[S-CG57] Hybrid Geochronology from Date to Age

[SCG57-02] Elucidation of the thermal history of parent asteroid based on textural observation and in-situ U–Pb dating of winonaite meteorites

*Ryosuke Sakai¹, Mizuho Koike¹, Ito Kengo², Ben G Rider-Stokes³, Takafumi Hirata² (1.Hiroshima University, 2.Geochemical Research Center, The University of Tokyo, 3.The Open University)

Keywords:Winonaite, U-Pb dating, Thermal history

Winonaites are a group of primitive achondrites that exhibit chondritic elemental compositions and partially melted textures. The winonaites may have originated from the same parent body as the IAB iron meteorites, due to reductive mineral compositions and similar oxygen isotope ratios (Benedix et al., 2000). These meteorites are considered to have formed via thermal metamorphism and/or partial melting caused by internal heat (e.g., 26Al decay) on their parent body, which accreted ~4.56 Ga. However, the detailed thermal history of the Winonaite-IAB parent body remains unclear due to their complicated textures. To elucidate this, we conduct petrological and mineralogical observations, including in-situ U–Pb dating of Ca-phosphate (apatite) in a winonaite meteorite, Northwest Africa (NWA) 13679.
Four polished samples of NWA 13679 were observed with an optical microscope and SEM-EDS (JSM-6390A) to obtain the backscattered electron (BSE) images and the elemental maps. Based on those images, false color maps of those sections were created to evaluate their lithologies and mineral distributions. Subsequently, The elemental compositions of major silicate minerals (i.e., olivine, pyroxene, and plagioclase), metal veins (Fe-Ni), sulfides (troilite), as well as trace phosphide (schreibersite) and apatite were measured by EPMA (JXA-iSP 100). We also measured the crystal orientations of apatite using electron backscatter diffraction (EBSD; ZEISS Crossbeam 550) attached to the SEM at the Open University. Finally, the in-situ U–Pb dating of the identified apatite grains was conducted using the femtosecond laser ablation multicollector inductively coupled plasma mass spectrometer (fsLA-MC-ICP-MS; NuPlasmaII) at the Geochemical Research Center, The University of Tokyo.
The results of mineralogical observations show the significant heterogeneity in grain sizes, mineral compositions, and volume ratios within the single meteorite. Some olivine grains showed inverse zoning, with Fe concentration decreasing toward the rim. The apatite grains were found adjacent to Fe-Ni metal, troilite and high Ca pyroxene. The EBSD analysis of the apatite grains showed no evidence of any fragmentation, recrystallization, or dramatic misorientation. Therefore, it is suggested that those apatites did not experience impact metamorphism after the silicate-metal mixing of the Winonaite-IAB parent body. In addition, some metal veins crosscut both the silicate and apatite grains, suggesting that the mixing of the metallic melt and their subsequent mobilization occurred at least two or more times.
The Pb–Pb age of the apatites is determined to be 4555 ± 26 Ma (2σ, n=5). This result is slightly older than a previous NanoSIMS study on the same sample, which reported an age of 4200 ± 330 Ma (Sakai et al., 2024, JpGU). However, our result is consistent with previous I–Xe and Pb–Pb dating studies of IAB meteorites (Bogard et al., 2005; Li et al., 2024). Based on these findings, along with pyroxene thermometry and cooling rate constraints, we propose a refined thermal evolution model for the Winonaite-IAB parent body.

Reference
Benedix et al. (2000) Meteoritics & Planetary Science 35, 1127-1141
Bogard et al. (2005) Meteoritics & Planetary Science 40, Nr 2, 207–224
Y. Li et al. (2024) Journal of Geophysical Research: PlanetsVolume 129, Issue 3
Zeng et al. (2019) Earth, Planets and Space 71:38