Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state, and a non-steady-state modification to account for zoned actinolite-hornblende

Ashworth, J. R.; Birdi, J. J.; Emmett, Trevor F.

doi:10.1007/bf00283321

Cited by 26 publications

(35 citation statements)

References 27 publications

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“…Carlson et al (1995) distinguish the diÂusional flux of an aluminous species as the ratelimiting factor in garnet crystallization for a sample from the Picuris Range, based on the success of normalized radius-rate relations for Mn, Fe and Mg (indicating rapid rock-wide equilibration) and evidence from several sources in the literature that indicate low Birdi, 1990;Johnson & Carlson, 1990;Carlson & Johnson, 1991;Ashworth et al, 1992). In these studies, Al and Si display limited mobility (although the however, considering the absence of quantitative determination of Si mobility is complicated by the determinations of rates of intergranular diÂusion of saturation of silica in some occurrences); Mg and Fe cations in fluid-saturated metamorphic environments are found to be far more mobile, with calculated (Fisher, 1977;Carlson et al, 1995) and the lack of field Onsager diÂusion coeÃcients indicating a relatively or textural data to indicate the exact garnet-forming rapid rate of diÂusion ( but see Foster, 1981, for an reactions responsible for porphyroblast crystallization opposing view).…”

Section: Systematic Changes In Mn Mg and Fe During Crystallizationmentioning

confidence: 99%

Disequilibrium for Ca during growth of pelitic garnet

Chernoff¹,

Carlson²

1997

Journal Metamorphic Geology

154

104

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A B S TR A CT Compositional zoning in hundreds of almandine-rich garnets in amphibolite by facies micaceous quartzites from the Picuris Range, north-central New Mexico USA, indicates that although Mn, Mg and Fe achieve chemical equilibrium at hand-sample scale during garnet growth, Ca does not. Instead, Ca concentrations at the surface of growing garnets appear to depend strongly on kinetic factors that govern the local chemical environment, yielding disequilibrium for Ca at scales larger than the region immediately surrounding an individual porphyroblast.Detailed zoning profiles were obtained for 371 garnet crystals in a small volume of a single sample of garnetiferous quartzite, and core analyses were made of 97 additional crystals. Each analysis was made on a section that passed precisely through the morphological centre of the crystal, located by means of 3-D imagery from computed X-ray tomography. The data reveal strong correlations between crystal size and concentrations of Mn, Mg and Fe (but not Ca) in garnet cores; a relationship between crystal size and isolation; rigorous cross-correlations among concentrations of Mn, Mg and Fe (but not Ca); and systematic variations in Ca concentrations as a function of crystal size and core composition that are anomalous in comparison to the behaviour of the other divalent cations.We interpret these observations as the result of thermally accelerated diÂusion-controlled garnet growth, in circumstances that promoted rapid intergranular diÂusion and thus rock-wide equilibration of Mn, Mg and Fe, but that prevented equilibration at similar scale for Ca because of its more sluggish intergranular diÂusion. The anomalous behaviour of Ca is made evident in these garnets by the presence of sharp spikes in Ca concentration, which are demonstrably not a consequence of any simultaneous rock-wide event, such as a change in pressure, temperature, or some other intensive parameter. Instead, Ca concentrations probably reflect the local extent of reaction in the immediate vicinity of each porphyroblast. To the degree that such kinetic factors introduce departures from chemical equilibrium for Ca, thermobarometric estimates that involve grossular contents of pelitic garnet will be in error.

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Section: Systematic Changes In Mn Mg and Fe During Crystallizationmentioning

confidence: 99%

Disequilibrium for Ca during growth of pelitic garnet

Chernoff¹,

Carlson²

1997

Journal Metamorphic Geology

154

104

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“…This inference has been validated in numerous cases by non-equilibrium thermodynamic models (Joesten, 1977;Ashworth & Birdi, 1990;Johnson & Carlson, 1990), which reproducewhen appropriate relative rates of intergranular diffusion for components participating in the reaction are specified -the observed assemblages, the modal amounts of product minerals and the sequential arrangements of layers in the coronal reaction zones (e.g. Joesten, 1977;Mongkoltip & Ashworth, 1983;Nishiyama, 1983;Swapp, 1986;Ashworth & Birdi, 1990;Johnson & Carlson, 1990;Carlson & Johnson, 1991;Ashworth et al, 1992Ashworth et al, , 1998Ashworth & Sheplev, 1997;Attoh, 1998;Markl et al, 1998;Abart et al, 2001;Fukuyama et al, 2006;Sartini-Rideout et al, 2007). A common, although not quite universal, finding of these models is that Al diffuses as slowly as, or more slowly than any other component, so that the overall rate of reaction is typically governed by the diffusivity of Al (e.g.…”

Section: Estimates Of Intergranular Diffusivities From Coronal Texturesmentioning

confidence: 95%

“…One of the first estimates of effective intergranular diffusion coefficients ( D eff ) for Al came from the analysis of Ashworth (1993) of coronal reaction zones formed at 600 ± 100 °C between plagioclase and olivine as described by Ashworth & Birdi (1990), and of coronal reaction zones formed at 550 ± 100 °C between plagioclase and clinopyroxene as described by Ashworth et al. (1992).…”

Section: Estimates Of Intergranular Diffusivities From Coronal Texturesmentioning

confidence: 99%

Dependence of reaction kinetics on H₂O activity as inferred from rates of intergranular diffusion of aluminium

Carlson¹

2010

Journal Metamorphic Geology

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Quantitative constraints on the accelerative effects of H 2 O on the kinetics of metamorphic reactions arise from a comparison of rates of intergranular diffusion of Al in natural systems that are fluidsaturated, hydrous but fluid-undersaturated, and nearly anhydrous. Widths of symplectitic reaction coronas around partially resorbed garnet crystals in the contact aureole of the Makhavinekh Lake Pluton, northern Labrador, combined with time-temperature histories from conductive thermal models, yield intergranular diffusivities for Al from $700-900°C under nearly anhydrous conditions. Those rates, when extrapolated down temperature, are approximately three orders of magnitude slower than rates derived from re-analysis of garnet resorption coronas formed under hydrous but fluidundersaturated conditions near 575°C in rocks of the Llano Uplift of central Texas, which are in turn approximately four orders of magnitude slower than rates at comparable temperatures derived from numerical simulations of prograde garnet growth in fluid-saturated conditions in rocks from the Picuris Range of north-central New Mexico. Thus, even at constant temperature, rates of intergranular diffusion of Al -and corresponding length scales and timescales of metamorphic reaction and equilibration -may vary by as much as seven orders of magnitude across the range of H 2 O activities found in nature.

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“…Johnson and Carlson 1990;Carlson and Johnson 1991;Ashworth et al 1992;Ashworth and Sheplev 1997;Markl et al 1998). The general trends in relative diffusivity probably reflect the relative solubilities determined by the chemistry of the most commonly encountered fluids, but a wide range of variation should be possible.…”

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confidence: 97%

Porphyroblast crystallization: linking processes, kinetics, and microstructures

Carlson

2010

International Geology Review

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Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state, and a non-steady-state modification to account for zoned actinolite-hornblende

Cited by 26 publications

References 27 publications

Disequilibrium for Ca during growth of pelitic garnet

Disequilibrium for Ca during growth of pelitic garnet

Dependence of reaction kinetics on H₂O activity as inferred from rates of intergranular diffusion of aluminium

Porphyroblast crystallization: linking processes, kinetics, and microstructures

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Diffusion in coronas around clinopyroxene: modelling with local equilibrium and steady state, and a non-steady-state modification to account for zoned actinolite-hornblende

Cited by 26 publications

References 27 publications

Disequilibrium for Ca during growth of pelitic garnet

Disequilibrium for Ca during growth of pelitic garnet

Dependence of reaction kinetics on H2O activity as inferred from rates of intergranular diffusion of aluminium

Porphyroblast crystallization: linking processes, kinetics, and microstructures

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Product

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Dependence of reaction kinetics on H₂O activity as inferred from rates of intergranular diffusion of aluminium