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Scientists often strive for a big-picture approach to understanding a given problem. Sometimes, this big-picture thinking requires first zooming into the microscopic level.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Madison Myers, Assistant Professor, and Behnaz Hosseini, Ph.D. candidate, both in Geochemistry and Volcanology in the Department of Earth Sciences at Montana State University.

Melt inclusions in a quartz crystal from the Huckleberry Ridge Tuff
Melt inclusions (<50 micrometers in diameter) in a quartz crystal from the Huckleberry Ridge Tuff, erupted 2.1 million years ago. Photomicrograph taken by Behnaz Hosseini at Montana State University.

Picture this: within the minerals that speckle volcanic rocks all over Yellowstone National Park reside minuscule pockets of melt that are frozen in time. These melty morsels represent snapshots of the subsurface history of the magmas that have fueled Yellowstone's massive eruptions. Now existing as volcanic glasses, these were once tiny pockets of liquid magma much smaller (1/10 millimeter) than the individual minerals that host them. We call these little blobs melt inclusions.

How do melt inclusions form within crystals of minerals in the first place? As magma cools and crystallizes, minerals such as sanidine and quartz start to form. However, crystal growth is sometimes non-perfect, with corners often growing faster than edges. As a mineral starts to correct this imbalance, small pockets of the surrounding melt become enclosed within the growing crystal lattice.

Although small, these melt inclusions are essentially time capsules provide invaluable insights into the composition, temperature, and volatile content (like water vapor, carbon dioxide, and sulfur dioxide—the things that form bubbles and gases) of the magma from which they were derived. By studying melt inclusions, scientists can reconstruct the earlier distribution of magma deep beneath Yellowstone's surface and better constrain the range of possible future hazards from the Yellowstone region today. In this way, these tiny melt pockets punch above their weight as a scientific tool for understanding magmatic systems.

Illustration of the crystal growth process that leads to melt inclusion entrapment
Illustration of the crystal growth process that leads to melt inclusion entrapment. (a) A crystal (gray area) nucleates and grows. (b) As the magma cools, the crystal corners will grow more efficiently than crystal edges. (c) This growth process will lead to the entrapment of surrounding melt (orange area). Figure modified from Barbee et al. (2020).

If these minerals grow in separate places beneath the surface, the composition of entrapped melts can be quite different. For instance, magma beneath the southern part of Yellowstone caldera has a slightly different composition than magma in the northern part, and the enclosed melt inclusions can be used to map out these differences in the magma chamber. Additionally, researchers have found that melt inclusions from the eruption of the Huckleberry Ridge Tuff suggest the presence of distinct magma bodies at depth prior to eruption. But how did we figure out how deep beneath the surface these magma bodies were located?

As mentioned earlier, melt inclusions contain information about the volatile content of the magma—that is, the amount of water, carbon dioxide, sulfur, chlorine, etc. These volatiles play a significant role both in our ability to understand where magmas were stored at depth, and also in the style of volcanic eruptions. In general, higher volatile concentrations can lead to more explosive eruptions, while lower concentrations often result in more gentle lava flows. Measuring all volatiles together can be used to calculate the pressure at which the host crystals were forming, because lesser amounts of volatiles can remain dissolved at lower pressures. From these principles and looking at data from melt inclusions from Yellowstone eruptions, we can infer that the magmas feeding caldera-forming eruptions at Yellowstone were stored at greater depths than those feeding lava flows.

Schematic of the Huckleberry Ridge Tuff magma storage configuration
Schematic of the Huckleberry Ridge Tuff magma storage configuration, consisting of discrete batches of magma. Analyzing the compositions of melt inclusions can help paint this type of big picture of the magmatic system. Figure modified from Myers et al. (2016).

The next time you are standing on the rim of the Yellowstone caldera or are examining an outcrop of the Lava Creek Tuff (from Yellowstone’s most recent caldera-forming eruption 631,000 years ago), remember that sometimes the mightiest scientific tools come in small packages, unable to be perceived by the naked eye. Just like drops of water make an ocean, small pockets of melt make a magma body. These melt inclusions provide powerful snapshots of the magmatic system that allow scientists to decipher not only where magma was stored, but also the factors that contributed to its eventual eruption. When it comes to the melt inclusion, small but mighty is an understatement.

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