In situ high-pressure and high-temperature bubble growth in silicic melts

We present the first investigation of in situ high-pressure and high-temper ature bubble growth in silicic melts. In a hydrothermal diamond-anvil cell, a haplogranite melt (79 Wt% SiO2) is hydrated then subjected to cooling an d decompression. With decreasing pressure, water exsolves from the melt and bubbles grow. The whole experiment is observed through an optical microsco pe and video-recorded, so that bubble nucleation, bubble growth, and the gl ass transition are directly monitored. Bubbles nucleate and expand in melt globules having radii from 15 to 70 mum. Bubbles reached 3.6-9.1 mum in rad ius within 6.1-11.7 s (until the glass transition is attained) while temper ature decreases from 709-879 degreesC to 482-524 degreesC, corresponding to decompressions from 7.0-21.9 to 3.4-15.2 kbar. Bubbles nucleated either in a single event occurring within the first second or in successive pulses o ver a period of up to 7 s when the melt globules are in contact with a diam ond culet of the cell. In these experiments, bubble growth can be fitted to the cube root or a logarithm of time, mainly ascribable to the combination of large water oversaturations due to rapid cooling and decompression. At pressures of 3.4-15.2 kbar, we measure glass transition temperatures that a re 20-80 degreesC higher than those calculated at atmospheric pressure. (C) 2001 Elsevier Science BN. All rights reserved.