Christy B. Till1 & Michael Krawczynski2
1School of Earth and Space Exploration, Arizona State University
2Dept. of Earth and Planetary Sciences, Washington University, St. Louis
One of the main techniques we use in our research is experimental petrology. For those who might not be familiar, experimental petrology is the study of geologic materials in the laboratory at simulated pressure and temperature conditions relevant to planetary interiors. High temperature experiments can be conducted in several different apparatus, each specializing in different pressure regimes. Experimental equipment range from those at ambient pressures (typically called “one atmosphere furnaces”) to those using gas/water as a pressure medium (“cold-seal” and “internally heated” pressure vessels) to those using solid pressure medium to attain pressures up to 4 gigapascals (GPa) (“piston cylinders”), up to ~40 GPa (“multi-anvil apparatus”) and ~100-200 GPa (“diamond anvil cells”). Depending on the experimental apparatus and duration of experiment, temperatures accessible in the lab range from 600-2000°C. More detailed explanations of these apparatus can be found here. The modern field of experimental petrology started in the early 1900’s with the construction of high pressure apparatus at Harvard University and high temperature apparatus (as well as the temperature scale) at the Geophysical Lab at the Carnegie Institute in Washington. By the 1960-1970’s the majority of the apparatus designs we use today had been developed.
The experimental geochemistry lab in the ESPM group at WUSTL. The lab houses 2 1-atm gas mixing furnaces, 2 piston cylinder devices, and 2 cold-seal pressure vessel furnaces allowing the lab to reach pressures from 1-atm to 35 GPa.
Today, experimental work plays much the same role that it did in the early 20th century; it provides the foundation for making all interpretations of geochemical and isotopic behavior at high temperature in planetary interiors. As technology advances and analytic and theoretical techniques push their boundaries to higher precision, smaller sample sizes, and more exotic element systems, experimental petrology must work along with these studies to provide information on the mechanisms for element transport within planets. Below we describe several exciting research topics where experimental petrology is a key tool, including some of the topics our labs focus on.
Mantle Volatile Storage & Effects on Melting Behavior
One of the scientific advances over the last ten years in which experimental petrology has played a critical role is an understanding of the volatile storage budget of the Earth’s mantle. This includes identifying and quantifying the capacity for nominally anhydrous upper mantle minerals such as olivine, pyroxene and garnet to store and diffuse hydrogen, as well as olivine’s higher pressure polymorphs including wadsleyite and ringwoodite (e.g., Smyth et al., 1991; Panero et al., 2013; Bucholz et al., 2013; Thomas et al., 2015). In addition, large strides have been made in understanding carbon storage in mantle minerals and the deep carbon cycle (e.g., Dasgupta, 2013) and noble gas storage and recycling in the mantle (e.g., Shcheka and Keppler, 2012; Cherniak and Watson 2012; Jackson et al., 2013a). A topic that goes hand-in-hand with storage of volatiles in the mantle is how these volatile concentrations change the melting and rheological behavior of the mantle. Much progress has been made in the last decade regarding our understanding of volatile depression of the mantle solidus, the effect of volatiles on the composition of mantle melts, and the solubility of volatiles in silicate melts. Specifically research has looked at H2O, CO2 and now work is expanding on SO2 and noble gases (e.g., Grove et al., 2006; Dasgupta and Hirschmann, 2010; Till et al., 2012; Schmandt et al., 2014; Green et al., 2014; Fiege et al., 2014; Jackson et al., 2013b).
A backscatter electron image of an example piston cylinder experiment (from Till et al., 2012). This experiment was held at 4 GPa for a week, where it grew peridotite minerals from a synthetic oxide mixture of a primitive mantle composition. The white area in the picture is the gold capsule material.
Constraining Diffusivities for Geospeedometry in Magmatic Systems
The technique of geospeedometry is founded on the idea that chemical diffusion of elements and/or isotopes within a crystal, or between a crystal and its matrix, will capture information about the duration of a geologic process as long as the host material has a predictable initial composition that is out of chemical equilibrium with its surroundings. Thus the power of geospeedometry lies in its potential to document rates of processes over a wide range of timescales in any rock that contains chemically zoned minerals. These techniques have been applied to a wide range of geologic problems including the cooling of meteorites (e.g., Yang et al. 2010; Goldstein et al. 2014; Beck et al., 2005; Van Orman et al. 2014), the exhumation of metamorphic provinces (e.g., Lasaga et al., 1977; Ducea et al., 2003; Ganguly et al., 2010) and the tempo of magmatic processes (e.g., Zhang, 1994; Costa et al., 2003; Druitt et al. 2012; Cooper and Kent, 2014;). These studies require experimental investigation into the diffusivities of the elements of interest in the minerals of interest (e.g., Watson and Dohmen, 2010). Diffusivity measurements for elements and isotopes that commonly exhibit zoning in minerals found in mafic to silicic magma compositions is an active area of experimental research (e.g., Cherniak and Watson, 2001; Brady and Cherniak, 2010; Müller et al., 2013; Johnson and Rossman, 2013; Faak et al. 2013; Padrón-Navarta et al., 2014). In addition to diffusion, crystal growth is another kinetic process that can be used to interpret the rates of geologic processes by studying its effects on crystal structure, melt inclusion incorporation and crystal chemistry, for example (e.g., Brugger and Hammer, 2015; Watson and Liang, 1995; Lofgren and Russell, 1986).
Constraints on the Origins of Magmas from Other Planets
Similar to how we are able to understand mantle melting processes by experimentally re-engineering primitive terrestrial basalts, we can also learn something about the melting processes and conditions in other planets by conducting experiments on the composition of volcanic rocks measured on planetary surfaces. Several decades of experimental work on the mare basalts and high-Ti glasses have refined our understanding of the origin of the volcanic and plutonic samples brought back by the Apollo missions (e.g., Elardo et al., 2011; Krawczynski and Grove, 2012). Similar experiments have been conducted to identify the melting scenarios appropriate for Martian meteorites (e.g., Agee and Draper, 2004; Johnson et al., 1991). And now we are able to use analyses from the Martian rovers and other remote sensing missions to collect compositions that can be used for experimental studies (e.g., Monders et al., 2010; Filiberto and Treiman. 2009). For example, measurement of major element ratios by the MESSENGER spacecraft were used to estimate the chemical composition of lavas on the surface of Mercury and experimental investigations of these samples suggest they formed at pressures less <10 kbar near Mercury’s crust-mantle boundary by melting two different mantle lithologies (Charlier et al., 2012; Vander Kaaden et al., 2014). Also experimenters are starting to look outside the solar system, where extra-solar planets may shed light on silicate liquid-vapor equilibrium (e.g., Fegley and Schaefer, 2014) and will likely be relevant to interpret isotopic anomalies in our early solar system (e.g., Richter et al., 2002; Pahlevan et al., 2011).
New Directions All The Time
The field of experimental petrology and geochemistry is constantly pushing new boundaries, collaborating with a wide variety of analysts, using experiments hand in hand with first principles simulations, doing experimental work at synchrotron facilities, and using data collected by remote sensing planetary missions, to name just a few. Other new exciting work includes:
- Experiments combined with molecular dynamics theoretical studies (e.g., Lacks et al., 2012)
- Oxygen fugacity and oxidations mechanisms (e.g., Crabtree and Lange, 2012; Trail et al., 2012; Sharp et al., 2013)
- Trace element behavior during melting (e.g., Matzen et al., 2013)
- Radiogenic isotopic evolution of the mantle (e.g., Bennett and Brenan, 2013)
- Experimental constraints on core formation (e.g., Shahar et al., 2015; Hin et al., 2014)
- Phase transformations at high pressures and temperatures for applications to super-Earths (e.g., McWilliams et al., 2012)
The field is so dynamic we could never hope to capture all of the exciting ongoing science here. We apologize that we only have space to cover a few topics and our list is not meant to be exhaustive or evaluative. To find experimental constraints relevant to your research interests you can visit the Library of Experimental Phase Relations (LEPR) here (Hirschmann et al., 2008). And keep your eyes out at your next scientific meeting to see the many exciting ways in which experimental petrology continues to forward our understanding of solar system geochemistry and beyond!
ESPM lab group, from left to right Zoe Lefebvre (UGrad ’16), Yuval Boneh (3rd year Grad), Kelsey Williams (1st year Grad), Phil Skemer, Emily Sexton (UGrad ’15), Mike Krawczynski, Helene Couvy (Research Engineer), Matt Guiang (UGrad ’15), Molly Chaney (UGrad ’16).
The members of the Experimental Petrology and Igneous processes Center at Arizona State University from left to right: Sarah Cichy (Postdoctoral Fellow), Michael Huh (Laboratory Manager), Meghan Guild (PhD Student), Christy Till (Principal Investigator/Faculty) and Kara Brugman (PhD Student).
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