Carbonation of porous rocks by interaction with magmatic and hydrothermal fluids - a case study on Unzen volcano, Japan

Unzen钻探项目是首次尝试通过在活火山喷发后不久钻探活火山来深入了解火山喷发的机制。该项目产生了几个意想不到的结果，即钻孔内的温度远远低于预期，钻探岩心被大量次生矿物改变，如碳酸盐、绿泥石和黄铁矿，这些矿物被认为是喷出的火山流体与寄主岩石反应的产物。这些令人惊讶的发现启发我们结合实验工作使用钻探岩心来更仔细地观察流体-岩石相互作用的机制，特别是岩石的碳化和脱碳。这项研究不仅对理解火山的深层脱气作用很重要，而且对洞穴或多孔/角砾岩中二氧化碳的储存也有重大影响。例如，碳酸盐的形成固定了二氧化碳，并可能通过关闭开放的路径强烈地改变岩石的渗透率。我们的研究包括：(I)钻探岩心的岩石学调查，特别是蚀变产物的结构和组成；(Ii)碳同位素和氧同位素的分析，以获得碳酸盐中CO2键的来源信息；(Iii)用Wood金属浸渍和分析预饱和样品的热释放水来表征不同蚀变岩石中的孔隙系统；(Iv)利用原位技术对挥发分在岩石孔隙空间中的运移和反应进行实验研究。以及(V)在与火山导管附近区域有关的条件下(200-700℃和高达150兆帕)进行热液-岩石实验。目标(一)至(三)已基本完成，任务(四)和(五)是今后两年的主要目标。在项目初期，通过各种分析技术测定了矿物成分和孔隙度，以便对渗透火山流体对矿物蚀变、孔隙几何形状和运移的影响有新的认识。尽管密集的碳化作用导致碳酸盐含量高达20%，但大多数毛孔是连通的。我们下一步的工作将是调查孔隙系统内的运输和岩石样品中碳酸盐的形成/溶解的实验研究。此外，通过将聚类标记的结果与矿物学分析的结果进行调整，将优化三维分析的岩石孔隙度和矿物赋存状态的成像和分析。3D分析还将用于确定连接孔体积和优先孔取向。我们的研究结果将与其他致力于云顶火山的研究小组的研究结果相结合，以提高我们对流体-岩石相互作用和火山脱气的理解。

The Unzen drilling project was the first attempt to get insights into the mechanisms of volcanic eruptions by drilling into an active volcano shortly after eruption. The project yielded a couple of unexpected results, i.e. the temperature in the borehole was much lower than expected, and the drilling cores were highly altered with large amounts of secondary minerals such as carbonates, chlorite and pyrite, supposed to be products of reactions of discharged volcanic fluids with the host rocks. These surprising findings inspired us to use the drilling cores in combination with experimental work to have closer look on the mechanisms of fluid-rock interaction, in particular the carbonation and decarbonation of rocks. This research is not only important for understanding the deep degassing of volcanoes, but it has also major impacts for storage of CO2 in cavities or in porous/brecciated volcanic rocks. For instance the formation of carbonate immobilizes CO2 and may strongly change the permeability of rocks by closing open paths. Our research involves: (i) a petrographical investigation of drilling cores with special focus on texture and composition of alteration products, (ii) the analyses of carbon isotopes and oxygen isotopes to get information about the origin of the CO2 bond in carbonates, (iii) the characterization of pore systems in differently altered rocks using impregnation with Wood’s metal and analyzing thermally released water from pre-saturated samples, (iv) an experimental study of transport and reaction of volatiles in the pore space of rocks using in situ techniques, and (v) hydrothermal fluid-rock experiments at conditions relevant to the near-conduit region of the volcano (200 – 700°C and up to 150 MPa). The objectives (i) to (iii) have been almost completed whereas tasks (iv) and (v) are the main aims for the next two years. In the initial period of the project mineralogical composition and porosity was determined by various analytical techniques in order to get new insights on the effect of penetrating volcanic fluids on mineral alteration, pore space geometry, and transport. Despite intensive carbonization, which resulted in carbonate contents of up to 20 %, most of the pores are connective. The next steps of our work will be to investigate the transport within the pore system and the experimental study of formation/dissolution of carbonate in rock samples. Additionally, imaging and analyzing of rock porosity and mineral occurrence by 3D analysis will be optimized by adjusting results from cluster-labeling with those from mineralogical analysis. 3D analysis will also be used for determining connective pore volume and preferred pore orientation. The results of our research will be combined with findings from other research groups working on Unzen volcano to improve our understanding of fluid-rock interaction and volcanic degassing.