In the last few years, considerable attention has been focused on carbon capture and storage to mitigate anthropogenic input of atmospheric CO2. One proposed option for mitigation is to increase conversion of CO2 gas to stable, solid carbonate minerals during chemical weathering of tectonically exposed mantle rock (or peridotite). While natural carbonation of peridotite is commonly observed, its rate, and therefore the rate of CO2 uptake via this weathering mechanism, is poorly known. Determining the natural rate of peridotite carbonation is critical for understanding the influence of this potentially important ?sink? in the global carbon cycle. The natural peridotite carbonation rate is also an essential, but poorly constrained, parameter in calculations evaluating the viability of using artificially-enhanced, in situ alteration of peridotite to mitigate the buildup of anthropogenic input to atmospheric CO2.
The primary objective of this research is to determine natural carbonation rates of exposed mantle peridotites. The Samail Ophiolite, Oman, is one of the largest and best-exposed ophiolites in the world, and hence is an ideal natural laboratory for investigating natural carbon storage in mantle peridotites. The research team has selected and carefully mapped eight sites that include roadcuts exposing carbonate veins, travertine deposits, and one site located on the natural peridotite weathering surface and containing abundant carbonate veins. The work plan consists of determining the ages of carbonates using two independent radiogenic dating techniques: 14C (for carbonates as old as ~50,000 years) and 238U-230Th (for carbonates as old as ~350,000 years). In addition, they will determine exposure ages for the host mantle peridotite to test the hypothesis that carbonate weathering primarily occurs in a thin weathering horizon that keeps pace with erosion rates. This project constitutes the Ph.D. thesis research for MIT/WHOI Joint Program student Evelyn Mervine. An improved understanding of natural carbonate formation in ultramafic rocks will aid efforts to develop CO2 capture and storage in ultramafic rocks to offset anthropogenic CO2 emissions and mitigate global climate change.
Alteration of ultramafic (peridotite) rocks is an important, but poorly quantified, category of silicate weathering. Peridotites are composed primarily of the minerals olivine and pyroxene and are far from equilibrium with H2O and CO2 on the Earth's surface. Thus, when uplifted and exposed, peridotites are easily altered to hydrous silicates, Fe-oxides, and carbonates (calcite, magnesite, dolomite). This alteration occurs naturally at low temperatures when peridotite is exposed to water. However, the rates at which these reactions occur are poorly quantified. Because weathering of silicate rocks provides an important sink for atmospheric CO2, determining peridotite weathering rates and the residence times of carbon in the alteration products is necessary to quantify the role of weathering of ultramafic rocks in the global carbon cycle. The natural peridotite carbonation rate is also an essential, but poorly determined, parameter in calculations evaluating the viability of using artificially-enhanced, in situ alteration of peridotite to help mitigate the build-up of anthropogenic input to atmospheric CO2 (Lackner et al., 1995; Kelemen and Matter, 2008; Wilson et al., 2006; 2009; Kelemen et al., 2011). The Samail Ophiolite, Oman is one of the largest and best-exposed ophiolites in the world, and hence is an ideal natural laboratory for investigating the rates of natural carbonation of altered peridotite. To determine timescales of natural carbonation of peridotite in the Samail Ophiolite, we combined 14C and 230Th-238U radiogenic dating to determine carbonate formation ages. This work was the PhD thesis of Eveline Mervine, successfully defended in April 2012. The following details the results of this study. 14C Dating: Detailed 14C dating as well as stable C and O isotope analyses were conducted on carbonate alteration products in the peridotite layer of the Samail Ophiolite, Sultanate of Oman. 14C results obtained in this and previous (Clark and Fontes, 1990; Clark et al., 1992; Kelemen and Matter, 2008; Kelemen et al., 2011; Kelemen et al., unpublished data) studies indicate that surface travertines range in age from modern to >45,000 yr BP, indicating long-term deposition and preservation. Travertine deposition rates in two localities were ~0.1-0.3 mm/yr between ~30,000-45,000 yr BP. Using an estimate of total travertine area from Kelemen and Matter (2008), this would result in a maximum of ~1,000-3,000 m3/yr of travertine being deposited throughout the ophiolite during this time period. This travertine deposition would have sequestered a maximum of ~1-3 x 106 kg CO2/yr. Ca-rich carbonate veins that are associated with the surface travertine deposits have ages ranging from ~4,000-36,000 yr BP (average: 15,000 yr BP). Mg-rich carbonate veins exposed in outcrops have ages ranging from ~8,000-45,000 yr BP (average: 35,000 yr BP). Sampling from numerous locations indicates that no carbonate veins from the natural peridotite weathering surface are older than ~50,000 years. However, 14C dating of Mg-rich carbonate veins from three roadcut exposures (Qafeefah, Fanja, and Al-Wuqbah) indicates that a significant number of roadcut veins are 14C dead (>50,000 yr BP). Mg-rich carbonate veins are estimated to sequester on the order of 107 kg CO2/yr throughout the ophiolite. 230Th/238U Dating: Due to their low U concentrations and relatively high Th/U ratios (0.01 to 5.6; average: 1.3), Samail carbonates are challenging to date with the 230Th-238U technique due to the sensitivity of the ages to corrections for initial 230Th. Uncorrected 230Th-238U ages for Ca-rich travertines are consistently older than previously obtained 14C ages. However, geologically reasonable initial 230Th corrections for the 230Th/238U ages bring the two sets of ages into concordance. In contrast, uncorrected 230Th-238U ages for Mg-rich carbonate veins sampled on the natural peridotite weathering surface are generally younger than previously obtained 14C ages. This is likely due to the addition of excess 238U to the carbonate outcrop veins during alteration by reducing serpentinization fluids. Two Mg-rich carbonate veins sampled from roadcuts have near-equilibrium (230Th/238U) and (234U/238U), which indicates that these veins are >375,000 years in age, consistent with their "14C Dead" (>50,000 yr BP) ages.
