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About Melt Inclusions

Silicate-melt inclusions (MI) are small (~1-300 µm) blebs of silicate melt that are trapped within igneous phenocrysts (Fig. 1). They are glassy or crystalline, and are found within both extrusive and intrusive rocks. Because they can form at high pressures and are contained within relatively incompressible phenocryst hosts, they may retain high concentrations of volatile elements that normally escape from magmas during degassing. As such, analysis of these inclusions provides direct information on the volatile contents of magmatic systems.

A number of recent papers have used silicate MI to understand magma degassing, to characterize the compositions of immiscible fluids separating from magmas, and to study petrogenetic processes such as magma mixing and crystal fractionation. Their use has allowed greater understanding of igneous geochemistry and magmatic processes (Table 1). This paper reviews the nature and characteristics of silicate MI, the tools and methods for their analysis, and some important findings of recent studies.

Several previous reviews have also addressed the general topic of MI and the interested reader is referred to these works. Clocchiatti (1975) reviewed optical and thermometric studies of MI in quartz and provides abundant observational and experimental data on the behavior of MI. Sobolev & Kostyuk (1975) synthesized the considerable volume of Russian literature on MI accumulated prior to 1975. Roedder (1979) gave a thorough discussion of silicate MI, their origins, and the methods of study and work to that date. Much of this information is repeated in his later, more general volume on fluid inclusions (Roedder 1984). Roedder (1992) summarized evidence for immiscible fluids in the magmatic environment. Johnson et al. (1994) discussed MI, and the information they provide about the pre-eruptive volatile contents of four magmatic systems; Kilauea, Mt. St. Helens, the Bishop tuff, and Mt. Pinatubo.

My review focuses on inclusions from andesitic to rhyolitic magmas and concentrates on those studies with information useful to economic geologists and hydrothermal geochemists who seek understanding of fluids emanating from igneous systems. The review consists of two parts; it begins with a primer on the characteristics of MI and methods by which they can be studied. The second half concentrates on recent studies and applications for petrologists and economic geologists.