Hf-W isotope systematics of enstatite chondrites: Parent body chronology and origin of Hf-W fractionations among chondritic meteorites
Jan L. Hellmann, James A. Van Orman, Thorsten Kleine
Earth and Planetary Science Letters
Volume 626, 15 January 2024, 118518
“Five enstatite chondrites, including two EH4 (Abee, Indarch) and three EL6 samples (Hvittis, Khairpur, Pillistfer), were investigated for their Hf-W isotope systematics to constrain the chronology and thermal evolution of their parent bodies and to assess the nature and extent of Hf-W fractionations among chondritic meteorites. The Hf-W ages range from ∼4.5 Ma to ∼10 Ma after CAI formation, where Pillistfer exhibits an older Hf-W age (∼4.5 Ma) than the other two EL6 chondrites (∼8.5–10 Ma), probably reflecting rapid cooling after impact excavation. The ∼8 Ma Hf-W age of the EH4 chondrite Indarch overlaps with those of the younger EL6 chondrites, indicating a similar cooling timescale despite the lower metamorphic grade. By contrast, Abee shows evidence for only partial resetting of the Hf-W system during metamorphism, and the 182W composition of Abee’s metal still records the time of chondrule formation at ∼2 Ma after CAI formation. Thus, the thermal and cooling histories of enstatite chondrites do not appear to be a simple function of their metamorphic grade.
Despite their distinct Fe-metal contents, EH and EL chondrites and their precursor material have uniform Hf/W ratios. This most likely reflects the early and proportional removal of refractory metal and silicate components from the enstatite chondrite formation region, which left the Hf/W ratio unchanged. As such, the Hf/W ratio of enstatite chondrites provides a good estimate for the average composition of primitive chondritic material from the inner solar system. All carbonaceous chondrites except CI chondrites have higher Hf/W ratios, reflecting admixture of CAIs or metal-silicate fractionation during chondrule formation. Using the Hf/W ratio of enstatite chondrites, rather than carbonaceous chondrites, in the calculation of Hf-W core formation model ages makes these ages up to ∼0.7 Ma younger, shifting the core formation ages for non-carbonaceous iron meteorites closer to those of carbonaceous iron meteorites.”