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

摘要

Fe-Ni isotope measurements of ferrous pyroxenes of the Piplia Kalan eucrite using Secondary Ion Mass Spectrometer revealed the presence of super(60)Ni excess corresponding to the initial super(60)Fe/ super(56)Fe of (5.2 c 2.4) x 10 super(-9). Combining this ratio with the inferred initial super(26)Al/ super(27)Al ratio of (7.5 c 0.9) x 10 super(-7) in plagioclase of the Piplia Kalan that has been already reported in the literature suggests initial super(60)Fe/ super(56)Fe of (5.2 c 2.4) x 10 super(-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 unequilibrated ordinary chondrites. We attribute the difference in the initial super(60)Fe/ super(56)Fe 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 super(60)Fe/ super(56)Fe values found in the Piplia Kalan, it is possible that the Fe-Ni isotope systematics was disturbed for another 65 Ma (million years) after closure of the Al-Mg isotope systematics. The closure temperature of 6450-650C for Fe-Ni isotope system seems feasible and we anticipate a cooling rate of 620-60C/Ma in the crust region of the parent body of Piplia Kalan, thereby matching the initial super(60)Fe/ super(56)Fe to 65 x 10 super(-7). This is consistent with the initial value found in the silicate phases in chondrules of least metamorphosed meteorites. However, the presence of super(60)Ni excess in Piplia Kalan does not confirm super(60)Fe 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, super(26)Al and super(60)Fe, to the early solar system.