Convective Melting and Water Behavior around Magmatic-Hydrothermal Transition: Numerical Modeling with Application to Krafla Volcano, Iceland

• Published in: Journal of Petrology
• Main Authors: Simakin, A G, Bindeman, I N
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2022
• Subjects: Iceland
• Online Access: http://dx.doi.org/10.1093/petrology/egac074; https://academic.oup.com/petrology/advance-article-pdf/doi/10.1093/petrology/egac074/45271392/egac074.pdf; https://academic.oup.com/petrology/article-pdf/63/8/egac074/45537693/egac074.pdf

Abstract Water is an essential component of rhyolitic magmas and nearly universally, silicic magmatism in the upper crust includes a transition from magma to water-saturated roof rocks. We have numerically simulated the effect of the addition of geothermal fluids to an intruded rhyolitic sill from the hydrothermal system contained within the porous felsitic roof rocks. Water uptake in the melt proceeds via its thermodynamically estimated saturation with a partial melt, corresponding to the fugacity of hydrothermal water in the melt-fluid zone at particular T–P-${X}_{H_2O}$ conditions. It is assumed that the exchange occurs until the melt fraction increases to the threshold melt fraction value εb ≈ 0.3–0.45. In this approximation, the amount of added water is the product of its solubility and the critical melt fraction εb. In two series of numerical experiments run at pressures of 200 and 50 MPa, the interaction of water-filled porous felsite with near liquidus rhyolite magma resulted in water absorption, induced partial melting creating a narrow several meters wide mushy zone, and sluggish convection below that distributed water across the intruded sill. At P = 200 MPa, the addition of about 1.5 wt% water results in stronger volume convection, causing the melting rate to increase to 20 m/year. However, the addition of <0.22 wt% water induced no melting on the magma/contact mush interface, and the intruded sill crystallizes without convection. We apply the results of these numerical experiments to hot and dry rhyolites of the Yellowstone hot spot track magmas and then to the 2009 AD rhyolite sampled by the IDDP-1 exploration well in Krafla (Iceland). An active contact between the hydrothermal system within felsite and hot 963°C rhyolite magma was accidentally crossed at the depth of 2100 m, with a very thin (<30 m) transition providing information for a partial verification of our theoretical model. With the parameters observed in 2009, including the water concentrations in the melt (1.8 wt%) and ...