Linking shock textures revealed by BSE,CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure,Brazil     

Natalia Hauser  Wolf Uwe Reimold  Aaron J. Cavosie  Alvaro P. Crsta  Winfried H. Schwarz  Mario Trieloff  Carolinna Da Silva Maia de Souza  Luciana A. Pereira  Eduardo N. Rodrigues  Matthews Brown 《Meteoritics & planetary science》2019,54(10):2286-2311

A silicious impact melt rock from polymict impact breccia of the northern part of the alkali granite core of the Araguainha impact structure, central Brazil, has been investigated. The melt rock is thought to represent a large mass of impact‐generated melt in suevite. In particular, a diverse population of zircon grains, with different impact‐induced microstructures, has been analyzed for U‐Pb isotopic systematics. Backscattered electron and cathodoluminescence images reveal heterogeneous intragrain domains with vesicular, granular, vesicular plus granular, and vesicular plus (presumably) baddeleyite textures, among others. The small likely baddeleyite inclusions are not only preferentially located along grain margins but also occur locally within grain interiors. LA‐ICP‐MS U‐Pb data from different domains yield lower intercept ages of 220, 240, and 260 Ma, a result difficult to reconcile with the previous “best age” estimate for the impact event at 254.7 ± 2.7 Ma. SIMS U‐Pb data, too, show a relatively large range of ages from 245 to 262 Ma. A subset of granular grains that yielded concordant SIMS ages were analyzed for crystallographic orientation by EBSD. Orientation mapping shows that this population consists of approximately micrometer‐sized neoblasts that preserve systematic orientation evidence for the former presence of the high‐pressure polymorph reidite. In one partially granular grain (#36), the neoblasts occur in linear arrays that likely represent former reidite lamellae. Such grains are referred to as FRIGN zircon. The best estimate for the age of the Araguainha impact event from our data set from a previously not analyzed type of impact melt rock is based on concordant SIMS data from FRIGN zircon grains. This age is 251.5 ± 2.9 Ma (2σ, MSWD = 0.45, p = 0.50, n = 4 analyses on three grains), indistinguishable from previous estimates based on zircon and monazite from other impact melt lithologies at Araguainha. Our work provides a new example of how FRIGN zircon can be combined with in situ U‐Pb geochronology to extract an accurate age for an impact event.  相似文献   

Decoding earth's plate tectonic history using sparse geochemical data     

Michael G.Tetley  Zheng-Xiang Li  Kara J.Matthews  Simon E.Williams  R.Dietmar Müller 《地学前缘(英文版)》2020,11(1):265-276

Accurately mapping plate boundary types and locations through time is essential for understanding the evolution of the plate-mantle system and the exchange of material between the solid Earth and surface environments.However,the complexity of the Earth system and the cryptic nature of the geological record make it difficult to discriminate tectonic environments through deep time.Here we present a new method for identifying tectonic paleo-environments on Earth through a data mining approach using global geochemical data.We first fingerprint a variety of present-day tectonic environments utilising up to 136 geochemical data attributes in any available combination.A total of 38301 geochemical analyses from basalts aged from 5-0 Ma together with a well-established plate reconstruction model are used to construct a suite of discriminatory models for the first order tectonic environments of subduction and mid-ocean ridge as distinct from intraplate hotspot oceanic environments,identifying 41,35,and 39 key discriminatory geochemical attributes,respectively.After training and validation,our model is applied to a global geochemical database of 1547 basalt samples of unknown tectonic origin aged between 1000-410 Ma,a relatively ill-constrained period of Earth’s evolution following the breakup of the Rodinia supercontinent,producing 56 unique global tectonic environment predictions throughout the Neoproterozoic and Early Paleozoic.Predictions are used to discriminate between three alternative published Rodinia configuration models,identifying the model demonstrating the closest spatio-temporal consistency with the basalt record,and emphasizing the importance of integrating geochemical data into plate reconstructions.Our approach offers an extensible framework for constructing full-plate,deeptime reconstructions capable of assimilating a broad range of geochemical and geological observations,enabling next generation Earth system models.  相似文献   

The steady-state structure of accretion discs in central magnetic fields     

O. M. Matthews †  R. Speith  M. R. Truss  G. A. Wynn 《Monthly notices of the Royal Astronomical Society》2005,356(1):66-76

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‘Little ice age’ variations of outlet glaciers from the jostedalsbreen ice-cap,Southern Norway: A regional lichenometric-dating study of ice-marginal moraine sequences and their climatic significance     

Richard W. Bickerton  John A. Matthews 《第四纪科学杂志》1993,8(1):45-66

Complex moraine-ridge sequences in front of seven outlet glaciers of the Jostedalsbreen ice-cap (Austerdalsbreen, Bergsetbreen, Fåbergstølsbreen, Lodalsbreen, Stegaholbreen, Tuftebreen, Bødalsbreen) are dated using families of lichenometric dating curves established previously at an eighth outlet (Nigardsbreen). Applicability of the Nigardsbreen curves at the regional level is tested using independent historical evidence: moraines deposited during the present century are dated to an accuracy of ± 9.4 yr (16.0%), and most of them are dated to an accuracy of ± 5.5 yr (9.4%). Results from the moraine sequences are combined to form a composite ‘Jostedalsbreen’ record. Median predicted dates for moraine ridges cluster around AD 1939 ± 2 yr, 1929 ± 3, 1908 ± 3, 1886 ± 5, 1875 ± 2, 1867 ± 4, 1855 ± 3, 1842 ± 5, 1822 ± 5, 1807 ± 4 and 1785 ± 5. At least four glaciers reached their ‘Little ice age’ maxima prior to AD 1780, two (Nigardsbreen and Bødalsbreen) at ca. 1750, one (Fåbergstølsbreen) at ca. 1705. Stegaholbreen attained its maximum ca. 1863. Since the ‘Little ice age’ maximum, and despite large differences in glacier size, frontal variations of the various outlets have exhibited a high degree of synchroneity, which suggests that the moraine sequences contain a sensitive record of high-frequency climatic variations over the last ca. 250 yr. During the early twentieth century, measured readvances of the order of 5–150 m over 1–10 yr led to moraine formation. Dendroclimatic evidence indicates that since the late eighteenth century, moraine ridges formed about 5 yr after summer temperature minima and correlate with runs of cool summers (temperature depression of 0.5–1.0°C below the AD 1700–1950 average). Almost simultaneous glacier advances appear to have been caused by reduced ablation. This near-immediate response to climatic variation, by glacier tongues that descend to relative low altitudes, is superimposed upon the longer-term dynamic response of the ice cap to climate.  相似文献   

Modelling the photopolarimetric variability of AA Tau     

Mark O'Sullivan  Michael Truss  Christina Walker  Kenneth Wood  Owen Matthews  Barbara Whitney  J. E. Bjorkman 《Monthly notices of the Royal Astronomical Society》2005,358(2):632-640

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Magnetically moderated outbursts of WZ Sagittae     

O. M. Matthews  R. Speith  G. A. Wynn  R. G. West 《Monthly notices of the Royal Astronomical Society》2007,375(1):105-114

We argue that the quiescent value of the viscosity parameter of the accretion disc in WZ Sge may be  α_cold∼ 0.01  , in agreement with estimates of α_cold for other dwarf novae. Assuming the white dwarf in WZ Sge to be magnetic, we show that, in quiescence, material close to the white dwarf can be propelled to larger radii, depleting the inner accretion disc. The propeller therefore has the effect of stabilizing the inner disc and allowing the outer disc to accumulate mass. The outbursts of WZ Sge are then regulated by the (magnetically determined) evolution of the surface density of the outer disc at a radius close to the tidal limit. Numerical models confirm that the recurrence time can be significantly extended in this way. The outbursts are expected to be superoutbursts since the outer disc radius is forced to exceed the tidal (3:1 resonance) radius. The large, quiescent disc is expected to be massive, and to be able to supply the observed mass accretion rate during outburst. We predict that the long-term spin evolution of the white dwarf spin will involve a long cycle of spin-up and spin-down phases.  相似文献   

Thermal response of Saturn's ring particles during and after eclipse     

Lucien Froidevaux  Keith Matthews  Gerry Neugebauer 《Icarus》1981,46(1):18-26

We present infrared (20 μm) observations of Saturn's rings for a solar elevation angle of 10° and phase angle of 6°. Scans across the rings yield information about the cooling of particles during eclipse and the subsequent heating along their orbits. All three rings exhibit significant cooling during eclipse, as well as a 20-μm brightness asymmetry between east and west ansae, the largest asymmetry occuring in the C ring (the brightest ring). The eclipse cooling is a simple and adequate explanation for 20-μm brightness asymmetries between the ansae of Saturn's rings. The relatively large C ring asymmetry is thought to be primarily due to the short travel time of the particles in that ring from eclipse exit to east ansa. We compare the B ring data to the theoretical models of H.H. Aumann and H.H. Kieffer (1973, Astrophys. J.186, 305–311) in order to set constraints on the average particle size and thermal inertia. The rather rapid heating after exit from eclipse points to low-conductivity-particle surfaces, similar to the water frost surfaces of Galilean satellites. If the surface conductivity is indeed low, one cannot determine an upper limit for the particle size through such infrared observations, since only the uppermost millimeters experience a thermal response during eclipse. However, based on these infrared data alone, it is clear that particles of radius equal to a few millimeters or less cannot occupy a significant fraction of the ring surface area, because-regardless of thermal inertia-their thermal response is much faster than observed.  相似文献   

The Oblique Seismic Experiment on DSDP Leg 52     

R. A. Stephen  K. E. Louden  D. H. Matthews 《Geophysical Journal International》1980,60(2):289-300

Summary. The Oblique Seismic Experiment (OSE) has been proposed to increase the usefulness of the IPOD crustal borehole as a means of investigating layer 2 of oceanic crust. Specific objectives are: to determine the lateral extent of the structure intersected by the borehole, to analyse the role of cracks in the velocity structure of layer 2, to look for anisotropy which may be caused by large cracks with a preferred orientation and, finally, to measure attenuation in oceanic crust.
The first successful Oblique Seismic Experiment in oceanic crust was carried out in 1977 March in a hole 400 miles north of Puerto Rico. An adequate study of lateral velocity variations was impossible because the hole was not deep enough, the hole was inadequately logged, and the small scale basement topography was not known. In general both P - and S -wave velocity profiles suggest that the crack density decreases with depth in layer 2. Velocities at the bottom of layer 2 are the same as matrix velocities for basalt, implying that crack density may be negligible at this depth. No convincing evidence for anisotropy in either layer 2 or 3 is found from travel time analysis. The hole was not deep enough to measure attenuation from normal incidence shots and amplitudes were not consistent enough to obtain a measure of attenuation from long range shots.  相似文献   

Evolution of a calcite marble shear zone complex on Thassos Island, Greece: microstructural and textural fabrics and their kinematic significance   总被引：1，自引：0，他引：1  

Michel Bestmann  Karsten Kunze  Alan Matthews 《Journal of Structural Geology》2000,22(11-12)

The deformation history of a monophase calcite marble shear zone complex on Thassos Island, Northern Greece, is reconstructed by detailed geometric studies of the textural and microstructural patterns relative to a fixed reference system (shear zone boundary, SZB). Strain localization within the massive marble complex is linked to decreasing P–T conditions during the exhumation process of the metamorphic core complex. Solvus thermometry indicates that temperatures of 300–350°C prevailed during part of the shear zone deformation history. The coarse-grained marble protolith outside the shear zone is characterized by symmetrically oriented twin sets due to early coaxial deformation. A component of heterogeneous non-coaxial deformation is first recorded within the adjacent protomylonite. Enhanced strain weakening by dynamic recrystallization promoted strong localization of plastic deformation in the ultramylonite of the calcite shear zone, where high strain was accommodated by non-coaxial flow. This study demonstrates that both a pure shear and a simple shear strain path can result in similar crystallographic preferred orientations (single c-axis maximum perpendicular to the SZB) by different dominant deformation mechanisms. Separated a-axis pole figures (+a- and −a-axis) show different density distributions with orthorhombic texture symmetry in the protolith marble and monoclinic symmetry in the ultramylonite marble consistently with the observed grain fabric symmetry.  相似文献   

Oxygen and hydrogen isotope study of high-pressure metagabbros and metabasalts (Cyclades, Greece): implications for the subduction of oceanic crust   总被引：1，自引：1，他引：0  

Benita Putlitz  Alan Matthews  John W. Valley 《Contributions to Mineralogy and Petrology》2000,138(2):114-126

Oxygen and hydrogen stable isotope ratios of eclogite-facies metagabbros and metabasalts from the Cycladic archipelago (Greece) document the scale and timing of fluid–rock interaction in subducted oceanic crust. Close similarities are found between the isotopic compositions of the high-pressure rocks and their ocean-floor equivalents. High-pressure minerals in metagabbros have low δ¹⁸O values: garnet 2.6 to 5.9‰, glaucophane 4.3 to 7.1‰; omphacite 3.5 to 6.2‰. Precursor actinolite that was formed during the hydrothermal alteration of the oceanic crust by seawater analyses at 3.7 to 6.3‰. These compositions are in the range of the δ¹⁸O values of unaltered igneous oceanic crust and high-temperature hydrothermally altered oceanic crust. In contrast, high-pressure metabasalts are characterised by ¹⁸O-enriched isotopic compositions (garnet 9.2 to 11.5‰, glaucophane 10.6 to 12.5‰, omphacite 10.2 to 12.8‰), which are consistent with the precursor basalts having undergone low-temperature alteration by seawater. D/H ratios of glaucophane and actinolite are also consistent with alteration by seawater. Remarkably constant oxygen isotope fractionations, compatible with isotopic equilibrium, are observed among high-pressure minerals, with Δ_{glaucophane−garnet} = 1.37 ± 0.24‰ and Δ_{omphacite−garnet} = 0.72 ± 0.24‰. For the estimated metamorphic temperature of 500 °C, these fractionations yield coefficients in the equation Δ = A * 10⁶/T ² (in Kelvin) of A_{glaucophane−garnet} = 0.87 ± 0.15 and A_{omphacite−garnet} = 0.72 ± 0.24. A fractionation of Δ_{glaucophane–actinolite} = 0.94 ± 0.21‰ is measured in metagabbros, and indicates that isotopic equilibrium was established during the metamorphic reaction in which glaucophane formed at the expense of actinolite. The preservation of the isotopic compositions of gabbroic and basaltic oceanic crust and the equilibrium fractionations among minerals shows that high-pressure metamorphism occurred at low water/rock ratios. The isotopic equilibrium is only observed at hand-specimen scale, at an outcrop scale isotopic compositional differences occur among adjacent rocks. This heterogeneity reflects metre-scale compositional variations that developed during hydrothermal alteration by seawater and were subsequently inherited by the high-pressure metamorphic rocks. Received: 4 January 1999 / Accepted: 7 July 1999  相似文献