The pressure and temperature stability limits of lawsonite: implications for H2O recycling in subduction zones     

Alison R. Pawley 《Contributions to Mineralogy and Petrology》1994,118(1):99-108

The stability relations of lawsonite, CaAl₂Si₂O₇(OH)₂H₂O, have been investigated at pressures of 6 to 14 GPa and temperatures of 740 to 1150°C in a multi-anvil apparatus. Experiments used the bulk composition lawsonite+H₂O to determine the maximum stability of lawsonite. Lawsonite is stable on its own bulk composition to a pressure of 13.5 GPa at 800°C, and between 6.5 and 12 GPa at 1000°C. Its composition does not change with pressure or temperature. All lawsonite reactions have grossular, vapour and two other phases in the system Al₂O₃-SiO₂-H₂O (ASH) on their high-temperature side. A Schreinemakers analysis of the ASH phases was used to relate the reactions to each other. At the lowest pressures studied lawsonite breaks down to grossular+kyanite+coesite+vapour in a reaction passing through 980°C at 6 GPa and 1070°C at 9 GPa. Above 9 GPa the reactions coesite=stishovite and kyanite+vapour=topaz-OH are crossed. The maximum thermal stability of lawsonite is at 1080°C, at 9.4 GPa. At higher pressures the lawsonite breakdown reactions have negative slopes. The reaction lawsonite=grossular+topaz-OH+stishovite+vapour passes through 1070°C at 10 GPa and 1010°C at 12 GPa. At 14 GPa, 740–840°C, lawsonite is unstable relative to the assemblage grossular+diaspore+vapour+a hydrous phase with an Al:Si ratio of 1:1. Oxide totals in electron microprobe analyses suggest that the composition of this phase is AlSiO₃(OH). Two experiments on the bulk composition lawsonite+pyrope [Mg₃Al₂Si₃O₁₂] show that at 10 GPa the reaction lawsonite=Gr-Py_ss+topaz-OH+stishovite+vapour is displaced down temperature from the end-member reaction by 200°C for a garnet composition of Gr₂₀Py₈₀. Calculations suggest similar temperature displacements for reaction between lawsonite and Gr-Py-Alm garnets of compositions likely to occur in high-pressure eclogites. Temperatures in subduction zones remain relatively low to considerable depth, and therefore slab P-T paths can be within the stability field of lawsonite from the conditions of its crystallisation in blueschists and eclogites, up to pressures of at least 10 GPa. Lawsonite contains 11.5 wt% H₂O, which when released may trigger partial melting of the slab or mantle, or be incorporated in hydrous phases such as the aluminosilicates synthesised here. These phases may then transport H₂O to an even greater depth in the mantle.  相似文献   

Factors that influence water exchange in spring-fed tidal canals     

Alison M. Fox  William T. Haller  Kurt D. Getsinger 《Estuaries and Coasts》1991,14(4):404-413

Environmental factors that influenced the rates of water exchange in a series of tidal, dead-end canals in Crystal River, Florida, were investigated by measuring the rates of dilution of the fluorescent dye Rhodamine WT. Water temperature regime was the most influential of these factors, with type of tidal cycle (spring or neap) and density of submersed vegetation (dominated byHydrilla verticillata) only affecting dye dilution rates in winter treatments. Dye half-lives were shortest in the summer (12–24 h), longest in the fall and spring (60–120 h), and most sensitive to water temperature patterns in the winter (26–60 h). These data provide useful information relating to the timing of hydrilla management using aquatic herbicides in spring-fed tidal systems. A mechanism is proposed that relates the canal water temperatures to tidal water movement and outputs from the adjacent Three Sisters Springs.  相似文献   

A new approach to detecting vegetation and land-use change using high-resolution lipid biomarker records in stalagmites   总被引：2，自引：0，他引：2  

Alison J. Blyth  Asfawossen Asrat  Andy Baker  Pauline Gulliver  Melanie J. Leng  Dominique Genty 《Quaternary Research》2007,68(3):314-324

A hundred-year stalagmite lipid biomarker record from Mechara, southeastern Ethiopia, is presented. The record has been recovered at a 10-yr temporal resolution, marking the first time this has been achieved in stalagmite biomarker work and providing the first opportunity to investigate the relationship between stalagmite lipid records and hydrological transport lags, a vital issue in interpreting palaeoenvironmental signals. Preserved plant-derived n-alkanes and n-alkanols show clear changes in composition over time, relating to known land-use changes in the area, particularly the expansion of agriculture in the early twentieth century. The level of environmental detail provided by this technique, combined with the long-term chronological framework offered by stalagmites, holds significant promise for the investigation of early human environments and their associated climatic and anthropogenic controls.  相似文献   

Preparing for Climatic Change: The Water, Salmon, and Forests of the Pacific Northwest     

Philip W. Mote  Edward A. Parson  Alan F. Hamlet  William S. Keeton  Dennis Lettenmaier  Nathan Mantua  Edward L. Miles  David W. Peterson  David L. Peterson  Richard Slaughter  Amy K. Snover 《Climatic change》2003,61(1-2):45-88

The impacts of year-to-year and decade-to-decade climatic variations on some of the Pacific Northwest's key natural resources can be quantified to estimate sensitivity to regional climatic changes expected as part of anthropogenic global climatic change. Warmer, drier years, often associated with El Niño events and/or the warm phase of the Pacific Decadal Oscillation, tend to be associated with below-average snowpack, streamflow, and flood risk, below-average salmon survival, below-average forest growth, and above-average risk of forest fire. During the 20th century, the region experienced a warming of 0.8 °C. Using output from eight climate models, we project a further warming of 0.5–2.5 °C (central estimate 1.5 °C) by the 2020s, 1.5–3.2°C (2.3 °C) by the 2040s, and an increase in precipitation except in summer. The foremost impact of a warming climate will be the reduction of regional snowpack, which presently supplies water for ecosystems and human uses during the dry summers. Our understanding of past climate also illustrates the responses of human management systems to climatic stresses, and suggests that a warming of the rate projected would pose significant challenges to the management of natural resources. Resource managers and planners currently have few plans for adapting to or mitigating the ecological and economic effects of climatic change.  相似文献   

Rugate filters for OH-suppressed imaging at near-infrared wavelengths     

Alison R. Offer  & Joss Bland-Hawthorn 《Monthly notices of the Royal Astronomical Society》1998,299(1):176-188

Ground-based observations at near-infrared wavelengths are severely affected by atmospheric OH bands. Many authors have recognized the potential gains in sensitivity from suppressing these features. Dispersive instruments show some promise but are both expensive and complicated to build. OH suppression filters using single or periodic notches have the advantage of simplicity but significant gains have not yet been realized.   Rugate filters (with graded index inhomogeneous coatings) offer key advantages for astronomical imaging. It is possible to produce a transmission profile comprising a series of irregular and sharply defined bandpasses. We demonstrate through numerical simulation of rugate filters that it should be possible to achieve 95 per cent suppression of the OH features in the J photometric band, while retaining roughly half of the spectral coverage. This would lead to extraordinary gains in sensitivity even for observations of continuum sources. In addition, these filters allow longer exposures before the detector saturates on the sky background. I - and z -band filters can also be envisaged.   In 1-arcsec seeing, a J -band rugate filter used in conjunction with a 4-m telescope would detect a J  = 23 continuum source at 5.0σ in a single 10-min exposure. In comparison, a conventional J filter requires multiple exposures for a 10-min integration time and achieves only a 2.5σ detection. For emission-line sources, the rugate filter has an even bigger advantage over conventional filters, with a fourfold increase in signal-to-noise ratio possible in certain instances.   Astrophysical studies that could benefit from rugate filters are searches for very low-mass stars and galaxy evolution out to z  = 3.  相似文献   

The high-pressure stability of Mg-sursassite in a model hydrous peridotite: a possible mechanism for the deep subduction of significant volumes of H<Subscript>2</Subscript>O     

Geoffrey D. Bromiley  Alison R. Pawley 《Contributions to Mineralogy and Petrology》2002,142(6):714-723

The stability of the high-pressure phase Mg-sursassite, previously MgMgAl-pumpellyite, in ultramafic compositions has been determined in experiments in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). The breakdown of Mg-sursassite + forsterite + enstatite to pyrope + vapour with increasing temperature was bracketed at 6.0 and 7.0 GPa. Below 6.0 GPa, Mg-sursassite + forsterite + vapour reacts to chlorite + enstatite. This reaction provides a mechanism for transfer of water from chlorite- to Mg-sursassite-bearing assemblages. At pressures of 7.0 GPa and above, the assemblage Mg-sursassite + phase A + enstatite was found. Phase relations involving Mg-sursassite and phase A are considered. For bulk compositions with a low water content, the vapour-absent reaction Mg-sursassite + forsterite = pyrope + phase A + enstatite determines the upper-pressure stability of Mg-sursassite, and provides a mechanism for the complete transfer of water from Mg-sursassite to phase A-bearing assemblages. Mg-sursassite plays an important role in peridotite compositions in the subducting slab because, at temperatures below 700 °C, it can transfer water from hydrous phases such as antigorite and chlorite to high-pressure stable phases such as phase A.  相似文献   

A laboratory analogue model study of ocean-wave induced magnetic fields for cases of non-uniform depths and sea-land interfaces     

T. Miles  H.W. Dosso 《Physics of the Earth and Planetary Interiors》1979,19(1):12-20

The laboratory analogue model described by Miles et al. (1977) for the case of a uniform depth ocean is extended to study ocean-wave induced magnetic fields for cases of non-uniform ocean depths and sea-land interfaces. The models studied are: (i) the step and shelf model, (ii) the wedge and shelf model, (iii) the dyke model, (iv) the sea mount model, (v) the sloping bay and shelf model, and (vi) the reef and shelf model. For shallow depths, the behaviour of the induced magnetic field is strongly dependent on the fluid depth over the structure. The shape of the leading edge of the submerged structure, and the contour of the coastline also significantly affect the induced horizontal magnetic field, and possible mechanisms for these effects include wave interference and current channelling. On the basis of the model results, it is apparent that an irregular ocean bottom, or a large submerged structure, should affect the induced magnetic field at the surface only for rather shallow ocean depths of less than 40 m for a fluid wavelength of 360 m.  相似文献   

Application of an oil spill vulnerability index to the shoreline of lower Cook Inlet,Alaska     

Jacqueline Michel  Miles O. Hayes  P. Jeffrey Brown 《Environmental Geology》1978,2(2):107-117

Offshore tracts in Alaska's lower Cook Inlet are scheduled to be opened for exploratory petroleum drilling in the near future. Because of the potential for oil spills resulting from this activity, a field study of the coastal zone was conducted in June 1976. A total of 1216 km of shoreline was classified into 3 types: erosional (45 percent), neutral (38 percent), and depositional (17 percent). These were further subdivided into 16 subclasses on the basis of small scale morphological features. This classification was used in conjunction with a vulnerability index of potential oil spill damage, developed through study of three major oil spills, to predict the longevity of oil in the different coastal environments of the Inlet. On a scale from 1 to 10, 45 percent of the shoreline has low values of 1 to 4, which means that oil would be dispersed by natural processes within less than six months after a spill on these coasts. Values from 4 to 6 were assigned to 13.4 percent of the shoreline, where oil residence time may be up to one year. A 6 to 10 rating was assigned to 41.5 percent of the shoreline, where oil contamination may remain for periods of two to ten years, or possibly longer should no major clean-up procedures be initiated. We propose that the use of this type of vulnerability indexing, in conjunction with a biological susceptibility index and oil spill trajectory models, would provide a rational basis for decision making concerning the location of on- and off-shore oil facilities and the design of oil spill contingency plans.  相似文献   

A simulation of inertial oscillation in drifting pack ice     

Miles G. McPhee 《Dynamics of Atmospheres and Oceans》1978,2(2):107-122

A simple model for simulating the motion of pack ice during periods of energetic inertial oscillation is developed by writing an integrated momentum equation for the ice and upper ocean driven by surface wind stress. Damping is provided by a term proportional to the component of mass transport parallel to wind stress, which is a measure of the departure from a steady-state balance of wind stress and Coriolis force. Oceanic boundary-layer transport is taken to be proportional to the square of the surface velocity on the basis of extensive current measurements made under drifting ice, providing a relationship between ice velocity and total transport. Ice drift velocities measured at AIDJEX stations during the summer of 1975 are simulated with some success using measured local winds. A simple superposition of waves generated at the corners of the triangular array of stations is considered and it is shown that at times the waves are coherent across the 150 km array and at other times there is considerable interference at lesser scales. The importance of the motions for the production of new ice is briefly discussed.  相似文献   

Earliest sea-floor spreading magnetic anomalies in the north Arabian Sea and the ocean-continent transition   总被引：2，自引：0，他引：2  

Peter R. Miles  Walter R. Roest 《Geophysical Journal International》1993,115(3):1025-1031

Magnetic and gravity data collected during a GLORIA survey of the Indus Fan provide new information on the earliest sea-floor spreading history of the Arabian Sea. A negative gravity anomaly correlates with the buried Laxmi Ridge. This ridge is interpreted here to be a sliver of continental crust adjacent to the oceancontinent transition which bounds thinned, probably intruded, transitional crust to the NE. The oldest sea-floor spreading anomaly is anomaly 28 (65-66 Ma), breakup occurring at the time of the Deccan Traps volcanic event. The earliest oceanic crust formed from two phases of rift propagation which accommodates the angular disparity between the E-W trending anomalies in the western Arabian Sea and the NE-SW trending western part of the Laxmi Ridge. Flow-line projection shows that the Laxmi ridge forms the conjugate structure to the northern Mascarene Plateau margin.  相似文献