A Mg isotopic perspective on the mobility of magnesium during
serpentinization and carbonation of the Oman ophiolite
Abstract
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.