Alteration of mantle peridotite in the Samail ophiolite forms secondary minerals, mainly serpentine and Mg-rich carbonates. Magnesium accounts for approximately 25 to 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 (d26Mg =-0.25±0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of d26Mg values for our suite of alteration products from the mantle section is ~4.5‰;, or ~60% of the total range of terrestrial variability in d26Mg values. Serpentine veins are typically enriched in Mg (up to 0.96‰) whereas Mg-carbonate veins are associated with low Mg/Mg ratios (magnesite d26Mg =-3.3‰, dolomite d26Mg =-1.91‰). Our preferred explanation for the range in d26Mg values involves co-precipitation of serpentine and carbonates at water-to-rock ratios >10^3. The coincidence of alteration products characterized by d26Mg values that are both lower and higher than bulk silicate Earth and the finite C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from D26Mgcal-fl suggest that travertine formation in Oman sequesters a total of 10^6-10^7 kg CO/yr, consistent with previous estimates.