A new type of high-pressure low-flow metering valve for continuous decompression: First experimental results on degassing of rhyodacitic melts

Abstract

A novel type of high-pressure low-flow metering valve has been designed for experiments at continuous decompression in internally heated pressure vessels (IHPV) at pressures up to 500 MPa and temperatures up to 1500 °C. It consists of a modified high-pressure valve coupled with a piezoelectric nanopositioning system. The piezoelectric actuator (1) positions the needle of the valve statically with very high precision on a nanometer scale for infinitesimally slow decompression and, alternatively, (2) opens and closes the valve completely, within milliseconds, to achieve a fast pressure drop only limited by the diameter of the high-pressure tubing and the entire volume of the autoclave system.The valve connected to an IHPV has been successfully tested to simulate continuous decompression of volatile-bearing magma. Water saturated rhyodacitic melt was synthesized at 1050 °C and 300 MPa and subsequently decompressed to 50 MPa at an integrated decompression rate of 0.28 MPa/s using continuous decompression as well as single-step and multi-step decompression techniques. The experimental results of the three methods show significant differences, having important implications for the interpretation of textures observed in natural volcanic rocks. Bubble number density (BND) values increase from continuous to multi-step and to single-step decompression by two orders of magnitude. Bubble size distribution (BSD) also differs significantly. The BSD curve of the singlestep decompression shows largest variation both in size and in population number density, whereas the BSD trend for the sample of multi-step decompression shows the smallest variation between size and population density of bubbles. The BSD of the continuously decompressed sample indicates similar proportions of bubbles with different size. Channel formation in the sample of single-step decompression may indicate fluid escape from over-pressurized bubbles through the melt. Singlestep and continuous decompression style represent two extreme cases of magma degassing in which bubble nucleation and bubble growth are predominant processes, respectively, whereas multi-step decompression resembles the intermediate case.

Degassing of H2O in a phonolitic melt: A closer look at decompression experiments

“…The IHPV is equipped with a special high-P valve construction (Nowak et al, 2011), facilitating both CD and SD. The super-liquidus starting conditions of 200 MPa and 1323 K as well as the range of nominal decompression rates from 0.0028 to 1.7 MPa·s − 1 were adopted from Iacono Marziano et al (2007).…”

“…To date, only few decompression experiments were performed using CD with reasonable decompression rates (e.g., Mangan and Sisson, 2000;Brugger and Hammer, 2010;Pichavant et al, 2013). The SD technique was therefore compared to true, continuous decompression by Nowak et al (2011) to determine the influence of decompression path on melt degassing. A hydrous, rhyodacitic melt was decompressed at a nominal decompression rate of 0.28 MPa·s −1 with SSD, MSD and CD.…”

“…One aim of this study is to compare the H 2 O degassing behavior of a phonolitic melt at SSD, MSD and CD at different decompression rates to verify the basic results of the single set of decompression experiments from Nowak et al (2011) for a less silicic melt composition from Vesuvius and a smaller step-size at MSD. The main experimental parameters were adopted from the decompression experiments of Iacono Marziano et al (2007).…”

The roles of decompression rate and volatiles (H2O + Cl ± CO2± S) on crystallization in (trachy-) basaltic magma

“…The samples were all heated at a rate of 50°C/min and annealed for 20 h at 300 MPa, 1030°C and QFM + 2 in IHPV to ensure similar starting conditions prior to the decompression. Isothermal decompression was performed at 1030°C by releasing P continuously from 300 to 70 MPa by using an appropriate decompression valve (Nowak et al, 2011). The r was adjusted to 0.01, 0.1 or 1 MPa/s, and was constant for each experiment.…”

“…Recent studies have shown that decompression style (i.e., single-step vs. multi-step vs. continuous decompression) can affect bubble formation and, thus, most likely also DIC to a significant extent (Brugger and Hammer, 2010;Nowak et al, 2011;Fiege et al, 2014b). Thus, we performed continuous decompression experiments at rates of 0.01, 0.1 and 1 MPa/s.…”