DECOMPRESSION OF FLUID-SATURATED SILICATE MELTS

Abstract

There exists a group of igneous rocks, generated in an environment of intense fluid activity, from specific fluid - silicate systems in which the concentrations of fluids are comparable to those of the petrogenic oxides. The fluid concentrations are even greater in cases in which the fluid pressure (Pfl) exceeds the total pressure (Pfl > P). The melts formed under such conditions are anomalously high in fluids. We have previously identified these melts as a special petrological type, which we call fluid-rich (or supersaturated) magmas. What happens in the decompression of such magmas is of interest. The most popular point of view holds that in such cases the fluid component separates from the melt, and that the residual melt then undergoes rapid crystallization because of the loss of volatiles. It is also assumed that the melt itself does not undergo significant deformation, nor does it significantly change volume. The exceptions to this rule are the volcanic systems, in which decompression is accompanied by explosive phenomena and blowout of the upper parts of the magmatic columns. To investigate the various modes of decompression, we ran experiments on fluid-rich acidic silicate melts in a high-pressure UVD-10,000 unit. Following the experiments, the samples were subjected to comprehensive examination by inspection of thin sections, x-ray analysis, scanning electron microscopy, thermal analysis, gas chromatography, and various methods for determining the contents of the petrogenic and ore components.