The effect of water on intergranular mass transport: new insights from diffusion-controlled reaction rims in the MgO–SiO2 system

We experimentally investigate the effect of water on intergranular mass transport. The growth of enstatite single rims between forsterite and quartz and forsterite-enstatite double rims between periclase and quartz was performed at 1,000°C and 1.5 GPa for incremental water–solid fractions ranging fr...

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Bibliographic Details
Published in:Contributions to mineralogy and petrology 2012-07, Vol.164 (1), p.1-16
Main Authors: Gardés, E., Wunder, B., Marquardt, K., Heinrich, W.
Format: Article
Language:English
Subjects:
Earth and Environmental Science
Earth Sciences
Geology
Geophysics
Kinetics
Metamorphic rocks
Mineral Resources
Mineralogy
Original Paper
Petrology
Water
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Summary:We experimentally investigate the effect of water on intergranular mass transport. The growth of enstatite single rims between forsterite and quartz and forsterite-enstatite double rims between periclase and quartz was performed at 1,000°C and 1.5 GPa for incremental water–solid fractions ranging from 0 to 5 wt%. A kinetics analysis based on time series ranging from 8 min to 18 h at 1 wt% H 2 O demonstrates that the growth of both the single and double rims is controlled by the transport of the chemical components in the intergranular medium. Inert platinum markers reveal that MgO is always the most mobile component, and the only one, except at the highest water fractions (>~2 wt%) where some mobility of SiO 2 is observed. Modelling yields that, in all of the rims from both the single and double rims, there is an increase by about six orders of magnitude between diffusivities in dry grain boundaries and in intergranular media with 5 wt% H 2 O of the bulk, therefore confirming that water/rock ratio is a parameter as important as temperature regarding reaction kinetics. The transition from dry to water-saturated conditions appears virtually instantaneous, occurring between 500 and 1,000 wt. ppm water–solid fraction. It corresponds to a jump of four to five orders of magnitude in diffusivities, in line with the gap between ‘dry’ and ‘wet’ enstatite single rim growth rates from various previous experimental investigations. At higher water–solid fractions, distinct increase in the diffusivities demonstrates the existence of two different intergranular transport mechanisms competing at water-saturated conditions. The first mechanism is diffusion in hydrous-saturated grain boundaries (HGBs), which dominates as long as pores are isolated. We estimate the Arrhenius law for MgO diffusion in enstatite HGBs to be with E  = 185 ± 20 kJ/mol and δ HGB D 0  = 10 −14.7±0.9  m 3 /s, where δ HGB is the effective width of the HGBs. The second mechanism is diffusion through fluid-filled channels, which dominates when pores are interconnected. A new description of the effect of water on intergranular mass transport is proposed.
ISSN:0010-7999
1432-0967
DOI:10.1007/s00410-012-0721-0