Before an eruption, magma tends to accumulate in a magma chamber, until the pressure is sufficiently high that an eruption begins
Phenocrysts (large crystals)
form in the magma chamber
Microlites (small crystals) form en route to the surface, in the conduit, and/or after the magma has erupted, whilst the deposits cool
Melt
inclusion
Whilst in the magma chamber, crystals grow. The longer the magma spends in the chamber, the more crystal growth occurs. This is why granite is full of crystals - it is formed from magma that never made it to the surface but rather cooled and solidified very slowly within a magma chamber.
Sometimes as the crystals grow, some of the melt from the magma chamber becomes trapped inside them. This trapped magma is called a melt inclusion. The crystal then protects this magma during ascent, preventing it from being affected by processes such as degassing which would serve to change its composition.
Melt inclusions
Consequently, melt inclusions are thought to preserve the composition of the magma chamber. Despite the fact that they are incredibly small (generally a tenth to a hundredth of a millimetre), there are various different machines that can measure the composition of these melt inclusions: electron microprobes (EMPA) can measure major elements and secondary ion mass spectrometers (SIMS), such as the one pictured (left) at Edinburgh university, can measure the amount of water, carbon dioxide and other volcanic gasses. Melt inclusions thus offer useful tools for investigating the initial composition of the magma, prior to eruption.
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Recommended reads:
Lowenstern, J. B. (1995) Applications of silicate-melt inclusions to the study of magmatic volatiles, in Magmas, Fluids and Ore Deposits, edited by J. F. H. Thompson, p. 71-99, Mineralogical Association of Canada.
Wallace, P. J. (2005) Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data, Journal of Volcanology and Geothermal Research, 140(1–3), 217-240.
Devine, J. D., Gardner, J. E., Brack, H. P., Layne, G. D., and Rutherford, M. J. (1995) Comparison of Microanalytical Methods for Estimating H2O Contents of Silicic Volcanic Glasses, American Mineralogist, 80(3-4), 319-328.