Zircon M257 ‐ a Homogeneous Natural Reference Material for the Ion Microprobe U‐Pb Analysis of Zircon     

Lutz Nasdala  Wolfgang Hofmeister  Nicholas Norberg  James M. Martinson  Fernando Corfu  Wolfgang Dörr  Sandra L. Kamo  Allen K. Kennedy  Andreas Kronz  Peter W. Reiners  Dirk Frei  Jan Kosler  Yusheng Wan  Jens Götze  Tobias Häger  Alfred Kröner  John W. Valley 《Geostandards and Geoanalytical Research》2008,32(3):247-265

We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for ²⁰⁶pb/²³⁸U and 0.7392 ± 0.0003 for ²⁰⁷pb/²³⁵U. Its ²⁰⁶pb/²³⁸U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ～ 840 μg g^?1 U (Th/U ～ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 10¹⁸ g^?1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.  相似文献   

Increased Terrestrial to Ocean Sediment and Carbon Fluxes in the Northern Chesapeake Bay Associated With Twentieth Century Land Alteration     

Casey Saenger  Thomas M. Cronin  Debra Willard  Jeffrey Halka  Randy Kerhin 《Estuaries and Coasts》2008,31(3):492-500

We calculated Chesapeake Bay (CB) sediment and carbon fluxes before and after major anthropogenic land clearance using robust monitoring, modeling and sedimentary data. Four distinct fluxes in the estuarine system were considered including (1) the flux of eroded material from the watershed to streams, (2) the flux of suspended sediment at river fall lines, (3) the burial flux in tributary sediments, and (4) the burial flux in main CB sediments. The sedimentary maximum in Ambrosia (ragweed) pollen marked peak land clearance (~1900 a.d.). Rivers feeding CB had a total organic carbon (TOC)/total suspended solids of 0.24?±?0.12, and we used this observation to calculate TOC fluxes from sediment fluxes. Sediment and carbon fluxes increased by 138–269% across all four regions after land clearance. Our results demonstrate that sediment delivery to CB is subject to significant lags and that excess post-land clearance sediment loads have not reached the ocean. Post-land clearance increases in erosional flux from watersheds, and burial in estuaries are important processes that must be considered to calculate accurate global sediment and carbon budgets.  相似文献   

Group Time Delay in the Reflection of P-Polarized Electromagnetic Plane Waves at a Stratigraphic Interface     

Fu-Ping Liu  Rui-Zhong Li  Jin-yao Li  Hui-Guo Chen  Chang-Chun Yang 《Mathematical Geosciences》2008,40(7):813-829

The group time delay (GTD) formulae of quasi-total reflection of inhomogeneous P-polarized electromagnetic waves (P-PEW, whose electric field parallels to the plane of incidence), caused by the Goos–Hänchen effect, is derived by the phase shift of the wave. A numerical example where the frequency equals to 1 GHz is given. The curves of calculating results show that there are three discontinuous points at the critical angle of phase shift vector, the critical angle of attenuation vector and 90^○. When the angle of incidence is equal to one of these three angles, the GTD will become infinite, and the electromagnetic wave will propagate along the interface. The GTD will be very large, when the angle of incidence is close to one of these three angles. The results indicate that there is an important relationship between the two critical angles and the conductivity and permittivity of the two strata, and that the GTD has an important relationship with the angle of incidence. These results can be used to determine the lithology of the strata and to delimit the interfaces of strata more effectively. It is suggested that this research may prove useful in electromagnetic logging analysis and, perhaps, in the design of logging instruments.  相似文献   

Book reviews     

Tapan Mukerji  Gary Mavko  Mark S. Bukowinski  Lucía Lovìson-Golob  J. J. Telega  Benoit Cushman-Roisin  Michael Manga  Yu. N. Korenkov 《Pure and Applied Geophysics》1996,146(2):393-404

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Petrogenesis of the late-Delamerian gabbroic complex at Black Hill,South Australia: Implications for convective thinning of the lithospheric mantle     

S. P. Turner 《Mineralogy and Petrology》1996,56(1-2):51-89

Summary A group of funnel-shaped gabbroic plutons at Black Hill, South Australia, consist of a lower series of layered peridotite, troctolite and olivine gabbro cumulates overlain by gabbronorites and potassic gabbronorites, the latter yielding a Sm-Nd isochron of 489 ± 39 Ma. Mineral assemblages in the gabbros record an olivine compositional hiatus (Fo_75-55) and a high temperature (1200-1000°C), low pressure (1 kbar), continental tholeiitic fractionation trend under moderate f_{O 2} (QFM) conditions. The liquid line of descent involved complex open system processes including recharge and crustal assimilation. In one pluton, fine-grained norites may reflect assimilation which resulted in an increaseda _SiO2 ^liquid causing orthopyroxene to crystallize prior to plagioclase.All the gabbros, including the most primitive peridotites, are LREE and incompatible element enriched. Moreover, the calculated parental magma composition in equilibrium with the most primitive troctolite has high La/Yb, La/Nb, Ti/Y and low Rb/Ba, similar to that of basaltic dykes which cut the gabbroic complex. Such compositions are untypical of melts derived from the asthenosphere suggesting that the incompatible element enrichment is not simply due to small degrees of melting. Given the isotopic constraints (Nd_i 3.4 to –4.6,⁸⁷Sr/⁸⁶Sr_i 0.7038-0.7065), this enrichment is not easily reconciled by crustal contamination either, and instead it is inferred to reflect an enriched lithospheric mantle source. Published data on mantle xenoliths from local Tertiary volcanoes overlap the isotopic and geochemical array of the gabbros and dykes, supporting this hypothesis.In conjunction with A-type granites and minor volcanic rocks, the gabbroic plutons form part of a high temperature, bimodal magmatic suite which intruded the Adelaide fold belt just after the cessation of convergent deformation during the Cambro-Ordovician Delamerian Orogeny. The appearance of such magmas is problematic since thick orogenic lithosphere severely restricts the likelihood of decompression melting in the asthenosphere. One solution to this dilemma is that convective thinning of the lithospheric mantle beneath the orogen promoted melting of hydrated, enriched regions within the lithospheric mantle. Such a model can reconcile the strong lithospheric mantle signature in the gabbros with the observation that their intrusion was coincident with uplift and the cessation of deformation.

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Die Petrogenese eines spätdelamerischen Gabbrokomplexes von Black Hill, Südaustralien: Schlußfolgerungen zur konvektiven Ausdünnung des lithospharischen Mantels

Zusammenfassung Eine Gruppe von trichterförmigen, gabbroiden Plutonen von Black Hill, Südaustralien, setzt sich aus einer unteren Serie von geschichteten Peridotiten, Troktoliten and Olivin-Gabbro-Kumulaten, die von Gabbro-Noriten und potassischen Gabbros überlagert werden, zusammen; letzgenannte Gesteine ergaben ein Sm-Nd Isochronenalter von 489 ± 39 Ma. Die Mineralzusammensetzungen der Gabbros belegen eine Mischungslücke bei Olivin (Fo_75-55). Die Gesteine folgen einem kontinentalen, tholeiitischen Fraktionierungstrend und kristallisierten bei hohen Temperaturen (1200-1000°C), unter niedrigem Druck ( 1 kbar) and moderatem fo₂ ( QFM). Wiederholte Magmenzufuhr und Krustenassimilation sind weitere zu beriicksichtigende, komplexe Prozesse, die sich in einem offenen System abspielten. Feinkörnige Norite von einem der Plutone könnten durch Assimilationsprozesse - she äußern sich in einer Erhöhung desa _SiO₂ ^liquid liquid - entstanden sein. In diesen Gesteinen kristallisierte Orthopyroxen vor Plagioklas.Alle Gabbros, einschließich der primitivsten Peridotite, sind an LREE und inkompatiblen Elementen angereichert. Die berechnete Zusammensetzung des Ausgangsmagmas, das im Gleichgewicht mit dem primitivsten Troktolit steht, weist hohe La/Yb, La/Nb, Ti/Y and niedrige Rb/Ba auf. Basaltische Dykes, die die Gabbros durchsetzen zeigen ähnliche Elementverhältnisse. Derartige Zusammensetzungen sind untypisch für astenosphärische Schmelzen und belegen, daß die Anreicherung an inkompatiblen Elementen nicht einfach nur mit geringen Aufschmelzungsraten zu erklären ist. Auf Grund der Isotopendaten (Nd_i 3.4 to -4.6,⁸⁷Sr/⁸⁶ Sr_i 0.7038–0.7065) kann these Anreicherung auch nicht leicht mit Krustenkontamination erklärt werden. Vielmehr wird eine angereicherte, lithosphärische Mantelquelle vermutet. Publizierte Daten an Mantelxenolithen von tertiären Vulkanen zeigen sehr ähnliche isotopische und geochemische Zusammensetzungen and belegen these Hypothese.Gemeinsam mit A-Typ Graniten und untergeordnet, vorkommenden, vulkanischen Gesteinen sind die Gabbros Teil einer hochtemperierten bimodalen, magmatischen Serie, die den Adelaide Fold Belt postdeformativ, während der kambro-ordovizischen delamerischen Orogenese, intrudierten. Das Vorkommen solcher Magmen ist problematisch, da die Präsenz dicker, orogen gebildeter Lithosphäre die Möglichkeit der Dekompressions-Aufschmelzung der Astenosphäre stark vermindert. Eine mögliche Lösung dieses Dilemmas ist, daß konvektives Ausdünnen des lithosphärischen Mantels unterhalb der Orogenzone das Aufschmelzen von hydratisierten, angereicherten Regionen innerhalb der Lithosphäre förderte. Dieses Modell bringt die Beobachtungen, daß die Gabbros einerseits eine lithosphärische Signatur zeigen, andererseits aber während der Hebungsphase und am Ende des Deformationsgeschehens intrudierten, in Einklang.

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With 15 Figures  相似文献   

The propagation of a single shear crack: A displacement discontinuity model     

J. -Cl. De Bremaecker  K. Wei 《Pure and Applied Geophysics》1994,142(3-4):567-585

Numerical studies using the displacement discontinuity method show that a single shear crack under compression propagates in its own direction, because such propagation results in the maximum release of strain energy. The methods of linear elastic fracture mechanics may not be used for such a closed crack, and the stress intensity factors are meaningless in that case. Laboratory observations of propagation by means of kinks at an angle of approximately 70° to the crack may be due to heterogeneities, to the effect of a preexisting crack, to end effects, to microcracking, or to some combination of these factors. Such kinks may thus be local phenomena which cannot release most of the strain energy, and are not incompatible with our numerical results which are based on a global energy balance.  相似文献   

Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window,Eastern Alps   总被引：1，自引：1，他引：0  

John N. Christensen  Jane Selverstone  John L. Rosenfeld  Donald J. DePaolo 《Contributions to Mineralogy and Petrology》1994,118(1):1-12

In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67 _–0.13 ^+0.19 mm/million years and 0.88 _–0.19 ^+0.34 mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7 _–0.7 ^+1.2 ×10^-14 s^-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at 30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4 _–2 ⁺³ mm/year and 2±1 mm/year, respectively.  相似文献   

肯尼亚Anza盆地东南部重力场及构造特征     

陈青  袁炳强  黄小宇  李君梅  陈浩  丁成艺  孙帅  覃雯 《地质力学学报》2021,27(6):928-937

肯尼亚Anza盆地东南部地处东非裂谷系，发育了巨厚的中—新生界沉积盖层。然而，该区域勘探程度较低，制约了对其构造体系的认识及油气勘探潜力的评价。文章基于研究区的重力异常数据，针对其构造特征的认识进行了数据处理及解释。研究结果表明，受中非剪切带右旋剪切应力的影响，研究区发育规模较大的北西向基底断裂和规模较小的北东向盖层断裂，且北东向断裂切断北西向断裂；基底深度差异大，总体呈"两凹夹一隆"的特征，凹陷区沉积了巨厚的中—新生界盖层；受北西向拉张断裂和沿构造软弱带发育的北东向断裂的控制，研究区划分为东部凹陷、中部凸起、南部隆起和西部凹陷4个构造单元，呈现"东西分带、南北分块"的构造格局。   相似文献   

Channel overflows of the Pōhue Bay flow,Mauna Loa,Hawai'i: examples of the contrast between surface and interior lava     

Zinzuni Jurado-Chichay  Scott K. Rowland 《Bulletin of Volcanology》1995,57(2):117-126

A number of overflows from a large lava channel and tube system on the southwest rift zone of Mauna Loa were studied. Initial overflows were very low viscosity gas-rich phoehoe evidenced by flow-unit aspect ratios and vesicle sizes and contents. Calculated volumetric flow-rates in the channel range between 80 and 890 m³/s, and those of the overflows between 35 and 110 m³/s. After traveling tens to hundreds of meters the tops of these sheet-like overflows were disrupted into a surface composed of clinker and phoehoe fragments. After these 'a' overflows came to rest, lava from the interiors was able to break out on to the surface as phoehoe. The surface structure of a lava flow records the interaction between the differential shear rate (usually correlated with the volumetric flow-rate) and viscosity-induced resistance to flow. However, the interior of a flow, being better insulated, may react differently or record a later set of emplacement conditions. Clefts of toothpaste lava occurring within fields of clinker on proximal-type 'a' flows also record different shear rates during different times of flow emplacement. The interplay between viscosity and shear rate determines the final morphological lava type, and although no specific portion of lava ever makes a transition from 'a' back to phoehoe, parts of a flow can appear to do so.  相似文献   

Wollastonite at Nuliyam,Kerala, southern India: a reassessment of CO2-infiltration and charnockite formation at a classic locality     

Simon L. Harley  M. Santosh 《Contributions to Mineralogy and Petrology》1995,120(1):83-94

The occurrence of a charnockitised felsic gneiss adjacent to a marble/calc-silicate horizon at Nuliyam, southern India, has been cited in recent literature as a classic example of the dehydration of crustal rocks resulting from the advective infiltration of CO₂-rich fluids generated from a local carbonate source. Petrographic study of the Nuliyam calc-silicate, however, reveals it to consist of abundant wollastonite and scapolite and contain locally discordant veins rich in wollastonite. At the pressure—temperature conditions proposed for charnockite formation in recent studies, 5 kbar and 725°C, this wollastonite-bearing mineral assemblage was stable in the presence of a fluid phase only if X _CO2 was near 0.25 and could not have coexisted with the fluid causing biotite breakdown and charnockite development in adjacent rocks (X _CO2>0.85). The stable coexistence of wollastonite and scapolite prohibits the calc-silicate from being a source for fluid driving charnockitisation at the required P-T conditions. Textural observations such as the limited replacement of wollastonite by calcite+quartz symplectites and mosaics, are consistent with late fluid infiltration into the calc-silicate. The extensive isotopic, chemical and mineral abundance data of Jackson and Santosh (1992) are re-interpreted and integrated with these observations to develop a model involving the infiltration of an externally derived CO₂-rich fluid during high-temperature decompression. Increased charnockite development next to the calc-silicate has arisen because the calc-silicate acted as a relatively unreactive and impermeable barrier to fluid transport and caused fluid ponding beneath antiformal closures. The Nuliyam charnockite/calc-silicate locality is an example of a structural trap in a metamorphic setting rather than a site where charnockite formation can be attributed to local fluid sources.  相似文献