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Geologists are very much like detectives—they use a variety of investigative tools to understand how a landscape develops. Geochemistry is one such tool and can help link volcanic deposits to their sources, even if those sources are dozens of miles away.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Natali Kragh, graduate student, and Madison Myers, Assistant Professor of Igneous Processes, both in the Department of Earth Sciences at Montana State University.

Volcanic deposits associated with the Absaroka volcanic province along the eastern and northern boundaries of Yellowstone National Park
Volcanic deposits associated with the Absaroka volcanic province along the eastern and northern boundaries of Yellowstone National Park. The left panel shows the spread of the Absaroka Volcanic Supergroup (AVS) throughout Wyoming and Montana. The intrusive rocks (dark blue) typically coincide with the main eruptive centers of the volcanic chain, and the rest of the material (light blue) is a mix of lavas, tuffs, and volcaniclastic units. The right panel shows the different extents of the primary volcanics, like lava and ash flows, and secondary volcaniclastics, made up of debris derived from the primary units, of the three groups of the AVS (the Washburn, Sunlight, and Thorofare groups) within Yellowstone National Park (YNP). This figure demonstrates how the products of the different groups interact and overlap. Geologic mapping interpretations have been made at an appropriate scale for this map within YNP, but mapping is too coarse outside of YNP to identify the differences between the primary volcanics and secondary volcaniclastics. Figure by Natali Kraugh, Montana State University.

Before Yellowstone was the volcano we know and love today, in its place, 50 million years ago, was a volcanic arc called the Absaroka volcanic province.  This ancient arc, similar to the modern-day Cascade range, produced volcanic material that covers more than 23,000 square kilometers (almost 9000 square miles). For reference, this is about the same size as the states of Vermont or New Hampshire!

Besides producing massive volcanic centers that still dot the landscape today (for example, Mount Washburn), erosion of the three main volcanic groups, referred to as the Washburn (oldest), the Sunlight (middle), and the Thorofare (youngest), has resulted in extensive volcaniclastic units—eroded volcanic material redeposited as mudstones, sandstones, and conglomerates. These deposits dominate the northern and eastern landscape in Yellowstone National Park, with some units stretching >80 kilometers (50 miles) away from their original source. This distance and the lack of distinct features make it difficult to distinguish what unit came from what source. But that’s nothing a little geological detective work can’t begin to sort out!

To start, some background information. Volcaniclastics and their classification remain a gray subject, as they fall in a nebulous world between sedimentary processes (for example, erosion) and volcanic processes (including eruptions). Lahars are a good example—is a volcanic mudflow a sedimentary process, a volcanic process, or both? A further challenge is linking such units to specific a volcanic center, and thus recreating the history of a region—a particularly difficult task when the units all look similar and can travel far from a clear volcanic source. Given these challenges, we need an identifier that is more powerful than what we can observe in the field.

Photo of volcaniclastic units of the Absaroka volcanic province in northeastern Yellowstone National Park
Photo of geologic units of the Absaroka volcanic province in northeastern Yellowstone National Park that shows volcaniclastic sandstones grading up into a conglomerate, followed by another sequence of sandstone to conglomerate. These repeating layers of the same-looking material make it difficult to assign them to a specific volcanic group. Photo by Natali Kragh, Montana State University, July 9, 2020.

One such idea is to fingerprint the minerals within these deposits to see if they can be traced back to their source volcanoes. In the world of geology, this is referring to using geochemistry, and more specifically, isotope geochemistry. Isotopes, which are different forms of the same element based on the number of neutrons in the element’s nucleus, have long been used to assess the conditions under which a rock unit forms because specific regions have distinctive isotope chemistry due to differences in the Earth’s composition. This method, if viable, could aid in better understanding the formation conditions and “provenance” (source material) for distal volcaniclastic units found not just in Yellowstone, but in other volcanic areas throughout the world.

To test this hypothesis, geologists from Montana State University collected geochemical data from both ‘primary’ volcanic products of the Washburn, Sunlight, and Thorofare units—that is direct eruptive products, like lava flows, ash, and intrusive rocks that solidified underground and did not erupt—and ‘secondary’ volcaniclastic samples, like sandstones and conglomerates that were derived from the primary deposits. As a case study, the geologists started with two primary volcanic samples and two volcaniclastic units that were thought to be related. The data for different isotopes of lead, which is one of the best indicators of source areas, for both the primary volcanics and secondary volcaniclastics are tightly clustered—a very helpful result! This strongly suggests that the volcaniclastics are not just a random mix of different source rock, like we would assume for typical sedimentary rocks, but rather that they come from specific primary volcanic deposits. In addition, the geochemistry of the secondary volcaniclastic units correlates strongly to the closest volcanic center. Thus, the initial data indicate that using isotopes to fingerprint volcaniclastic units may indeed be possible! If applied to enough deposits, it might even be possible to reconstruct the paleo valleys and rivers of the landscape 50 million years ago, providing a window into the topography of a region before the Yellowstone hotspot arrived. 

Isotopic composition of units within the Absaroka volcanic province
Isotopic composition of the primary volcanic groups of the Absaroka volcanic province (the Washburn, Sunlight, and Thorofare groups) and two volcaniclastic units, the Sepulcher formation and the Daly formation. Based on their location (seen on the map), the two volcaniclastic units were expected to correlate with the Washburn group data (all other data were collected from samples off the map). Both volcaniclastic rocks isotopically match up with the Washburn group data, indicating we can better understand the origin of volcaniclastic units in Yellowstone National Park using isotopes. Figure by Natali Kraugh, Montana State University.

There are still many samples to collect, and many correlations to be made, but it seems clear that detailed isotope analysis of the Absaroka volcanic units—both primary and secondary—can help geologists to better understand the history of the region that is now Yellowstone National Park. With these investigative tools, the real detective work can now begin!

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