Overview: The goal of our research is to understand the mechanisms responsible for stable isotope fractionation. 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.
Silicon 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.
It has been known for some time that seismic velocities are inconsistent with an outer core made of pure iron and nickel and that it is hypothesized that there is at least one light element in the core that accounts for the seismic deficit. Silicon is one of the elements that has been considered to partially reside in the core, based on mineral physics experiments showing that silicon alloys with iron easily at reduced conditions and cosmochemical arguments based on the sub-chondritic Mg/Si ratio on Earth.
Previous studies tested the idea that there is substantial Si in the core by comparing the Si isotopic composition of Earth’s mantle to chondrites. Most researchers have found that the Earth has a higher 30Si/28Si than chondrites and concluded that the difference is a consequence of Si fractionation between core and mantle. In order to test this idea we conducted the first silicon isotope experiments at high P/T. Our results showed that there is a ~2‰ 30Si/28Si fractionation between iron metal (core analog) and silicate (mantle) resulting in ~4 wt% Si in the core.
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 has helped to constrain the melting history of each object, and more specifically the time and pressure at which each CAI was molten. The figure below shows the calculated time-pressure paths for evaporation of two igneous CAIs and a FUN inclusion, along with the relevant astrophysical environment.
The Effect of Sulfur 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. To address this possibility, we began a project that studied the effect of sulfur in this system. With the help of a college intern, Juliana Garcia-Mesa, we conducted many experiments in both the piston cylinder and multi-anvil apparatus with varying concentrations of sulfur in the iron alloy.
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. The similarity in Fe content of the glasses and metals in all the experiments suggests that all experiments have nearly the same fO2 as demonstrated by a simple calculation comparing the mole fraction of FeO in the silicate to the mole fraction of Fe in the metal. While fO2 has been argued to be the leading mechanism for iron isotopic fractionation at high temperature, in this case, the addition of a light element into the metal seems to be causing the fractionation. It is extremely fascinating that one mechanism that can cause iron isotope fractionation is simply the addition of a light element to the alloy. 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. By measuring the iron isotope ratios in iron meteorites or Martian meteorites, for example, we hope to be able to calculate the amount of sulfur that is in the core.
The Effect of Pressure on Iron Isotope Fractionation
The Effect of Nickel on Iron Isotope Fractionation