High‐ to ultrahigh‐temperature metamorphism in the lower crust: An example resulting from Hikurangi Plateau collision and slab rollback in New Zealand          下载免费PDF全文

Jean‐Baptiste Jacob  James M. Scott  Rose E. Turnbull  Matthew S. Tarling  Matthew W. Sagar 《Journal of Metamorphic Geology》2017,35(8):831-853

Lower crustal xenoliths erupted from an intraplate diatreme reveal that a portion of the New Zealand Gondwana margin experienced high‐temperature (HT) to ultrahigh‐temperature (UHT) granulite facies metamorphism just after flat slab subduction ceased at c. 110–105 Ma. P–T calculations for garnet–orthopyroxene‐bearing felsic granulite xenoliths indicate equilibration at ~815 to 910°C and 0.7 to 0.8 GPa, with garnet‐bearing mafic granulite xenoliths yielding at least 900°C. Supporting evidence for the attainment of HT and UHT conditions in felsic granulite comes from re‐integration of exsolution in feldspar (~900–950°C at 0.8 GPa), Ti‐in‐zircon thermometry on Y‐depleted overgrowths on detrital zircon grains (932°C ± 24°C at aTiO₂ = 0.8 ± 0.2), and correlation of observed assemblages and mineral compositions with thermodynamic modelling results (≥850°C at 0.7 to 0.8 GPa). The thin zircon overgrowths, which were mainly targeted by drilling through the cores of grains, yield a U–Pb pooled age of 91.7 ± 2.0 Ma. The cause of Late Cretaceous HT‐UHT metamorphism on the Zealandia Gondwana margin is attributed to collision and partial subduction of the buoyant oceanic Hikurangi Plateau in the Early Cretaceous. The halt of subduction caused the fore‐running shallowly dipping slab to rollback towards the trench position and permitted the upper mantle to rapidly increase the geothermal gradient through the base of the extending (former) accretionary prism. This sequence of events provides a mechanism for achieving regional HT–UHT conditions in the lower crust with little or no sign of this event at the surface.  相似文献   

ACCURATE MAP OF MOSCOW ISSUED     

Matthew J. Sagers 《地理信息系统科学与遥感》2013,50(1)

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Book Review     

Matthew Bate 《地球物理与天体物理流体动力学》2013,107(4):435-436

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Food habits and distribution of wintering canvasbacks,Aythya valisineria,on Chesapeake Bay     

Matthew C. Perry  Francis M. Uhler 《Estuaries and Coasts》1988,11(1):57-67

Baltic clams (Macoma balthica) were the predominant food items of 323 canvasbacks (Aythya valisineria) collected throughout Chesapeake Bay during 1970–1979. Natural vegetation constituted 4% of the food volume. Widgeongrass (Ruppia maritima) and redhead grass (Potamogeton perfoliatus) constituted the greatest percent volume and frequency of occurrence among the plant species, whereas wild celery (Vallisneria americana) constituted only a trace of the food volume. These results contrast with historical records of food habits of canvasbacks in Chesapeake Bay. Canvasback population estimates during the 1970’s were examined to detect annual and seasonal changes in distribution. Linear regression analyses of winter canvasback populations in the bay showed a significant decline in the upper-bay and middle-bay populations, but no significant changes in the lower-bay and Potomac River populations. The changes in winter distribution and abundance of the canvasback appear related to changes in natural food availability, which is the result of altered environmental conditions.  相似文献   

Argon diffusion domains in K-feldspar II: kinetic properties of MH-10   总被引：2，自引：0，他引：2  

Oscar M. Lovera  Matthew T. Heizler  T. Mark Harrison 《Contributions to Mineralogy and Petrology》1993,113(3):381-393

Viewing K-feldspars as containing a discrete distribution of diffusion domain sizes reconciles otherwise disconsonant features common in their ⁴⁰Ar/³⁹Ar age spectra and Arrhenius plots but raises a fundamental question. What are the features in K-feldspar that endow it with this behavior? We report here the results of two different kinds of experiments that help isolate the nature of the responsible diffusion properties. To assess the thermal stability of the diffusion domains during laboratory treatment, MH-10 K-feldspar was step-heated to 850°C, removed from the furnace and split. One split was reirradiated and the other returned to the furnace and completely degassed. Following re-irradiation, the original heating schedule was used to degas the second aliquot. Apart from the first 5% of gas released, the diffusion properties show little change relative to the original result but, it appears, the physical character of a portion of the smallest domain has been altered. Results of duplicate step-heating experiments of samples treated at 750°C, 950°C and 1100°C prior to irradiation are consistent with the conclusions of the double irradiation experiment. In a second series of experiments, sized aggregates of MH-10 K-feldspar were analyzed by the ⁴⁰Ar/³⁹Ar step-heating method. The resultant log(r/r _o) plots reveal that the largest domain is annihilated when the particle size is reduced to about 50 μm. From this result we infer that the largest diffusion domain size is between 60 and about 130 μm in diameter. This estimate, together with knowledge of the relative domain size distribution obtained from modeling the log(r/r _o) plot, sets the size of the smallest domain to be less than about 1 μm. Microstructural examination of MH-10 K-feldspar identifies sub-grain features that correspond in size to our independent estimates for the largest and smallest diffusion domains. These results strongly support the view that low-temperature K-feldspars contain a distribution of diffusion length scales that are well approximated as discrete domain sizes and that laboratory heating below the onset of melting does not significatly affect the ability to obtain thermal reconstructions from the ⁴⁰Ar/³⁹Ar systematics.  相似文献   

Long-term variations of annual and growing season rainfalls in Nigeria   总被引：1，自引：0，他引：1  

O. J. Olaniran  G. N. Sumner 《Theoretical and Applied Climatology》1990,41(1-2):41-53

Summary Evidence for changes in the annual and growing season rainfall series for the period 1919 to 1985 in Nigeria are examined on a regional basis, using power-spectral and lowpass filter techniques, and the Mann-Kendall rank statistic. Four regions, the Coastal Zone, the Guinea-Savanna Zone, the Midland area and the Sahel, are used in the investigation of rainfall variation from south to north across the country.Quasi-periodic oscillations in the annual and growing season rainfall series are found to be concentrated in four spectral bands: 2.0–2.4, 2.7–2.9, 3.2–3.6 and 5.6–6.3 years. The spatial coherence of the fluctuations in annual and growing season rainfall is found to be limited to Nigeria south of 11 degrees north latitude. Evidence also emerges of a progressive decline in annual and growing season rainfall for northern Nigeria, north of nine degrees north latitude, for the period 1939–1985.With 5 Figures  相似文献   

A National Evaluation of the Leaching Potential of Aldicarb Part 2. An Evaluation of Ground Water Monitoring Data     

Matthew N. Lorber  Stuart Z. Cohen  George D. DeBuchananne 《Ground Water Monitoring & Remediation》1990,10(1):127-141

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Dependence of Effective Porosity on Fracture Continuity in Fractured Media     

Matthew J. Gordon 《Ground water》1986,24(4):446-452

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Progress in isotope paleohydrology using lake sediment cellulose     

Brent B. Wolfe  Matthew D. Falcone  Ken P. Clogg-Wright  Cherie L. Mongeon  Yi Yi  Bronwyn E. Brock  Natalie A. St. Amour  William A. Mark  Thomas W. D. Edwards 《Journal of Paleolimnology》2007,37(2):221-231

Recent advances in sample preparation techniques and mass spectrometry have fostered more routine oxygen isotope analysis of aquatic cellulose in lake sediment cores, a proxy for lake water oxygen isotope history. These methodological developments have significantly increased the feasibility of incorporating this approach into high-resolution, multi-site, and multi-proxy studies, which are frequently necessary to answer complex hydrological, hydroecological and hydroclimatic questions requiring a paleoenvironmental perspective. Direct translation of lake sediment aquatic cellulose oxygen isotope composition into lake water oxygen isotope composition offers appreciable opportunity for quantitative paleohydrological reconstructions, as evidenced by studies conducted over the past 15 years that span Holocene and pre-historical timescales.  相似文献   

The gas content and buoyancy of strombolian ash plumes     

Matthew Patrick 《Journal of Volcanology and Geothermal Research》2007

Plinian plumes erupt with a bulk density greater than that of air, and depend upon air entrainment during their gas-thrust phase to become buoyant; if entrainment is insufficient, the column collapses into a potentially deadly pyroclastic flow. This study shows that strombolian ash plumes can be erupted in an initially buoyant state due to their extremely high initial gas content, and in such cases are thus impervious to column collapse. The high gas content is a consequence of decoupled gas rise in the conduit, in which particles are ultimately incidental. The relations between conduit gas flow, eruption style and plume density are explored here for strombolian scenarios and contrasted with conventional wisdom derived from plinian eruptions. Considering the inherent relation between gas content and initial plume density together with detailed measurements of plume velocities can help unravel ambiguities surrounding conduit processes, eruption styles and hazards at poorly understood volcanoes. Analysis of plume dynamics at Santiaguito volcano, Guatemala adds further support for a model involving decoupled gas rise in the conduit.  相似文献