Abstract
Hydrothermal activity in the crust results in the precipitation of large volumes of silica and often involves the formation of ore deposits, the shaping of geothermal systems, and recurring earthquakes. Pore fluid pressures fluctuate between lithostatic and hydrostatic, depending on seismic activity, and some models suggest the possibility of flash vaporization, given that fluid pressures can drop to the level of vapour at fault jogs during seismic slip. The phase changes of water could create extremely high supersaturations of silica, but the mechanisms of quartz vein formation under such extreme conditions remain unclear. Here we describe flash experiments conducted with silica-saturated solutions under conditions ranging from subcritical to supercritical. We found that amorphous silica is produced instantaneously as spherical nano- to micron-scale particles via nucleation and aggregation during the evaporation of water droplets. The nanoparticles are transformed to microcrystalline quartz very rapidly by dissolution and precipitation in hydrothermal solutions, with this process requiring less than one day under supercritical conditions because of the huge surface areas involved. We suggest that such short-lived silica nanoparticles have significant impacts on the dynamic changes in mechanical behaviour and hydrology of hydrothermal systems in volcanic areas.
Highlights
Fluid flow along fractures and faults plays an essential role in the transport of energy and the redistribution of elements within the upper crust, and the chemical interactions between hydrothermal fluids and rocks influence the mechanical and hydrological properties of the crust, as is evident in the formation of hydrothermal ore deposits[1,2], the shaping and maintenance of geothermal systems[3,4,5], and recurring earthquakes[6]
The aim of this study was to understand the mechanism of silica precipitation induced by flash vaporization, and to evaluate whether the formation of amorphous silica is a key step in the formation of quartz veins in the crust
The vaporization of pore water requires drastic changes in the crustal environment, and such changes during seismic slip have been proposed to occur in two ways: by instantaneous decompression at a fault jog[13], and by frictional heating on a fault surface[32]
Summary
Fluid flow along fractures and faults plays an essential role in the transport of energy and the redistribution of elements within the upper crust, and the chemical interactions between hydrothermal fluids and rocks influence the mechanical and hydrological properties of the crust, as is evident in the formation of hydrothermal ore deposits[1,2], the shaping and maintenance of geothermal systems[3,4,5], and recurring earthquakes[6]. Weatherley and Henley[13] proposed a model in which gold–quartz veins form by the flash vaporization of fluids at the very instant of earthquake rupturing In their model, the fluids could be decompressed to the level of vapour at a fault jog. The fluids could be decompressed to the level of vapour at a fault jog They speculated that gold and silica particles could be co-precipitated by the flash vaporization. Flow-through experiments[18,19,20] have revealed that various types of silica precipitation occur via a range of mechanisms under supercritical conditions including epitaxial quartz overgrowth, formation of metastable silica minerals, and 3-dimensional homogeneous (or heterogeneous) nucleation of quartz in fluids, depending on temperature and solution chemistry. In the field of engineering, silica nanoparticles have been synthesized by the rapid expansion of silica-dissolved solutions[21] or the evaporation of silica gel dispersed droplets[22], but their geological significance has not been considered
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