16:30 〜 16:45
[PPS07-04] The first fall-observed and recovered meteorite in Japan: Narashino meteorite
キーワード：習志野隕石、流星観測、普通コンドライト
At 2:32 am JST on July 2, 2020, a fireball was observed over the Kanto region, Japan. Several amateur astronomers recorded the fireball by automated meteor-observation cameras belonging to the SonotaCo low-light video camera network [1] and calculated its trajectory and a strewn field [2-4]. The recovered meteorite is named “Narashino” to be the 53rd meteorites found in Japan and the first one whose trajectory is well-investigated. Here, we report a recovering history and a petrological description of the Narashino meteorite.
Until today, fragments are recovered from three locations in the northern part of the predicted strewn field. A first fragment (63 g) was found on a floor of a corridor in an apartment. Two days later, another 70 g fragment was found in the garden of the same apartment. On the basis of their shapes, they were originally one mass (No. 1) and were crushed when it hit on the wall of the apartment. Later, several small fragments are collected on the top of the apartments, and the total weight of the mass No. 1 becomes more than 156 g. On July 22, the second mass (No. 2) was found in Funabashi city about 1 km northeast of the No. 1 site. The mass broke a roofing tile of an apartment and spread into 2 large (95 g and 73 g) and 11 small fragments. The total weight of mass No. 2 is more than 194 g. The last one (No. 3) was found on October 25, about three months later after the last recovery, at the top of a supermarket located further 1 km north than the second location. The third one is small, weighing 14.67 g, and maybe a piece of a larger unrecovered mass. Gamma rays emitted from cosmogenic radionuclides (52Mn, 48V, 54Mn, 22Na, and 26Al) were measured by the pure Ge detector at the National Museum of Nature and Science, confirming that all of these fragments are recently fallen meteorites. The total weight of the Narashino meteorite so far is about 365 g, which is smaller than the expected mass based on the fireball observation.
A half surface of two large fragments of mass No. 1 is covered by the fusion crust. The first fragment of mass No. 1 is extremely fresh, but the second one looks slightly weathered by rain. The mass No. 2 is also half-covered by fusion crust. The weathering of the mass No. 2 surface is severer than the mass No.1 due to the longer exposure to the weather (20 days), and the same is true for No. 3. However, the weathering reaches just on the surface or in small cracks, thus the interiors of mass No.1 and 2 looks fresh (No. 3 is not investigated except for gamma rays).
For petrological analysis, polished thick and thin sections were made from the mass No. 1 and No. 2 at the National Institute of Polar Research. We observed the sections by FE-SEM and analyzed chemical compositions of constituent minerals by electron microprobe. Narashino meteorite is mainly composed of ferromagnesian silicate (olivine+pyroxene, 63.9 vol.%), albite (10.1 %), Fe-Ni metal (8.7 %), sulfides (3.6 %) and accessory minerals such as chromite (0.53 %) and phosphate (0.47 %). One of the characteristic features of Narashino is its high porosity (12.7 vol%). Olivine compositional ranges are Fa17.0-18.1 (mean=Fa17.7, N=27) for No.1 and Fa16.9-18.0 (mean Fa17.6, N=25) for No.2. Compositional ranges of low-Ca pyroxene are Fs15.4-16.5 (mean=Fs15.8, N=13) and Fs15.5-16.2 (mean=Fs15.8, N=19), respectively. The size of albite is >50 µm. The above observation indicates that Narashino is H5 chondrite. Since no shock feature was observed, the shock stage of Narashino is S1. Noble gas analysis performed at Kyushu University using fragments of No. 1 and No. 2 yielded the K-Ar gas retention age of about 4.5 Ga, assuming the K content of 780 ppm.
This work is supported by Project to Build an International Collaborative Research Network for Pre-modern Japanese Texts (NIJL-NW project), “Hoshi-ishi 4D project”. In the project, we are continuing to seek a 200-year-old meteorite based on the literature survey. Narashino meteorite fall is similar to the event written in the literature.
Reference
[1] Central Bureau for Astronomical Telegrams, CBET No.4810, July 2020, 12.
[2] Astronomical Circular No.939, Nippon Meteor Society, Oct. 2020, 19-22.
[3] The Heavens, No.1149, Oriental Astronomical Association, Feb. 2021, 64-67.
[4] Astronomical Circular No.938, Nippon Meteor Society, Sep. 2020, 19-22.
Until today, fragments are recovered from three locations in the northern part of the predicted strewn field. A first fragment (63 g) was found on a floor of a corridor in an apartment. Two days later, another 70 g fragment was found in the garden of the same apartment. On the basis of their shapes, they were originally one mass (No. 1) and were crushed when it hit on the wall of the apartment. Later, several small fragments are collected on the top of the apartments, and the total weight of the mass No. 1 becomes more than 156 g. On July 22, the second mass (No. 2) was found in Funabashi city about 1 km northeast of the No. 1 site. The mass broke a roofing tile of an apartment and spread into 2 large (95 g and 73 g) and 11 small fragments. The total weight of mass No. 2 is more than 194 g. The last one (No. 3) was found on October 25, about three months later after the last recovery, at the top of a supermarket located further 1 km north than the second location. The third one is small, weighing 14.67 g, and maybe a piece of a larger unrecovered mass. Gamma rays emitted from cosmogenic radionuclides (52Mn, 48V, 54Mn, 22Na, and 26Al) were measured by the pure Ge detector at the National Museum of Nature and Science, confirming that all of these fragments are recently fallen meteorites. The total weight of the Narashino meteorite so far is about 365 g, which is smaller than the expected mass based on the fireball observation.
A half surface of two large fragments of mass No. 1 is covered by the fusion crust. The first fragment of mass No. 1 is extremely fresh, but the second one looks slightly weathered by rain. The mass No. 2 is also half-covered by fusion crust. The weathering of the mass No. 2 surface is severer than the mass No.1 due to the longer exposure to the weather (20 days), and the same is true for No. 3. However, the weathering reaches just on the surface or in small cracks, thus the interiors of mass No.1 and 2 looks fresh (No. 3 is not investigated except for gamma rays).
For petrological analysis, polished thick and thin sections were made from the mass No. 1 and No. 2 at the National Institute of Polar Research. We observed the sections by FE-SEM and analyzed chemical compositions of constituent minerals by electron microprobe. Narashino meteorite is mainly composed of ferromagnesian silicate (olivine+pyroxene, 63.9 vol.%), albite (10.1 %), Fe-Ni metal (8.7 %), sulfides (3.6 %) and accessory minerals such as chromite (0.53 %) and phosphate (0.47 %). One of the characteristic features of Narashino is its high porosity (12.7 vol%). Olivine compositional ranges are Fa17.0-18.1 (mean=Fa17.7, N=27) for No.1 and Fa16.9-18.0 (mean Fa17.6, N=25) for No.2. Compositional ranges of low-Ca pyroxene are Fs15.4-16.5 (mean=Fs15.8, N=13) and Fs15.5-16.2 (mean=Fs15.8, N=19), respectively. The size of albite is >50 µm. The above observation indicates that Narashino is H5 chondrite. Since no shock feature was observed, the shock stage of Narashino is S1. Noble gas analysis performed at Kyushu University using fragments of No. 1 and No. 2 yielded the K-Ar gas retention age of about 4.5 Ga, assuming the K content of 780 ppm.
This work is supported by Project to Build an International Collaborative Research Network for Pre-modern Japanese Texts (NIJL-NW project), “Hoshi-ishi 4D project”. In the project, we are continuing to seek a 200-year-old meteorite based on the literature survey. Narashino meteorite fall is similar to the event written in the literature.
Reference
[1] Central Bureau for Astronomical Telegrams, CBET No.4810, July 2020, 12.
[2] Astronomical Circular No.939, Nippon Meteor Society, Oct. 2020, 19-22.
[3] The Heavens, No.1149, Oriental Astronomical Association, Feb. 2021, 64-67.
[4] Astronomical Circular No.938, Nippon Meteor Society, Sep. 2020, 19-22.