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 similar to 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 (delta Mg-26 = -0.25 +/- 0.04 parts per thousand). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of delta Mg-26 values for our suite of alteration products from the mantle section is similar to 4.5 parts per thousand (from -3.39 parts per thousand to 1.19 parts per thousand), or >60% of the total range of terrestrial variability in delta Mg-26 values. Serpentine veins are typically enriched in Mg-26 (max delta Mg-26 value = 0.96 parts per thousand) whereas Mg-carbonate veins are associated with low Mg-26/Mg-24 ratios (magnesite delta Mg-26 = -3.3 parts per thousand, dolomite delta Mg-26 = -1.91 parts per thousand). Our preferred explanation for the range in delta Mg-26 values involves coprecipitation of serpentine and carbonates at water-to-rock ratios >10(3). The coincidence of alteration products characterized by delta Mg-26 values that are both lower and higher than bulk silicate Earth and the finite C-14 ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Delta Mg-26(cal-fl) suggest that travertine formation in Oman sequesters a total of 10(6)-10(7) kg CO2/yr, consistent with previous estimates.

Automated summary

Research in the Samail ophiolite in Oman shows that alteration of mantle peridotite leads to the formation of secondary minerals such as serpentine and Mg-rich carbonates. Partially serpentinized peridotites have similar Mg isotope ratios to the average mantle, while more extensively altered samples show significant shifts in Mg isotopic composition. Serpentine veins are enriched in Mg-26, whereas Mg-carbonate veins have lower Mg-26/Mg-24 ratios. These findings suggest that ongoing serpentinization and carbonation processes are occurring in Oman, with implications for CO2 sequestration.