Evidence for lithium and boron from star‐forming regions implanted in presolar SiC grains

Abstract

Abstract-We report the first measurements of lithium and boron isotope ratios and abundances measured in "gently separated" presolar SiC grains. Almost all analyses of presolar SiC grains since their first isolation in 1987 have been obtained from grains that were separated from their host meteorite by harsh acid dissolution. We recently reported a new method of "gently" separating the grains from meteorites by using freeze-thaw disaggregation, size, and density separation to retain any nonrefractory coatings or alteration to the surfaces of the grains that have been acquired in interstellar space. Nonrefractory coats or amorphized surfaces will almost certainly be removed or altered by the traditional acid separation procedure. High Li/Si and B/Si ratios of up to ~10 -2 were found implanted in the outer 0.5 μm of the grains dropping to ~10 -5 in the core of the grains. 7 Li/ 6 Li and 11 B/ 10 B ratios indistinguishable from solar system average values were found. Analyses obtained from SiC grains from the acid dissolution technique showed isotope ratios that were the same as those of gently separated grains, but depth profiles that were different. These results are interpreted as evidence of implantation of high velocity (1200-1800 km s -1 ) Li and B ions into the grains by shock waves as the grains traveled through star-forming regions some time after their condensation in the outflow of an AGB star that was their progenitor. The results are in line with spectroscopic measurements of Li and B isotope ratios in star-forming regions and may be used to infer abundances and isotopic sources in these regions.

Trace element depth profiles in presolar silicon carbide grains

“…(2009) measured Li abundances as low as approximately 10 −8 . In contrast, Lyon et al. (2007) measured Li/Si up to approximately 10 −2 in the outer regions of both acid and gently separated SiC grains.…”

“…This is likely due to an insufficient deconvolution of the 28 SiH + interference from the 29 Si‐peak. Lyon et al. (2007) showed that the presence of the 28 SiH + interference can add a 20–30‰ uncertainty to the δ 29 Si value when using the peak deconvolution technique.…”

“…1996; Truelove and McKee 1999). Lyon et al. (2007) argued that the relative uniformity of 7 Li/ 6 Li approximately 12 recorded within a range of extra‐terrestrial samples (e.g., Sephton et al.…”

“…The isotope ratio is given by the scaling factor between the two peaks. The presence of the 28 SiH + interference has previously been shown to add 20–30‰ uncertainty to the δ 29 Si value (Lyon et al. 2007).…”

“…There is evidence that some presolar SiC grains did experience ion implantation in the ISM. Lyon et al. (2007) presented Li and B isotopic compositions, and Li/Si and B/Si abundance ratios in eleven acid‐extracted and ten pristine (isolated using the gentle separation procedure of Tizard et al.…”