Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in 26Mg and 44Ca, which can be attributed to the in situ decay of radioactive and now extinct 26Al and 44Ti, respectively. Previous work on presolar SiC and graphite had shown that 26Al/27Al and 44Ti/48Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. 26Mg/24Mg and 44Ca/40Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial 26Al/27Al and 44Ti/48Ti ratios of approximately 1 × 10−3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The 26Al/27Al and 44Ti/48Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred 26Al/27Al and 44Ti/48Ti ratios considerably.