A Mg Isotopic Perspective on the Mobility of Magnesium During Serpentinization and Carbonation of the Oman Ophiolite. Issue 2 (3rd February 2021)

• Record Type: Journal Article
• Authors: de Obeso, Juan Carlos; Santiago Ramos, Danielle P.; Higgins, John A.; Kelemen, Peter B.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-03
• Subjects: carbonation -- Mg isotopes -- Oman -- ophiolite -- peridotite -- serpentinization; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB020237
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: 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 ( δ 26 Mg = −0.25 ± 0.04‰). However, more extensively altered peridotite samples show large shifts in Mg isotopic composition. The range of δ 26 Mg 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 δ 26 Mg values. Serpentine veins are typically enriched in 26 Mg (max δ 26 Mg value = 0.96‰) whereas Mg‐carbonate veins are associated with low 26 Mg/ 24 Mg ratios (magnesite δ 26 Mg = −3.3‰, dolomite δ 26 Mg = −1.91‰). Our preferred explanation for the range in δ 26 Mg values involves coprecipitation of serpentine and carbonates at water‐to‐rock ratios >10 3 . The coincidence of alteration products characterized by δ 26 Mg values that are both lower and higher than bulk silicate Earth and the finite 14 C ages of the carbonates suggest that both serpentinization and carbonation are ongoing in Oman. Rates of calcite precipitation in travertines inferred from Δ 26 Mgcal‐fl suggest that travertine formation in Oman sequesters a total of 10 6 –10 7 kg CO2 /yr, consistent with previous estimates. Key Points: The range of δ 26 Mg from samples of the mantle section in the Oman ophiolite is ∼4.5‰, or >60% of the total range of terrestrial variability The range in δ 26 Mg values involves coprecipitation of serpentine and carbonates at high water‐to‐rock ratios Serpentinization and carbonation are ongoing in the mantle section of the Oman ophiolite