal were also made. It was found that, for the SESA samples, TT-OSL dating results do not agree with previous independent measurements above 200 ka. The results for the young Baldina Creek sample were within the expected range.; Thesis (M.Phil.) -- University of Adelaide...

Luminescence dating: an Australian perspective

Prescott, J.; Robertson, G.
Fonte: Taylor & Francis Ltd. Publicador: Taylor & Francis Ltd.
Tipo: Artigo de Revista Científica
Publicado em //2008 EN
Relevância na Pesquisa
37.02%
Applications of luminescence dating to Quaternary earth science are reviewed, the examples being from Australia. The methods of thermoluminescence (TL) and optical dating using optically stimulated luminescence (OSL) are briefly described, together with their limitations. Prominent results are: (i) the distribution of ages of Australian desert dunes shows peak activity at about the time of the last glacial maximum; (ii) the lake-full episodes of Lake Eyre are traced back to 140 ka; (iii) the ages of relict beach dunes in South Australia, when compared with independent geological ages based on δ18O records, show that luminescence dating can be used for ages to 500 ka and older; and (iv) the development of interior river systems has been traced. Among less common applications are: (i) the dating of Australia's youngest volcanoes at 5-4 ka (in these cases the luminescence signal was reset by heat); (ii) the determination of age vs depth of a deep-sea core; and (iv) the age of earthquakes (from the age of the infilling of fault scarps). The emphasis is on recent work and is illustrative rather than exhaustive, the object being to show the scope of luminescence dating. There are suggestions where further research might be directed.; J. R. Prescott and G. B. Robertson

Assessing the uncertainty on particle size and shape: implications for ESR and OSL dating of quartz and feldspar grains

Duval, M.; Campaña, I.; Guilarte, V.; Miguens, L.; Iglesias, J.; González Sierra, S.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica Formato: 7 pages
Relevância na Pesquisa
36.97%
Grain size and shape are two important parameters in Electron Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL) dating of sediment, since they have a direct impact on several correction factors that are used for evaluating the dose rate. These parameters were evaluated by static image analysis for a wide range of quartz and feldspar grain samples. Our results indicate that there are many factors that may influence the final grain size and shape. Overall, grains are not perfect spheres, but should be rather approximated to smooth slightly elongated ellipsoids, with a width that is on average about 25% smaller than the length. For multiple grains dating, this may have an influence of a few percents on the beta dose rate evaluation, and thus even less on the total dose rate. However, in the case of single grain dating, the impact may be somewhat more significant given the large variability in size and shape between grains that may be encountered in a natural sediment. For beta micro-dosimetry purpose, it may be thus useful to better characterize the single grain that is going to be dated.; This study was partially sponsored by a project from the Spanish Ministry of Science and Innovation (Grant number: CGL2010- 16821). MD is currently the recipient of an International Outgoing Fellowship from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007- 2013) under REA grant agreement PIOF-GA-2013-626474. IC is the beneficiary of a PhD scholarship from the Spanish MINECO (project CGL2012-38434-C03-02).

BREAKING THE CYCLE OF VIOLENCE: AN EXPLORATION INTO DATING VIOLENCE PREVENTION CURRICULUM

Runciman, Sarah
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
Relevância na Pesquisa
37.06%
In 2004 the Ontario Ministry of Education created the Safe Schools Action Team (SSAT) to advise on the development of a comprehensive approach to bullying prevention. When four years later in 2008 the SSAT re-engaged in order to review the issues of gender-based violence, homophobia, sexual harassment, and inappropriate sexual behaviour, recommendations were made for addressing these issues (Ministry of Education, 2008). One of the most significant dimensions of the SSAT report was their statement that the most effective method to learn about healthy relationships is through school curriculum (Ministry of Education, 2008). Taking the SSAT recommendation that the most effective method for educating adolescents on healthy relationships is through curriculum, the current study seeks to describe, from the teacher’s perspective, experiences with and motivations for using curriculum advocating healthy relationships, specifically dating violence prevention programming. The purpose of this thesis is to examine four female Ontario Physical and Health Education teachers’ personal and professional experience with teen dating violence and their knowledge about and use of dating violence prevention curriculum that has been approved by the Ontario Ministry of Education. The participants related their experiences with dating violence within their school communities and discussed how they approached these issues within their own classroom. The results of this study indicate that dating violence continues to be a prevalent issue for Ontario secondary schools...

Online dating - aspectos comportamentais e emocionais: um estudo exploratório

Vaz, Carolina Lima
Fonte: Universidade da Beira Interior Publicador: Universidade da Beira Interior
Tipo: Dissertação de Mestrado
Publicado em //2010 POR
Relevância na Pesquisa
37.02%
Os sites e canais de online dating são plataformas online, através das quais os utilizadores efectuam um registo que lhes permite aceder a uma enorme rede de utilizadores, com diferentes perfis, através da qual escolhem perfis desejados, com a finalidade de estabelecer relações românticas. O principal objectivo desta investigação é explorar e compreender os processos associados aos encontros virtuais, nomeadamente de que forma a utilização de sites e canais de online dating influencia a conduta e vida pessoal dos seus utilizadores. Desta forma, foram estabelecidos três objectivos específicos, sendo que 1) explorar os comportamentos e emoções associados ao online dating; 2) avaliar diferenças relativamente às variáveis sócio-demográficas; e, 3) determinar as correlações entre comportamentos e emoções dos utilizadores de sites e canais de online dating. Os dados foram obtidos através da aplicação do Questionário de Avaliação dos Comportamentos e Emoções associados ao Online Dating a 240 utilizadores deste tipo de sites e canais. Através da análise do questionário foi possível verificar que a utilização de sites e canais de encontros virtuais não influencia de forma significativa a vida dos seus utilizadores...

Issues in radiocarbon and U-series dating of corals from the last glacial period

Esat, Tezer; Yokoyama, Yusuke
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.97%
Radiocarbon calibration beyond the extent of tree-ring records depends on U-series dating of fossil corals or speleothem, both of which can provide independent calendar ages. Less direct methods rely on layer counting and comparison with other well-dated

Dating fault-generated pseudotachylytes: comparison of Ar-40/Ar-39 stepwise-heating, laser ablation and Rb-Sr microsampling analyses

Mueller, Wolfgang; Kelley, Shari K; Villa, Igor M
Fonte: Springer Publicador: Springer
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.97%
Three different geochronological techniques (stepwise-heating, laser-ablation 40Ar/39Ar, Rb-Sr microsampling) have been evaluated for dating fault-generated pseudotachylytes sampled along the Periadriatic Fault System (PAF) of the Alps. Because pseudotachylytes are whole-rock systems composed of melt, clast and alteration phases, chemical control from both Ar isotopes (Cl/K, Ca/K ratios) and EMPA analyses is crucial for their discrimination. When applied to stepwise-heating 40Ar/39Ar analyses, this approach yields accurate melt-related ages, even for complex age spectra. The spatial resolution of laser-ablation 40Ar/39Ar analyses is capable of contrasting melt, clast and alteration phases in situ, provided the clasts are not too fine grained, the latter of which results in integrated "mixed" ages without geological information. Elevated Cl/K and Ca/K ratios were found to be an invaluable indicator for the presence of clast admixture or inherited 40Ar. Due to incomplete isotopic resetting during frictional melting, Rb-Sr microsampling dating did not furnish geologically meaningful ages. On the basis of isotopic disequilibria among pseudotachylyte matrix phases, and independent Rb-Sr microsampling dating of cogenetic (ultra) mylonites...

Optical Dating of Deep-Sea Sediments using Single Grains of Quartz: a Comparison with Radiocarbon

Olley, Jonathon; De Deckker, Patrick; Roberts, Richard; Fifield, L Keith; Yoshida, Hiroyuki; Hancock, Gary
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
37.06%
In this paper, we demonstrate that optical dating of single grains of quartz offers an alternative means of dating deep-sea sediments. The precision and accuracy of the technique, which has the potential to date sediments deposited during the last 500,000 years or so, is limited by the random and systematic uncertainties associated with producing optical ages. These result in total relative age uncertainties of between 10% and 20% at the 68% confidence interval, which are similar in size to those associated with Late Quaternary oxygen-isotope chronologies. We analysed single grains of quartz from several depth intervals down core Fr10/95-GC17, which was collected offshore from Cape Range Peninsula, Western Australia, from a water depth of 1093 m in the eastern Indian Ocean. The single-grain optical ages are shown to be consistent with AMS radiocarbon ages obtained from planktonic foraminifera from the same core. We also show that marine sediments are not immune from partial or heterogeneous bleaching (incomplete resetting) of the optical dating signal. Where partial or heterogeneous bleaching of the optical dating signal is indicated, we recommend that single-grain dating be employed and the burial dose estimated from the population of grains with the lowest absorbed radiation dose.

He-Pb double dating of detrital zircons from the Ganges and Indus Rivers: Implication for quantifying sediment recycling and provenance studies

Campbell, Ian; Reiners, Peter; Allen, Charlotte M; Nicolescu, Stefan; Upadhyay, Rajeev
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
37.15%
He-Pb double dating of detrital zircons is more reliable than conventional U-Pb dating for tracing the source of detritus in sediments and can be used to constrain the percentage of recycled material in sediments. Conventional U-Pb dating can be used to constrain the provenance of sediments if the U-Pb zircon age pattern for potential source regions is known but can only be used to trace the source of individual zircons if they are first-cycle grains. The advantages of He-Pb double dating are demonstrated using examples from the Indus and Ganges rivers, and previously published data from the Navajo sandstone. Conventional U-Pb dating can unambiguously identify only 2.5% of the Ganges zircons, and 18% of the Indus zircons as coming from the Himalayan Mountains or Tibet Plateau and only 23% of the Navajo zircons as coming from the Appalachian Mountains. The correct figure, as determined from double dating, is over 95% from the Himalayan Mountains or Tibet Plateau in the case of the Indus and Ganges rivers and at least 70% from the Appalachian Mountains in the case of the Navajo Sandstone. This result casts doubt on the reliability of the U-Pb method when used in the absence of other techniques, such as He dating, to identify the true provenance of sediments...

Comparison of ESR and TIMS U/Th dating of marine isotope stage (MIS) 5e, 5c, and 5a coral from Barbados - implications for palaeo sea-level changes in the Caribbean

Schellmann, Gerhard; Radtke, Ulrich; Potter, Emma-Kate; Esat, Tezer; McCulloch, Malcolm
Fonte: Pergamon-Elsevier Ltd Publicador: Pergamon-Elsevier Ltd
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.97%
Electron spin resonance (ESR) dating of coral has become an efficient geochronological tool in supporting morphostratigraphic studies carried out on Barbados during the last 10 years. The newly developed approach for DE determination (DE- Dmax plot procedure) improves the precision of ESR dating of Pleistocene coral, and therefore permits differentiation between the main marine isotope stages (MIS) 5, 7, 9 and 11 and also between sub-stages 5e, c and a. This study compares results of ESR and TIMS Uranium series dating (U/Th) of emergent Last Interglacial coral reef terraces from Barbados, and presents some implications for the timing and extent of sea-level changes during marine isotope stages (MIS) 5e, c and a. Both dating methods indicate a distinct formation of up to three coral reef terraces during MIS 5e, at approximately 132 ka (ESR) to 128 ka (U/Th), at c. 128 ka (ESR) and at c. 120 ka (U/Th) to 118 ka (ESR). It is also highly probable that three reef terraces were developed during MIS 5c between c. 103 ka (U/Th) and 105 ka (ESR). The formation of two separate coral reefs during MIS 5a is recognized for the first time on Barbados, with an age estimate for the older MIS 5a-2 reef of 85 ka (ESR) or 84 ka (U/Th), and an age estimate for the younger MIS 5a-1 reef terraces of 74 ka (ESR) or 77 ka (U/Th). Assuming a constant uplift rate of 0.276 m/ka at the south coast of Barbados...

Gem-bearing basaltic volcanism, Barrington, New South Wales: Cenozoic evolution, based on basalt K-Ar ages and zircon fission track and U-Pb isotope dating

Sutherland, F L; Fanning, Christopher
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
37.11%
Barrington shield volcano was active for 55 million years, based on basalt K-Ar and zircon fission track dating. Activity in the northeast, at 59 Ma, preceded more substantial activity between 55 and 51 Ma and more limited activity on western and southern flanks after 45 Ma. Eruptions brought up megacrystic gemstones (ruby, sapphire and zircon) throughout the volcanism, particularly during quieter eruptive periods. Zircon fission track dating (thermal reset ages) indicates gem-bearing eruptions at 57, 43, 38, 28 and 4-5 Ma, while U-Pb Isotope SHRIMP dating suggests two main periods of zircon crystallisation between 60 and 50 Ma and 46-45 Ma. Zircons show growth and sector twinning typical of magmatic crystallisation and include low-U, moderate U and high-U types. The 46 Ma high-U zircons exhibit trace and rare-earth element patterns that approach those of zircon inclusions in sapphires and may mark a sapphire formation time at Barrington. Two Barrington basaltic episodes include primary lavas with trace-element signatures suggesting amphibole/apatite-enriched lithospheric mantle sources. Other basalts less-enriched in Th, Sr, P and light rare-earth elements have trace-element ratios that overlap those of HIMU-related South Tasman basalts. Zircon and sapphire formation is attributed to crystallisation from minor felsic melts derived by incipient melting of amphibole-enriched mantle during lesser thermal activity. Ruby from Barrington volcano is a metamorphic type...

≥ 3850 Ma BIF and mafic inclusions in the early Archaean Itsaq Gneiss Complex around Akilia, southern West Greenland? The difficulties of precise dating of zircon-free protoliths in migmatites

Nutman, Allen; McGregor, Victor; Shiraishi, Kazuyuki; Friend, Clark; Bennett, Victoria; Kinny, Peter D
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
37.06%
The southern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on Akilia and adjacent islands consists of polyphase dioritic-tonalitic-granitic injection components with inclusions of metavolcanic amphibolites, chemical sediments such as banded iron formation (BIF), gabbros and ultramafic rocks. Incipient in situ partial melting and strong deformation during several Archaean tectonothermal events strongly modified these injection components, so that they are now mostly banded, schlieric migmatites with neosome produced during several events. The margins of many inclusions have been loci for either segregation of neosome and/or higher strainobliterating the relationship between the inclusions and the older components of the migmatites. An added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon, which would provide precise, direct dates. Instead, minimum ages of inclusions are obtained by dating invasive components of the granitic (sensu lato) migmatites. Previous age determinations of the inclusions were centred on a ∼ 200 m long body of amphibolites, BIF and ultramafic rocks on the southwestern corner of Akilia. Geochronology of this locality has been controversial...

Detailed mass spectrometric U-series analyses of two teeth from the archaeological site of Pech de lAze II: implications for Uranium Migration and Dating

Grun, Rainer; Yan, Ge; McCulloch, Malcolm; Mortimer, Graham
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.97%
We report Th/U ages obtained by thermal ionization mass spectrometry of cross-sections of two teeth from the archaeological site of Pech de l'Aze II. The aim of this study is to reconstruct the pathways and time dependency of uranium uptake into different constituents of the two teeth. A previous ESR dating study of the two teeth yielded age estimates in the range of about 130,000 to 200,000 years, which was in agreement with the geological assignment of Riss for layers from which the specimens were recovered. The closed system U-series ages of all analyses range from 48 to 137 ka, thus yielding age-estimates which are between 30 and 90 ka too young. The apparent closed system U-series ages of the dentine and cement fractions at the top of the teeth are the oldest, becoming gradually younger towards the interior of the two teeth. The difference between apparent U-series ages of dentine at the outer and inner parts of the two teeth is about 35 ka. The enamel sections show the same patterns in U concentration and progression of apparent Th/U ages as the adjacent dentine samples, but with generally younger ages. Our results indicate that uranium migrates both from the top and the base into the dentine of the teeth and from there into the adjacent enamel. Depending on the position of the sample...