Differential alteration of basaltic lava flows and hyaloclastites in Icelandic hydrothermal systems

Abstract

Geological field observations evidence that active and fossil Icelandic hydrothermal systems are typically embedded into an intercalation of almost completely altered and nearly unaltered volcanic rock layers. We investigated the reasons for this finding with help of geochemical reaction path calculations, by studying the mineralogical evolution of contrasting lithofacies-basalt flows and hyaloclastites at various temperatures and pressures, different recharge water composition, and gas content. From this study, we conclude that the initial porosity of protoliths and volume changes due to their transformation into secondary minerals are sufficient to explain the different extents of alteration as observed in field studies. In addition, we present a generalized kinetic model to estimate the alteration time of glassy fragments in hyaloclastite as a function of grain size, surface roughness, and temperature. This time was found to be rather short, ranging from a few hours to a few years.

Three‐Dimensional Electrical Resistivity Tomography of the Solfatara Crater (Italy): Implication for the Multiphase Flow Structure of the Shallow Hydrothermal System

“…Indeed, the Solfatara crater has been hosting a sustainable hydrothermal system for the last ~4,000 years without eruptions (Isaia et al, ). Chemical weathering and leaching produced by the circulation of hot acid hydrothermal fluids, and meteoric water infiltration, are well‐known processes with the capacity to almost completely alter volcanic rocks (e.g., Keller, ; Thien et al, ).…”

Permeability Changes Resulting from Quartz Precipitation and Dissolution around Upper Crustal Intrusions

“…The magnitude of permeability reduction induced by mineral deposition strongly depends on the initial or "primary" porosity of the host rock [10,23,24]. A given volume of secondary mineral precipitates will reduce the permeability of initially low porosity rocks more than in high porosity rocks.…”

“…A given volume of secondary mineral precipitates will reduce the permeability of initially low porosity rocks more than in high porosity rocks. While the clogging of pore space by secondary mineral precipitates in low porosity rocks limits the maximum extent of alteration, alteration in high porosity rocks may proceed to completion, with a total replacement of the host rock by secondary minerals [24]. The porosity of rocks hosting geothermal reservoirs can vary over a wide range, from <0.05 to >0.6 [25,26].…”

Fluid Chemistry of Mid-Ocean Ridge Hydrothermal Vents: A Comparison between Numerical Modeling and Vent Geochemical Data

“…Previous experimental and modeling work on the alteration of both glassy and crystalline mafic rocks suggests that although basaltic glass dissolves faster than more crystalline basaltic rocks [77], the crystallinity of the protolith has only a limited influence on the formation of secondary minerals when approaching fluid/rock equilibria at hydrothermal conditions [51,71,[78][79][80][81][82]. Similar observations have been made in Iceland, where low temperature alteration products of volcanic glasses have similar compositions to those formed during the alteration of crystalline rocks on longer geological timescales [83,84].…”

“…For S S S S S P S P,S S S S P 200 250 300 350 400°C Mineral formula Mineral 2 50 Figure 1: Temperature stability constraints for minerals occurring in hydrothermally altered mid-ocean ridge basalts. Full black lines: upper and/or lower stability constraints of the phase added in the code according to alteration mineralogy data after 1 Arnold et al [39], 2 Lonsdale et al [40], 3 Henley and Ellis [41], 4 Jenkins et al [42], 5 Reyes [43], 6 Hannington et al [44], 7 White and Hedenquist [45], 8 Jové and Hacker [46], 9 Chipera and Apps [47], 10 Bird and Spieler [48], 11 Monecke et al [49], 12 Hodgkinson et al [50], and 13 Thien et al [51]. Dashed grey lines: nonconstrained mineral phase.…”