Presentation Information
[R5-12]Aqueous Alteration Experiments Using GEMS Analog Materials: Toward Understanding Aqueous Alteration in the Parent Bodies of Ryugu and Bennu
*Akira TSUCHIYAMA1,2, Haiyang Xian2, Mingqi Sun2, Xing Ding2, Satomi Enju3, Yuan Xu2, Yao Xiao2, Shengdong Chen2, Jiarui Lin2, Miaomiao Zhang2, Chenyi Zhao2, Kirika Otono4, Megumi Matsumoto5, Junya Matsuno6, Shogo Tachibana4 (1. Ritsumeikan Uni. ROST., 2. CAS GIG, 3. Ehime Univ. Sci., 4. Univ. Tokyo Sci., 5. Tohoku Univ. Sci., 6. Kyoto Univ. ICR)
Keywords:
Carbonate~phyllosilicate interaction,Amorphous silicate,Organics,Redox conditions,CI chondrites
Ryugu and Bennu samples are similar to CI carbonaceous chondrites. Most are aqueously altered and consist mainly of phyllosilicate-rich matrix with magnetite, pyrrhotite, and carbonates. Minor porous particles dominated by minimally-altered amorphous silicates also occur. As they resemble cometary dust and primitive carbonaceous chondrite matrices, they are likely parent-body protoliths. Thus, GEMS or GEMS-like materials mixed with organics and ice probably accreted, where water from melted ice drove alteration.
We conducted experiments using GEMS analogs of CI-like or average GEMS-like composition by condensation using an induction thermal plasma furnace. The products are submicron spherical amorphous silicate particles containing FeS and (Fe,Ni) nanoparticles, resembling natural GEMS. About 20-30 mg of analog was reacted with pure water (water/rock = 5 by mass) in sealed gold capsules or Teflon vessels at 200 C for two months. Some runs included organics or HMT and ammonium bicarbonate. Products were analyzed by XRD, Raman, SEM/EDS, and TEM/EDS.
Poorly crystalline phyllosilicates (M-S-H) formed from amorphous silicates. Teflon-vessel runs produced hematite and anhydrite, indicating that H2 from Fe-water reaction escaped while H2O was retained, causing Fe-S oxidation. Such assemblages are absent from Ryugu and Bennu, suggesting H2 retention during parent-body alteration. In contrast, gold-capsule runs produced phyllosilicates with magnetite, Fe(-Ni) sulfides, and carbonates, reproducing Ryugu-Bennu minerals. With increasing CO2, carbonates changed from Ca carbonate to dolomite and magnesite, and saponite increased relative to serpentine; thus carbonate formation controlled phyllosilicate mineralogy, consistent with Bennu analyses. Products also contain two Ryugu-Bennu-like matrix textures (porous and less-porous), implying accretionary-texture preservation. Diverse magnetite morphologies were not reproduced except for framboidal-like magnetite.
We conducted experiments using GEMS analogs of CI-like or average GEMS-like composition by condensation using an induction thermal plasma furnace. The products are submicron spherical amorphous silicate particles containing FeS and (Fe,Ni) nanoparticles, resembling natural GEMS. About 20-30 mg of analog was reacted with pure water (water/rock = 5 by mass) in sealed gold capsules or Teflon vessels at 200 C for two months. Some runs included organics or HMT and ammonium bicarbonate. Products were analyzed by XRD, Raman, SEM/EDS, and TEM/EDS.
Poorly crystalline phyllosilicates (M-S-H) formed from amorphous silicates. Teflon-vessel runs produced hematite and anhydrite, indicating that H2 from Fe-water reaction escaped while H2O was retained, causing Fe-S oxidation. Such assemblages are absent from Ryugu and Bennu, suggesting H2 retention during parent-body alteration. In contrast, gold-capsule runs produced phyllosilicates with magnetite, Fe(-Ni) sulfides, and carbonates, reproducing Ryugu-Bennu minerals. With increasing CO2, carbonates changed from Ca carbonate to dolomite and magnesite, and saponite increased relative to serpentine; thus carbonate formation controlled phyllosilicate mineralogy, consistent with Bennu analyses. Products also contain two Ryugu-Bennu-like matrix textures (porous and less-porous), implying accretionary-texture preservation. Diverse magnetite morphologies were not reproduced except for framboidal-like magnetite.