Magnesium isotopic compositions of a profile through saprolites developed on a diabase dike from South Carolina have been measured in order to study the behavior of Mg isotopes during continental weathering. As weathering progresses, Mg isotopes are greatly fractionated and are correlated with Mg concentration, clay mineral proportions and density of the saprolites. δ 26 Mg values increase from −0.22 in the unweathered diabase to + 0.65 in the most weathered saprolite. These observations are consistent with the release of light Mg to the hydrosphere and formation of isotopically heavy Mg in the weathered products. The loss of Mg during weathering can be modeled by Rayleigh distillation with an apparent fractionation factor between the saprolite and fluid (α) of 1.00005 to 1.0004, i.e., up to 0.4‰ fractionation in the 26 Mg/ 24 Mg ratio between the saprolite and fluid. The large variation in α value reflects a mineralogical control on Mg isotope fractionation during primary dissolution of Mg-rich minerals and formation of secondary minerals during continental weathering. Like Mg isotopes, Li isotopes in the saprolite profile are also greatly fractionated, with δ 7 Li values ranging from −6.7 down to −20. The large Li isotope fractionation and variation in Li concentration, as well as irregularities in the δ 7 Li profile with depth, however, cannot be explained by Li loss during weathering alone. Instead, Li can be modeled by a two-step process: (1) equilibrium isotope fractionation during continental weathering, which lowered δ 7 Li and Li concentrations and produced a Li concentration gradient in the saprolites like that seen in Mg, and (2) subsequent kinetic isotope fractionation produced by diffusion of Li in the saprolites, possibly across a paleo-water table. The results presented here suggest that continental weathering will shift the Mg isotopic composition of the continental crust to values higher than the mantle value, whereas crustal recycling over the history of the Earth will have no discernible effect on the Mg isotopic composition of the mantle.

Contrasting lithium and magnesium isotope fractionation during continental weathering

Volcanoes are unstable structures that deform laterally and frequently experience mass wasting events. Hydrothermal alteration is often invoked as a mechanism that contributes significantly to volcano instability. We present a study that combines laboratory experiments, geophysical data, and large-scale numerical modeling to better understand the influence of alteration on volcano stability, using La Soufriere de Guadeloupe (Eastern Caribbean) as a case study. Laboratory experiments on variably altered (advanced argillic alteration) blocks show that uniaxial compressive strength, Young's modulus, and cohesion decrease as a function of increasing alteration, but that the internal friction angle does not change systematically. Simplified volcano cross sections were prepared (a homogenous volcano, a volcano containing the alteration zone identified by a recent electrical survey, and a volcano with an artificially enlarged area of alteration) and mechanical properties were assigned to zones corresponding to unaltered and altered rock. Numerical modeling performed on these cross sections, using a hydro-thermo-mechanical modeling code, show (a) the importance of using upscaled values in large-scale models and (b) that alteration significantly increases volcano deformation and collapse volume. Finally, we combined published muon tomography data with our laboratory data to create a 3D strength map, exposing a low-strength zone beneath the southern flank of the volcano coincident with the hydrothermal system. We conclude that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide.

/pdf/alteration-induced-volcano-instability-at-la-soufriere-de-23bn0zyk5r.pdf

Alteration-Induced Volcano Instability at La Soufrière de Guadeloupe (Eastern Caribbean)

http://manuscript.elsevier.com/S037702732200107X/pdf/S037702732200107X.pdf

The tensile strength of hydrothermally altered volcanic rocks

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazu volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazu crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of Turrialba and Irazu volcanoes into two main rivers. Rio Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazu, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazu. All thermal spring waters are of SO4-type (i.e. steam-heated waters), none of the springs has, however, a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year (~ 2400 m3/year by San Cayetano-Rio Sucio). Despite Irazu being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazu’s crater lake in April 2010.

/pdf/response-of-a-hydrothermal-system-to-escalating-phreatic-1ldau5gvkd.pdf

Response of a hydrothermal system to escalating phreatic unrest: the case of Turrialba and Irazú in Costa Rica (2007–2012)

Hydrothermally induced edifice destabilisation: The mechanical behaviour of rock mass surrounding a shallow intrusion in andesitic lavas, Pinnacle Ridge, Ruapehu (New Zealand)

Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering

Lithium isotopes and Cu-Au concentrations in hydrothermal alterations from Solfatara Volcano, Campi Flegrei caldera complex, and La Fossa volcano, Vulcano Island, Italy: Insights into epithermal ore forming environments

Emily Mick

Papers

Hydrothermal alteration and sealing at Turrialba volcano, Costa Rica, as a mechanism for phreatic eruption triggering

Lithium isotopes and Cu-Au concentrations in hydrothermal alterations from Solfatara Volcano, Campi Flegrei caldera complex, and La Fossa volcano, Vulcano Island, Italy: Insights into epithermal ore forming environments