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461.
Cliff S. J. Shaw 《Contributions to Mineralogy and Petrology》1999,135(2-3):114-132
A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the
upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in
Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions
are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments
designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO2-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar
in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments
using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the
saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence
with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures
of 1 GPa and below under anhydrous and CO2-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and
CO2-bearing experiments is only slightly enriched in SiO2 and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass
of similar composition whereas the glass from CO2-bearing experiments is less SiO2 rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene,
combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene
saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K
also causes minor SiO2 enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable
phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal
and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively
dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting
the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there
are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility
of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline
glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within
the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor.
Received: 24 August 1998 / Accepted: 2 December 1998 相似文献
462.
The value of applying a regional level, Ecosystem Based Fisheries Management (EBFM) framework, which is a step-wise, hierarchical, risk-based approach, was tested on the West Coast Bioregion of Western Australia. With structured stakeholder input, over 600 ecological, social, economic and governance issues were initially identified for the region. This complexity was reduced to a level useful for management by consolidating the individual risks into 60 regional-level risks, with a multi-criteria analysis used to integrate the ecological, social and economic risks into just 24 Departmental-level priorities, which ranged from very low to urgent. Given this success, EBFM-based priorities now form the basis for the Department’s budget planning process, plus the framework is providing a critical link between fishery level issues and the broader processes undertaken by other marine based agencies. The EBFM framework is a significant step forward for the integrated management of natural resources by enabling all assets and issues relevant to stakeholders and government to be holistically considered at a regional level. 相似文献
463.
464.
The Aegean island of Thera (Santorini) was covered by tephra from its cataclysmic Late Bronze Age (ca. 3600 yr B.P.) eruption. Vertical exposures of the eruptive sequence show secondary, nonvolcanic, circular (in cross section) features composed of stratified sediment. Many are inaccessible from the floors of modern quarries and appear to be caves filled with younger sediment, but show no connection to the land surface. A filled cave was found in the wall of a modern gully outside the modern quarries, and a filled cave was found in a terrace scarp, well above the modern gully. Natural (and probably rapid) incision by gullies into the thick tephra deposit left many locations with lateral access to tephra. Inhabitants from post‐Minoan to recent times excavated tephra for materials and buildings, and caves were subsequently filled by sporadic (possibly seasonal) flood events that deposited sediment. These gullies may have provided access for modern tephra removal that isolated the filled caves high on the modern quarry walls. © 2003 Wiley Periodicals, Inc. 相似文献