Carbonate Formation in the Samail Ophiolite, Oman: A Case Study for CO2 Sequestration Through Alteration of Mantle Peridotites
AbstractIn the last few years, considerable attention has been focused on carbon capture and storage to mitigate anthropogenic input to atmospheric CO2. One proposed option for mitigation is to increase conversion of CO2 gas to stable, solid carbonate minerals during alteration of tectonically exposed mantle rock (or peridotite).
While natural carbonation of subaerial and submarine peridotite is commonly observed, its rate, and therefore the rate of CO2 uptake via this alteration 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 goal of the proposed research is to determine the natural rate of carbonation of peridotite in the Samail Ophiolite, Oman – one of the largest and best-exposed ophiolites in the world. We will use a combination of field mapping to obtain alteration volume estimates, radiogenic dating to determine carbonate formation ages, and cosmogenic dating to determine carbonate (and ultramafic bedrock) weathering rates in the ultramafic layer of the ophiolite. This will enable us to quantify the residence time of carbon in three types of carbonate alteration products (veins, travertine deposits, carbonate-rich cements) forming in the ophiolite. We will determine 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). This will provide a powerful approach to de-convolving the effects of open-system behavior. In addition, we will determine weathering rates for both the carbonates and the host ultramafic bedrock by cosmogenic dating of exposed surfaces. We will use 36Cl to date carbonate weathering surfaces and 3He to date ultramafic weathering surfaces. Combining volume estim ates from field mapping with the radiogenic ages of the carbonate alteration will allow us to determine natural rates of carbonate formation in the ophiolite.
This project constitutes the Ph.D. thesis research for MIT/WHOI Joint Program student Evelyn Mervine – the funds requested are to cover the radiogenic and cosmogenic dating of samples. This proposal is relevant to the new DOEI theme: “Role of the Deep Earth and Ocean in Elemental Cycles” because it addresses carbon cycling and storage in deep Earth (mantle) rocks exposed at Earth’s surface. Peridotites similar to those found in Oman are known to be exposed along the global mid-ocean ridge system, particularly at slow and ultraslow spreading ridges. Seawater circulation through seafloor peridotites gives rise to pervasive carbonate veining as well as extensive seafloor carbonate deposits, such as those seen at the Lost City hydrothermal site. However, finding and sampling these exposures is expensive and challenging. Studying carbonate formation in oceanic peridotites that have been uplifted onto land provides an excellent analog for carbonate formation in peridotites on the seafloor and a case study for their potential for carbon sequestration.