Carbide-metal assemblages in a sample returned from asteroid 25143 Itokawa: Evidence for methane-rich fluids during metamorphism

摘要: Abstract We found that the particle RA-QD02-0115 returned by Hayabusa spacecraft from near-Earth asteroid 25143 Itokawa contains iron carbide haxonite (Fe 21.9-22.7 Co 0.2-0.3 Ni 0.2-0.8 )C 6 and several Fe,Ni alloys, including multi-domain tetrataenite spinodally decomposed taenite. Ellipsoidal to nearly spherical voids occur throughout suggest presence of a fluid phase during textural chemical equilibration host rock within parent Itokawa. The calculated solubility carbon in metal indicates formed at temperatures larger than 600 °C thermal metamorphism LL-chondritic mineral assemblage. Haxonite metastably with respect graphite cohenite, probably due its high degree lattice match neighboring taenite, low cooling rate peak metamorphic temperatures, hindered nucleation graphite. Thermodynamic equilibrium calculations indicate present was dry (H 2 O-poor) dominated methane. reactive most plausibly had an atomic H/C ratio 4–5 derived reduction macromolecular, insoluble organic matter (IOM) initially co-accreted water ice. initial is necessary assumption provide sufficient hydrogen for formation methane hydrolyzed IOM. Metallic turn partially oxidized incorporated into ferromagnesian silicates high-temperature stage metamorphism. An exemplary bulk reaction unequilibrated material on left equilibrated assemblage right may be written as: 330 CH 0.8 O 0.2(IOM)  + 500 H (ice/g)  + 681 Fe (in alloy)  + 566 FeSiO 3(in Opx)  → 300 4(g)  + 32 2(g)  + 5 23 C 6(in Hx) SiO 4(in Ol) (Opx = orthopyroxene, Hx = haxonite, Ol = olivine, g = fluid species). best estimate fluid/rock region LL body where about 3 × 10 −3 corresponds ice/rock 7 × 10 (both mass).