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Dust grains that formed around ancient stars and in stellar explosions seeded the early solar protoplanetary disk. While most of such presolar grains were destroyed during solar system formation, a fraction of such grains were preserved in primitive materials such as meteorites. These grains can provide constraints on stellar origins and secondary processing such as aqueous alteration and thermal metamorphism on their parent asteroids. Here, we report on the nature of aqueous alteration in the Miller Range (MIL) 07687 chondrite through the analysis of four presolar silicates and their surrounding material. The grains occur in the Fe‐rich and Fe‐poor lithologies, reflecting relatively altered and unaltered material, respectively. The O‐isotopic compositions of two grains, one each from the Fe‐rich and Fe‐poor matrix, are consistent with formation in the circumstellar envelopes of low‐mass Asymptotic Giant Branch (AGB)/Red Giant Branch (RGB) stars. The other two grains, also one each from the Fe‐rich and Fe‐poor matrix, have O‐isotopic compositions consistent with formation in the ejecta of type‐II supernovae (SNe). The grains derived from AGB/RGB stars include two polycrystalline pyroxene grains that contain Fe‐rich rims. The SNe grains include a polycrystalline Ca‐bearing pyroxene and a polycrystalline assemblage consistent with a mixture of olivine and pyroxene. Ferrihydrite is observed in all focused ion beam sections, consistent with parent‐body aqueous alteration of the fine‐grained matrix under oxidizing conditions. The Fe‐rich rims around presolar silicates in this study are consistent with Fe‐diffusion into the grains resulting from early‐stage hydrothermal alteration, but such alteration was not extensive enough to lead to isotopic equilibration with the surrounding matrix.

This paper reports a systematic study of the collection efficiency of an X-ray energy dispersive spectrometer (EDS) equipped on a newly installed 200 kV TEM/STEM. Understanding of the collection efficiency is critical to quantitative compositional measurements, and several factors affect the overall collection efficiency. Solid angle is arguably the most important parameter, which describes the angular extent of X-rays emitted by a point source and collected by the detector system [1][2]. Once a particular EDS system is installed on a specific TEM, the solid angle remains a constant. However, there are several variables that affect the performance, such as the specimen holder penumbra (shadowing), specimen eucentric height, probe current, accelerating voltage, and number and location of the detectors used (in the case of multiple-detector configuration).Herein, we evaluate the collection efficiency of the twin Oxford X-Max 100TLE EDS system on the probe-corrected Hitachi HF5000 TEM/STEM at the University of Arizona. Shown in Figure 1, two rectangular-shaped 100 mm 2 window-less X-Max 100TLE detectors are oriented normal to the axis of the specimen holder, 180 o apart. Following Zaluzec formula [2], the solid angle of each detector is calculated to be 0.85 sr, providing a total angle of 1.70 sr for the twin configuration. We measured the penumbra of the Gatan 646 double-tilt low-background (Be) holder using the EDS "Test specimen" recommended by Zaluzec [3]. The Test Specimen is a 20 nm Ge/20 nm SiN/20 nm thick membrane (500 µm x 500 µm) with arrays of 2 µm-diameter holes on a Si wafer (TEMwindows/SiMPore Inc.) [3]. With the known thickness, using N K, Si K, Ge K and L peaks, this specimen can be used to calibrate the energy resolution over a wide range. It can also be used to monitor the possible deterioration due to detector contamination over time. We undertook all the measurements at 200 kV at a probe current of 1.40 nA unless otherwise stated. Fig. 2 plots the Ge Kα peak intensity as a function of the α-tilt from -10° to +10° while using Detector 1 alone (positive tilt is in the clockwise direction towards Detector 1) and in twin configuration. It shows that using only Detector 1, the penumbra decreased linearly with increasing α-tilt. The counts almost doubled when tilted from -10 to +10 °. However, the penumbra profile in the twin-detector setting is much flatter throughout out the range, with less than 10% intensity variation. We are going to further investigate the penumbra in the single and twin-configuration as a function of the full α and β tilt range. We also acquired EDS measurements at various probe currents. Fig. 3 presents the Ge Kα peak intensity as a function of the probe current. The X-ray counts increased linearly with the probe current except the first data point. We will repeat the experiment and verify whether it was a measuring error. In summary, the findings from this study will enable us to optimize the experimental setup for quantitative X-ray analysis at its maximal collection efficie...

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