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101.
Cliff S. J. Shaw 《Contributions to Mineralogy and Petrology》2000,139(5):509-525
Mineral dissolution is an important factor in many magmatic processes such as melting, assimilation and magma mixing. Since
it is not possible to determine dissolution rates or mechanisms from natural samples, experimental measurements are very useful.
However, the geometry of the crystal–melt system can have a large effect on the measured rate, depending on whether the contaminated
melt formed during dissolution is gravitationally stable or unstable. This study examines the effects of the crystal–melt
geometry on the dissolution rate and mechanism. The experiments were performed using basanite melt and cylinders and spheres
prepared from a single crystal of natural quartz. All of the experiments were performed in the piston cylinder apparatus at
0.5 GPa and 1350 °C. Four crystal–melt geometries were used: (1) quartz cylinders on top of a column of melt; (2) quartz cylinders
beneath a column of basanite melt; (3) quartz cylinders in the middle of column of melt; (4) quartz spheres on top of a column
of basanite melt. These geometries allow an examination of non-convective, convective and mixed non-convective/convective
dissolution. Sphere experiments were included, as this has been the most commonly used geometry in previous experimental studies.
In all of the experiments quartz dissolves directly into the basanite without formation of cristobalite or tridymite. Quartz
on top of a column of melt dissolves at a rate almost proportional to the square root of time and forms a silica-rich compositional
boundary layer that is gravitationally stable. All of the samples show well-defined compositional gradients in the boundary
layer; however, the melt at the interface varies in composition with time and plots of concentration as a function of distance
normalized to time show that the diffusion rate of SiO2 increases with time. These data suggest that the rate-controlling step during quartz dissolution is interface reaction rather
than cation diffusion. Quartz on the bottom of a column of basanite dissolves much more quickly than in the quartz-on-top
experiments and the dissolution rate is linear, due to the periodic gravitational instability and resultant convection of
the boundary layer. Even though interface kinetics are the rate-controlling step in quartz dissolution, convection causes
an increase in dissolution rate because it replenishes the boundary layer with new, silica-undersaturated melt, which dissolves
the quartz more quickly than the contaminated melt. These data suggest that the interface reaction rate is controlled by the
degree of undersaturation of the solvent melt in the dissolving component. Both quartz-in-middle and quartz sphere experiments
dissolve at a rate intermediate between the two extremes and both show a power law rate. Both dissolve by a combination of
convective and non-convective dissolution but the sphere experiments are affected by an additional factor. During the experiment
the sphere can sink through the capsule causing forced convection which adds another complication to the interpretation of
the dissolution rate data. The results of this study indicate that the choice of experiment geometry plays a major role in
determining the observed dissolution rate. Mineral spheres, which have been widely used in the past, are not ideal for dissolution
studies. Instead, dissolution rates and mechanisms are best determined in the absence of convection. These experiments have
an additional advantage in that for diffusion-controlled dissolution, they allow determination of cation diffusivity.
Received: 2 March 2000 / Accepted: 11 April 2000 相似文献
102.
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
103.
The geometry and timing of orogenic extension: an example from the Western Italian Alps 总被引:3,自引:0,他引:3
Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low-P upon high-P contacts are commonly interpreted as extensional (i.e. material points on either side of the contact moved apart relative to the palaeo-horizontal), but dating of deformation and metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophiolite consists of eclogites (T ≈550–600 °C and P≈18–20 kbar) structurally beneath greenschist facies rocks (T ≈400 °C and P≈9 kbar). Mapping shows that the latter form a kilometre-wide shear zone (the Gressoney Shear Zone, GSZ) dominated by top-SE movement related to crustal extension. Rb–Sr data from micas within different GSZ fabrics, which dynamically recrystallized below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c. 45–36 Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. The discrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movements were therefore not a major component of eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have been dominated by erosion. Key words: deformation age, eclogite, exhumation, Rb–Sr dating, tectonic. 相似文献