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

点击翻译按钮获取中文摘要

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.