Potassium isotopic compositions measured by MC-ICP-MS in cold-plasma, high-resolution mode indicate terrestrial variability in ⁴¹K/³⁹K ratios.

Alteration of mantle peridotite in the Samail ophiolite forms secondary minerals, mainly serpentine and Mg‐rich carbonates. Magnesium accounts for ∼25 – 30% of peridotite mass and its mobility can be used to trace this alteration. We report the first set of Mg isotope measurements from peridotites and their alteration products in Oman. Partially serpentinized peridotites have Mg isotope ratios that are indistinguishable from estimates for the average mantle and bulk silicate earth (δ²⁶Mg = −0.25 ± 0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of δ²⁶Mg values for our suite of alteration products from the mantle section is ∼4.5‰ (from −3.39‰ to 1.19‰), or >60% of the total range of terrestrial variability in δ²⁶Mg values. Serpentine veins are typically enriched in ²⁶Mg (max δ²⁶Mg value = 0.96‰) whereas Mg‐carbonate veins are associated with low ²⁶Mg/²⁴Mg ratios (magnesite δ²⁶Mg = −3.3‰, dolomite δ²⁶Mg = −1.91‰). Our preferred explanation for the range in δ²⁶Mg values involves coprecipitation of serpentine and carbonates at water‐to‐rock ratios >10³. The coincidence of alteration products characterized by δ²⁶Mg values that are both lower and higher than bulk silicate Earth and the finite ¹⁴C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Δ²⁶Mg_cal‐fl suggest that travertine formation in Oman sequesters a total of 10⁶–10⁷ kg CO₂/yr, consistent with previous estimates.

Background 13 Scientific objectives 23 Operational strategy 24 Preexpedition engineering and coring operations 25 Expedition 390C and 395E