Gabbro‐eclogite reaction rate and its geophysical significance

Ahrens, Thomas J.; Schubert, G.

doi:10.1029/rg013i002p00383

Cited by 196 publications

(81 citation statements)

References 64 publications

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“…In order to extrapolate experimental kinetic data to geological conditions, the effects of variables such as temperature, pressure, grain size, and the catalytic role of water must be understood. The effectiveness of water as a catalyst is qualitatively well known, and is so pronounced that the availability of water, rather than temperature, has been suggested, for example, as the most important factor controlling the rate of the gabbro to eclogite transformation during subduction (Ahrens and Schubert, 1975). However, apart from preliminary investigations of the quartz + periclase ~ forsterite reaction (Fyfe et al, 1958, pp.…”

Section: Water As a Catalyst In Mineral Reactionsmentioning

confidence: 99%

“…Although it is often assumed that this value is applicable to diffusion of ions through thin aqueous films in intergranular regions (e.g. Ahrens and Schubert, 1975;Walther and Wood, 1984), Rutter (1976) has suggested that the diffusivity of ions in thin films is reduced by about five orders of magnitude relative to the diffusivity in a bulk fluid. The basis of this estimate is first the inverse relation between diffusivity and fluid viscosity, the StokesEinstein equation…”

Section: Diffi4sion In Fluid Filmsmentioning

confidence: 99%

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The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism

1986

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ABSTRACT, The problems of characterizing intergranular regions and of estimating rates of intergranular diffusion in metamorphic rocks are discussed. Intergranular regions can be anhydrous, hydrated but undersaturated with H20, or saturated with H20, but only in the latter case can a free aqueous fluid phase be present. Estimates of intergranular diffusion coefficients (D mR) at 550 ~ derived from a variety of published experimental work, vary from ~ 10 -8 m 2 s -1 for diffusion of species through an intergranular fluid film to ~< 4 • 10-24 m 2 s 1 for diffusion of SiO2 or O in anhydrous grain boundaries in quartzite. Estimates of D mR for hydrated grain boundaries vary from ~ 10 -13 m 2 s -1 to ~ 10 -zl m 2 s 1; the concentration of H20 in the grain boundaries and the identity of the diffusing species (generally unknown) may be important controlling factors, and there exists the possibility of a spectrum of values between these two extremes.Using available kinetic data it is shown that a free aqueous fluid could never have been present in parts of the basement terrane of the Sesia Zone (Western Alps) during uplift from the eclogite facies, except possibly late in the cooling history. The breakdown of sodic pyroxene+ quartz occurred in response to the localized infiltration of catalytic aqueous fluid, possibly over a time interval as short as 6-6000 a, and possibly under conditions remote from equilibrium. H20-present conditions during a dehydration reaction in metapelites of the Adula nappe (central Alps) could also have been of short duration. These examples are consistent with a model in which basement rocks at deep crustal levels are dry for long periods of time and in which the development of equilibrium mineral assemblages and microstructures generally occurs over relatively short periods of time under transitory fluid-present conditions (caused by devolatilization and/or infiltration).

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Section: Water As a Catalyst In Mineral Reactionsmentioning

confidence: 99%

Section: Diffi4sion In Fluid Filmsmentioning

confidence: 99%

The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism

1986

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“…For the lowermost section of thick igneous crust, there is a possibility of garnet growth [Ringwood and Green, 1964;Ringwood and Green, 1966]. For anhydrous igneous rocks with a typical cooling history, however, this garnet growth is unlikely because of kinetic barriers to reaction [Ahrens and Schubert, 1975]. Even if garnet forms, the velocity and density of resultant garnet granulites and eclogitic rocks both increase following Birch's law [e.g., Sobolev and Babeyko, 1994].…”

Section: The Paradox Of the Margin Gravity Highmentioning

confidence: 99%

Magmatism and dynamics of continental breakup in the presence of a mantle plume

Korenaga¹

2000

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This thesis studies the dynamics of mantle melting during continental breakups by geophysical, geochemical, and numerical analyses. The first part focuses on the mantle melting and crustal accretion processes during the formation of the Southeast Greenland margin, on the basis of deep-crustal seismic data. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a compressional velocity structure, and a long-wavelength structure with strong lateral heterogeneity is successfully recovered, including 30-to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone. A nonlinear Monte Carlo analysis is also conducted to establish the absolute uncertainty of model parameters. The derived crustal structure is first used to resolve the origin of a margin gravity high, with new inversion schemes using both seismic and gravity constraints. Density anomalies producing the gravity high seem to be confined within the upper crust, not in the lower crust as suggested for other volcanic margins. A new robust framework is then developed for the petrological interpretation of the velocity structure of igneous crust, and the thick igneous crust formed at the continentocean transition zone is suggested to have resulted from vigorous active upwelling of mantle with only somewhat elevated potential temperature. In the second part, the nature of mantle melting during the formation of the North Atlantic igneous province is studied on the basis of the major element chemistry of erupted lavas. A new fractionation correction scheme based on the Ni concentrations of mantle olivine is used to estimate primary melt compositions, which suggest that this province is characterized by a large degree of major element source heterogeneity. In the third part, the nature of preexisting sublithospheric convection is investigated by a series of finite element analyses, because the strength of such convection is important to define the "normal" state of mantle, the understanding of which is essential to identify any anomalous behavior of mantle such as a mantle plume. The results suggest that small-scale convection is likely in normal asthenosphere, and that the upwelling velocity in such convection is on the order of 1-10 cm/yr. 271 Introductory NotesThis thesis is composed of three parts, all of which are related to the dynamics of mantle melting during continental breakups. In the first part, rifting magmatism associated with the opening of the North Atlantic (at -60 Ma) is investigated on the basis of deep-crustal seismic data collected across the Southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure (Chapter 1). Several key issues in travel time tomography such as velocity-depth ambiguity and absolute parameter uncertainty are identified, and a practical strategy is presented to resolve them. The derived crustal velocity structure is the...

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“…The release of H 2 O from the serpentine in the mantle would cause H 2 O saturation in the crust just above the dehydration zone, and might have resulted in the retardation of the dehydration reactions in the crust (see Ahrens and Schubert, 1975), reducing seismicity in the subducting crust. To discuss this effect more qualitatively, we have to know better the processes of aqueous fluid migration in the crust and mantle associated with dehydration-hydration reactions, which is beyond the scope of the present study.…”

Section: Slab Seismicity Beneath Kantomentioning

confidence: 99%