Open AccessMelt pockets in phenocrysts and decompression rates of silicic magmas before fragmentation
Author(s) -
Liu Yang,
Anderson Alfred T.,
Wilson Colin J. N.
Publication year - 2007
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2006jb004500
Subject(s) - phenocryst , silicic , melt inclusions , geology , magma , mineralogy , quartz , pumice , decompression , petrology , volcano , geochemistry , volcanic rock , thermodynamics , paleontology , physics
A new method is presented to estimate the decompression rate of silicic magma prior to fragmentation and is applicable to rates of 1–100 kPa/s. This method uses the remaining concentrations of H 2 O and CO 2 in bubble‐free cylindrical melt (now glass) pockets in phenocrysts that are connected to the surrounding bubbly melt. These melt pockets are common in volcanic rocks. During magma ascent, bubbles grow in the surrounding melt, and the concentrations of dissolved H 2 O and CO 2 decrease, establishing a boundary condition of low volatile concentration at the outlet of the melt pocket. In turn, volatiles diffuse from the melt pocket into the surrounding bubbly magma and form a gradient that is time dependent. The volatile concentration at the inner limit of the melt pocket likewise diminishes with time. Knowing the diffusivity and solubility of H 2 O and CO 2 permits the remaining H 2 O and CO 2 contents to be estimated for different decompression rates using diffusion equations. We apply this approach to the measured H 2 O and CO 2 contents in quartz‐hosted melt pockets in pumice from the phreatomagmatic 26.5 ka Oruanui eruption, Taupo, New Zealand. A decompression rate of 1–7 kPa/s (∼5–35 cm/s) was obtained for Oruanui magma.