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1.
Transgressive Upper Cretaceous Chalk terminates (between SW Wales and SE Ireland) at approximately latitude 52°N as thinly
bedded marginal facies; while the Tertiary sequences, previously considered to extend uninterrupted into both the South Celtic
Sea area and the Nymphe Bank basin are preserved as isolated subcrops separated by Jurassic. The distinct subsidence history
of St. Georges Channel basin, as compared to the Nymphe Bank basin which both belong to the North Celtic Sea graben, is attributed
to inversion activity with the final phase occurring during the Paleogene. 相似文献
2.
An origin for laminated glacimarine sediments through sea-ice build-up and suppressed iceberg rafting 总被引：4，自引：0，他引：4
Laminated glacimarine sediments are observed in visual core logs and x-radiographs from Scoresby Sund and Nansen Fjord, east Greenland. They are mostly underlain and overlain by massive or stratified glacimarine diamicton (Dmm or Dms), which is a product of iceberg delivery of heterogeneous debris and, in Scoresby Sund, reworking by deep-drafted iceberg keels. The laminated sediments are AMS radiocarbon dated to two cold periods since the last, Late Weichselian deglaciation: the Younger Dryas stadial (Milne Land Stadial in east Greenland) and the Little Ice Age. During cold climatic events, multiyear shorefast sea ice ('sikussak') formed in these fjords and trapped the icebergs. Fine-grained, laminated muds (Fl) were deposited in Scoresby Sund when the flux of icebergs was suppressed, but turbid meltwater continued to provide some sediment flux to the fjord systems, varying through time to produce laminations. In Nansen Fjord, thinner and often massive mud layers (Fm) resulted from shorter intervals of sea-ice cover with no ice rafting. Stratified diamicton layers (Dms), which alternate with mud deposition to produce a laminated unit, probably represent intervening times of more open conditions with iceberg rafting. In Scoresby Sund, foraminifera are either absent from the laminated unit or begin to appear towards the end of its deposition. The absence of both benthic and planktonic foraminifera also suggests that multiyear sea ice was covering the core sites. There is no evidence of macrofaunal activity, and bioturbation is absent from the laminated sediments. Satellite data show that multiyear shorefast sea ice is present in several areas of the high Arctic today, and this traps icebergs calved from interior ice-cap drainage basins. Thus, the process of laminated glacimarine sediment formation is likely to be applicable to a number of areas of the modern and Quaternary Arctic. 相似文献
3.
Peter I. Nabelek Alan G. Whittington Mona-Liza C. Sirbescu 《Contributions to Mineralogy and Petrology》2010,160(3):313-325
Granite pegmatite sheets in the continental crust are characterized by very large crystals. There has been a shift in viewing
pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled
liquids. With this shift there has been a renewed debate about the role of H2O in the petrogenesis of pegmatites. Based on data on nucleation of minerals and new viscosity models for hydrous granite
melts, it is argued that H2O is the essential component in the petrogenesis of granite pegmatites. H2O is key to reducing the viscosity of granite melts, which enhances their transport within the crust. It also dramatically
reduces the glass transition temperature, which permits crystallization of melts at hundreds of degrees below the thermodynamic
solidus, which has been demonstrated by fluid inclusion studies and other geothermometers. Published experimental data show
that because H2O drastically reduces the nucleation rates of silicate minerals, the minerals may not be able to nucleate until melt is substantially
undercooled. In a rapidly cooling intrusion, nucleation starts at its highly undercooled margins, followed by inward crystal
growth towards its slower-cooling, hotter core. Delay in nucleation may be caused by competition for crystallization by several
minerals in the near-eutectic melts and by the very different structures of minerals and the highly hydrated melts. Once a
mineral nucleates, however, it may grow rapidly to a size that is determined by the distance between the site of nucleation
and the point in the magma at which the temperature is approximately that of the mineral’s liquidus, assuming components necessary
for mineral growth are available along the growth path. Granite pegmatites are apparently able to retain H2O during most of their crystallization histories within the confinement of their wall rocks. Pegmatitic texture is a consequence
of delayed nucleation and rapid growth at large undercooling, both of which are facilitated by high H2O (±Li, B, F and P) contents in granite pegmatite melts. Without retention of H2O the conditions for pegmatitic textural growth may be difficult to achieve. Loss of H2O due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in
rhyolites. In contrast, slow cooling within a large magma chamber promotes continuous exsolution of H2O from crystallizing magma, growth of equant crystals, and final solidification at the thermodynamic solidus. These are the
characteristics of normal granites that distinguish them from pegmatites. 相似文献
4.
5.
Maik Pertermann Alan G. Whittington Anne M. Hofmeister Frank J. Spera John Zayak 《Contributions to Mineralogy and Petrology》2008,155(6):689-702
Thermal diffusivity (D) was measured using laser-flash analysis from oriented single-crystal low-sanidine (K0.92Na0.08Al0.99Fe3+
0.005Si2.95O8), and three glasses near KAlSi3O8. Viscosity measurements of the three supercooled liquids, in the range 106.8 to 1012.3 Pa s, confirm near-Arrhenian behavior, varying subtly with composition. For crystal and glass, D decreases with T, approaching a constant near 1,000 K: D
sat ∼ 0.65 ± 0.3 mm2 s−1 for bulk crystal and ∼0.53 ± 0.03 mm2 s−1 for the glass. A rapid decrease near 1,400 K is consistent with crossing the glass transition. Melt behavior is approximated
by D = 0.475 ± 0.01 mm2 s−1. Thermal conductivity (k
lat) of glass, calculated using previous heat capacity (C
P) and new density data, increases with T because C
P strongly increases with T. For melt, k
lat reaches a plateau near 1.45 W m−1 K−1, and is always below k
lat of the crystal. Melting of potassium feldspars impedes heat transport, providing positive thermal feedback that may promote
further melting in continental crust. 相似文献
6.
Pascal Richet Alan Whittington François Holtz Harald Behrens Susanne Ohlhorst Max Wilke 《Contributions to Mineralogy and Petrology》2000,138(4):337-347
A review of published and newly measured densities for 40 hydrous silicate glasses indicates that the room-temperature partial
molar volume of water is 12.0 ± 0.5 cm3/mol. This value holds for simple or mineral compositions as well as for complex natural glasses, from rhyolite to tephrite
compositions, prepared up to 10–20 kbar pressures and containing up to 7 wt% H2O. This volume does not vary either with the molar volume of the water-free silicate phase, with its degree of polymerization
or with water speciation. Over a wide range of compositions, this constant value implies that the volume change for the reaction
between hydroxyl ions and molecular water is zero and that, at least in glasses, speciation does not depend on pressure. Consistent
with data from Ochs and Lange (1997, 1999), systematics in volume expansion for SiO2–M2O systems (M=H, Li, Na, K) suggests that the partial molar thermal expansion coefficient of H2O is about 4 × 10−5 K−1 in silicate glasses.
Received: 30 June 1999 / Accepted: 5 November 1999 相似文献
7.
Andesitic–dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315?K, at low shear stress (0.085 to 0.42?MPa), low strain rate (between 1.1?×?10?8 and 1.9?×?10?5?s?1), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate- and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226?K and strain rates lower than 1.8?×?10?4?s?1, is $ \log {\eta_{\text{app}}} = - 0.738 + 9.24 \times {10^3}{/}T(K) - 0.654 \cdot \log \dot{\varepsilon } $ where η app is apparent viscosity and $ \dot{\varepsilon } $ is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa?s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances. 相似文献
8.
William L. Romine Alan G. Whittington Peter I. Nabelek Anne M. Hofmeister 《Bulletin of Volcanology》2012,74(10):2273-2287
Thermal diffusivity (D) was measured using laser-flash analysis on pristine and remelted obsidian samples from Mono Craters, California. These high-silica rhyolites contain between 0.013 and 1.10?wt% H2O and 0 to 2?vol% crystallites. At room temperature, D glass varies from 0.63 to 0.68?mm2?s?1, with more crystalline samples having higher D. As T increases, D glass decreases, approaching a constant value of ??0.55?mm2?s?1 near 700?K. The glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity. Dissolved water contents up to 1.1?wt% have no statistically significant effect on the thermal diffusivity of the glass. Upon crossing the glass transition, D decreases rapidly near ??1,000?K for the hydrous melts and ??1,200?K for anhydrous melts. Rhyolitic melts have a D melt of ??0.51?mm2?s?1. Thermal conductivity (k?=?D·??·C P) of rhyolitic glass and melt increases slightly with T because heat capacity (C P) increases with T more strongly than density (??) and D decrease. The thermal conductivity of rhyolitic melts is ??1.5?W?m?1?K?1, and should vary little over the likely range of magmatic temperatures and water contents. These values of D and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures, suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor. Numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian, and those that crystallize, results from the release of latent heat of crystallization. Latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies. The time to solidification is similar in both cases, however, because solidification requires cooling through the glass transition in the first case, and cooling only to the solidus in the second. 相似文献
9.
Adrian M. Hall John E. Gordon Graeme Whittington Geoffrey A. T. Duller Hendrik Heijnis 《第四纪科学杂志》2002,17(1):51-67
New stratigraphical, palynological and dating evidence is presented for pre‐Late Devensian/Weichselian sediments at Fugla Ness and Sel Ayre, Shetland. The Fugla Ness Peat rests on till and formed during an interglacial that saw the development of maritime heaths, with scattered trees and shrubs, including Pinus and possibly Ilex. A decline into stadial conditions is marked by overlying periglacial breccia and till. The Sel Ayre Organic Sands and Gravels lie between periglacial breccias and beneath till and appear to record a changing interstadial environment in which trees were absent and the vegetation comprised largely heaths, with Bruckenthalia, and grasslands. The Fugla Ness Peat is dated to 110+40/?35 ka by uranium series disequilibrium, suggesting that it formed during the Ipswichian/Eemian Interglacial (Marine Isotope Substage 5e). Luminescence ages of ca. 98–105 ka on intercalated sands within the Sel Ayre Organic Sands and Gravels place these deposits in Marine Isotope Substage 5c (Brørup Interstadial). The two sites provide the first detailed record of Marine Isotope Stage 5 environments on Shetland. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
10.