Isotopic analysis of tochilinite (carbonate and magnetite) in Winchcombe: Temperature constraints on early‐stage aqueous alteration in the CM parent body

We report the first oxygen isotope measurements of tochilinite (δ 17 O: 11.0 ± 2.1‰, δ 18 O: 23.5 ± 4.0‰ and Δ 17 O: −1.1 ± 1.2‰) in a CM chondrite (Winchcombe, lithology C [CM2.2/2.3]). We analyzed type‐I tochilinite‐cronstedtite intergrowths (TCIs)—formed by pseudomorphic replacement of kamacite. Alongside T1 and T2 calcite and magnetite, these secondary phases define a linear trendline in δ 17 O‐δ 18 O isotope space with a slope of 0.50, slightly shallower than the mass‐dependent slope (0.52). This demonstrates that, in addition to dominant mass‐dependent fractionation (controlled by mineral‐specific and temperature‐dependent equilibrium processes), mass‐independent mixing between 16 O‐rich anhydrous silicates, and 16 O‐poor water influenced the evolving Δ 17 O composition of alteration fluids. Petrographic evidence shows tochilinite and T1 calcite formed early and are closely associated in the alteration sequence. Assuming isotopic equilibrium between these phases, we estimate formation temperatures of approximately 135°C and a δ 18 O water value of 28‰. These findings align with previous hydrothermal synthesis experiments and underscore the value of multi‐phase isotopic measurements for reconstructing the fluid history of chondritic parent bodies.