The case of the Lava Creek Tuff and the empty reentrants

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Geoscientists have never observed an active magma reservoir firsthand because magmas are stored inaccessibly deep underground. However, crystals are born, grow, and live within magma reservoirs. The physical textures and internal composition of these crystals preserve evidence for the nature of the host magma. 

ellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Kenneth Befus, assistant professor in the Department of Geosciences at Baylor University.

Geoscientists have never observed an active magma reservoir firsthand because magmas are stored inaccessibly deep underground. At best, a few magma chambers have been encountered by drilling, or observed after they have long since frozen and been exposed by erosion, but these glimpses don't provide a full picture of a molten magma chamber. We are thus left with many questions about how magma chambers work. For example, Yellowstone has been the source of some huge eruptions, but how could magma chambers grow so large to feed these events?

We can also learn about magma chambers from geophysics, like the seismic imaging that has been used to study the Yellowstone system. That work discovered that a shallow reservoir exists in the modern system that extends from 5 to 17 km depth and is composed of a crystal-rich mush with only 5 to 15% melt. Such crystal-rich magma is locked up and cannot erupt, so we thus must look to other datasets to get an improved view of an eruptible magma reservoir from Yellowstone's past. One method? Crystals!

Crystals are born, grow, and live within magma reservoirs. The physical textures and internal composition of these crystals preserve evidence for the nature of the host magma. For example, zircon crystals indicate that the magmas at Yellowstone exist in the crystal-rich mush state for many tens of thousands of years, and they only become mostly molten just prior to eruption. Compositions of melt contained within quartz crystals provide evidence that Yellowstone's Huckleberry Ridge supereruption was sourced from a number of isolated magma reservoirs. Collections of other crystals let us know that the temperatures of Yellowstone's magmas prior to eruption hover around ~800 degrees C.

Microtomography 3D image and cathodluminescence slice from quartz crystal Lava Creek Tuff Yellowstone

Synchrotron X-Ray microtomography 3D image (a) and cathodoluminescence slice (b) from the same reentrant-bearing quartz crystal from the Lava Creek Tuff. The reentrants are in darker blue in (a) and the black cavities in (b). Note their relationship to quartz growth bands. Red domains are small magnetite crystals. The plane of the cathodoluminescence slice is shown by the gray plane in (a) (Credit: Kenneth Befus, Baylor University Department of Geosciences. Public domain.)

To learn more about Yellowstone's most recent super eruption, we studied crystals from the Lava Creek Tuff. To do this, we collected pumice from outcrops of the Lava Creek Tuff. We then crushed the pumices and used a microscope to hand pick quartz crystals. To our surprise, the crisp faces of the quartz crystals are riddled with hollow "reentrants"—small cavities that resemble worm holes in an apple or a narrow bay on a coastline.

After surveying hundreds of quartz crystals and their reentrants, we then analyzed select specimens with methods called synchrotron X-Ray microtomography and cathodoluminescence. Synchrotron X-Ray microtomography generates 3D images that we used to look at the structure of the reentrant-bearing quartz crystals. Cathodoluminescence produced 2D maps of crystal interiors and, like tree rings, preserve detailed information on crystal growth.

We discovered that hollow reentrants occur in ~20% of the quartz crystals. They are small features, ranging from a few microns to 400 ?m wide (a micron is one-millionth of a meter). They have bulging interiors that narrow to necks at the crystal surface. Rare examples tunnel through entire crystals, creating hollow pathways that connect opposite sides. Many reentrants cut across the alternating light and dark growth bands shown in cathodoluminescence images.

The textures and distribution of hollow reentrants provide new insight into the physical state of the magma reservoir from which the Lava Creek Tuff erupted, suggesting that the reservoir was so full of gas that it started to make bubbles that were trapped in the magma. The reentrants likely formed as the quartz crystals dissolved when and where they came into contact with bubbles of magmatic gas. This creates a picture of the pre-eruptive magma reservoir: free-floating bubbles were distributed throughout the Lava Creek magma chamber, some of which attached themselves to quartz and other crystals.

The discovery of the hollow reentrants demonstrates that the magma chamber that was the source of the Lava Creek Tuff existed as a bubbly, gas-saturated reservoir before the eruption. This is an important new step in in our understanding of magmas at Yellowstone caldera. The presence of gas bubbles means that the magma is compressible, like a sponge. This can insulate magma from changes in pressure, allowing for very large volumes of magma to accumulate before finally erupting. This is a story that could only be told by crystals—their compositions, shapes, and inclusions preserve critical information about how magma chambers work!

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