Petrologic Study of Peridotite Carbonation in Oman: Temperature, Timing and Fluid Composition

该项目是对阿曼Samail蛇绿岩地幔部分的大型陆上橄榄岩块的热液蚀变和风化的研究。橄榄岩是一种主要由矿物橄榄石（宝石名称，“橄榄石”）组成的岩石，它包含了地球上地幔的大部分，但通常被海洋和大陆地壳屏蔽，不与表面岩石反应。板块构造碰撞加上侵蚀使表面的橄榄岩暴露出来，在那里它与地表水迅速反应。建议通过橄榄岩与近地表水的反应来研究水合矿物和固体碳酸盐矿物的形成(a)活跃的，持续的低t系统，可能在30至60°C，涉及到大气水；(b)更古老的，化石高t系统，可能达到~ 200°C，形成完全碳化的橄榄岩，其中所有的镁和钙都在固体碳酸盐矿物中，称为李氏石。高t系统可能涉及蛇绿岩下含碳酸盐的变质沉积层的流体。他们将通过化学和同位素分析来限制活跃的低t系统和化石的高t系统的温度、时间和流体组成。目标是限制控制橄榄岩有效碳酸化的因素，扩展相平衡的热力学模型，纳入固溶体模型，并提供同位素温度计的交叉校准。假设橄榄岩的完全碳酸化发生在压力、温度和流体成分接近橄榄石碳酸化速率最大的条件下，并且快速的碳酸化速率促进了体积变化、反应引起的裂缝、渗透率和反应速率之间的正反馈。验证这一假设的关键一步是确认该小组关于橄榄岩碳酸化条件的初步推论。人们可能会期望渗透率的降低和反应表面的装甲会限制反应的程度，但天然橄榄岩碳酸化可以并且确实会进行完井，其中所有的镁和钙（以及大部分铁）都被碳酸盐矿物吸收。同样，在大体积的岩石中，加入H2O通常会产生100%的Mg水化。这项研究将描述导致橄榄岩迅速、广泛水化和碳酸化的化学和物理过程。地下水或海水中的二氧化碳与橄榄岩发生反应，形成丰富的碳酸盐矿物，这一过程是由巨大的可用化学势能库驱动的。丰富的矿物橄榄石的碳化作用比其他丰富的造岩矿物要快。最近，有几篇论文强调了橄榄岩的原位碳酸化潜力，无论是通过与注入的富含二氧化碳的流体反应，还是仅仅通过与表层海水的强化反应。最佳的橄榄岩碳酸化条件可产生约10亿吨/km3 /年的二氧化碳吸收量，并可提供高达约100万亿吨二氧化碳的巨大储层。这项研究的结果将有助于未来设计用于原位地质二氧化碳捕获和储存（CCS）的工程技术。特别是，我们需要了解天然橄榄岩碳酸化过程如何避免由于碳酸盐矿物在孔隙空间沉淀过程中渗透率降低和活性表面积损失而造成的潜在限制。如果在工程系统中能够克服这些负反馈，那么在固体碳酸盐矿物中就地储存二氧化碳的成本可能比传统的将二氧化碳注入地下孔隙空间进行CCS更具竞争力。与将流体注入孔隙空间相比，原位矿物碳酸化带来的性质问题和泄漏危险更少，提供了稳定、惰性、无毒的储存。

This project is a study of hydrothermal alteration and weathering of large, subaerial peridotite massifs that form the mantle section of the Samail ophiolite in Oman. Peridotite is a rock comprised mainly of the mineral olivine (gemstone name, 'peridot'), and it comprises most of the Earth's upper mantle but is normally shielded from reaction with surface rocks by the oceanic and continental crust. Plate tectonic collisions coupled with erosion expose peridotite at the surface, where it reacts rapidly with surface waters. It is proposed to investigate formation of hydrated minerals and solid carbonate minerals via reaction of peridotite with near-surface waters in (a) active, ongoing low-T systems, probably at 30 to 60°C and involving meteoric water, and (b) older, fossil high-T systems which may have reached ~ 200°C, formed completely carbonated peridotites - in which all Mg and Ca are in solid carbonate minerals - called listwanites. The high-T system may have involved fluids from carbonate bearing metasediments beneath the ophiolite. They will constrain the temperatures, timing, and fluid composition of both the active low-T systems and fossil high-T systems through chemical and isotopic analysis. The goal is to constrain factors that control efficient carbonation of peridotite, expand thermodynamic models of phase equilibria incorporating solid solution models, and provide cross-calibration of isotope thermometers. It is hypothesized that complete carbonation of peridotite occurred at pressures, temperatures and fluid compositions close to the conditions at which olivine carbonation rates are maximized, and that rapid rates facilitate a positive feedback between volume change, reaction-induced cracking, permeability, and reaction rate. A key step in testing this hypothesis is to confirm preliminary inferences by the group about the conditions of peridotite carbonation. One might expect decreasing permeability and armoring of reactive surfaces to limit the extent of reaction, but natural peridotite carbonation can and does go to completion, in which all Mg and Ca (and much of the Fe) are incorporated in carbonate minerals. Similarly, addition of H2O commonly produces 100% hydration of Mg in large volumes of rock. This study will characterize the chemical and physical processes that lead to rapid, extensive peridotite hydration and carbonation.Reaction of CO2 from ground water or seawater with peridotite forms abundant carbonate minerals, in processes that are driven by a vast reservoir of available, chemical potential energy. Carbonation of the abundant mineral, olivine, is faster than for other abundant, rock-forming minerals. Recently, several papers have emphasized the potential for in situ carbonation of peridotite, either via reaction with injected, CO2-rich fluids, or simply via enhanced reaction with surface-derived seawater. Optimal peridotite carbonation conditions could yield CO2 uptake of ~ 1 billion tons/km3 of peridotite/yr, and could provide an enormous reservoir for up to ~ 100 trillion tons of CO2. Results of this study will facilitate future design of engineered, in situ techniques for in situ geological CO2 capture and storage (CCS). In particular, we need to learn how natural peridotite carbonation processes avoid potential limitations due to decreasing permeability and loss of reactive surface area during precipitation of carbonate minerals in pore space. If these negative feedbacks can be overcome in an engineered system, in situ storage in solid carbonate minerals may be cost competitive with more conventional injection of CO2 into subsurface pore space for CCS. In situ mineral carbonation poses fewer property problems and leakage hazards than injection of fluid into pore space, providing stable, inert, non-toxic storage.