11:45 AM - 12:00 PM
[PPS08-11] Elucidation of thermal history of winonaite parent asteroid based on textural observation and in-situ U-Pb dating of Northwest Africa 13679
Keywords:Winonaite, Petrology , in-situ U-Pb dating, thermal history
Winonaites are a group of primitive achondrites with near-chondritic elemental compositions and partially melted microstructures. The winonaite group is characterized by reductive mineral compositions and oxygen isotope ratios and has been suggested to originate from the same parent body as IAB, a group of iron meteorites (Benedix et al., 2000). The formation and thermal history of the winonaite-IAB parent body remains largely unrevealed. To elucidate this, we conduct the petrological and mineralogical observations and the in-situ U-Pb dating of Ca-phosphate mineral (apatite) in a recently found winonaite meteorite, Northwest Africa (NWA) 13679.
A polished thin section sample of NWA 13679 were observed with an optical microscope and SEM-EDS (JSM-6390A) to obtain the backscattered electron (BSE) images. False color maps were made from the BSE images to evaluate the lithologies and mineral distributions of the whole rock. Then, the elemental compositions of the major silicate minerals (olivine, pyroxene, and plagioclase), metal veins (Fe-Ni), sulfides (troilite), as well as the trace nitrides (schreibersite) and apatite were measured by EPMA (JXA-iSP 100). Finally, the in-situ U-Pb dating of the identified apatite grains was conducted using a high-spatial resolution secondary ion mass spectrometer (NanoSIMS 50) at the Atmosphere and Ocean Research Institute, The University of Tokyo.
The EPMA measurements reveal that the iron contents (Fa#) of olivine present reverse zoning. Almost all apatite grains co-exist with Fe-Ni metal, troilite, and high-Ca pyroxene. These textures suggest that the apatite had grown during thermal metamorphism of the host rock. It is inferred that NWA 13679 experienced the silicates mixing associated with the intrusion of metallic melt, followed by the significant thermal metamorphism under the reductive conditions.
The 18 apatite grains determine a 207Pb-206Pb isochron age of 4216 ± 330 Ma, a 238U-206Pb isochron age of 3680 ± 750 Ma, and a total Pb/U (3D) age of 4186 ± 330 Ma, respectively. Those values are consistent with or slightly younger than the reported Ar-Ar ages of other winonaite members (Pontlyfni; 4.530-4.535 Ga, Winona; ≧4.45 Ga Mt. Morris; 4.40 Ga; Benedix et al. 1998). Considering the metamorphic origin of apatite, the U-Pb age of ca. 4200 ± 330 Ma may have recorded the slow cooling process after the earlier metallic melt mixing. We further estimate the peak temperature for NWA 13679 based on the pyroxene thermometry (Nakamuta et al., 2017), which respectively results in 655 ± 18°C (590 °C to 817 °C) for the high Ca pyroxene, and 728 ± 17°C (603 °C to 818 °C) for the low-Ca pyroxene. This temperature difference probably reflects the mineral formation sequences. The peak temperature of the low-Ca pyroxene is consistent with the closure temperature of the U-Pb in apatite (731 to 844 °C) estimated from the Dodson’ model (Dodson, 1973).
Consequently, the winonaite parent body may have experienced (1) the ancient accretion and partial melting of the chondritic precursor, (2) the silicates and metallic melts mixing, and (3) the subsequent slow cooling and the thermal metamorphism. At the final stage, the secondary apatite and high Ca pyroxene may have grown at 600-800 °C by ca. 4200 ± 330 Ma.
A polished thin section sample of NWA 13679 were observed with an optical microscope and SEM-EDS (JSM-6390A) to obtain the backscattered electron (BSE) images. False color maps were made from the BSE images to evaluate the lithologies and mineral distributions of the whole rock. Then, the elemental compositions of the major silicate minerals (olivine, pyroxene, and plagioclase), metal veins (Fe-Ni), sulfides (troilite), as well as the trace nitrides (schreibersite) and apatite were measured by EPMA (JXA-iSP 100). Finally, the in-situ U-Pb dating of the identified apatite grains was conducted using a high-spatial resolution secondary ion mass spectrometer (NanoSIMS 50) at the Atmosphere and Ocean Research Institute, The University of Tokyo.
The EPMA measurements reveal that the iron contents (Fa#) of olivine present reverse zoning. Almost all apatite grains co-exist with Fe-Ni metal, troilite, and high-Ca pyroxene. These textures suggest that the apatite had grown during thermal metamorphism of the host rock. It is inferred that NWA 13679 experienced the silicates mixing associated with the intrusion of metallic melt, followed by the significant thermal metamorphism under the reductive conditions.
The 18 apatite grains determine a 207Pb-206Pb isochron age of 4216 ± 330 Ma, a 238U-206Pb isochron age of 3680 ± 750 Ma, and a total Pb/U (3D) age of 4186 ± 330 Ma, respectively. Those values are consistent with or slightly younger than the reported Ar-Ar ages of other winonaite members (Pontlyfni; 4.530-4.535 Ga, Winona; ≧4.45 Ga Mt. Morris; 4.40 Ga; Benedix et al. 1998). Considering the metamorphic origin of apatite, the U-Pb age of ca. 4200 ± 330 Ma may have recorded the slow cooling process after the earlier metallic melt mixing. We further estimate the peak temperature for NWA 13679 based on the pyroxene thermometry (Nakamuta et al., 2017), which respectively results in 655 ± 18°C (590 °C to 817 °C) for the high Ca pyroxene, and 728 ± 17°C (603 °C to 818 °C) for the low-Ca pyroxene. This temperature difference probably reflects the mineral formation sequences. The peak temperature of the low-Ca pyroxene is consistent with the closure temperature of the U-Pb in apatite (731 to 844 °C) estimated from the Dodson’ model (Dodson, 1973).
Consequently, the winonaite parent body may have experienced (1) the ancient accretion and partial melting of the chondritic precursor, (2) the silicates and metallic melts mixing, and (3) the subsequent slow cooling and the thermal metamorphism. At the final stage, the secondary apatite and high Ca pyroxene may have grown at 600-800 °C by ca. 4200 ± 330 Ma.