Critical Parameters In Magmatic Degassing

Abstract

Decompression experiments conducted at pressures up to 200MPa and temperatures of 825�C-880�C on hydrated K-phonolite and rhyolite melts were used to explore the critical parameters controlling nucleation, exsolution and degassing behavior. Experiments on the low viscosity/surface tension K-Phonolite melt highlighted the role of melt properties. Although the sample porosities deviated below equilibrium values for pressures less than ~40MPa, the melt exsolved water in equilibrium over all the pressures and decompression rates studied. Melt shearing is proposed to have caused bubble deformation and alignment, lowering the porosity at which extensive permeability develops and significant degassing occurs compared to rhyolite. Experiments on a rhyolite melt decompressed slowly from 100 MPa and then held at 10 MPa for up to 900 s highlighted the critical parameters controlling the formation and stability of a highly vesicular magma: bubble number density, bubble size distribution and porosity. The porosity of the interconnected, highly vesicular network decreased during "Stage I" degassing and the bubble size distribution evolved from a unimodal population to include a population of much larger bubbles. During Stage II degassing, the network collapsed. Pre-collapse and collapse degassing rates were obtained and a coalescence-induced coalescence model proposed to explain the rapid destabilization. The ability of a melt to efficiently exsolve volatiles and the ease of bubble coalescence are both a function of the initial distribution of nucleated bubbles. The development of a new method for quantifying this distribution using spatial statistics will allow future researchers to explore the underlying controls on nucleation such as melt structure and the occurrence of a prior nucleation event. To investigate the critical parameters controlling shallow dike intrusion and therefore magmatic ascent rate, the fracture mechanics of intrusion into homogeneous and layered (weak sandstone/strong granite) particle models under lithostatic, compressive and extensional regimes were examined. Although the scale of the model intrusions were an order of magnitude greater than field observations, extensive microfracturing across the weaker layers, parallel dike jointing in the stronger layers and a length scale dependence to fracture toughness were observed suggesting that the use of a particle code is a promising approach to intrusion modeling.