Schematic diagram of Yellowstone

A schematic diagram shows Yellowstone's current magmatic system. A silicic crystal-mush (gray region with white crystals) is thought to underlie the caldera at a depth of about 5 to 15 kilometers, with more mafic basalt (in black) underneath.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

That a magma reservoir exists beneath present-day Yellowstone caldera has long been known, and we have some idea of what it may look like from seismology. But how long has the magma chamber existed? And how can we explore such a complex question, given that we can't directly see several kilometers deep beneath the ground? It turns out that, in a way, we can use a crystal ball — a zircon crystal ball — to look back in time at Yellowstone's magma chamber.

A previous issue of Yellowstone Caldera Chronicles, "Yellowstone's Mushy Past", discussed the physical nature of Yellowstone's magmatic system. As pointed out in that article, techniques such as geophysical imaging can provide us with a picture of the magma chamber. These images show us that the magma reservoir underlying the Yellowstone caldera is not a large tank of molten rock but instead a "crystal mush" — a semi-rigid region that is made up of 85 to 95% solid, interlocking crystals with 5 to 15% melt distributed within a crystalline framework. These geophysical images show us what Yellowstone's magma reservoir looks like today, but how old is the crystal mush that makes up the magma reservoir?

Before considering this question, it is important to understand that magmatic systems in places like Yellowstone are static phenomena. In other words, although the greater Yellowstone area has been erupting magma for 2.1 million years, the magma reservoir underneath Yellowstone today is not the same as the one that existed 2.1 million years ago. As new magma is generated deep within the earth and moves into shallower levels of the crust, the memory of older magma reservoirs can be erased, and new magma reservoirs form.

Zircon aging

Sections of a zircon crystal, only 100 millimeters across, show stages of the crystal's growth. The left panel shows the outer surface of the crystal, which is 131,000 years old based on 238U-230Th dating. This is when the crystal stopped growing. The right panel shows the interior of the same zircon crystal. The dark central growth zone of the zircon is 196,000 years old, which is when the zircon crystal started growing. Dating of the intermediate, gray growth zone yielded an age of 154,000 years old. Together, these data demonstrate that this crystal grew over 65,000 years, although this growth was not necessarily continuous and may have occurred in spurts.

To determine the age of Yellowstone's magma reservoir, researchers turn to a field of geology called geochronology, which is the study of the age of earth materials, like volcanic rocks and the crystals they contain, and is typically based on radioactive decay. Magma contains radioactive atoms that are incorporated into crystals that grow from the magma. Over time these radioactive atoms (the parent) will transform into another atom (the daughter) at a known rate via radioactive decay. The relative proportions of parent and daughter atoms, typically measured on an instrument called a mass spectrometer, can be used to determine the age of the mineral in which they are contained.

Some minerals can exist within the crystal-mush for very long periods of time before being involved in an eruption that brings the mineral grain to the surface. During this long residence in the magma reservoir, these crystals continue to grow, and as new layers are added, they record the age of the reservoir by their proportion of parent and daughter atoms. Once these crystals are erupted, scientists analyze these growth layers to determine how long the crystals existed within the magma reservoir. Keep in mind that these recorded growth histories do not mean the magma reservoir was molten during this time. Rather, it was a crystal-mush for most of this history, and only mostly molten around the time of eruptions.

At Yellowstone and other large volcanic systems, zircon is one of the most useful minerals for geochronology. Zircon has high concentrations of uranium, which is a radioactive element, and zircon crystals can survive for long periods of time in a magmatic system, making it useful for determining how long a magma reservoir has existed. Using a technique called Secondary Ionization Mass Spectrometry, scientists have measured the age of zircon crystals hosted in Yellowstone lavas that erupted over the past 160,000 years. Results from these studies show that zircon crystals in these lavas can record as much as 150,000 years of growth in a magma chamber before eruption. These data suggest that the modern crystal mush that underlies Yellowstone Caldera may have existed in a state similar to what is imaged today since about 300,000 years ago.

Yellowstone is a dynamic place and changes frequently at the surface. The magma chamber beneath the surface, however, apparently hasn't changed much in hundreds of thousands of years. How do we know? We looked into our zircon crystal ball.

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