Yes, we can sample the stars! Presolar grain samples isolated from their primitive host meteorites, preserve a record of their stellar progenitors including stellar condensation conditions (pressure, temperatures, compositions) and nucleosynthetic processes. As the only physical material from other stars available for study, presolar grains provide unique and critical constraints on our understanding of stars and the formation of the elements.

Presolar grains (PSGs) are small, with the largest grains generally less than a few microns in size. Since each grain can have origins in a different star and/or at different stages of stellar evolution, each grain must be analyzed and interpreted individually. Due to the small size of these samples, however, analytical techniques are limited and often result in the eventual destruction of the grain. Some of my current research has focused on presolar SiC grains, and their non-destructive analytical characterization prior to the application of more destructive techniques. This includes scanning electron microscopy (SEM), electron microprobe analyses (EPMA) and microbeam synchrotron X-ray fluorescence (SXRF) experiments, the latter of which can quantify a wide range of trace elements present in individual grains. Synchrotron experiments are conducted using the synchrotron beam lime at the Advanced Photon Source (Argonne National Laboratory) with the GeoSoilEnviro- Consortium for Advanced Radiation Sources (GSE-CARS) group. We are following this work up with isotopic (destructive) determinations of these same individual grains using RIMS (see below) to determine the degree of s-process nucleosynthesis and other nucleosynthetic signatures. We can then begin to deconvolve the condensation and nucleosynthetic contributions to grain compositions, and interpret these data to better understand stellar parent conditions and the formation of elements in stars.

Resonant ionization mass spectrometry (RIMS) is a technique only recently applied to problems in cosmochemistry. Thus far, RIMS has been used to characterize isotopic compositions of select trace elements including Ba, Mo and Zr in presolar grains, allowing the detection of preserved nucleosynthetic signatures of parent stars (r- and s-processes), as well as supernovae signatures. We are currently developing RIMS methods aimed at the determination of B, Be and Li in early solar system materials such as Ca-Al-rich inclusions (CAIs) and chondrules using two RIMS instruments (CHARISMA and SARISSA) designed, built and optimized at Argonne National Laboratory for the determination of trace element isotopic compositions with high spatial resolution.

¹⁰Be and ⁷Be are both are short-lived radionuclides (half lives of 1.5 Ma and 53 days, respectively) primarily produced through energetic particle irradiation. While heterogenous initial incorporation of Be and Li into early solar system condensates such as Ca-Al-rich inclusions (CAIs) and chondrules may prevent them from utility as early solar system chronometers (in contrast to other short-lived nuclides such as ²⁶Al), the presence of the decay products of ¹⁰Be and ⁷Be provides some of the only records of irradiation conditions in the early solar system, and can improve our understanding of the evolution of the proto sun and surrounding environments. RIMS offers the prospect of smaller spacial resolution in samples (important for samples such as finer grained CAIs) without a loss of analytical precision. We are also exploring simultaneous RIMS determinations of two and three elements from the same sputtered volume.

Ca-Al rich inclusions (CAIs), relicts of the earliest condensates of our solar system, are a unique record of early solar system conditions and processes. Several studies have demonstrated that CAIs likely survived significant post-condensation heating event(s) prior to incorporation into their host meteorites, and that these processes caused significant evaporation of the pristine CAIs as well as alteration of their original elemental and isotopic compositions. Understanding the extent of evaporation-driven fractionation in CAI-like systems is of paramount importance for interpretation of these condensates. Following studies quantifying evaporation-driven kinetic fractionation of Mg isotopes in CAI-composition glasses, we obtained complimentary Si isotope data from the CAI-composition glasses through development of Si isotope secondary ionization mass spectrometry (SIMS) at the University of Wisconsin-Madison.

We are currently applying Si and Mg isotope measurements with SIMS to the study of isotopic fractionation within specific CAIs. Determination of the fractionation of and relationship between Mg and Si within individual CAIs, alongside an understanding of the fractionation behavior of these elements in CAI-like melts permits us to better constrain the post-condensation processes which affected these materials, and thus CAI processing in the early solar system. We can also begin to realistically reconstruct and interpret the original isotopic compositions of these earliest solar system records.

P. Renne (2000) demonstrated that measurement of accumulated cosmogenic ³⁸Ar is possible in terrestrial samples, and that ³⁸Ar is viable as a cosmogenic nuclide. Cosmogenic argon is an important addition to the family of applied cosmogenic nuclides as it is less prone to diffusive loss than other stable nuclides. Additionally, sample analysis takes advantage of pre-existent instrumentation and theory developed for application of the ⁴⁰Ar/³⁹Ar method. Production mechanisms and rates for cosmogenic argon, as well as the relative contributions from different targets are poorly constrained. This project seeks to isolate mineral-specific production rates, taking advantage of diverse lithology and mineralogy and the significant (1-10 Ma) exposure history of the Dry Valleys of Antarctica. The result will be target-specific production rates that are calibrated by and comparable with presently known production rates for cosmogenic ³He and ²¹Ne.

This project explores the application of 2.5 MeV neutrons to geochronology using a deuteron - deuteron fission reaction in contrast with in traditional ⁴⁰Ar/³⁹Ar dating, which makes use of irradiation in fission reactors. The benefits of 2.5 MeV neutrons, which are energetic enough to drive the ³⁹K(n,p)³⁹Ar reaction, include minimizing the effects of lattice recoil and the elimination of several interfering argon producing corrections. Ongoing developments in fission technologies may allow generation of sufficient neutron fluence to make such advancement possible, significantly broadening the scope of problems uniquely addressable by ⁴⁰Ar/³⁹Ar methods.

The Central Atlantic Magmatic Province (CAMP) is the largest known LIP (Large Igneous Province) erupted in the Phanerozoic prior to the opening of the Atlantic Ocean at the Triassic-Jurassic boundary ca. 200 Ma. Paleomagnetic sampling targeted packages of CAMP basalt flows preserved in Morocco in conjunction with geochemical and paleontological sampling. We evaluated the magnetic polarity and secular variation record of these lava packages, enhancing constraints on their timing and duration. Select samples were additionally dated using the ⁴⁰Ar/³⁹Ar method to place the paleomagnetic record in geologic time. Dating of samples constrain the duration of volcanism to less than 2 Ma, while paleomagnetic results show the presence of at least one brief reversal and a distinct, pulsed eruptive mode.

Bimodal Oligocene volcanism in Yemen, contemporaneous with traps preserved in Ethiopia, has been inferred to be associated with an underlying plume and the rifting of Arabia from Africa, as well as (more speculatively) climate perturbations. Recent dating supports the onset of silicic volcanism in Ethiopia at ca. 30 Ma, contemporaneous with the onset of silicic volcanism in Yemen. We applied the ⁴⁰Ar/³⁹Ar method to feldspar separates from a selection of the Yemen silicic units and several interbedded basalt flows complimenting paleomagnetic investigations of these same units to better constrain the onset and duration of silicic volcanism on the Yemeni margin and clarify the chronology of the Oligocene.

Along the eastern coast of India an isolated outcrop of basalts, the Rajahmundry Traps, are speculatively associated with the Deccan Traps. The timing of emplacement of the Deccan Traps, covering a preserved area of 10⁶ km², has remained a hotly debated topic in part because of their proximity to Cretaceous-Tertiary boundary events. This project completed ⁴⁰Ar/³⁹Ar dating of feldspar separates and geochemical studies designed to pinpoint the timing of the Rajahmundry Traps and explore the possibility of an evolutionary relationship with the more infamous Deccan Traps. The Rajahmundry Trap samples reveal a mean age of 64.7 Ma with a total duration of <2 Ma. Geochemical analyses indicate strong similarities with eastern Deccan Trap flows.