The oxygen isotope evolution of CM chondrites during aqueous alteration

We investigate three unresolved questions in CM chondrite research: (1) what was the initial O-isotope composition of their accreted water? (2) How much water was accreted? And (3) Did aqueous alteration proceed under open or closed-system conditions? We approach this problem through numerical modelling of the aqueous alteration process using an O-isotope perspective. Our model simulates dissolution and mixing between dry rock and the fluid reservoir, secondary mineralisation (phyllosilicate and calcite) and, in some scenarios open-system evaporative loss. Previously published O-isotope measurements (CM chondrite bulk rock, and T1 and T2 calcite) and a suitable alteration extent (65–85 vol%) are used as constraints against which viable alteration scenarios must match. We find the most likely conditions for alteration were the accretion of water-ice with an approximately Earth-like composition (plotting on or close to the intersection between the TFL and the PCM) and with a δ 18 O value <30 ‰ and likely at ∼5 ‰, prograde conditions (increasing temperature with time), T1 formation before significant protolith was dissolved, an initial water-to-rock (mass) ratio of ∼0.6 and moderate open system evaporative water loss. The most surprising result of this study is the isotopically light (relatively 16 O-rich) initial water composition. We show that this is consistent with fluid compositions independently determined from several other studies. This implies that outer Solar System water was not ubiquitously 16 O-poor, as might have been expected based on the composition of isotopically anomalous cosmic symplectite found in Acfer 094.