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Origin and migration of fluoride in the area of the Aluto Volcanic Complex (Main Ethiopian Rift)
(2022)
Fluoride-enriched ground and surface waters represent a major health risk for the local population in many areas along the East African Rift.
The present study investigates the origin of fluoride and the reason for its accumulation in the rift waters, following two hypotheses: (i) fluid-rock-interactions release fluoride from minerals into the water and (ii) magmatic-derived fluoride-containing liquids and gases migrate along permeable fault zones until they dissolve in ground-and surface water or be released to the atmosphere.
Rock-, gas, water-, soil-, and plant samples were collected from the area within and close by the Aluto Volcanic Complex, which is part of the Main Ethiopian Rift. Most analyzed waters showed fluoride concentrations above the drinking water limit (> 1.5 mg/L) with the highest values in hot springs (up to 70 mg/L) and the geothermal well (76 mg/L).
In the solid phase, a high fluoride content was found in acid volcanic rocks (ignimbrite: 4391 ppm; rhyolite: 3248 ppm) as well as in pumice (up to 1955 ppm). The fluoride content of soil samples collected within the volcanic complex varied between 82 and 1036 ppm, whereas former lake sediments from outside the Aluto Volcanic Complex showed higher fluoride contents ranging from 674 to 8171 ppm. Identified fluoride-rich minerals are various amphiboles (about 3 wt.-% F-), fluor-apatite (about 3 wt % F-), minerals of the fluorcaphite group (about 5 wt.-% F-), parisite (up to 9 wt.-% F-), and fluorite (CaF2).
Elevated fluoride concentrations were also measured in some gas samples from fumaroles (up to 50 ppm) and in plant samples collected next to the fumaroles (up to 65 ppm). Leaching experiments of solid samples with deionized water demonstrated that fluoride can easily be mobilized from pumice and sediments but hardly from consolidated rocks.
This fluoride release increased with temperature (up to 150 C) and correlated roughly with dis-solved silica indicating the binding of some fluoride to the amorphous or weakly crystalline silica fraction. Based on these results it was concluded that fluoride migrates via different pathways through the environment: At the depth of the magma chamber during magmatic differentiation fluoride enriches initially in the magmatic melt and accumulates in some late-crystallizing minerals of igneous rocks such as fluorite or mica. Upon volcanic eruption fluoride is predominantly incorporated in the glass (ignimbrite) and ash phase (pumice).
On the surface, these rocks are exposed to weathering and fluoride leaches partly out into the aqueous phase. Soft and porous rocks such as pumice release fluoride first whereas hard extrusive/igneous rocks are less prone to weathering and retain the fluoride. Pumice and (lake) sediments might act both, as source and as sink for fluoride in the area.
Although some fluoride might drain from the surface into the deeper subsoil, we conclude that magmatic fluids (liquid and gas) contribute more substantially to the overall fluoride ground water content because (i) of the much higher fluoride content in deep geothermal waters and hot springs as compared to surface-near waters; (ii) active geothermal surface manifestations located along fault zones indicate that fluids migrate from deep magmatic intrusions (as gas and liquid) towards the surface, where fluoride dissolves in groundwater; (iii) and the good correlation between bicarbonate (deriving from dissolution of magmatic CO2) and fluoride content in all analyzed water samples.
Monitoring the response of volcanic CO2 emissions to changes in the Los Humeros hydrothermal system
(2021)
Carbon dioxide is the most abundant, non-condensable gas in volcanic systems, released into the atmosphere through either diffuse or advective fluid flow. The emission of substantial amounts of CO2 at Earth's surface is not only controlled by volcanic plumes during periods of eruptive activity or fumaroles, but also by soil degassing along permeable structures in the subsurface. Monitoring of these processes is of utmost importance for volcanic hazard analyses, and is also relevant for managing geothermal resources. Fluid-bearing faults are key elements of economic value for geothermal power generation. Here, we describe for the first time how sensitively and quickly natural gas emissions react to changes within a deep hydrothermal system due to geothermal fluid reinjection. For this purpose, we deployed an automated, multi-chamber CO2 flux monitoring system within the damage zone of a deep-rooted major normal fault in the Los Humeros Volcanic Complex (LHVC) in Mexico and recorded data over a period of five months. After removing the atmospheric effects on variations in CO2 flux, we calculated correlation coefficients between residual CO2 emissions and reinjection rates, identifying an inverse correlation of rho = - 0.51 to - 0.66. Our results indicate that gas emissions respond to changes in reinjection rates within 24 h, proving an active hydraulic communication between the hydrothermal system and Earth's surface. This finding is a promising indication not only for geothermal reservoir monitoring but also for advanced long-term volcanic risk analysis. Response times allow for estimation of fluid migration velocities, which is a key constraint for conceptual and numerical modelling of fluid flow in fracture-dominated systems.
The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and multiparameter observation of earthquake swarms and related phenomena, such a laboratory comprises a set of shallow boreholes with high-frequency 3-D seismic arrays as well as modern continuous real-time fluid monitoring at depth and the study of the deep biosphere.
This laboratory is located in the western part of the Eger Rift at the border of the Czech Republic and Germany (in the West Bohemia–Vogtland geodynamic region) and comprises a set of five boreholes around the seismoactive zone. To date, all monitoring boreholes have been drilled. This includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline units north and east of the major Nový Kostel seismogenic zone, borehole F3 in the Hartoušov mofette field and borehole S4 in the newly discovered Bažina maar near Libá. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological research. At each of these sites, a borehole broadband seismometer will be installed, and sites S1, S2 and S3 will also host a 3-D seismic array composed of a vertical geophone chain and surface seismic array. Seismic instrumenting has been completed in the S1 borehole and is in preparation in the remaining four monitoring boreholes. The continuous fluid monitoring site of Hartoušov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.
The new in situ geodynamic laboratory established in the framework of the ICDP Eger project aims to develop the most modern, comprehensive, multiparameter laboratory at depth for studying earthquake swarms, crustal fluid flow, mantle-derived CO2 and helium degassing, and processes of the deep biosphere. In order to reach a new level of high-frequency, near-source and multiparameter observation of earthquake swarms and related phenomena, such a laboratory comprises a set of shallow boreholes with high-frequency 3-D seismic arrays as well as modern continuous real-time fluid monitoring at depth and the study of the deep biosphere.
This laboratory is located in the western part of the Eger Rift at the border of the Czech Republic and Germany (in the West Bohemia-Vogtland geodynamic region) and comprises a set of five boreholes around the seismoactive zone. To date, all monitoring boreholes have been drilled. This includes the seismic monitoring boreholes S1, S2 and S3 in the crystalline units north and east of the major Novy Kostel seismogenic zone, borehole F3 in the Hartousov mofette field and borehole S4 in the newly discovered Bazina maar near Liba. Supplementary borehole P1 is being prepared in the Neualbenreuth maar for paleoclimate and biological research. At each of these sites, a borehole broadband seismometer will be installed, and sites S1, S2 and S3 will also host a 3-D seismic array composed of a vertical geophone chain and surface seismic array. Seismic instrumenting has been completed in the S1 borehole and is in preparation in the remaining four monitoring boreholes. The continuous fluid monitoring site of Hartousov includes three boreholes, F1, F2 and F3, and a pilot monitoring phase is underway. The laboratory also enables one to analyze microbial activity at CO2 mofettes and maar structures in the context of changes in habitats. The drillings into the maar volcanoes contribute to a better understanding of the Quaternary paleoclimate and volcanic activity.
In this paper we present densely sampled fumarole temperature data, recorded continuously at a high-temperature fumarole of Mt. Merapi volcano (Indonesia). These temperature time series are correlated with continuous records of rainfall and seismic waveform data collected at the Indonesian - German multi-parameter monitoring network. The correlation analysis of fumarole temperature and precipitation data shows a clear influence of tropical rain events on fumarole temperature. In addition, there is some evidence that rainfall may influence seismicity rates, indicating interaction of meteoric water with the volcanic system. Knowledge about such interactions is important, as lava dome instabilities caused by heavy-precipitation events may result in pyroclastic flows. Apart from the strong external influences on fumarole temperature and seismicity rate, which may conceal smaller signals caused by volcanic degassing processes, the analysis of fumarole temperature and seismic data indicates a statistically significant correlation between a certain type of seismic activity and an increase in fumarole temperature. This certain type of seismic activity consists of a seismic cluster of several high-frequency transients and an ultra-long-period signal (< 0.002 Hz), which are best observed using a broadband seismometer deployed at a distance of 600 m from the active lava dome. The corresponding change in fumarole temperature starts a few minutes after the ultra-long-period signal and simultaneously with the high-frequency seismic cluster. The change in fumarole temperature, an increase of 5 degreesC on average, resembles a smoothed step. Fifty-four occurrences of simultaneous high-frequency seismic cluster, ultra-long period signal and increase of fumarole temperature have been identified in the data set from August 2000 to January 2001. The observed signals appear to correspond to degassing processes in the summit region of Mt. Merapi. (C) 2004 Elsevier B.V. All rights reserved
During the second phase of the Alpine Fault, Deep Fault Drilling Project (DFDP) in the Whataroa River, South Westland, New Zealand, bedrock was encountered in the DFDP-2B borehole from 238.5–893.2 m Measured Depth (MD). Continuous sampling and meso- to microscale characterisation of whole rock cuttings established that, in sequence, the borehole sampled amphibolite facies, Torlesse Composite Terrane-derived schists, protomylonites and mylonites, terminating 200–400 m above an Alpine Fault Principal Slip Zone (PSZ) with a maximum dip of 62°. The most diagnostic structural features of increasing PSZ proximity were the occurrence of shear bands and reduction in mean quartz grain sizes. A change in composition to greater mica:quartz + feldspar, most markedly below c. 700 m MD, is inferred to result from either heterogeneous sampling or a change in lithology related to alteration. Major oxide variations suggest the fault-proximal Alpine Fault alteration zone, as previously defined in DFDP-1 core, was not sampled.