5:15 PM - 7:15 PM
[MZZ42-P01] U-Pb geochronology of zircons by ID-TIMS on two coarse-grained eucrites
Keywords:Eucrite, ID-TIMS U-Pb dating, Zircon
The Howardite-Eucrite-Diogenite (HED) meteorites, the largest achondrite group, are believed to originate from the asteroid Vesta. They provide key insights into the protoplanetary thermal evolution. Eucrites, representing the upper crust of Vesta, are classified into basaltic and cumulate types, with the former occurring at the surface and the latter underlying them. Basaltic eucrites are enriched in incompatible elements and permit various dating methods, which indicate an upper crust crystallization before 4553 Ma (e.g., Iizuka et al., 2015; Haba & Wotzlaw, 2021). In contrast, chronological studies on cumulate eucrites are limited and suggest potentially younger crystallization ages (e.g., Moore County: 4542 ± 85 Ma; Boyet et al., 2010), restricting the applicability of some short-lived chronometers (e.g., 26Al-26Mg, 53Mn-53Cr). However, constraining the age of the lower eucritic crust is essential for understanding the internal thermal history of Vesta. This study presents high-precision U-Pb zircon ages from two unbrecciated gabbroic eucrites, Northwest Africa (NWA) 11455 and NWA 11247.
This study reports bulk elemental analysis, zircon mineralogical observations, and isotope dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon dating. For whole-rock compositions, powdered samples were dissolved in concentrated HF-HNO3 and analyzed using a triple quadrupole ICP-MS (iCAP TQ) at Science Tokyo. Zircon grains were recovered from acid residues after high-temperature annealing and acid digestion of separate meteorite fragments. Subsequent zircon preparation included a single HF leaching step before complete dissolution and U-Pb separation. The U-Pb dating of single zircon grains was conducted using a Thermo TRITON plus TIMS at ETH Zurich. All ages are reported at the 2σ level.
The whole-rock REE abundances of NWA 11455 and NWA 11247 are significantly lower than those of basaltic eucrites, with positive Eu anomalies resembling the cumulate eucrite Moore County. However, NWA 11247 exhibits notable LREE enrichment, likely due to terrestrial weathering. Zircons in these eucrites reach up to 200 μm, and are significantly larger than those in basaltic eucrites (typically <30 μm). FE-EPMA detected no apparent overgrowth textures or zoning but revealed vein-like features along fractures, suggesting possible reheating after crystallization.
The ID-TIMS U-Pb dating shows that most zircon data plot along the concordia curve. Unlike previous ID-TIMS Pb-Pb dating of basaltic eucrite zircons, which yielded a single concordant age, NWA 11455 and NWA 11247 exhibit significant age dispersion. NWA 11455 (n = 7) ranges from 4540.05 ± 0.16 Ma to 4553.11 ± 0.20 Ma, while NWA 11247 (n = 8) spans 4539.39 ± 0.16 Ma to 4550.59 ± 0.17 Ma. The older ages suggest magma crystallization at the cumulate eucrite formation sites around 4550 Ma, whereas the younger ages reach down to 4539 Ma. These ages combined with zircon microstructural observations indicate one or more reheating events above the solidus temperature (~1060°C) extending well beyond the early crystallization recorded by basaltic eucrites.
According to the thermal evolution model (Neumann et al., 2014), the ~10 km depth where Moore County is thought to have resided (Miyamoto & Takeda, 1992) would have cooled well below the solidus temperature by 4539 Ma. At this depth, a surface impact is unlikely to generate enough heat for zircon crystallization or U-Pb system resetting in cumulate eucrites. However, the large-scale impact at 4525 Ma that formed the mesosiderites (Haba et al., 2019) may have provided both a heat source and a thick regolith, sustaining a prolonged high-temperature state. Alternatively, an early thermally insulating crust on Vesta (Roszjar et al., 2016) may explain extended magmatic activity. If so, this crust with regolith may have formed through early high-frequency impacts associated with gas dissipation in the protoplanetary disk (e.g., Hunt et al., 2022).
This study reports bulk elemental analysis, zircon mineralogical observations, and isotope dilution thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon dating. For whole-rock compositions, powdered samples were dissolved in concentrated HF-HNO3 and analyzed using a triple quadrupole ICP-MS (iCAP TQ) at Science Tokyo. Zircon grains were recovered from acid residues after high-temperature annealing and acid digestion of separate meteorite fragments. Subsequent zircon preparation included a single HF leaching step before complete dissolution and U-Pb separation. The U-Pb dating of single zircon grains was conducted using a Thermo TRITON plus TIMS at ETH Zurich. All ages are reported at the 2σ level.
The whole-rock REE abundances of NWA 11455 and NWA 11247 are significantly lower than those of basaltic eucrites, with positive Eu anomalies resembling the cumulate eucrite Moore County. However, NWA 11247 exhibits notable LREE enrichment, likely due to terrestrial weathering. Zircons in these eucrites reach up to 200 μm, and are significantly larger than those in basaltic eucrites (typically <30 μm). FE-EPMA detected no apparent overgrowth textures or zoning but revealed vein-like features along fractures, suggesting possible reheating after crystallization.
The ID-TIMS U-Pb dating shows that most zircon data plot along the concordia curve. Unlike previous ID-TIMS Pb-Pb dating of basaltic eucrite zircons, which yielded a single concordant age, NWA 11455 and NWA 11247 exhibit significant age dispersion. NWA 11455 (n = 7) ranges from 4540.05 ± 0.16 Ma to 4553.11 ± 0.20 Ma, while NWA 11247 (n = 8) spans 4539.39 ± 0.16 Ma to 4550.59 ± 0.17 Ma. The older ages suggest magma crystallization at the cumulate eucrite formation sites around 4550 Ma, whereas the younger ages reach down to 4539 Ma. These ages combined with zircon microstructural observations indicate one or more reheating events above the solidus temperature (~1060°C) extending well beyond the early crystallization recorded by basaltic eucrites.
According to the thermal evolution model (Neumann et al., 2014), the ~10 km depth where Moore County is thought to have resided (Miyamoto & Takeda, 1992) would have cooled well below the solidus temperature by 4539 Ma. At this depth, a surface impact is unlikely to generate enough heat for zircon crystallization or U-Pb system resetting in cumulate eucrites. However, the large-scale impact at 4525 Ma that formed the mesosiderites (Haba et al., 2019) may have provided both a heat source and a thick regolith, sustaining a prolonged high-temperature state. Alternatively, an early thermally insulating crust on Vesta (Roszjar et al., 2016) may explain extended magmatic activity. If so, this crust with regolith may have formed through early high-frequency impacts associated with gas dissipation in the protoplanetary disk (e.g., Hunt et al., 2022).