New Atomic Spectroscopy–Based Approaches in Geochronology: An Interview with the 2018 Emerging Leader in Atomic Spectroscopy

Geochronology is an exciting area of atomic spectroscopy and earth science research. One of the goals is to answer tectonic questions, and in particular, how the crust responds to continent–continent collision. John M. Cottle, a professor of earth science at the University of California, Santa Barbara, is one of the scientists on that mission. Cottle and his research group are at the forefront of discovery in geochronology, combining both laboratory and field-based research. In particular, Cottle is a leader in the development of novel laser-ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS) measurements and their application to tectonic questions in convergent orogens, which are mountain ranges formed when a continental plate crumples and is pushed upwards. Here, Cottle dicusses his work, perspective, and goals for the future.

Where or how did your interest in analytical chemistry and atomic spectroscopy begin?

During my master's studies I was exposed to both thermal ionization and laser ablation mass spectrometry which initially got me interested in geochronology methods and spectroscopy in general. Soon after starting my doctorate at Oxford, I was fortunate enough to be able to work with a great group of researchers at the Natural Environment Research Council (NERC) Isotope Geoscience Facilities of the British Geological Survey (BGS)-including Randy Parrish, Matt Horstwood, Steve Noble, and Dan Condon. All of these people fostered my interest in geochronology and mass spectrometry. They allowed me to try things in the LA-ICP-MS lab that I'm sure they all knew were doomed to fail, but that presented some of the best learning opportunities I've ever had. It also gave me plenty of experience in fixing mass spectrometers when I (inevitably) broke them.

How does your work with geochronology, tectonics, and geochemistry influence your atomic spectroscopy work, or vice versa?

My research centers on answering tectonic questions about large mountain belts, and in particular, how the deep crust responds to continent–continent collision. A key aspect of this work is understanding both the absolute timing and rate of geologic processes such as melting and metamorphism of the crust. To establish geologic ages of rocks, we use laser ablation–based ICP-MS to measure uranium–lead isotope ratios in accessory minerals. This technique is extremely useful because we can often measure multiple ages, and therefore multiple geologic events, recorded in single crystals. The desire to answer fundamental tectonic questions drives my work to improve LA-ICP-MS methods. For example, better precision and accuracy, combined with increases in spatial resolution enable even more detail to be extracted from rocks.

You have done some significant work developing novel LA-ICP-MS measurements and their application to tectonic questions in convergent orogens. Can you please briefly describe that work and what led you to develop the LA-ICP-MS system for those measurements?

I think there are two main areas where I have contributed to developments in LA-ICP-MS. Typical analyses are conducted using static spots and continuous pulsing of the laser for 20–45 s. In cases where there is intracrystal heterogeneity in age or elemental concentrations, this method results in the production of "mixed" ages. It also limits users' ability to target very thin domains within crystals. In 2009, my colleagues at the BGS and I developed an alternative approach in which data were obtained from consecutive single laser pulses separated in time (instead of the typical 80–200 pulses). This single-shot approach (SS-LA-ICP-MS) dramatically increases sample throughput, enabling very large numbers of grains to be dated, and it uses only ~1% of the mineral compared to "conventional" LA-ICP-MS methods. One of the key things we discovered was that it is possible to generate accurate and precise data by integrating the entire transient peak shape from each pulse, rather than measuring just peak height. This alternative method eliminates effects from mixed detector arrays.

Secondly, after much work in the Himalayas and elsewhere, it became apparent to us (and many others), that it is necessary to collect additional geochemical information as well as isotopic ages from accessory minerals to match a measured date to a specific geologic process-whether it be melt crystallization, responses to changes in pressure, temperature, or fluid–melt infiltration, with or without accompanying deformation. To meet this demand, my colleagues at UCSB and I developed a new method in which two ICP mass spectrometers are used for simultaneous measurement of a laser stream, enabling simultaneous isotope–isotope or isotope–element measurements. The breakthrough came from realizing that the flow of particles into the two spectrometers could be controlled by modulating the carrier-gas flow and that the ionization efficiency of the coupled mass spectrometers is only slightly reduced. This LASS-ICP-MS (where SS stands for split stream) method is now gaining wide usage, particularly for the simultaneous measurement of U/Th-Pb dates and petrologically informative elements like Ti, Zr, and rare earth elements (REEs) and isotopic tracers from the same mineral volume to allow dates to be tied closely to petrologic processes.

Can you tell us about the extension of that method to single-pulse depth-profiling and three-dimensional (3D) mapping of zircon, monazite, titanite, and rutile?

In a recent paper (1), we were able to combine the advantages of the LASS and SS-LA-ICP-MS methods-simultaneous collection of complementary tracers, increased spatial resolution, and shortened analytical time-are combined to establish a hybrid approach, that of single-shot laser-ablation split-stream (SS-LASS) ICP-MS. We optimized instrumental parameters to maximize accuracy and precision and were then able to rapidly construct two-dimensional (2D) and 3D images of individual crystals and time-scales of crystal growth or recrystallization as well as characterize intracrystal elemental and isotopic zoning in one-dimensional, 2D, and 3D. I think there is still a lot to be gotten out of this technique, and this paper really just sets the stage for applying this technique to many different problems.

What research are you most proud of thus far?

I'm most proud of the contribution my colleagues and I have been able to make to various aspects of Himalayan tectonics. I think we have been able to understand more about previously known geologic features, but also through the use of new analytical techniques, such as LASS, we have been able to recognize and explain new features that force a revision to our understanding of how the Himalayas were formed. I also like the fact that we came up with large-scale tectonics questions, and were then able to adapt and develop new analytical techniques that, in their own way, prompted more geologic questions, and further spurred more method development.

You have done research all over the world, including Nepal, Tibet, Antarctica, and New Zealand. What have you learned from these different places, either professionally or personally? Is there one spot in particular that you think requires more research attention than the others?

I really enjoy getting into the field and looking at the geology and landscape-particularly in places like the Himalayas where the geology is complicated. I'm always seeing and learning new things about how the earth works. For me, laboratory work is equally fun. I still get excited when I see the data coming off the mass spectrometers and being able to think about how it relates to the observations we make in the field, to pull together a bigger picture of how the earth works.

Each of the places I have done research has taught me something different, but if I had to pick one, I would say we still know the least about the Himalayas, especially the early history of this mountain belt.

You are currently supervising four graduate students and in total have supervised 15 graduate or postdoctoral students. How do their theses and research projects impact your work? What kind of collaboration is there between your students' work and your own?

I try to start my students off on a project for which we already have some interesting data or results. They can use this as a starting point to develop their own interests and find out what really excites them. As they progress, I try and give my students enough time and space to develop their own ideas and run with them. Through this approach, I learn a lot from my graduate students. I hope by the end of their time with me that they are able to teach me things or help me view problems through a different lens. It's always exciting to watch students come up with a solution to a problem or develop a new method, and see how enthusiastic they are about pushing the boundaries of science.

You have published more than 70 peer-reviewed manuscripts and given many talks and poster presentations. How do you balance the demands of research with those of teaching?

I have a great family who continue to be very supportive, putting up with way too many requests for "just 30 more minutes in the lab." They bring a balance to my life for which I'm extremely appreciative. My department is very accommodating, allowing me a flexible teaching schedule. I also work to involve students in all aspects of my research. I try to structure my working time so I have short and medium team goals. I use this approach as a way to work on different projects while not getting bogged down if something isn't working. I also try to write something every day.

What do you plan to focus on next? Is there one big problem that you really want to tackle?

We are only really beginning to attempt to quantitatively link isotopic ages to trace element data in the field of petrochronology. My feeling is that a lot of our interpretations are ad hoc and we could do a much better job of applying rigorous statistical treatment to our data. We could also do a better job of storing and interrogating data from many different places to answer some bigger-picture geologic questions. Analytically, I think there is also a lot to be achieved still. Mass spectrometer design, and, in particular, laser design has come a long way since I first made measurements more than 15 years ago. Building instruments that can measure a wider range of isotopes simultaneously and to higher precision should be one of our top priorities for the geochronology/geochemistry community, in my opinion.

Reference

(1) J.M. Cottle and M.A. Stearns, Microstructural Geochronology: Planetary Records Down to Atom Scale 232, 91 (2017).