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Type 1 aqueous alteration in CM carbonaceous chondrites: Implications for the evolution of water‐rich asteroids
Author(s) -
King A. J.,
Schofield P. F.,
Russell S. S.
Publication year - 2017
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/maps.12872
Subject(s) - chondrite , chondrule , parent body , calcite , meteorite , enstatite , olivine , asteroid , magnetite , geology , carbonaceous chondrite , geochemistry , mineralogy , astrobiology , formation and evolution of the solar system , mineral , chemistry , physics , paleontology , organic chemistry
The CM carbonaceous chondrite meteorites experienced aqueous alteration in the early solar system. They range from mildly altered type 2 to almost completely hydrated type 1 chondrites, and offer a record of geochemical conditions on water‐rich asteroids. We show that CM 1 chondrites contain abundant (84–91 vol%) phyllosilicate, plus olivine (4–8 vol%), magnetite (2–3 vol%), Fe‐sulfide (<5 vol%), and calcite (<2 vol%). The CM 1/2 chondrites contain phyllosilicate (71–88 vol%), olivine (4–20 vol%), enstatite (2–6 vol%), magnetite (2–3 vol%), Fe‐sulfides (1–2 vol%), and calcite (~1 vol%). As aqueous alteration progressed, the abundance of Mg‐serpentine and magnetite in the CM chondrites increased. In contrast, calcite abundances in the CM 1/2 and CM 1 chondrites are often depleted relative to the CM 2s. The modal data support the model, whereby metal and Fe‐rich matrix were the first components to be altered on the CM parent body(ies), before further hydration attacked the coarser Mg‐rich silicates found in chondrules and fragments. Based on the absence of tochilinite, we suggest that CM 1 chondrites experienced increased alteration due to elevated temperatures (>120 °C), although higher water/rock ratios may also have played a role. The modal data provide constraints for interpreting the composition of asteroids and the mineralogy of samples returned from these bodies. We predict that “ CM 1‐like” asteroids, as has been proposed for Bennu—target for the OSIRIS ‐ RE x mission—will have a high abundance of Mg‐rich phyllosilicates and Fe‐oxides, but be depleted in calcite.