A Mg Isotopic Perspective on the Mobility of Magnesium During Serpentinization and Carbonation of the Oman Ophiolite

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