Search

Primitive meteorites preserve remnants of evolved stars as presolar grains, from asymptotic and red giant branch stars, novae and supernovae. These grains retain records of the formation mechanisms of the stellar environment from which they condensed, as well as physical and chemical processes in the interstellar medium (ISM), solar nebula and the parent bodies in which they are found. Consequently, their study can refine our understanding of the solar system and external stellar and planetary systems. However, presolar grains and their records can be altered, re-equilibrated with the surrounding matrix, and eventually lost through aqueous and thermal processing on the meteorite parent body. NanoSIMS is the most rapid and effective method for in situ identification of presolar grains via their highly anomalous isotopic ratios. Previous studies have measured variations in presolar grain populations between different matrix regions within the same meteorite thin section. However, most of these studies do not identify if there are any ties between these variations in presolar grain populations and the chemical and mineralogical differences in measured matrix regions. Therefore, the goal of this study is to determine if there is measurable variations in the degree of alteration of different matrix regions within the same thin section, and if mineralogical and chemical changes in matrix regions can be used as indicators for presolar grain preservation. Such a correlation could improve the efficiency of presolar grain identification during searches across large surface areas. To do this, we carried out coordinated scanning electron microscope (SEM) and NanoSIMS analyses, using a new SEM method which measures the ppm concentration of 100 nm to 5 μm sized Mg-, S-, Al-, C- and Ca-rich matrix grains.

HIDALGO, a ~300×300 μm, hexagonally shaped, chemically and mineralogically simple hibonite crystal residing in the matrix of the CO3 chondrite Dar al Gani 027 (DaG027), is the newest member in the FUN-hibonite family. Based on the mass-dependent fractionations in O, Ca, and Ti isotopes, large negative anomalies in δ48Ca (= −30‰) and δ50Ti (= −20‰), and the abundances of short-lived radioisotopes of 10Be, 26Al, 41Ca, which have all been reported previously, we have inferred that HIDALGO evolved from a condensate that most likely formed before 26Al was built up and homogenized to 26Al/27Al = 5.2×10−5 in the solar system. This precursor also incorporated 10Be at the level of 10Be/9Be = 7.5×10−4, possibly through local irradiation of gas and/or dust or inheritance from the molecular cloud, but the amount of 41Ca present at that time and location is uncertain. Strong heating of the precursor occurred ~0.5 Myr after the initial condensation, leading to total melting accompanied by extensive distillation (losing Mg, Si, and other moderately volatile elements) resulting in a stoichiometrically pure hibonite crystal with strongly mass fractionated O, Ca, and Ti isotope compositions and an extreme Ce depletion relative to other REEs. Here we report the results of Li isotope analyses of HIDALGO and discuss the irradiation history of this inclusion.

Introduction: The NASA Stardust mission captured thousands of particles from the Jupiter-family comet 81P/Wild 2 in a collector composed of aluminum foil and blocks of silica aerogel [1]. To date, most Wild 2 particles available for study are relatively large and coherent particles extracted individually from the ends of hollow, carrot-shaped impact tracks produced during impact into aerogel. However, >65% of the impacting mass can be found in the ‘bulb’ of the track, including nearly all of the <1 μm size fraction [2]. This fraction contains organic-rich material and is likely to include presolar grains, representing a critical opportunity to constrain the organic and presolar inventory of primitive outer solar system materials. However, the small size and susceptibility of this fraction to melting or alteration during capture poses significant analytical challenges. Previous attempts to extract and concentrate fine-grained material from the bulb of Stardust tracks have attempted to develop techniques that efficiently destroy aerogel and leave impactor particles relatively unharmed. The low density and high porosity of silica aerogel makes it more susceptible to attack by etching with hydrofluoric acid (HF) [3] or CF4 plasma ashing [4] than collected cometary silicates. Previous studies of HF vapor etching used HF solutions varying between 5 to 49% and noted that at high concentrations and etch rates, a liquid droplet was produced according to the etching reaction 4HF + SiO2 → SiF4 (g) + 2H2O [3]. SiF4 readily decomposes into HF and silicic acid in water, which has the potential to alter the freed cometary silicates. The plasma ashing technique avoids production of a liquid droplet and minimizes damage to embedded silicates but requires specialized equipment [4]. In this abstract, we present recent improvements to the HF etching procedure with the aim of constructing an etching chamber capable of slowly etching silica aerogel using small quantities of dilute HF. The etching chamber can be assembled using readily available materials.