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[PPS08-12] U–Pb dating of phosphate minerals in a brecciated eucrite Juvinas and a mesosiderite Tanezrouft 091.
Keywords:HED meteorite, Mesosiderite, U-Pb chronology, Phosphate, Collisional history
Asteroid 4 Vesta is a rocky differentiated body with layered structures of metallic core, mantle, and basaltic crust. It is known that crusts on Vesta have been preserved for ~4.5 billion years and have recorded the early Solar System evolution. Howardite-eucrite-diogenite (HED) meteorites are considered as the crustal samples from Vesta, because of their similarities in the reflectance spectra [1]. Mesosiderite, a stony-iron meteorites, are mixtures of Fe-Ni metal and brecciated HED-like silicates [2]. Both Vesta and MPB have experienced heavy collisional events, which related to dynamical processes of the asteroid belts. To reveal the thermal and collisional history of asteroids, uranium-lead (U–Pb) dating of the phosphate minerals, such as apatite [Ca5(PO4)3(F,Cl,OH)] and merrillite [Ca9NaMg(PO4)7], is informative [3]. In this study, we have conducted the in-situ U–Pb dating on phosphate minerals in a basaltic eucrite Juvinas and a mesosiderite Tanezrouft 091.
Three polished sections of Juvinas and a polished section of Tanezrouft 091 were observed with FE-SEM-EDS (SU6600) at the Astromaterials Science Research Group, ISAS, JAXA and SEM-EDS (JSM-6390) at Hiroshima University, respectively, to identify phosphate grains suitable for the following U–Pb dating. Their U–Pb dating was conducted using a high-resolution secondary ion mass spectrometer (NanoSIMS 50) at Atmosphere and Ocean Research Institute (AORI), The University of Tokyo. To calculate isochron ages, the Isoplot Ver. 4 software was used [4].
In Juvinas, the 31 spots on the 27 apatite grains define a concordia line in the three-dimensional 238U/206Pb–207Pb/206Pb–204Pb/206Pb space (Figure 1). Their total U–Pb isochron age is calculated as 4503 ± 16 Ma (MSWD = 2.8). In contrast, the seven spots on the 7 merrillite grains define a different discordia plane in the three-dimensional space. Their total U–Pb isochron age is calculated as 4153 ± 130 Ma and 260 ± 470 Ma (MSWD = 0.91). Obviously, the determined ages of the apatite (~4500 Ma) is older than that of the merrillite (~4150 Ma), highlighting the difference of their closure temperatures. Although the closure temperature is uncertain for U–Pb in merrillite because of lack of experimental data, it may be lower than that of co-existing apatite.
In Tanezrouft 091, 13 spots on the 12 merrillite grains define a concordia line in the three-dimensional space. Their total U–Pb isochron age is calculated as 3938 ± 290 Ma (MSWD = 2.0). This age is identical to the reported phosphates in other mesosiderites [5] and some brecciated eucrites [e.g. 3, 6, 7], indicating the considerable thermal metamorphism due to collisional event(s), occurred on the parent bodies (i.e., Vesta and Mesosiderite Parent Body) at this timing.
In summary, our study shows that multiple collisional events have occurred on the parent body between ~4500–4000 Ma.
REFERENCES: [1] McCord et al. (1970) Science, 168 (3938), 1445-1447. [2] Greenwood et al. (2006) Science, 313(5794), 1763-1765. [3] Koike et al. (2020) Earth and Planetary Science Letters, 549, 116497. [4] Ludwig (2012). [5] Kouvatsis et al. (2022) Geochimica et Cosmochimica Acta, 348, 369-380. [6] Liao et al. (2017) Geochimica et Cosmochimica Acta, 204, 159-178. [7] Sumiya et al. (2022) 53rd Lunar and Planetary Science Conference [abstract #1791].
Three polished sections of Juvinas and a polished section of Tanezrouft 091 were observed with FE-SEM-EDS (SU6600) at the Astromaterials Science Research Group, ISAS, JAXA and SEM-EDS (JSM-6390) at Hiroshima University, respectively, to identify phosphate grains suitable for the following U–Pb dating. Their U–Pb dating was conducted using a high-resolution secondary ion mass spectrometer (NanoSIMS 50) at Atmosphere and Ocean Research Institute (AORI), The University of Tokyo. To calculate isochron ages, the Isoplot Ver. 4 software was used [4].
In Juvinas, the 31 spots on the 27 apatite grains define a concordia line in the three-dimensional 238U/206Pb–207Pb/206Pb–204Pb/206Pb space (Figure 1). Their total U–Pb isochron age is calculated as 4503 ± 16 Ma (MSWD = 2.8). In contrast, the seven spots on the 7 merrillite grains define a different discordia plane in the three-dimensional space. Their total U–Pb isochron age is calculated as 4153 ± 130 Ma and 260 ± 470 Ma (MSWD = 0.91). Obviously, the determined ages of the apatite (~4500 Ma) is older than that of the merrillite (~4150 Ma), highlighting the difference of their closure temperatures. Although the closure temperature is uncertain for U–Pb in merrillite because of lack of experimental data, it may be lower than that of co-existing apatite.
In Tanezrouft 091, 13 spots on the 12 merrillite grains define a concordia line in the three-dimensional space. Their total U–Pb isochron age is calculated as 3938 ± 290 Ma (MSWD = 2.0). This age is identical to the reported phosphates in other mesosiderites [5] and some brecciated eucrites [e.g. 3, 6, 7], indicating the considerable thermal metamorphism due to collisional event(s), occurred on the parent bodies (i.e., Vesta and Mesosiderite Parent Body) at this timing.
In summary, our study shows that multiple collisional events have occurred on the parent body between ~4500–4000 Ma.
REFERENCES: [1] McCord et al. (1970) Science, 168 (3938), 1445-1447. [2] Greenwood et al. (2006) Science, 313(5794), 1763-1765. [3] Koike et al. (2020) Earth and Planetary Science Letters, 549, 116497. [4] Ludwig (2012). [5] Kouvatsis et al. (2022) Geochimica et Cosmochimica Acta, 348, 369-380. [6] Liao et al. (2017) Geochimica et Cosmochimica Acta, 204, 159-178. [7] Sumiya et al. (2022) 53rd Lunar and Planetary Science Conference [abstract #1791].