Figure 3: TOF-SIMS elemental maps of the SFI from Fig. 2b.

(a) Photomicrograph of the SFI. (b–g) Elemental maps of different regions of the SFI, labelled I to VII, to show the ROI (regions of interest) where the TOF–SIMS analyses were performed (Supplementary Table 2). (b–d) ²⁵Mg⁺, Na⁺ and ²⁵Mg/(²⁵Mg+Fe) maps showing compositional differences between the host and the reaction products, labeled as ROI I and II. The ²⁵Mg/(²⁵Mg+Fe) map is obtained from the secondary ion yield map ²⁵Mg⁺/(²⁵Mg⁺+Fe⁺) by using a multiplication factor that ensures the olivine background yields a ratio of 0.8. (e–g) ²⁵Mg⁺, OH⁻ and ²⁵Mg/(²⁵Mg+Fe) maps showing compositional differences between the host and the reaction products labeled as ROI IV. The Na⁺ is contained in the frozen aqueous phase in the SFI. (f) OH⁻ maps showing the presence of serpentine and H₂O ice. (g) ²⁵Mg/(²⁵Mg+Fe) map of ROI IV.