Volcanoes and Geophysics and Isotopes… Oh My!

Despite the appearance of being a concrete and known science, Geology, due to the interpretive nature of the discipline, has plenty of differing interpretations that keeps the field alive and kicking for generations to come. Since it is designed to be a seminar that explores the current views and most up to date research in the field, the Van Tuyl lecture series often serves as a stage for these debates.

This past week was no different as Dr. Aaron Pietruszka of the United States Geological Survey presented on the nature of the magma chambers that feed one of the most spectacular volcanoes in the world, Kīlauea. Through geophysical findings, isotopic analyses, and volumetric calculations Pietruszka walked the attentive audience through his steps to determining that two small chambers of magma, rather than one, feed Hawaii’s fiery masterpiece.

Pietruszka said, “[We have] known for a number of years a lot about the plumbing system [of Kīlauea], primarily from geophysics.” In the system, the melt is primarily from the mantle with a few xenoliths thrown in for good measure, the magma rises from there through a primary conduit starting at about 60 kilometers down, then it spends time in a summit magma chamber that sits at 2-4 kilometers down. From there, the magma either finds its way out at Halema’uma’u or it travels down a set of faults to erupt at Pu’u’Ō’ō. Since the movement of fluid in the subsurface causes minor tectonic activity, the pathways used by the magma can be determined by compiling seismic activity over time. As it would be revealed later, there are some considerations to take into account, namely whether or not the fluid is magma or if it is just simply hydrothermal fluids.

Since determining the exact nature of these systems takes a lot of data and more than a few assumptions, there is wiggle room in terms of interpretation. Pietruszka put up models for the two major end members of these interpretations, one being a fairly simple system with only one magma chamber, and the other being the model that he was there to defend, the two chamber idea. Like a well seasoned performer before a show, Pietruszka announced, “[I am] going to try to convince you that there are two small magma chambers,” before leading off into the history of the competing models. The first step was to introduce one of the crucial factors that is needed to understand a magma system, the size of the magma chamber. For the most part this is done by looking at where there are earthquakes and finding a spot without any. Since a fluid will not carry any sort of earthquake through it, a blank zone in the data indicates where there is a lack of solid rock.

According to the data, there is an area below Kīlauea that ranges from 40 cubic kilometers, if the whole area is fluid, to a measly 0.08 cubic kilometers if there is slightly molten material surrounding the chamber. For his own PhD, Pietruszka used geochemical data and came up with a volume of around 2-3 cubic kilometer.

Beyond the size of the chamber, shape is another important constraint. This data is compiled from ground deformation over time, but due to the resolution of the data, there is still room for multiple interpretations. “They saw these inflation centers rising and falling,” stated Pietruszka as he displayed the data, “it seemed like there were two main areas where activity was focused.” Of course other people have looked at this data and have come up with entirely different interpretations. Rather than two vague areas, data reduction can point to one main spot. Pietruszka hinted that this may not be correct given that recent geophysical studies have leaned towards two magma chambers. The studies also added in a mark of confusion to the interpretation, rather than having two chambers at normal depths, only one was deep while the other was interpreted to be extremely shallow. Pietruszka admitted that while it would help his interpretation to assume that the signature represented a magma chamber, it may also be an active hydrothermal system below the surface. To his dismay Pietruszka announced that the group that did the study “[was] really agnostic to if [the data] were hydrothermal [activity] or magma.”

The most poignant geophysical data-set “is kinda the smoking gun for magma,” revealed Pietruszka. Microgravity measurements at Halema’uma’u indicated that mass was increasing, which generally indicates magma is intruding into the system. The current thought behind the interpretation is that a while back an earthquake created a small void which is currently filling with magma, this explains why it was not seen before. As the chamber fills up, it drives the eruption at Halema’uma’u.
With the geophysics out of the way, Pietruszka asked, “how can we use lead isotopes to confirm these findings.” Given that his specialty is in lead isotopes he was well prepared to back up the geophysical work. “In Hawaii, the lead isotope signature indicates mantle derived magmas,” said Pietruszka. On top of the basic average signature, each of the eruptions has an individual signature which can be used to determine some of the basic properties of how it behaved in the system. Beyond this, for some of the longer eruptions, the signature will fluctuate based on the actual source from which the eruption is drawing, which may change over the course of an eruption. The fundamental principle that Pietruszka drew upon was that if there is a secondary magma chamber, the isotope ratios should reflect a different signature than the primary chamber. During the early stage of the recent large eruption of the Kīlauea caldera, the isotopic ratios followed a singular path that was reflected at all of the sites. Then as Pietruszka revealed, something unusual happened in the early 1970’s, the trend split into two different groups based on location. “[The] interpretation is that there are two different magma bodies, [which] looks to correspond to the geophysics,” said Pietruszka, then he added, with an air of relief, “this is encouraging.” The isotopic data can also be used along side a few well thought-out assumptions to find out more about the magma chamber. According to Pietruszka the chamber is likely cooled to some extent by hydrothermal activity near the surface. The residence times that can be ascertained by eruptions to help determine the size of the chamber, which is around 0.2 cubic kilometers provided the chamber exists.

To finish up the presentation, Pietruszka took time to address a few remaining questions about the Kīlauea system as a whole. The first venture outside the main topic focused around figuring out if there could be another magma chamber deeper than the main system, unfortunately the data that can be used to make this interpretation is sparse. “We can put up all the evidence for this on two slides,” said Pietruszka. The main evidence surrounding the idea concerns xenoliths. In igneous geology, xenoliths are artifacts of the rocks which a magma passes through. The longer a magma is present in a system, the more likely it is for xenoliths to be accumulated into the fluid. If there is a magma body at depth, xenoliths should be present from this chamber. Since the rock types at depth are different than the shallow rocks, an analysis of xenoliths is handy in a system such as the Kīlauea volcano. Unfortunately for Pietruszka’s curiosity, the current system will not reveal it through the magmas; “Even if a deeper chamber is there, we [will not] see these effects because [the magma] is too hot.” To add another nail to the coffin, geophysical studies have not seen any evidence for it. Pietruszka would reveal later that despite that, there is data that helps prove that there is not a deep chamber.

The other concern brought up that was somewhat secondary to the main focus of the presentation and concentrated on the current state of the plumbing system. Pietruszka put up multiple slides of isotopic data and worked through what it could mean. When the recent eruptions at Halema’uma’u began, the isotopic data was very similar to what was erupting at Pu’u ‘Ō’ō. This would mean that the system possibly did not have a secondary chamber, or that the Pu’u ‘Ō’ō magmas are a much later version of the Halema’uma’u material. As the magmas kept erupting, their composition moved away from that of the Pu’u ‘Ō’ō magmas and started their own trend. If the trends stay separate, it could indicate that the rift zone that ends in the Pu’u ‘Ō’ō eruptive site is not as related to the summit zone as one thought. On the other hand if the magmas at Pu’u ‘Ō’ō started reflecting what is erupting at the summit, it could help reinforce the idea of the system having one main plumbing pipeline. “Of course,” added Pietruszka, “this is something we will work on in the future.”

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