Tracing alteration of ultramafic rocks in the Samail ophiolite
Creators: de Obeso, Juan Carlos
Publisher: name: Columbia University
Year: 2019
Subjects: subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: Ultrabasic rocks
subject: Hydrothermal alteration
subject: Peridotite
subject: Geology

description: Alteration of ultramafic rocks is ubiquitous to their occurrence near the surface. Primary mantle minerals like olivine and pyroxenes are unstable at low pressure and temperatures and undergo hydration (serpentinization), carbonation and weathering reactions forming hydrated and carbonated minerals. Employing a variety of analytical techniques including: electron microprobe, X-Ray diffraction, major and trace element geochemistry, Mg isotopes geochemistry and geochemical modelling this work seeks to contrain conditions ofa lteration and trace changes in composition of peridotite during alteration. In Wadi Fins in the southeast of Oman peridotites outcrop at the bottom of a canyon overlaid by a thick sequence 1.5 km of Cretaceous to Eocene shallow oceanic limestones and dolostones. The peridotites exhibit different types of alteration. While the common view is that serpentinization and carbonation of peridotites is isochemical this is not the case for alteration in Wadi Fins. Peridotites tens of meters below the unconformity are characterized by a striking grid of carbonate and serpentine veins. The calcite veins and relatively low MgO/SiO2 suggest that the peridotites reacted with a hydrous fluid derived from interaction of seawater with the overlying sediments composed of limestones with minor amounts of chert. This is further affirmed by average δ13C, δ18O and 87Sr/86Sr from carbonate veins in the peridotites that are similar to values of the sediments. Clumped isotope thermometry on calcite veins in peridotite establish that they formed at 25–60 ℃. Reaction path modeling of carbonate- quartz derived fluids with peridotite reproduces the observed mineral assemblage composed of carbonate and serpentine with similar Mg and MgO/SiO2 at high water to rock ratios, with carbon, H2O and silica added to the rock by the reacting fluid. Close to the unconformity the altered peridotites are characterized by concentric alteration halos recording variable fO2 and fS2. The partially serpentinized cores preserved primary minerals and record extremely low oxygen fugacities (fO2~10^(-75) bars). Two alteration zones are present evident from the alteration color. These zones exhibit nonisochemical alteration characterized by intergrowths of stevensite/lizardite. The alteration zones record progressively higher (fO2) recorded by Ni-rich sulfides and iron oxides/hydroxides. The alteration zones lost 20-30% of their initial magnesium content, together with mobilization of iron over short distances from inner green zones into outer red zones, where iron was reprecipitated in goethite intermixed with silicates due to higher fO2. The loss of magnesium in this peridotites motivated the final section of work. Mg isotopic compositions of partially serpentinized harzburgites and dunites in Oman are identical to average mantle and bulk silicate Earth (δ26Mg=-0.25‰) while altered periodites from Wadi Fins get heavier with increasing alteration. Analyses of peridotite alteration products including samples from Wadi Fins and carbonates from Wadi Tayin were used to show that isochemical serpentinization at low W/R does not fractionate Mg isotopes. I propose a mechanism that with increasing W/R and co-precipitation of Mg-carbonates and serpentine leads to carbonates with light isotopic compositions (Magnesite δ26Mg =-3.3 and dolomite δ26Mg=-1.91) and serpentine with heavy compositions (up to δ26Mg =-0.96 in serpentine veins). This complementary enrichment-depletion and the finite 14C ages of the carbonates suggest that serpentinization is ongoing along carbonation in Oman at ambient temperatures. Rates of calcite precipitation in travertines inferred from Δ26Mgcal-fl suggest that travertine formation in Oman sequesters a total of 10^6-10^7 kg CO2/yr.