Type 1 aqueous alteration in <scp>CM</scp> carbonaceous chondrites: Implications for the evolution of water‐rich asteroids

<jats:title>Abstract</jats:title><jats:p>The <jats:styled-content style="fixed-case">CM</jats:styled-content> 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 <jats:styled-content style="fixed-case">CM</jats:styled-content>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 <jats:styled-content style="fixed-case">CM</jats:styled-content>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 <jats:styled-content style="fixed-case">CM</jats:styled-content> chondrites increased. In contrast, calcite abundances in the <jats:styled-content style="fixed-case">CM</jats:styled-content>1/2 and <jats:styled-content style="fixed-case">CM</jats:styled-content>1 chondrites are often depleted relative to the <jats:styled-content style="fixed-case">CM</jats:styled-content>2s. The modal data support the model, whereby metal and Fe‐rich matrix were the first components to be altered on the <jats:styled-content style="fixed-case">CM</jats:styled-content> 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 <jats:styled-content style="fixed-case">CM</jats:styled-content>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 “<jats:styled-content style="fixed-case">CM</jats:styled-content>1‐like” asteroids, as has been proposed for Bennu—target for the <jats:styled-content style="fixed-case">OSIRIS</jats:styled-content>‐<jats:styled-content style="fixed-case">RE</jats:styled-content>x mission—will have a high abundance of Mg‐rich phyllosilicates and Fe‐oxides, but be depleted in calcite.</jats:p>