Nucleosynthetic vanadium isotope heterogeneity of the early solar system recorded in chondritic meteorites

Abstract

Vanadium (V) isotopes have been hypothesized to record irradiation processes in the early solar system through production of the minor 50V isotope. However, because V only possesses two stable isotopes it is difficult to distinguish irradiation from other processes such as stable isotope fractionation and nucleosynthetic heterogeneity that could also cause V isotope variation. Here we perform the first detailed investigation of V isotopes in ordinary and carbonaceous chondrites to investigate the origin of any variation. We also perform a three-laboratory inter-calibration for chondrites, which confirms that the different chemical separation protocols do not induce V isotope analytical artifacts as long as samples are measured using medium resolution multiple collector inductively coupled plasma mass spectrometry (MCICPMS). Vanadium isotope compositions (51V/50V) of carbonaceous chondrites correlate with previously reported nucleosynthetically derived excesses in 54Cr. Both 51V and 54Cr are the most neutron-rich of their respective elements, which may suggest that pre-solar grains rich in r-process isotopes is the primary cause of the V-Cr isotope correlation. Vanadium isotope ratios of ordinary chondrite groups and Earth form a weaker correlation with 54Cr that has a different slope than observed for carbonaceous chondrites. The offset between carbonaceous and non-carbonaceous meteorites in V-Cr isotope space is similar to differences also reported for chromium, titanium, oxygen, molybdenum and ruthenium isotopes, which has been inferred to reflect the presence in the early solar system of two physically separated reservoirs. The V isotope composition of Earth is heavier than any meteorite measured to date. Therefore, V isotopes support models of Earth accretion in which a significant portion of Earth was formed from material that is not present in our meteorite collections.