A melt inclusion is a small parcel or "blobs" of melt(s) that is entrapped by crystals growing in magma and eventually forming igneous rocks. In many respects, it is analogous to a fluid inclusion within magmatic hydrothermal systems. Melt inclusions tend to be microscopic in size (10-80 µm) and can be analyzed for volatile contents that are used to interpret trapping pressures of the melt at depth.
Characteristics Melt inclusions are generally small - most are less than 80 micrometres across (a micrometre is one thousandth of a millimeter, or about 0.00004 inches). They may contain a number of different constituents, including glass (which represents melt that has been quenched by rapid cooling), small crystals and a separate vapour-rich bubble. They occur in the crystals that can be found in igneous rocks, such as for example quartz, feldspar, olivine, pyroxene, nepheline, magnetite, perovskite and apatite. Melt inclusions can be found in both volcanic and plutonic rocks. In addition, melt inclusions can contain immiscible (non-miscible) melt phases and their study is an exceptional way to find direct evidence for presence of two or more melts at entrapment.
Analysis Although they are small, melt inclusions can provide an abundance of useful information. Using microscopic observations and a range of chemical microanalysis techniques geochemists and igneous petrologists can obtain a range of unique information from melt inclusions. Various techniques are used in analyzing melt inclusion compositions. Volatile elements (H2O and CO2) are analyzed with Secondary Ion Mass Spectroscopy (SIMS), double-sided transmission FTIR, single-sided reflectance FTIR. These volatile compositions track storage depths of magmas, crystallization, volcanic degassing, and rates of ascent or decompression during eruption. Major, minor, and trace elements are often analyzed by electron microprobe analysis (EMPA), Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS), Scanning Electron Microscopy (SEM), and Secondary Ion Mass Spectroscopy (SIMS). If there is a vapor bubble present within the melt inclusion, analysis of the vapor bubble with Raman spectroscopy or microthermometry, or alternatively experimental rehomogenization must be performed when reconstructing the total volatile budget of the melt inclusion.
Microthermometry Microthermometry is the process of reheating a melt inclusion to its original melt temperature and then rapidly quenching to form a homogenous glass phase free of daughter minerals or vapor bubbles that may have been originally contained within the melt inclusion.
Microscope-mounted high temperature stage heating Stage heating is the process of heating a melt inclusion on a microscope-mounted stage and flowing either helium gas (Vernadsky stage) or argon gas (Linkam TS1400XY) over the stage and then rapidly quenching the melt inclusion after it has reached its original melt temperature to form a homogenous glass phase. Use of a heating stage allows for observation of changing phases of the melt inclusion as it is reheated to its original melt temperature.
One atmosphere vertical furnaces This process allows for reheating of one or more melt inclusions in a furnace held at a constant pressure of one atmosphere to their original melt temperatures and then rapidly quenching in water to produce a homogenous glass phase.
Fourier transform infrared spectroscopy (FTIR) FTIR is an analytical method which uses an infrared laser focused on a spot on the glass phase of the melt inclusion to determine the concentrations of H2O (dissolved as OH⁻ or molecular H2O) and CO2 (dissolved as CO2 or CO3²⁻). The absorption (or extinction) coefficient associated with wavelengths for each species of H2O and CO2 depending on the parent lithology that contained the melt inclusion is used to reconstruct concentrations, with the Beer-Lambert Law. Determining the baseline to these volatile peaks has been done with linear baselines, splines, flexicurves, and more recently with Bayesian sampling for error analysis.
Raman spectroscopy Raman spectroscopy is similar to FTIR in using a focused laser on the glass phase of the melt inclusion or a vapor bubble that may be contained in the melt inclusion to identify wavelengths associated with the Raman vibrating bands of volatiles, such as H2O and CO2. Raman spectroscopy can also be used to determine the density of CO2 contained in a vapor bubble if present at a high enough concentration within a melt inclusion.
Secondary Ion Mass Spectrometry (SIMS) SIMS is used to determine volatile and trace element concentrations by aiming an ion beam (16O− or 133Cs+) at the melt inclusion to produce secondary ions that can be measured by a mass spectrometer.
Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) LA-ICP-MS can determine major and trace elements, however, with LA-ICPMS, the melt inclusion and any accompanying materials within the melt inclusion are ionized, thus destroying the melt inclusion, and then analyzed with a mass spectrometer. This is often used as a final stage of analysis, as the melt inclusion subsequently cannot be analyzed.
Scanning Electron Microscopy (SEM) Scanning electron microscopy is a useful tool to employ before any of the above analyses that may result in loss of the original material since it can be used to check for daughter minerals or vapor bubbles and help determine the best technique that should be chosen for melt inclusion analysis.
Electron Microprobe Analysis (EPMA) Electron microprobe analysis is ubiquitous in the analysis of major and minor elements in melt inclusions and provide oxide concentrations used in determining parental magma types of the melt inclusions and phenocryst hosts. Compositional information is useful in determining the extent of post-entrapment crystallization or melting.
X-ray microtomography Melt inclusions have been imaged in three dimensions using X-ray microtomography. This method can be used to determine the dimensions of different phases present in melt inclusions more precisely than by using visible light microscopy.
Interpretation
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