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

云仙钻探项目是首次尝试通过在火山爆发后不久对活火山进行钻探来了解火山爆发的机制。该项目产生了几个意想不到的结果，即钻孔中的温度比预期的低得多，钻井岩心被大量的次生矿物如碳酸盐、黄铁矿和黄铁矿高度改变，这些次生矿物被认为是排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将钻井岩心与实验工作相结合，以更仔细地研究流体-岩石相互作用的机制，特别是岩石的碳酸化和脱碳作用。这项研究不仅对了解火山的深层脱气很重要，而且对洞穴或多孔/角砾状火山岩中CO2的储存也有重大影响。例如，碳酸盐的形成使CO2固定，并可能通过关闭开放通道而强烈改变岩石的渗透性。我们的研究涉及：（i）钻探岩心的岩相学研究，特别关注蚀变产物的结构和成分，（ii）碳同位素和氧同位素分析，以获得有关碳酸盐中CO2键起源的信息，（iii）使用伍德金属浸渍和分析预饱和样品中热释放水，表征不同蚀变岩石中的孔隙系统，（四）利用现场技术对挥发物在岩石孔隙空间中的迁移和反应进行实验研究;（五）在与火山近导管区有关的条件下（200 - 700 ° C和高达150 MPa）进行热液流体-岩石实验。目标（i）至（iii）已接近完成，而任务（iv）和（v）是未来两年的主要目标。在项目初期，通过各种分析技术确定了矿物成分和孔隙度，以便对渗透火山流体对矿物蚀变、孔隙空间几何形状和运输的影响有新的认识。尽管强烈的碳化，导致碳酸盐含量高达20%，但大多数孔隙是连通的。我们下一步的工作将是研究孔隙系统内的传输和岩石样品中碳酸盐形成/溶解的实验研究。此外，通过3D分析对岩石孔隙度和矿物产状的成像和分析将通过调整来自聚类标记的结果与来自矿物学分析的结果来优化。3D分析还将用于确定连通孔隙体积和优选孔隙方向。我们的研究结果将与其他研究云仙火山的研究小组的发现相结合，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.