9:30 AM - 9:45 AM
[PPS08-03] Primary anhydrous minerals in CI chondrites and Ryugu samples: Their similarities and the relationship to Ca-carbonate
Keywords:CI chondrites, Ryugu, primary minerals, Ca carbonate
Introduction: There are only five unheated CI chondrites among >73,000 meteorites [1]. They experienced severe aqueous alteration [e.g., 2]. The chemistry, isotopic compositions, and mineralogy of the Ryugu return samples show a close relationship to the CI chondrites [e.g., 3-9]. In this study, we refined the inventory list of the primary anhydrous minerals and clarified the relationship between the occurrences of the primary minerals and the secondary minerals in CI chondrites and Ryugu samples.
Samples and methods: Five fragments of Ivuna (total area: ~18.1 mm2), eight fragments of Orgueil (~57.3 mm2), and seven fragments of Ryugu samples (~5.4 mm2) were investigated using petrographic microscopes, scanning electron microscopes, electron microprobe analyzer, and scanning transmission electron microscope. Oxygen isotopic analysis of olivine in Ivuna using a secondary ion mass spectrometer is now in progress.
Results: Olivine, pyroxene, and spinel (with/without hibonite and perovskite) are commonly found as shown in the previous studies [e.g., 2-9, 13-15]. Fe-rich phosphides were identified commonly: schreibersite (Fe, Ni)3P, transjordanite (Ni, Fe)2P or barrigerite (Fe, Ni)2P. Ivuna contains FeCr2S4. Its selected area electron diffraction pattern indicates that it is daubréelite. Both daubéelite and its high-pressure form zolenskyite were reported from Ryugu samples [6, 10]. (Fe, Cr, Ni)2P, which may be andreyivanovite, was found in CIs and Ryugu samples. In addition, some minerals were identified only in CI chondrites or Ryugu samples although it is not clear whether the difference is inherent or not due to their rarity. A 200-µm across porphyritic olivine chondrule-like object was identified in Ivuna. Olivine contains Si- and P-bearing Fe-Ni metal inclusions with chromium sulfide, which may be brezinaite Cr3S4 considering the coexistence with Fe-Ni metal. Platinum-group metals occurred as refractory metal nuggets in Fe-Ni sulfide grains showing pyrrhotite–pentlandite intergrowth [11] in Orgueil. These various rare primary minerals were found in Ca-carbonate-bearing areas.
Discussion and conclusion: Because CI chondrites are breccias [e.g., 12], most previous studies of CI chondrites classified clasts based on secondary minerals or on the assumption that sulfates and ferrihydrites were formed by aqueous alteration [e.g., 13, 14] although studies of Ryugu samples suggest that both phases may be terrestrial weathering products [e.g., 3-7]. Some previous studies pointed out the coexistence of Ca-carbonate, olivine, and pyroxene in CI chondrites and Ryugu samples [e.g., 6, 8, 9, 15]. This study shows that not only olivine and pyroxene but also various rare primary minerals are related to the presence of Ca-carbonate. We think that the presence of the primary phases coexisting with Ca-carbonate is important to infer the history of their parent bodies. Ca-carbonate crystallized earlier than other carbonates in CI chondrites and Ryugu samples [16]. If so, the areas containing these primary minerals might preserve the early stage of aqueous alteration except for some cases [e.g., 17]. Although the abundances of the phosphides and sulfides are very low in both CI and CM chondrites [18], these species commonly occur in both of them.
References: [1] the Meteorite Bulletin Database. [2] Brearley & Jones 1998. Planetary Materials. [3] Yokoyama+ 2023. Sci. 379, eabn7850. [4] Nakamura+ 2022. PJA B98, 227. [5] Ito+ 2022. Nat. Astron. 6, 1163. [6] Nakamura + 2023. Sci. 379, eabn8671. [7] Yamaguchi + 2023. Nat. Astron. 7, 398. [8] Kawasaki+ 2023. Sci. Adv. 8, 1. [9] Nakashima + 2023. Nat. Comm. 14, 532. [10] Tomioka+ 2023. Nat. Astron. 7, 669. [11] Singerling & Brearley 2018. MAPS 53, 2078. [12] Richardson 1978. Met. 13, 14. [13] Endreß & Bischoff 1996. GCA 60, 489. [14] Morlok+ 2006. GCA 70, 5371. [15] Lee & Nicholson 2009. GCA 280, 268. [16] Fujiya+ 2023. Nat. Geosci. 16, 675. [17] Frank+ 2023. MAPS 58, 1495. [18] Nazarov+ 2009. Pet. 17, 101.
Samples and methods: Five fragments of Ivuna (total area: ~18.1 mm2), eight fragments of Orgueil (~57.3 mm2), and seven fragments of Ryugu samples (~5.4 mm2) were investigated using petrographic microscopes, scanning electron microscopes, electron microprobe analyzer, and scanning transmission electron microscope. Oxygen isotopic analysis of olivine in Ivuna using a secondary ion mass spectrometer is now in progress.
Results: Olivine, pyroxene, and spinel (with/without hibonite and perovskite) are commonly found as shown in the previous studies [e.g., 2-9, 13-15]. Fe-rich phosphides were identified commonly: schreibersite (Fe, Ni)3P, transjordanite (Ni, Fe)2P or barrigerite (Fe, Ni)2P. Ivuna contains FeCr2S4. Its selected area electron diffraction pattern indicates that it is daubréelite. Both daubéelite and its high-pressure form zolenskyite were reported from Ryugu samples [6, 10]. (Fe, Cr, Ni)2P, which may be andreyivanovite, was found in CIs and Ryugu samples. In addition, some minerals were identified only in CI chondrites or Ryugu samples although it is not clear whether the difference is inherent or not due to their rarity. A 200-µm across porphyritic olivine chondrule-like object was identified in Ivuna. Olivine contains Si- and P-bearing Fe-Ni metal inclusions with chromium sulfide, which may be brezinaite Cr3S4 considering the coexistence with Fe-Ni metal. Platinum-group metals occurred as refractory metal nuggets in Fe-Ni sulfide grains showing pyrrhotite–pentlandite intergrowth [11] in Orgueil. These various rare primary minerals were found in Ca-carbonate-bearing areas.
Discussion and conclusion: Because CI chondrites are breccias [e.g., 12], most previous studies of CI chondrites classified clasts based on secondary minerals or on the assumption that sulfates and ferrihydrites were formed by aqueous alteration [e.g., 13, 14] although studies of Ryugu samples suggest that both phases may be terrestrial weathering products [e.g., 3-7]. Some previous studies pointed out the coexistence of Ca-carbonate, olivine, and pyroxene in CI chondrites and Ryugu samples [e.g., 6, 8, 9, 15]. This study shows that not only olivine and pyroxene but also various rare primary minerals are related to the presence of Ca-carbonate. We think that the presence of the primary phases coexisting with Ca-carbonate is important to infer the history of their parent bodies. Ca-carbonate crystallized earlier than other carbonates in CI chondrites and Ryugu samples [16]. If so, the areas containing these primary minerals might preserve the early stage of aqueous alteration except for some cases [e.g., 17]. Although the abundances of the phosphides and sulfides are very low in both CI and CM chondrites [18], these species commonly occur in both of them.
References: [1] the Meteorite Bulletin Database. [2] Brearley & Jones 1998. Planetary Materials. [3] Yokoyama+ 2023. Sci. 379, eabn7850. [4] Nakamura+ 2022. PJA B98, 227. [5] Ito+ 2022. Nat. Astron. 6, 1163. [6] Nakamura + 2023. Sci. 379, eabn8671. [7] Yamaguchi + 2023. Nat. Astron. 7, 398. [8] Kawasaki+ 2023. Sci. Adv. 8, 1. [9] Nakashima + 2023. Nat. Comm. 14, 532. [10] Tomioka+ 2023. Nat. Astron. 7, 669. [11] Singerling & Brearley 2018. MAPS 53, 2078. [12] Richardson 1978. Met. 13, 14. [13] Endreß & Bischoff 1996. GCA 60, 489. [14] Morlok+ 2006. GCA 70, 5371. [15] Lee & Nicholson 2009. GCA 280, 268. [16] Fujiya+ 2023. Nat. Geosci. 16, 675. [17] Frank+ 2023. MAPS 58, 1495. [18] Nazarov+ 2009. Pet. 17, 101.