10:05 〜 10:20
[PPS08-05] Investigation of the collisional history of asteroid 4 Vesta based on U-Pb dating of a melt-breccia eucrite, Northwest Africa 13166.
キーワード：HED隕石、U-Pb 年代、NanoSIMS、ベスタの衝突史
Howardite-eucrite-diogenite (HED) meteorites originated from asteroid 4 Vesta recorded the collisional and thermal history of the parent body. Previous chronological studies have reported precise dating of HED meteorites using various radiometric systems, which help to understand the formation and metamorphic process of Vesta’s crust (e.g., Kleine et al., 2004, Lugmair and Shukolyukov. 1998, Iizuka et al., 2015, Haba et al., 2019). However, owing to the complexity of thermal metamorphism of HED meteorites, the collisional history of Vesta is not yet well understood. Uranium-lead dating of phosphate minerals (apatite [Ca5(PO4)3(F,Cl,OH)] and merrillite [Ca9NaMg(PO4)7]) is informative, as it records moderately high temperature thermal event (Koike et al., 2020). In this study, we conducted in-situ U-Pb dating of zircon and phosphates in a highly melt-breccia eucrites, Northwest Africa (NWA) 13166, using a NanoSIMS.
A polished thin section (PTS) of NWA 13166 was observed with SEM-EDS (JSM-6390) at Hiroshima University, to identify zircon and phosphates. Their U-Pb dating was performed using a secondary ion mass spectrometer (NanoSIMS 50) at Atmosphere and Ocean Research Institute (AORI), The University of Tokyo. A 2nA O- primary beam was focused to make a spot with a size of approximately 8 µm in diameter. To obtain 238U-206Pb and 207Pb-206Pb ages, the secondary ions of 31P+, 43Ca+, 204Pb+, 206Pb+, 207Pb+, 238U16O+, and 238U16O2+ were simultaneously measured for about 1 hour. To calibrate 238U/206Pb ratios, a natural apatite with known age and homogenous compositions (Sano et al., 1999) was analyzed as a standard. Model 207Pb*/206Pb* ages for the individual data were calculated assuming the initial 207/206/204Pb compositions as those of CDT (Tatsumoto et al., 1973) and the 238U/235U ratio of basaltic eucrites as 137.8 (Goldman et al., 2015). Quantitative analyses of the identified zircons, phosphates, and surrounded silicate minerals were conducted using EPMA (JXA-iSP 100) after the U-Pb dating.
From SEM-EDS observation of the PTS, NWA 13166 consists of lithic clasts and melt rock matrix. The pyroxene located the lithic clasts in NWA 13166 show exsolution texture and do not have remnant Ca-zoning, indicating NWA 13166 is classified into type 5 (i.e., well equilibrated by thermal metamorphism).
We found 14 anhedral grains of merrillite in the lithic clasts and a single anhedral grain of apatite in the recrystallized area. Among them, 11 grains were dated. The others are too small for NanoSIMS analysis. A single grain of zircon was also dated. The 10 merrillite grains with 11 spot data define a three-dimensional total U-Pb isochron age of 57 ± 35 Ma and 4141 ± 170 Ma (errors are 2-sigma) with the mean squared weighted deviation (MSWD) of 1.4. These data plot as discordia. The model 207Pb*/206Pb* ages of the older 3 grains (#07, #08, #20) and an apatite grain are calculated to be ~3923 – 4221 Ma, with a weighted mean of 4161 ± 75 Ma. The zircon provides the oldest model 207Pb*/206Pb* age of 4543 ± 26 Ma, which is consistent with the global thermal metamorphism model of Vesta’s crust (Iizuka et al., 2015). The older merrillites and apatite indicate that moderate thermal metamorphism due to a collisional event occurred at ~4160 Ma, which was identical to the other brecciated eucrites (Koike et al., 2020). On the other hand, we found the considerably young metamorphic age (57 ± 35 Ma) from the merrillites. One of possible causes of this young age is the recent Pb-loss because of the severe weathering (Liao and Hsu. 2017) at Sahara Desert and/or reheating during the atmosphere entry onto the Earth. However, this metamorphic age of 57 ± 35 Ma is slightly older than the present (0 Ma). Another presumable interpretation is that a collisional event occurred on the parent body (Vesta or vestoids) at 57 Ma, when the meteorite was ejected.
A polished thin section (PTS) of NWA 13166 was observed with SEM-EDS (JSM-6390) at Hiroshima University, to identify zircon and phosphates. Their U-Pb dating was performed using a secondary ion mass spectrometer (NanoSIMS 50) at Atmosphere and Ocean Research Institute (AORI), The University of Tokyo. A 2nA O- primary beam was focused to make a spot with a size of approximately 8 µm in diameter. To obtain 238U-206Pb and 207Pb-206Pb ages, the secondary ions of 31P+, 43Ca+, 204Pb+, 206Pb+, 207Pb+, 238U16O+, and 238U16O2+ were simultaneously measured for about 1 hour. To calibrate 238U/206Pb ratios, a natural apatite with known age and homogenous compositions (Sano et al., 1999) was analyzed as a standard. Model 207Pb*/206Pb* ages for the individual data were calculated assuming the initial 207/206/204Pb compositions as those of CDT (Tatsumoto et al., 1973) and the 238U/235U ratio of basaltic eucrites as 137.8 (Goldman et al., 2015). Quantitative analyses of the identified zircons, phosphates, and surrounded silicate minerals were conducted using EPMA (JXA-iSP 100) after the U-Pb dating.
From SEM-EDS observation of the PTS, NWA 13166 consists of lithic clasts and melt rock matrix. The pyroxene located the lithic clasts in NWA 13166 show exsolution texture and do not have remnant Ca-zoning, indicating NWA 13166 is classified into type 5 (i.e., well equilibrated by thermal metamorphism).
We found 14 anhedral grains of merrillite in the lithic clasts and a single anhedral grain of apatite in the recrystallized area. Among them, 11 grains were dated. The others are too small for NanoSIMS analysis. A single grain of zircon was also dated. The 10 merrillite grains with 11 spot data define a three-dimensional total U-Pb isochron age of 57 ± 35 Ma and 4141 ± 170 Ma (errors are 2-sigma) with the mean squared weighted deviation (MSWD) of 1.4. These data plot as discordia. The model 207Pb*/206Pb* ages of the older 3 grains (#07, #08, #20) and an apatite grain are calculated to be ~3923 – 4221 Ma, with a weighted mean of 4161 ± 75 Ma. The zircon provides the oldest model 207Pb*/206Pb* age of 4543 ± 26 Ma, which is consistent with the global thermal metamorphism model of Vesta’s crust (Iizuka et al., 2015). The older merrillites and apatite indicate that moderate thermal metamorphism due to a collisional event occurred at ~4160 Ma, which was identical to the other brecciated eucrites (Koike et al., 2020). On the other hand, we found the considerably young metamorphic age (57 ± 35 Ma) from the merrillites. One of possible causes of this young age is the recent Pb-loss because of the severe weathering (Liao and Hsu. 2017) at Sahara Desert and/or reheating during the atmosphere entry onto the Earth. However, this metamorphic age of 57 ± 35 Ma is slightly older than the present (0 Ma). Another presumable interpretation is that a collisional event occurred on the parent body (Vesta or vestoids) at 57 Ma, when the meteorite was ejected.