Affiliation: Lunar and Planetary Laboratory, The University of Arizona, 1629 E University Blvd, Tucson, AZ 85721-0092, USA
Presolar, or stardust, grains are small (typically nanometers to a few microns in diameter) amorphous or crystalline dust particles that condensed in the outflows of asymptotic giant branch (AGB) stars and supernova explosions that predate the Sun. These grains were a part of the materials contained within the giant molecular cloud that collapsed to form the solar protoplanetary disk, and, having survived the formation of the solar system, are found within the fine-grained materials (e.g., inter-chondrule matrix, fine-grained chondrule rims (FGRs)) of primitive astromaterials, including meteorites (Figure 1). Due to their origins in previous generations of stars, presolar grains provide a unique opportunity to study the thermonuclear processes and thermodynamic conditions occurring within stars and stellar explosions in the laboratory. As remnants of the original materials in the solar protoplanetary disk, the study of presolar grains can greatly inform our understanding of the conditions and dust grain distribution during the formation of the Solar System. Furthermore, their presence in meteorites can help place constraints on the secondary processes that occurred on asteroids, such as aqueous alteration.
Due to their small size and physical resemblance to the surrounding material, identifying presolar grains within meteorites is a challenging process that has been described as “finding a needle in a haystack, where the hay looks like needles”. However, presolar grains can be identified from the surrounding material by their extreme isotopic anomalies, relative to all Solar System materials, that result from the unique nuclear processes in their parent star. These anomalies are so extreme that they cannot be explained by processes that occur within the Solar System and must be nucleosynthetic in origin. For example, within the Solar System the less abundant oxygen isotopes, 17O and 18O, show a maximum variation of ~100‰ (“per mille”, parts per thousand difference) relative to the more abundant 16O, while the same isotopes in presolar grains can vary in abundance by upwards of 100,000‰. Two primary methods are used to identify these characteristic anomalies. The first uses an analytical technique called “Nanoscale Secondary Ion Mass Spectrometry” (NanoSIMS) to collect isotopic maps of fine-grained materials at high spatial resolution (~50 to 100 nm) and identify anomalies in-situ (Figure 2). This preserves the petrologic context of the surrounding material and allows for easy calculations of presolar grain abundances, both of which are critical for comparative studies of presolar grains between different samples. The second method involves dissolving samples in harsh acids to leave a residue consisting only of robust phases, and then using SIMS analysis to identify grains that are presolar. This second technique is often described as “burning down the haystack to find the needle”, as it destroys the surrounding petrologic context, but results in a higher concentration of more durable presolar grain phases and can yield more accurate isotopic measurements, due to a lack of contribution from the surrounding material. For this reason, grains identified in acid residues are often used in studies that focus on constraining nucleosynthetic processes and the stellar origins of presolar grains.
Thus far, we have identified a variety of presolar grain phases, with oxygen- (silicates and oxides) and carbon-rich (graphite, silicon carbide, nanodiamonds) grains being by far the most common. The different presolar grain phases having varying degrees of susceptibility to thermal metamorphism and aqueous alteration. For this reason, the abundance of presolar grains can be used as an indicator of the degree of secondary processing on meteorite parent bodies. In particular, presolar silicates, the most abundant of the O-rich PSGs, are highly sensitive to aqueous processing. As the degree of aqueous alteration increases, the abundance of O-rich PSGs in a sample progressively decreases due to the destruction and O-isotopic dilution of presolar silicates. As a result, the abundance of O-rich presolar grains is the highest in petrologic type 3 chondrites, with abundances of over 200 ppm, and are almost non-existent in petrologic type 1 materials. Furthermore, localized differences in the degree of aqueous alteration can lead to significant variations in the O-rich presolar grain abundance within a sample. Presolar SiC and oxide grains are more resistant to aqueous alteration and are found in even the most aqueously altered astromaterials, including CI1 chondrites, but are found in lower abundance within heated astromaterials. For this reason, with detailed petrologic context, a comparison of presolar grain abundances between meteorites and within different lithologies of the same meteorite provides insight into the alteration histories of their parent-bodies, as well as the distribution of presolar grains in the solar protoplanetary disk. However, previous in situ studies of presolar grains have mostly focused on less altered samples. For example, only a single study has included an in-situ survey of presolar grains in CI1 chondrites. This lack of data on presolar grain abundances in aqueously altered samples significantly inhibits our ability to make such comparisons.
My research is focused on addressing this knowledge gap through in-situ investigations of presolar grains in aqueously altered carbonaceous chondrites, particularly focusing on meteorites that share similarities with samples of asteroid Bennu returned by NASA’s OSIRIS-REx mission, such as the C2-ungrouped chondrites Tarda and Tagish Lake. The objectives of my research are focused around improving our understanding of how aqueous alteration affects the mineralogy, chemical composition, structure, and abundance of different presolar grain phases. To do this I employ what we refer to as “coordinated analyses”, which utilizes several separate, but complementary, analytical techniques to characterize the sample mineralogy/petrology, identify presolar grains, and study individual presolar grains.
The first task in my analytical approach involves a detailed investigation into the mineralogy and petrology of the sample using optical light microscopy, scanning electron microscopy (SEM), and electron probe micro-analysis (EPMA). I begin this task by using a digital microscope to collect optical images under different lighting conditions (reflected, transmitted, cross-polarized light), a tabletop SEM to collect back-scattered electron (BSE) images of the sample, and EPMA to collect X-ray elemental maps of the entire meteorite thin sections. These images and maps help me to begin identifying different lithologies and act as a guide for selecting areas for quantitative chemical analysis of minerals and fine-grained matrix using EPMA. The local matrix composition and mineral chemistry obtained from EPMA analysis provides a quantitative means for understanding the mineralogic and petrologic variations between different lithologies. Collectively, this information is used to identify multiple locations with varying degrees of alteration to carry out my presolar grain search and provides the petrologic context that is necessary to interpret presolar grain abundances.
My second task is to search for isotopically anomalous presolar grains in locations selected from the data collected during the first phase of my routine. To do this, I use NanoSIMS to collect carbon (12C,13C) and oxygen (16O,17O,18O) isotopic maps and secondary electron images from 10 μm × 10 μm squares of fine-grained matrix at a time. The simultaneous collection of both carbon and oxygen isotopes allows me to identify the vast majority of presolar grain phases from a single data set. This is critical, as at least 10,000 μm2 of material (or 100 images) need to be mapped to achieve a statistically meaningful data set, which requires several weeks of constant analysis to collect. I can then produce isotopic ratio maps from these analyses, such as the 12C/13C ratio map in Figure 2. From these ratio maps, I identify presolar grains as isotopically anomalous hotspots with isotopic compositions that differ from the surrounding material by at least 5σ, where σ is the measurement uncertainty. I can then calculate an abundance of presolar grains by normalizing the total area occupied by anomalies to the total area mapped in each area.
The third and final task in my coordinated analysis is to directly study the mineralogy, structure, and composition of individual presolar grains, which can provide valuable information on the conditions under which the grains condensed, as well as how the grains have been affected by aqueous alteration. This requires the use of an instrument known as a focused-ion-beam (FIB)
to carefully extract cross-sections of individual presolar grains (Figure 3), which must be thinned to electron transparency (~100 nm). The extraction of FIB-cross sections is an inherently destructive technique, which requires a significant amount of time and care to mitigate the risk of sample damage. For this reason, I prioritize the preparation of cross-sections the most unique grains in a data set, based on size, shape, and isotopic composition. I am then able to analyze the cross-sections using a transmission electron microscope (TEM). The TEM is a very powerful analytical tool, which is capable of achieving imaging resolution down to the atomic scale and provides a wealth of structural and chemical information through different imaging and spectroscopic techniques. For presolar grain analyses, I primarily utilize scanning-TEM (STEM) and TEM imaging, energy dispersive X-ray spectroscopy (EDS) to obtain chemical information, and collect electron diffraction patterns to identify specific mineral phases (Figure 4). This approach allows me to investigate the presolar grain, the surrounding matrix, and any interactions that exist between the two at the nanometer scale. From this I can begin to identify the characteristics of the presolar grain that may be the result of alteration, such as the presence of an Fe-rich rim.
In summary, the coordinated study of presolar grains can provide us with valuable insights into several distinct processes and environments. The isotopic signatures and primary mineralogy of the presolar grains grants a unique opportunity to investigate the conditions under which they formed in their parent star, while the abundance of presolar grains in a sample, when combined with mineralogic and petrologic context of their host meteorite, can reveal details about the degree of secondary processing on asteroids. By continuing this work on presolar grains in aqueously altered meteorites with similarities to samples collected from asteroid Bennu, we can form a basis for comparing presolar grain abundances between meteorites and pristine asteroid samples, which in turn can assist in understanding genetic relationships between meteorites and asteroids.
Acknowledgement: This work is funded by the University of Arizona Research, Innovation and Impact Office and Arizona Technology and Research Initiative Fund (PI: Haenecour). All analyses were carried out in the University of Arizona’s Kuiper-Arizona Laboratory For Astromaterials Analysis (K-ALFAA), which is funded by NASA grants #80NSSC23K0327, #NNX12AL47G, #NNX15AJ22G and #NNX07AI520, and NSF grants #1531243 and #EAR-0841669. I would like to express my gratitude towards my advisor Pierre Haenecour and to Jess Barnes for their continued support and guidance as I carry out this work. Additionally, I would like to acknowledge NanoSIMS lab manager Eli Bloch, TEM lab manager Yao-Jen Chang, FIB-SEM manager Zoë Zeszut, and EPMA lab manager Ken Domanik for their assistance with the instrumentation.
Lastly, I would like to express my utmost gratitude to the International Meteorite Collectors Association and the Brian Mason Travel award for supporting my travel to the 86th Annual Meeting of the Meteoritical Society, which was held in Brussels, Belgium. Attending this meeting was extremely beneficial for my professional development, and the support I received allowed me to attend this conference at a very critical time in my graduate career.
