Numerical modeling of convective melting of the crust by hot rhyolite intrusions with the variable water contents

Abstract:

[Hot rhyolite] /another rhyolite contact may cause fast generation of large volumes of rhyolitic magma if convection (e.g. Simakin and Bindeman 2012) is involved. Here we further developed this high resolution (2-3 cm for composition and 20-30 cm for heat transfer and flow rate) finite element numerical model to simulate convective melting of the silicic substrate by including variable water contents (0-4 wt%), both in the magma and the substrate and explore the water behavior and influence on the melting processes using the haplogranitic Qz-Fsp-H₂O phase diagram. Melting is modeled via pseudo-kinetic procedure. Quartz and feldspar fractions in the melt are changed with rates proportional to the deviation from equilibrium liquidus surfaces expressed in the form C_Qz,i (T,C_H2O) corresponding to either superheating or supercooling. Our grid size of 57x27 m and high spatial resolution allow accounting water diffusion in the magma during the simulation lasting up to a month of the sill evolution history. Water content in the mushy substrate was controlled by melt fraction and water solubility in the melt and at P=200 MPa equals to 1.1-1.2 wt. %. Physical estimates demonstrate that in the rigid mush with melt content less than 30-35 vol.% stresses are induced due to thermoelastic and volume change effects, allowing water transport by fissures into the melting zone. Thus additional water enhances melting and allow formation of crystal rich plumes. We run-test the new model with Yellowstone (e.g. Befus and Gardner, 2016) and Iceland-like conditions: the activation of 80% crystalline mush held at T=698^oC reactivated by intrusion of 18 m thick sill of superheated rhyolite with T=950^oC and Cw=2.5 wt.%). Convection with roof-rock partial melting in two-phase plumes regime practically stops when magma temperature drops below T=875^oC in 1.5-2 months. Magma formed in these numerical experiments is similar to post LCT lavas containing low 1.0-2.5 wt.% of water.