11:00 〜 13:00
[PPS08-P08] Petrology of anomalous eucrites, NWA 011 and NWA 2976: Implications for the evolutionary history of differentiated planetesimals in the outer solar system
キーワード：分化エコンドライト、ユークライト、熱史、分化微惑星
Eucrites are the most common type of achondrites. They are basalts or gabbro mainly composed of low-Ca pyroxene and plagioclase. Most of them are believed to be derived from the same parent body (asteroid 4 Vesta), but a handful of eucrites came from distinct sources. NWA 011 is an anomalous eucrite and has eight paired stones. NWA 011 and paired stone are petrologically similar to eucrites. However, their FeO/MnO ratios of pyroxenes and isotopic compositions are largely different from other normal and anomalous eucrites [e.g., 1]. NWA 011 and paired stones may have been derived from the differentiated planetesimals or protoplanets that existed in the outer solar system [2]. We studied the NWA 011 and a paired stone, NWA 2978, to understand the origin and thermal history of the parent body. We examined the polished sections of NWA 011/2978 using an FE-SEM equipped with EDS, an EPMA, and a Raman spectrometer.
NWA 011/2978 are composed of large grains of pyroxene (low-Ca pyroxene and augite) and finer grains of plagioclase. Aggregates of smaller plagioclase grains occur interstitially along larger anhedral grains of pyroxene. Minor minerals include silica minerals, Ca-phosphates, ilmenite, chromite, troilite (mostly weathered), and weathering products. Ca-phosphates are merrillite and Cl-apatite, which are closely associated with plagioclase. Silica minerals also occur in plagioclase and along boundaries between oxide minerals and pyroxene. Silica minerals are cristobalite and tridymite (MC). Pyroxene grains are low-Ca pyroxene and augite, which are finely exsolved. Few large plagioclase grains show complicated chemical zoning and have inclusions of Na-rich plagioclase. Ilmenite and chromite are scattered in the sections. In NWA 2978, we found a large (a few mm) assemblage of Ti-chromite and ilmenite which poikilitically enclose pyroxene and plagioclase, similar to that found in a eucrite EET 90020. There are rims of Fe-rich olivine (partly weathered) along with the inclusions. NWA 2978 is petrologically identical to NWA 011 except for the large oxide assemblages.
The equilibration temperatures estimated from bulk augite compositions are ~1190 °C [3], which may correspond to the crystallization temperature from the melt. The presence of closely-spaced thin lamellae in low-Ca pyroxene and augite indicates relatively rapid cooling rates. The cooling rate would be several °C/yr (~1190-1000 °C) judging from similar pyroxene textures of Erg Chech 002 [4]. The metamorphic degree of NWA 011/2978 is comparable to type 3-5 eucrites. However, the presence of cristobalite and tridymite imply a complicated post-crystallization history. Tridymite (MC) can be formed at a slow cooling rate comparable to type 4-5 eucrites. However, the presence of cristobalite indicates that NWA 011/2978 were reheated and cooled very rapidly (>0.1-1 °C/hr below ~900 °C) [5].
NWA 011/2978 contain a significant amount of siderophile elements likely carried by the impactor. The incorporation of FeNi metal requires a large degree of melting. Fine-grained grains of plagioclase and some pyroxene crystallized at this stage. The impactor was suggested to be chondritic materials from the PGE compositions [6], although the compositions may have been modified by weathering. The preservation of silica minerals indicates that the impactor did not carry chondritic silicate materials (e.g., olivine). If the impactor was chondritic materials, silica minerals were reacted away. Thus, it is likely that the impactor was an iron meteorite. The petrogenetic history of NWA 011/2978 is similar to those of mesosiderites and anomalous eucrites, EET 92023 and Dho 007 [e.g., 7], whose parent bodies likely formed in the inner solar system. We conclude that the NWA 011/2978 parent body experienced igneous activity, metamorphism, large-scale collisional events similar to those in differentiated planetesimals in the inner solar system.
References: [1] Yamaguchi A. et al. (2002) Science 296, 334-336. [2] Warren P.H. (2011) Geochim. Cosmochim. Acta 75, 6912-6926. [3] Nakamuta Y. et al. (2017) Meteor. Planet. Sci. 52, 511-521. [4] Barrat J.A. et al. (2021) Proc. Natl. Acad. Sci. USA.118, e2026129118. [5] Ono H. et al. (2020) Meteor. Planet. Sci. 56, 1086-1108. [6] Isa J. et al. (2008) Meteor. Planet. Sci. Suppl. 43, 5204. [7] Yamaguchi A. et al. (2017) Meteor. Planet. Sci. 52, 709-721.
NWA 011/2978 are composed of large grains of pyroxene (low-Ca pyroxene and augite) and finer grains of plagioclase. Aggregates of smaller plagioclase grains occur interstitially along larger anhedral grains of pyroxene. Minor minerals include silica minerals, Ca-phosphates, ilmenite, chromite, troilite (mostly weathered), and weathering products. Ca-phosphates are merrillite and Cl-apatite, which are closely associated with plagioclase. Silica minerals also occur in plagioclase and along boundaries between oxide minerals and pyroxene. Silica minerals are cristobalite and tridymite (MC). Pyroxene grains are low-Ca pyroxene and augite, which are finely exsolved. Few large plagioclase grains show complicated chemical zoning and have inclusions of Na-rich plagioclase. Ilmenite and chromite are scattered in the sections. In NWA 2978, we found a large (a few mm) assemblage of Ti-chromite and ilmenite which poikilitically enclose pyroxene and plagioclase, similar to that found in a eucrite EET 90020. There are rims of Fe-rich olivine (partly weathered) along with the inclusions. NWA 2978 is petrologically identical to NWA 011 except for the large oxide assemblages.
The equilibration temperatures estimated from bulk augite compositions are ~1190 °C [3], which may correspond to the crystallization temperature from the melt. The presence of closely-spaced thin lamellae in low-Ca pyroxene and augite indicates relatively rapid cooling rates. The cooling rate would be several °C/yr (~1190-1000 °C) judging from similar pyroxene textures of Erg Chech 002 [4]. The metamorphic degree of NWA 011/2978 is comparable to type 3-5 eucrites. However, the presence of cristobalite and tridymite imply a complicated post-crystallization history. Tridymite (MC) can be formed at a slow cooling rate comparable to type 4-5 eucrites. However, the presence of cristobalite indicates that NWA 011/2978 were reheated and cooled very rapidly (>0.1-1 °C/hr below ~900 °C) [5].
NWA 011/2978 contain a significant amount of siderophile elements likely carried by the impactor. The incorporation of FeNi metal requires a large degree of melting. Fine-grained grains of plagioclase and some pyroxene crystallized at this stage. The impactor was suggested to be chondritic materials from the PGE compositions [6], although the compositions may have been modified by weathering. The preservation of silica minerals indicates that the impactor did not carry chondritic silicate materials (e.g., olivine). If the impactor was chondritic materials, silica minerals were reacted away. Thus, it is likely that the impactor was an iron meteorite. The petrogenetic history of NWA 011/2978 is similar to those of mesosiderites and anomalous eucrites, EET 92023 and Dho 007 [e.g., 7], whose parent bodies likely formed in the inner solar system. We conclude that the NWA 011/2978 parent body experienced igneous activity, metamorphism, large-scale collisional events similar to those in differentiated planetesimals in the inner solar system.
References: [1] Yamaguchi A. et al. (2002) Science 296, 334-336. [2] Warren P.H. (2011) Geochim. Cosmochim. Acta 75, 6912-6926. [3] Nakamuta Y. et al. (2017) Meteor. Planet. Sci. 52, 511-521. [4] Barrat J.A. et al. (2021) Proc. Natl. Acad. Sci. USA.118, e2026129118. [5] Ono H. et al. (2020) Meteor. Planet. Sci. 56, 1086-1108. [6] Isa J. et al. (2008) Meteor. Planet. Sci. Suppl. 43, 5204. [7] Yamaguchi A. et al. (2017) Meteor. Planet. Sci. 52, 709-721.