Presence of ⁶⁰Fe in eucrite Piplia Kalan: a new perspective to the initial ⁶⁰Fe/⁵⁶Fe in the early solar system

Abstract

Fe-Ni isotope measurements of ferrous pyroxenes of the Piplia Kalan eucrite using Secondary Ion Mass Spectrometer revealed the presence of ⁶⁰Ni excess corresponding to the initial ⁶⁰Fe/⁵⁶Fe of (5.2 ± 2.4) x 10^-9. Combining this ratio with the inferred initial ²⁶Al/²⁷Al ratio of (7.5 ± 0.9) X 10~7 in plagioclase of the Piplia Kalan that has been already reported in the literature suggests initial ⁶⁰Fe/⁵⁶Fe of (5.2 ± 2.4) x 10^-8 in the early solar system, which is significantly lower than the value inferred from Fe-Ni isotope measurements of chondrules in silicates and sulphides from unequili-brated ordinary chondrites. We attribute the difference in the initial ⁶⁰Fe/⁵⁶Fe to be solely due to the closure temperature of Fe-Ni isotope systematics, which is less compared to Al-Mg isotope systematics. With the initial ⁶⁰Fe/⁵⁶Fe values found in the Piplia Kalan, it is possible that the Fe-Ni isotope systematics was disturbed for another ~ 5 Ma (million years) after closure of the Al-Mg isotope systematics. The closure temperature of ~ 450-650°C for Fe-Ni isotope system seems feasible and we anticipate a cooling rate of ~20-60°C/Ma in the crust region of the parent body of Piplia Kalan, thereby matching the initial ⁶⁰Fe/⁵⁶Fe to ~ 5 X 10^-7. This is consistent with the initial value found in the silicate phases in chondrules of least metamorphosed meteorites. However, the presence of ⁶⁰Ni excess in Piplia Kalan does not confirm ⁶⁰Fe to be a major heat source for the early thermal evolution of meteorite parent bodies. Also, it seems that a massive star probably has contributed the two short-lived nuclides, ²⁶Al and ⁶⁰Fe, to the early solar system.