Overview: The goal of our research is to understand the mechanisms responsible for stable isotope fractionation at a range of conditions. We are particularly interested in how pressure and temperature effect the isotopic ratios of materials during planetary formation, differentiation and evolution.
Stable isotope geochemistry is the study of how physical and chemical processes can cause isotopic fractionation in natural substances. Experimental petrology is a lab-based approach to increasing the pressure and temperature of materials to simulate natural conditions within the Earth or other planetary bodies. In our current research we combine methods from these two fields in order to determine fractionation mechanisms that will enhance our understanding of how the Solar System evolved prior to planet formation and how planets and planetesimals formed and differentiated.
Some examples of previous and current research are below:
Iron Isotope Fractionation in the Mantle
We developed a method to identify the equilibrium isotopic fractionation of two minerals at high pressure and temperature, using a novel application of the three-isotope technique to piston-cylinder experiments. High temperature iron isotope studies are scarce and there is a lack of experimental studies aimed at understanding the isotopic fractionation at high temperature observed in nature. Our iron isotope research is one of the first studies of high temperature fractionation among non-traditional stable isotopes (Shahar, Young, and Manning, 2008). This experimental approach shows considerable promise for establishing equilibrium fractionation factors in high temperature and pressure experiments. (Shahar et al. 2017 gives a throrough explanation of the techniques used for conducting isotope experiments at high pressure and temperature.)
Isotope Fractionation during Core Formation
One of the broader goals in planetary science is to determine the chemical composition of the constituents of the solar system. Since the 1950’s it has become apparent that chondrites provide the best estimates for the mean abundances of condensable elements in the solar system as they were not (for the most part) affected by physiochemical processes. Since then, many studies have compared the chemical and isotopic compositions of meteorites with that of the Earth in order to better understand processes such as solar system formation and planetary differentiation. Several studies showed that comparisons between meteorites and terrestrial stable isotope ratios can be used to elucidate the composition of the Earth’s core. This is only the case however, if fractionation factors between the minerals of choice are known and at the conditions of core formation. While there are some theoretical calculations for these fractionation factors, experiments are the most effective way of determining them. We have tackled this problem so far by investigating the silicon, sulfur, and iron isotope fractionatoin during core formation. (Shahar et al. 2009, 2011; Labidi et al. 2016)
The Effect of Sulfur and Nickel on Iron Isotope Fractionation
One of the possible light elements in the core of planetesimals/planets is sulfur. While most studies have assumed that redox is the most important variable on iron isotope fractionation we hypothesized that the addition of a light element to the iron metal would alter the isotopic fractionation between the metal and the silicate. We found that there is a strikingly strong dependence of the fractionation factor between metal and silicate with sulfur content in the iron alloy. These results were highly unanticipated but show that the amount of sulfur in the system dramatically changes the bonding environment of the iron in the alloy and in the melt and has a significant effect on the equilibrium iron isotopic fractionation. Another fundamental aspect of this work is that it opens up a new and independent constraint on the sulfur content in planetary or asteroidal cores. (Shahar et al. 2015)
This work was continued and expanded by Steve Elardo who determined that nickel also has a large effect on iron isotope fractionation and published two papers showing how these data can be used to undertand the iron isotopic ratios of planetary mantles throughout the solar system. (Elardo and Shahar, 2017; Elardo et al. 2019)
The Effect of Pressure on Iron Isotope Fractionation
Initial studies in the 1970's and earlier suggested that pressure was not an important variable for understanding isotope fractionation factors. It was thought that pressure was negligible in fractionating isotopes. However in 2016 we performed an extensive study on how pressure effects iron isotopic ratios in a range of metallic alloys. We found that pressure has an important effect on isotopes and that it varies depending on the light elment alloyed with the iron. (Shahar et al. 2016)
Silicon and Magnesium Isotope Fractionation in CAIs
Evaporation and diffusion processes are ubiquitous in both terrestrial and extra-terrestrial settings and give rise to kinetic isotope fractionations. In order to study the fundamental physical chemistry of these mechanisms, it is advantageous to start with a simple system. Calcium-aluminum-rich inclusions (CAIs) in carbonaceous chondrite meteorites are amongst the simplest of natural samples in terms of their chemistry and process of formation. They are particularly valuable for tracing the processes that occurred in the young Solar System. CAIs are thought to be the oldest objects in the Solar System, and they preserve isotopic evidence of processes that lead to formation of rock from gas and dust.
One of the outstanding questions regarding the history of the Solar System is determining the astrophysical environment (i.e. temperature, pressure) in which CAIs formed. The stable isotopic composition of these once-molten objects was controlled by their evaporation history, which is in turn controlled by their astrophysical origin. It is therefore possible to constrain CAI astrophysical origin by exploring the origins of their stable isotope ratios. In order to do this, we developed silicon isotope laser ablation MC-ICPMS at UCLA and used this new technique to obtain silicon isotopic data on several CAIs in chondritic meteorites (Shahar and Young, 2007). Our research helped to constrain the melting history of each object, and more specifically the time and pressure at which each CAI was molten.