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

Abstract

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 d HGB D MgO HGB ¼ d HGB D 0 e ÀE=RT with E = 185 ± 20 kJ/mol and d HGB D 0 = 10 -14.7±0.9 m 3 /s, where d 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.

Experimental determination of melt interconnectivity and electrical conductivity in the upper mantle

“…However, according to Chantel et al (2016), the very low amounts of melt that could be expected in the pure olivine aggregate (< 0.1%) would not wet the grain boundaries, as reflected in the very high EC compared to samples with a low added basalt fraction (Table 2 and Fig. SI 7) even though intergranular mass transport is strongly influenced by minor amounts of hydroxyl as proven by the work of Gardés et al (2012). It is shown that activation energy of diffusion in hydrous-saturated grain boundaries is reduced compared to dry grain boundaries.…”

Textural and compositional complexities resulting from coupled dissolution–reprecipitation reactions in geomaterials

“…Fluid-saturated grain boundaries provide such a means for extensive mass transport by providing diffusional shortcuts relative to dry systems (Gardés et al 2012;Rubie 1986). A fluid capable of forming an interconnected network along grain boundaries is thus important for regional-scale mineral replacements (Harlov et al 2005).…”

New Occurrence of Pyroxenites in the Veria-Naousa Ophiolite (North Greece): Implications on Their Origin and Petrogenetic Evolution

“…Natural and experimental data reveal that Si-rich melts [56,57], or aqueous [58] or supercritical fluids [59] are released from the underlying subducted oceanic slab into the sub-arc region driving the dissolution of mantle olivine and the neoformation of pyroxenes and Al-phases in a range of pressures between 0.8 and 3.4 GPa [60][61][62][63][64][65][66][67][68]. Experimental results of Grant et al [69] at 800 MPa suggest that neoblasts of orthopyroxene and clinopyroxene form at a relatively narrow temperature range of around 800 to 900 • C (with few exceptions up to 1100 • C).…”