Degassing conditions for permeable silicic magmas: Implications from decompression experiments with constant rates

摘要

The permeability in silicic magmas through connected bubble networks is an important physical property controlling magmatic degassing. In order to understand the condition of permeable degassing, we performed permeability measurements on vesicular rhyolitic glass products of decompression experiments with constant rates. The rhyolitic melts with ca. 4.7 wt.% water were decompressed at 900 degrees C from 180 MPa to a final pressure of 30, 16, 10, or 5 MPa with constant rates of 0.05, 0.005, or 0.002 MPa/s. Based on steady-state gas flow measurements for the quenched products, viscous and inertial permeabilities were calculated using the Forchheimer equation. The viscous permeabilities of products with > 80 vol.% total porosity were detectable: the permeability increased up to 10(-11.7) m(2) with increasing total porosity. In contrast, products with < 80 vol.% were almost impermeable (< 10(-15) m(2)). The rapid permeability increase around 80 vol.% total porosity corresponds to the rapid increase of connected porosity to values similar to the total porosity. This correlation suggests that high permeability occurs in isotropically-vesiculated silicic magmas during decompression under the condition that almost all bubbles in the whole sample interconnect. Using a criterion based on the mass conservation of vertical gas and melt flows, we inferred the degassing conditions for the magmas with the permeability variation and magma ascent velocity simulated in the decompression experiments. The results suggest that silicic magma ascending at 0.08-2 m/s (decompression at 0.002-0.05 MPa/s) does not satisfy degassing conditions at pressures of >= 5 MPa. Extrapolating the experimental results to smaller ascent velocity suggests the possibility that degassing conditions can be achieved at a final pressure of 5-10 MPa with a magma ascent velocity of < 10(-2) m/s (< 10(-4) MPa/s for the decompression rate): this is consistent with geophysical and petrological estimates of ascent velocities for degassed magmas in non-explosive eruptions (10(-5)-10(-2) m/s) and for non-degassed magmas in explosive eruptions (10(-2)-10(2) m/s). This suggests that a low magma ascent velocity of < 10(-2) m/s is required for degassing at shallow levels in the conduits, causing the non-explosive eruption of degassed magmas.