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40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration
VASCONCELOS, PAULO M.;ONOE, ARTUR T.;KAWASHITA, KOJI;SOARES, ADALBERTO J.;TEIXEIRA, WILSON;
Anais da Academia Brasileira de Ciências , 2002, DOI: 10.1590/S0001-37652002000200008
Abstract: laser heating 40ar/39ar geochronology provides high analytical precision and accuracy, mm-scale spatial resolution, and statistically significant data sets for the study of geological and planetary processes. a newly commissioned 40ar/39ar laboratory at cpgeo/usp, s?o paulo, brazil, equips the brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes. detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the cpgeo/usp 40ar/39ar suitability to a diverse range of applications. details about analytical procedures, including mineral separation, irradiation at the ipen/cnen reactor at usp, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study. finally, the results of calibration tests using ca and k salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the 40ar/39ar laboratory at cpgeo/usp are comparable to results obtained in the most respected laboratories internationally. the extensive calibration and standardization procedures undertaken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility, enabling brazilian students and scientists to conduct forefront research in earth and planetary sciences.
40Ar/39Ar geochronology at the Instituto de Geociências, USP: instrumentation, analytical procedures, and calibration  [cached]
VASCONCELOS PAULO M.,ONOE ARTUR T.,KAWASHITA KOJI,SOARES ADALBERTO J.
Anais da Academia Brasileira de Ciências , 2002,
Abstract: Laser heating 40Ar/39Ar geochronology provides high analytical precision and accuracy, mum-scale spatial resolution, and statistically significant data sets for the study of geological and planetary processes. A newly commissioned 40Ar/39Ar laboratory at CPGeo/USP, S o Paulo, Brazil, equips the Brazilian scientific community with a new powerful tool applicable to the study of geological and cosmochemical processes. Detailed information about laboratory layout, environmental conditions, and instrumentation provides the necessary parameters for the evaluation of the CPGeo/USP 40Ar/39Ar suitability to a diverse range of applications. Details about analytical procedures, including mineral separation, irradiation at the IPEN/CNEN reactor at USP, and mass spectrometric analysis enable potential researchers to design the necessary sampling and sample preparation program suitable to the objectives of their study. Finally, the results of calibration tests using Ca and K salts and glasses, international mineral standards, and in-house mineral standards show that the accuracy and precision obtained at the 40Ar/39Ar laboratory at CPGeo/USP are comparable to results obtained in the most respected laboratories internationally. The extensive calibration and standardization procedures undertaken ensure that the results of analytical studies carried out in our laboratories will gain immediate international credibility, enabling Brazilian students and scientists to conduct forefront research in earth and planetary sciences.
U-Pb and 40Ar/39Ar geochronology of the coastal Sonora batholith: New insights on Laramide continental arc magmatism
Ramos-Velázquez, Ernesto;Calmus, Thierry;Valencia, Victor;Iriondo, Alexander;Valencia-Moreno, Martín;Bellon, Hervé;
Revista mexicana de ciencias geológicas , 2008,
Abstract: the coastal sonora batholith comprises a series of cretaceous granitoids that intruded a metasedimentary basement of possible mesozoic age. they are partially covered by tertiary volcanic flows and pyroclastic rocks. in order to elucidate the crystallization and cooling history of the granitoids, nine rock samples -were collected from bahía kino to punta tepopa. eight samples dated by u-pb zircon geochronology show that the coastal sonora batholith was emplaced during the late cretaceous, between 90.1 ± 1.1 and 69.4 ± 1.2 ma. the interval of 20 ma between the different stages of crystallization indicate that magmatism was relatively static within coastal sonora, although the magmatic arc recorded an eastward migration as a whole during cretaceous and paleogene. in addition, three of these samples were also dated by 40ar/39ar in biotite and k-feldspar separates. ages vary from 74 to 67 ma in biotite and from -68 to 42 ma in k-feldspar. we interpret these ages as the cooling progression of the batholith, associated with exhumation of the region before the basin and range extension. furthermore, these results show a local trend towards younger ages to the north of the batholith, and they are in good agreement with the model of a general eastward migration of the cretaceous-tertiary magmatic arc in northwestern mexico. in general, the available ages suggest that the arc moved slowly across baja california between 140 and 105 ma, and continued its eastward migration across the eastern portion of baja california and sonora between 105 and 60 ma. according to the isotopic ages, the coastal sonora batholith would be the westernmost part of the laramide magmatic event (90 - 40 ma). thus, on the basis of new and available geochronologic, petrographic, and geochemical data, we propose that the coastal sonora batholith and the eastern portion of the peninsular ranges batholith belong to a single magmatic arc, which was separated during the continental breakup and rifting of the
Progress and Challenges Using 40Ar/39Ar Geochronology in Costa Rica and Nicaragua
Saginor1,Ian; Gazel,Esteban; Michael J,Carr; Swisher III,Carl C; Turrin,Brent;
Revista Geológica de América Central , 2011,
Abstract: to better estimate the extrusive flux of the central american arc, from 2002-2008, we obtained sixty one high precision 40ar/39ar ages on geographically well-situated lavas and tephra from costa rica and nicaragua. here, we describe a number of observations encountered during this study using four examples that well document the precision, accuracy and general reliability of the 40ar/39ar ages. first, low k2o values, particularly in samples from nicaragua, is a major limitation in or attempts to obtain reliable dates on samples under 1 my. second, extensive weathering of samples due to the tropical climate of central america has resulted in various levels of argon loss even when the hand sample appeared unaltered. third, our field and geochronological data lead us to conclude that eruptive rates have not been constant over the past 15 to 20 my, but rather appears punctuated by gaps of up to several million years. we attempted to address the temporal gaps in several ways. first, geochemical analyses were used to identify samples that may have erupted during time periods without known volcanism. for example, u/th values in the active central american arc are significantly higher than those obtained from the miocene coyol group except for four samples with intermediate values that were dated to determine if their ages were intermediate as well. however, all of these samples were found to be from a period with known volcanism. second, we sought to locate the oldest sections of the active arc and the youngest sections of the coyol group in order to better constrain the timing and duration of the apparent gap in volcanic productivity. this approach also failed to locate samples from periods without known volcanism. when these methods proved largely unsuccessful, our focus shifted to dating regions of minor volcanism between the active and coyol volcanic fronts as well as between cosigüina and san cristóbal, the longest stretch of the central american volcanic front withou
40Ar/39Ar Geochronology of mafic dykes in north Xinjiang
新疆北部基性岩脉40Ar/39Ar年代学研究

ZHOU Jing,JI JianQing,HAN BaoFu,MA Fang,GONG JunFeng,XU QinQin,GUO ZhaoJie,
周晶
,季建清,韩宝福,马芳,龚俊峰,徐芹芹,郭召杰

岩石学报 , 2008,
Abstract: 晚古生代以来,新疆北部地区侵入了一系列的基性岩脉,成为了解该地区地壳生长过程的重要窗口.本文采用激光40Ar/39Ar年代学方法,对整个北疆地区出露的基性岩脉进行了精细年代学研究.从数据特征上分析.低本底激光加40Ar/39Ar方法能对低钾含量、极少量样品(0.1mg~5mg)进行精细定年,所以非常适合进行基性岩脉的精细定年.本文展示的基性岩脉激光40Ar/39Ar方法的高质量数据显示新疆北部基性岩脉群的活动时限跨越332~174Ma,为石炭纪-侏罗纪.由数据分析可以看出,新疆北部基性岩脉群在时间上,呈现脉动式侵位特征,在332~174Ma之间,有多达6~7次脉动式活动,时间间隔大致为20~30Ma.从基性岩脉所揭示的大地构造含义来分析,新疆北部地区从石炭纪到侏罗纪处在一个相似的地壳演化阶段.为一个陆壳垂向生长、区域构造应力场显示拉张的构造环境,并在时间、空间上具有相对统一的特征.
El Chichón volcanic complex, Chiapas, México: Stages of evolution based on field mapping and 40Ar/39Ar geochronology
Layer, P. W.;García-Palomo, A;Jones, D;Macías, J. L.;Arce, J. L.;Mora, J. C.;
Geofísica internacional , 2009,
Abstract: a new interpretation of the evolution of el chichón volcano is presented in this paper based on photogeology, fieldwork, 40ar/39ar dating and chemistry of juvenile products. el chichón volcano belongs to a volcanic complex formed by craters and peripheral domes with a total estimated volume of ~26 km3. our data suggest that inception of magmatism began around 370 ka with the emission of lava domes buried by younger products. the activity continued with the formation of a large andesitic dome complex between 209 and 276 and associated block and ash flows and lahars. the dome complex was subsequently destroyed by a major eruption that left a 1.5-km wide somma-type crater. the activity continued with the extrusion of the sw dome at 217 ka, that partially disrupted the southwestern somma crater wall. later on a series of dome extrusions occurred beyond the northwestern sector of the somma crater at about 95 ka. juvenile blocks found in 1982 products yielded ages of 44 and 29 ka attesting to heretofore unidentified late pleistocene activity. the onset of holocene activity occurred both outside the somma crater with explosive eruptions that formed the guayabal tuff cone and inside the somma crater with the formation of a tuff cone that has been repeatedly reactivated during the holocene, lastly during the 1982 eruption. all magmas produced during the past 370,000 years are k-alkaline andesites that exhibit minor variation in their chemical composition. an average discharge rate of 0.5 km3/ka is calculated during the past 8,000 years (~4 km3).
Paleomagnetism, structure and 40Ar/39Ar geochronology of the Cerro Mercado pluton, Coahuila: Implications for the timing of the Laramide orogeny in northern Mexico
Molina-Garza, Roberto S.;Chávez-Cabello, Gabriel;Iriondo, Alexander;Porras-Vázquez, Mario Alberto;Terrazas-Calderón, Guillermo Daniel;
Revista mexicana de ciencias geológicas , 2008,
Abstract: the cerro mercado pluton, emplaced in the coahuila fold belt, contains magmatic, ductile, and brittle fabrics that suggest local and regional deformation during igneous emplacement, later affected by regional deformation in the area. the cerro mercado pluton intruded, deformed and uplifted upper cretaceous strata in the southern edge of the central depression of the sabinas basin. a hornblende mineral separate from the pluton yielded a 40ar/39ar plateau age of 44.29 ± 0.19ma (1σ), interpreted as an approximation to the crystallization age of the monzonite. a younger biotite total fusion age of 41.23 ± 0.21 ma (1σ) from a different rock sample is interpreted to represent slow cooling (80°c/ma) of the cerro mercado pluton. paleomagnetic analyses of the pluton yieldwell defined remanent magnetizations of nearly uniform reverse polarity, with a grand mean of d=178.2° and i=-61.7° (n=9 acceptable sites; k=57.5, α.95 = 6.8°). these data are discordant with respect to the eocene reference direction, indicating apparent clockwise rotation (8°) and inclination steepening (21°). the simplest interpretation of the discordance is that northwestward tilting of the pluton occurred during basin inversion within a weakly right lateral transpressive regime with nnw directed contraction. the strain field is consistent with contraction during the laramide orogeny. also, remanence acquisition is contemporaneous with both tectonic and magmatic fabrics in the pluton indicating that it is syntectonic. this suggests - in agreement with stratigraphic data - that the laramide orogeny in central coahuila ended some time after about 44 ma. we propose that reactivation of the san marcos fault and other basement faults in the sabinas basin represents the last manifestation of shortening produced by the laramide orogeny. this was a short episode of deformation that ended by the time of emplacement of younger plutons of the candela-monclova magmatic belt ca. 41 ma. this indicates that culmination
El Chichón volcanic complex, Chiapas, México: Stages of evolution based on field mapping and 40Ar/39Ar geochronology
P. W. Layer,A. García Palomo,D. Jones,J. L. Macías
Geofísica internacional , 2009,
Abstract: A new interpretation of the evolution of El Chichón volcano is presented in this paper based on photogeology, fieldwork, 40Ar/39Ar dating and chemistry of juvenile products. El Chichón volcano belongs to a volcanic complex formed by craters and peripheral domes with a total estimated volume of ~26 km3. Our data suggest that inception of magmatism began around 370 ka with the emission of lava domes buried by younger products. The activity continued with the formation of a large andesitic dome complex between 209 and 276 and associated block and ash flows and lahars. The dome complex was subsequently destroyed by a major eruption that left a 1.5-km wide Somma-type crater. The activity continued with the extrusion of the SW dome at 217 ka, that partially disrupted the southwestern Somma crater wall. Later on a series of dome extrusions occurred beyond the northwestern sector of the Somma crater at about 95 ka. Juvenile blocks found in 1982 products yielded ages of 44 and 29 ka attesting to heretofore unidentified late Pleistocene activity. The onset of Holocene activity occurred both outside the Somma crater with explosive eruptions that formed the Guayabal Tuff Cone and inside the Somma crater with the formation of a tuff cone that has been repeatedly reactivated during the Holocene, lastly during the 1982 eruption. All magmas produced during the past 370,000 years are K-alkaline andesites that exhibit minor variation in their chemical composition. An average discharge rate of 0.5 km3/ka is calculated during the past 8,000 years (~4 km3).
Progreso y retos de la geocronología 40Ar/39Ar en Costa Rica y Nicaragua Progress and Challenges Using 40Ar/39Ar Geochronology in Costa Rica and Nicaragua  [cached]
Ian Saginor1,Esteban Gazel,Carr Michael J,Carl C Swisher III
Revista Geológica de América Central , 2011,
Abstract: Con el principal objetivo de realizar mejores cálculos de la producción volcánica, en el Arco Volcánico de América Central, realizamos 61 dataciones 40Ar/39Ar en tefras y lavas en localidades de frente volcánico de Costa Rica y Nicaragua por medio de varias campa as de campo y análisis de laboratorio (preparación de muestras, envió al reactor nuclear, espectroscopía de masas de las muestras radiactivas, al laboratorio de gases nobles del Departamento de Ciencias de la Tierra y Planetarias de la Universidad Rutgers) del 2002-2008. En este artículo presentamos diferentes observaciones que resultaron de este estudio donde se describe la precisión, exactitud y la confiabilidad de las edades 40Ar/39Ar. El primer resultado muestra las limitaciones de esta técnica en muestras baja en K2O con edades < 1 Ma, especialmente en Nicaragua. El segundo resultado muestra los efectos de la pérdida de Ar por los procesos de meteorización típicos del trópico, inclusive afectando las muestras sanas a nivel críptico. El tercer resultado sugiere que las razones eruptivas del frente volcánico no han sido constantes durante los últimos 15-20 Ma, sino más bien representan ciclos eruptivos. Con el propósito de evaluar los hitos entre diferentes ciclos eruptivos, evaluamos los datos geoquímicos a lo largo del frente volcánico, en especial las relaciones U/Th, cuyos valores son inferiores (Grupo Coyol) en Nicaragua, comparados con el frente volcánico activo. Sin embargo, las muestras seleccionadas con base en geoquímica, resultaron ser de edades conocidas y no produjeron datos que llenan los hiatos entre ciclos de actividad volcánica. La siguiente estrategia que manejamos fue extender el muestreo en el Grupo Coyol, inmediatamente detrás del frente volcánico activo, sin embargo, esta estrategia no fue exitosa para encontrar muestras que llenaran los hiatos de actividad. No obstante, obtuvimos edades de 1.1-3,6 Ma entre los volcanes Cosiguina and San Cristóbal, los cuales limitan el hiato de actividad y mejoran nuestra percepción de los ciclos de actividad volcánica en América Central. To better estimate the extrusive flux of the Central American Arc, from 2002-2008, we obtained sixty one high precision 40Ar/39Ar ages on geographically well-situated lavas and tephra from Costa Rica and Nicaragua. Here, we describe a number of observations encountered during this study using four examples that well document the precision, accuracy and general reliability of the 40Ar/39Ar ages. First, low K2O values, particularly in samples from Nicaragua, is a major limitation in or attempts to obtain
^40Ar/^39Ar Geochronology, geochemical characteristics and genesis of the Linzhou basic dikes,Tibet
西藏林周基性岩脉的^40Ar/^39Ar年代学、地球化学及其成因

YUE YaHui~ DING Lin~ Institute of Geology,Geophysics,Chinese Academy of Sciences,Beijing,China Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing,China,
岳雅慧
,丁林

岩石学报 , 2006,
Abstract: 在西藏林周盆地林子宗火山岩地层中发现一套始新世钾玄质基性岩脉.测定了它们的主量元素、微量元素和稀土元素含量以及Sr-Nd-O同位素和^40Ar/^39Ar同位素地质年代.测试结果表明,岩脉以低硅(SiO2为45.42%~48.32%)、高镁(MgO为2.84%~5.35%,平均4.57%)、高钾高碱(K2O+Na2O为4.60%~6.23%、K2O/Na2O为0.71~1.18)为特征,属于钾玄质基性岩脉;稀土和微量元素呈现陆缘火山弧的特征,即相对富集轻稀土元素、亏损重稀土元素,具有弱的Eu负异常(δEu=0.79~0.94,平均0.83);强烈富集K、Rb、Sr、Ba、Th、Sr等大离子不相容元素,相对亏损Th、Nb、Ta、P、Zr、Hf、HREE等高场强元素,具有明显的Nb、Ta、Ti的负异常;Sr-Nd-O同位素组成上,具有相对原始的Sr、Nd同位素比值,初始^87Sr/^86Sr比值为0.70487~0.70543,143Nd/144Nd比值为0.51266~0.51273,εNd值为0.5~1.8,相对低的δ^18OV-SMOW值(4.8‰~7.6‰);^40Ar/^39Ar角闪石单矿物年龄为52.9Ma.利用地幔地球化学示踪理论,推断林周钾玄质基性岩脉的形成与新特提斯洋岩石圈向欧亚大陆俯冲过程中的回转、断离以及由此引起的软流圈地幔的上涌等地球动力学过程有关,是软流圈地幔流体与先前被俯冲带流体交代过的上覆岩石圈地幔的部分熔融产物的混合,指示冈底斯带在古新世-始新世(55~50Ma)时期一次平行于造山带的伸展作用.
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