Carbonation of porous rocks by interaction with magmatic and hydrothermal fluids - a case study on Unzen volcano, Japan
Final Report Abstract
The degassing of magmas during ascent is strongly affected by the permeability of the surrounding rocks. An efficient release of volatiles from the magma before reaching the surface reduces the explosivity of the volcano. The rock porosity, however, is not a fix property but it may change by reaction with fluids which may induce hydrothermal alteration and carbonation of rocks. Such processes can both close and open paths for gases and the overall effect is difficult to predict. Effects of hydrothermal alteration on rock properties - primary mineral dissolution, precipitation of secondary phases - depends on various parameters including permeability and porosity of the rock, aqueous diffusion coefficients, thermodynamic equilibrium constants, and mineral dissolution/recrystallisation rates. Our project aimed to provide detailed insights on the effect of penetrating volcanic fluids on mineral alteration, pore space geometry, and transport of rocks from Unzen volcano. Changes of porosity during progress of alteration were analyzed by application of mercury intrusion porosimetry, Woods metal intrusion, N2-adsorption, and tomography. By penetration of volcanic fluids into coherent andesites and dacites with relatively low porosity, carbonates, up to 19,3 %, were formed, which are together with pyrite and chlorite important neoformed phases affecting pore space geometry and transport in low porosity coherent andesites and dacites from Unzen. Despite marked precipitation of secondary minerals extensive connective pore networks are present in all investigated samples from conduit and surrounding rocks, allowing fluid exchange and degassing. Weathering micromorphology shows that dissolution of primary minerals, in particular hornblende, contributes to formation of new pores. Qualitative pore space classification resulted in identification of four different types. Vuggy porosity observed in two types of pore spaces is clearly related to carbonate precipitation resulting in less pore network. Mostly well connected pore networks were preserved due to precipitation of carbonates in the neighboring matrix. Fracture dominated porosity has a relative bad connectivity. The comparison of calculated connectivity values with median pore diameter indicates a correlation between pore space classes and main hydraulic pathways. Due to the typically low porosity of coherent dacitic and andesitic rocks, primary mineral dissolution and precipitation of secondary phases lead to significant changes in transport properties. Diffusion experiments at 5, 20, 30, 40, and 50°C were performed with samples where the carbonate content is ranging from 2,9-19,3 % and porosity from 3,9 to 17,2 vol.% in order to identify and quantify the influence of pore space characteristics on intra-rock transport. Effective diffusion of water in Unzen samples is 1-3 orders of magnitude slower than the diffusion of protons and 1,5-2,5 orders of magnitude slower than diffusion of H2O and D2O in liquid water, indicating that porosity and pore characteristics exert a significant control on the effectiveness of mass transfer. For the diffusion coefficient (D eff), the experimental trend of log "D eff" vs. reciprocal temperature is steeper, indicating higher activation energy than for diffusion in liquid H2O and D2O implying that temperature has a smaller effect on diffusion. Including findings from previous studies on the hydrothermal system, these data can be used to improve our understanding of the evolution of circulating fluids and progression of alteration in these rocks. For carbonate formation two reaction scenarios are in agreement with the observed isotopic pattern: (1) Quantitative reaction between magmatic CO2 and silicates in the presence of negligible amounts of meteoric water. The generated carbonate inherits its C completely from magmatic CO2 and its O almost completely from the silicates. (2) Quantitative reaction between magmatic CO2 and silicates in the presence of fumarolic water. The C isotopic composition of the evolved carbonate would be defined by reaction stoichiometry. Hydrothermal fluid-rock experiments conducted at temperatures from 300 to 700°C and at pressures from 100 to 150 MPa, yielded in the formation of typically very small new phases, which could be analyzed by Raman microscopy. More work is needed to optimize the spectroscopic method for analyses of the experimental products and further experiments are required to reproduce natural mineral assemblages and to determine typical conditions for carbonate formation in Unzen conduit. Combining our results with those from other hydrothermal systems in magmatic rocks, a general model for predicting changes in permeability of rocks by reactions between penetrating gases and rock minerals can be established.
Publications
- 2010. Carbonation of porous volcanic rocks by interaction with
magmatic and hydrothermal fluids - a case study on Unzen volcano, Japan. IODP/ICDP Kolloquium "Geozentrum" Frankfurt, 2010, pp. 136-137.
Simonyan, A.V., Dultz, S., Behrens, H., Fiebig, J.
- 2011. Carbonation of porous rocks by interaction with magmatic and hydrothermal fluids – a case study on Unzen volcano, Japan. IODP/ICDP Kolloquium Münster 2011, pp. 167-169.
Simonyan, A.V., Dultz, S., Behrens, H., Fiebig, J., Voges, K.
- 2012 Carbonation of porous volcanic rocks by interaction with fluids at Unzen volcano, Japan. IODP/ICDP Kolloquium, GEOMAR Kiel, 2012, pp. 153-155.
Simonyan, A.V., Dultz, S., Behrens, H., Fiebig, J., Voges, K.
