Collaborative Research: Rates of Carbonate Formation in the Samail Ophiolite, Oman: Implications of Ultramafic Weathering for the Carbon Cycle

在过去的几年里，人们非常关注碳捕获和封存，以减少大气中二氧化碳的人为输入。一种提出的缓解方案是，在构造裸露的地幔岩石(或橄榄岩)的化学风化过程中，增加二氧化碳气体向稳定的固体碳酸盐矿物的转化。虽然橄榄岩的自然碳化作用经常被观察到，但它的速度，以及通过这种风化机制吸收二氧化碳的速度，却鲜为人知。确定橄榄岩碳化的自然速率对于理解这一潜在的重要下沉的影响是至关重要的。在全球碳循环中。天然橄榄岩碳化速率也是评估利用人工增强的就地改变橄榄岩以减少人为输入大气CO2的可行性的基本参数，但限制很少。这项研究的主要目的是确定暴露在地幔中的橄榄岩的自然碳化速率。阿曼的Samail蛇绿岩是世界上最大、暴露最好的蛇绿岩之一，因此是研究地幔橄榄岩中天然碳储量的理想天然实验室。研究小组选择并仔细绘制了8个地点，其中包括露出碳酸盐矿脉、钙华矿床的路障，以及位于天然橄榄岩风化面的一个地点，其中包含丰富的碳酸盐矿脉。工作计划包括使用两种独立的放射性测年技术确定碳酸盐的年龄：14C(碳酸盐年代为~50 000年)和238U-230 Th(碳酸盐年代为~350 000年)。此外，他们还将确定寄主地幔橄榄岩的暴露年龄，以检验碳酸盐风化主要发生在与侵蚀速率同步的薄风化层中的假设。该项目构成了麻省理工学院/世界卫生组织联合项目学生伊夫林·默文的博士论文研究。加深对超镁铁岩中天然碳酸盐形成的了解，将有助于开发超镁铁岩中二氧化碳捕获和储存的努力，以抵消人为二氧化碳排放和减缓全球气候变化。

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.