Volcanic Crystal Forensics: What Minerals Tell Us About Evolution

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Volcanic Crystal Forensics: WhatMinerals Tell Us About theEvolution of Mount St. Helens andLong ValleyBy Erik Klemetti May 31, 2012 | 11:56 am | Categories: Eruptions, Science Blogs

Students examining part of the Bishop Tuff, erupted from Long Valley in California. Image: Erik Klemetti

One of the major reasons I am a geologist is that I love history. I majored in both history and geology asan undergraduate because I am fascinated by unraveling what has happened in the past and what wasthe evidence that we can use to see those events. For me, it is the crystals in volcanic rocks that holdthe key to understanding the evolution of magma at volcanoes they record events in crystallinestructure through crystal growth, changing compositions of the crystals or incorporation of radioactiveelements that can be used as a stopwatch. Even after the crystal forms, the elements are redistributed to

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About Erik KlemettiErik Klemetti is an assistant professor of Geosciences

at Denison University. His passionin geology is volcanoes, and hehas studied them all over theworld. You can follow Erik onTwitter, where you'll get volcanonews and the occasional baseballcomment.

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show how time has passed. Two studies that came out this week examining St. Helens and Long Valleyuse these tools to unlock the unseen history of the volcanoes. These crystals hold the story of thevolcano, in both the long and short term, and reading that history is what fascinates me.

To read the history in crystals, you need to know that ages in geology dont all come the same. Thereare two types of ages when we consider almost any geochronologic information relative and absoluteages. The latter is straightforward an absolute age is one where you can assign a specific date to theevent in question. For example, if Im looking at the core of a zircon crystal (see example below) and Imeasure the U and Th content of that core, I can use the radioactive decay of these elements todetermine the age of the core is 41,900 years. This age comes with some error based on the quality ofyour analysis, but it is a specific number of years that fixes that zircon in time. Absolute ages are derivedtypically through radiometric clocks, so using elements that decay like U, Th, C and K.

On the other hand, relative ages cant tell us specifically when an event occurred, but rather how long ithas been since some event occurred. One way that relative time can be determined by using thediffusion of elements in a crystal. Crystals suck in specific elements based on the composition of themagma and the structure of the crystal itself. If there is a dramatic change in the composition of themagma, the composition of some elements in the crystal change as well, creating a gradient. If you havea concentration gradient, you know, from even basic chemistry, that elements from the higherconcentration side will move to the lower concentration side, taking a sharp boundary and making itmore relaxed. In crystals, this mainly occurs at high temperatures (magmatic conditions) and veryslowly, typically the elements move at rates of 10 to 22 m2/s. (Diffusion is seen as a surface, thus themeter squared.) That is something between a zepto- and yoctometer, or, in other words, about 1sextillionth to septillionth of a meter. However, when we have geologic timescales to do things, then wecan actually see diffusion of elements in crystals if they sit in magma for years or more. This diffusionprofile wont give us the absolute age of the crystal, but it does tell us the time since the compositionalgradient formed and that crystal was sitting at magmatic temperatures (note: at the surface conditions,diffusion in crystals is so slow that it can, for all intents and purposes, be assumed to have stopped).

A zircon from the Kaharoa eruption of Tarawera in New Zealand, showing compositional zoning and a core age. Image:Klemetti et al.

Crystals can also be used to fingerprint geologic events in the magmatic system beneath a volcano.Much like tree rings, crystals will grow, adding new layers. If you can measure the compositional

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changes in those rings, then you can try to match them to geologic events that youve examined outsideof the crystal record. For example, if you have the compositional changes in a large volcanic system asmeasured in the whole rock composition of the erupted material, you could analyze the zoning in crystalsto see those changes and match populations of crystals to specific events. An example is what I studiedin zircon from the Okataina Caldera Complex in New Zealand, where the crystals recorded changes inthe composition of the magma through time (see above), especially when looking at the Yttrium contentof the zircon. In that study that came out last year in Earth and Planetary Science Letters, we couldcould absolute ages taken in the cores of the zircon with relative ages from the growth of the zircon tomatch the ups and downs in the crystal zones with those in magmas being erupted. However, thesezircon came from the ~1300 A.D. eruption of Tarawera, so from a single eruption, you can look at thecrystals to deduce the compositional history of the whole system as far back at 350,000 years.

In the past week, two studies have garnered a lot of media attention for their application of what JonDavidson might call crystal forensics. One looked at how the compositional zoning and diffusion inpyroxene, another common volcanic mineral, can be linked to the seismic record (and thus magmaticintrusions) during the 1980s at Mt. St. Helens. The other looks at the Long Valley Caldera and usesdiffusion in quartz (and other stopwatches) to determine that the accumulation of the large volume ofmagma that formed the Bishop Tuff likely only occurred hundreds to thousands of years before theeruption. Both of these studies use these concepts of reading the record in crystals to examine thehistory of the volcanic system and thus unlocking information that can unravel what leads up to aneruption.

Mount St. Helens

The first study by KateSaunders and others inScience examined pyroxenecrystals erupted in lavas from1980 to 1986 at Mt. St.Helens in Washington. Bylooking at the composition ofthe zones in the pyroxenecrystals and how elementsdiffused in the crystals, theydetermined relative ages forthe growth of rims on thepyroxene. Specifically, theyexamined iron andmagnesium diffusion andcalculated relative ages of thecrystal zones based on whenlava that the crystal wassampled erupted, assumingthat diffusion stopped noearlier than the eruption of thelava. They also looked atwhether the crystal wasnormally zoned (from high Mgcore to high Fe rim) orreversely zoned (from high Fecore to high Mg rim). Thiscorrelates with temperature,where high Mg occurs duringperiods of higher temperature,so a reversely zonedpyroxene might mean that themagma heated back up. Ifyou combine the diffusionages and the zoning with theseismic record at St. Helensover that period (see right),

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Correlating seismicity and sulfur dioxide emissions from Mt. St. Helens from1980 to 1986 with diffusion ages from pyroxene. Image: Saunders et al.

you notice that rims grewmost abundantly duringperiods surrounding seismicswarms likely new magmainjection.

Now, a lot of media attention on this study has been saying that this could be used as a predictive toolfor eruptions at a volcano. That is stretching it way too far. Remember, these crystals need to besampled from an erupted lava, so the volcano has to be erupting already! Not much of a predictive tool ifthe volcano is already erupting, now is it? It does show that the activity at St. Helens was caused bymultiple intrusions over the 6-year span, which is an important piece of information when consideringhow long an eruption might last.

Long Valley

The second study by Guilherme Gualda in PLoS One tackled the Bishop Tuff that erupted from theLong Valley caldera ~750,000 years ago one of the largest eruptions in the past few million years(what some might call a supereruption.) Gualda covers a lot of ground in the study, but I wanted tofocus on the diffusion of titanium in quartz, which he uses to determine the time between the initialaccumulation of the large volume of magma that became the Bishop Tuff and its eruption. By looking atthe boundary between the high Ti cores of quartz crystals and lower Ti rims and how Ti diffused (seebelow), the time that the quartz sat at magmatic temperatures can be estimated. What they find is thatthe quartz crystals were likely only at magmatic temperatures for a few hundred to up to 10,000 years, soa relatively short period of time (geologically). This contrasts with the zircon ages from the Bishop Tuff



(from earlier studies) that date back 100,000s years. The study also looks at how melt inclusions inquartz crystals can be used to determine relative ages and how modeling the thermal conditions of themagma can be used to support the short timescales that the quartz crystals suggest. All of the data pointto the conclusion that the large body of magma couldnt have accumulated more than a few thousands ofyears before the eruption.

Ti zoning in quartz used to determine diffusion ages in the Bishop Tuff. Image: Gualda et al.

Much of the media coverage for this study has been implying that the shorter timescales are for thegeneration of the magma leading to these large eruptions (along with the usual supervolcanofearmongering). However, that isnt really the case what this study talks about is the accumulation ofmagma into a large body, so the magma likely already existed. This is a concept that many in thevolcano community support, where magma exists as pods and in-between crystals in a mush and isthen extracted prior to the eruption. That extraction can be caused by an earthquake or new injection ofmagma below the mush, but the magma is there. However, once the magma has been extracted andaccumulates into a larger body, the clock is ticking for an eruption. As new crystals form in the magma,gas accumulates (as it doesnt go into crystals, so it is left behind and builds up in the liquid portion ofthe magma), leading to overpressure the recipe for an eruption.

So, why the difference in zircon ages and quartz ages? Well, this has become a bit of a strawman insome articles Ive seen in the media about this study. Most geologists who work with zircon would agreethat zircon doesnt give us magma residence time, that is the time since the magma first formed. Instead,zircon is recycled repeatedly and records an integrated history of the magmatic system. So, those agesfrom the Bishop Tuff that date back 100,000 years tell us about how long it might take to generate allthat magma.

Crystals are incredible sources of information to understand volcanoes. From a single mineral that mightbe only half a millimeter across, we can examine hundreds of thousands of years of magmatic activity.By combining information from minerals that allow for absolute ages (zircon) and relative ages (like



quartz and pyroxene), we can begin to really unravel the complexity that lies beneath volcanoes andhopefully better understand what leads up to an eruption.

References

Gualda, G. and others, 2012. Timescales of Quartz Crystallization and the Longevity of the BishopGiant Magma Body. PLoS One.

Klemetti, E. and others, 2011. Magmatic perturbations in the Okataina Volcanic Complex, New Zealandat thousand-year timescales recorded in single zircon crystals. Earth and Planetary Science Letters 305,185-194.

Saunders, K. and others, 2012. Linking Petrology and Seismology at an Active Volcano. Science 336,1023-1027.

Image 1: Bishop Tuff, by Erik Klemetti.Image 2: Figure 5 from Klemetti et al. (2011)Image 3: Figure 4 from Saunders et al. (2012)Image 4: Figure 1 from Gualda et al. (2012)

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Erik Klemetti is an assistant professor of Geosciences at Denison University. His passion in geology is

volcanoes, and he has studied them all over the world. You can follow Erik on Twitter, where you'll get volcano

news and the occasional baseball comment.

Read more by Erik Klemetti

Follow @eruptionsblog on Twitter.

Tags: California, Chemistry, crystals, isotope dating, Long Valley, New Zealand, Okataina, quartz, St. Helens, Tarawera,Volcano research, washington, zirconPost Comment | 74 Comments | Permalink

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Volcanic Crystal Forensics: What Minerals Tell Us About Evolution