Low-temperature Water Rock Interaction Research Articles

Reconstruction of Phanerozoic climate using carbonate clumped isotopes and implications for the oxygen isotopic composition of seawater

The oxygen isotope ratio 18O/16O (expressed as a δ18OVSMOW value) in marine sedimentary rocks has increased by ~8‰ from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the δ18Oseawater, and the effects of postdepositional diagenesis. Carbonate clumped isotope measurements, a temperature proxy, offer constraints on this problem. This thermometer is thermodynamically controlled in cases where carbonate achieves an equilibrium internal distribution of isotopes and is independent of the δ18O of the water from which the carbonate grew; therefore, it has a relatively rigorous chemical-physics foundation and can be applied to settings where the δ18O of the water is not known. We apply this technique to an exceptionally well-preserved Ordovician carbonate record from the Baltic Basin and present a framework for interpreting clumped isotope results and for reconstructing past δ18Oseawater. We find that the seawater in the Ordovician had lower δ18Oseawater values than previously estimated, highlighting the need to reassess climate records based on oxygen-isotopes, particularly where interpretations are based on assumptions regarding either the δ18Oseawater or the temperature of deposition or diagenesis. We argue that an increase in δ18Oseawater contributed to the long-term rise in the δ18O of marine sedimentary rocks since the early Paleozoic. This rise might have been driven by a change in the proportion of high- versus low-temperature water-rock interaction in the earth's hydrosphere as a whole.

Mines to moana: Hydrochemical legacies in a historically mined watershed

This study investigates contemporary freshwater contamination originating from old mine workings in the Thames-Coromandel region of Te Ika-a-Māui (North Island), Aotearoa New Zealand. We employed a multi-method approach, including stable H and O isotope determinations of surface water, geochemical modelling, and diffusive gradients in thin films (DGT) deployments. Atypically, spring-summer (Sept–Nov) surface water stable oxygen isotope ratios (δ18O) were negatively displaced from meteoric values (ca. −0.28 ± 0.13 ‰), indicating a remarkable degree of low-temperature water-rock interaction, likely enhanced by dry antecedent conditions. Isotope ratios in tributaries showed less alteration than the main stem, suggesting greater meteoric inputs and shorter residence times in areas of high topographic relief. Conversely, in the main stem, isotope ratios revealed higher mineral weathering, accompanied by elevated dissolved metal concentrations, consistent with dominant inputs from shallow groundwater. Weathering of primary sulfides contributed pronounced acidity in one tributary (pH ca. 3.8), but overall, carbonate buffering ameliorated acid mine drainage across the catchment (pH ca. 7–8). Nevertheless, our results confirm exceedances of ecological guideline values (>80% protection threshold) for several toxic metals including Al, Zn, Cd and Pb; with implications for fresh and coastal water quality in Tikapa Moana-o-Hauraki, the Firth of Thames.

The onset of deep recycling of supracrustal materials at the Paleo-Mesoarchean boundary.

The recycling of supracrustal materials, and in particular hydrated rocks, has a profound impact on mantle composition and thus on the formation of continental crust, because water modifies the physical properties of lithological systems and the mechanisms of partial melting and fractional fractionation. On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is still uncertain. Here, we use zircon oxygen (O) isotopes to track recycling of supracrustal materials into the magma sources of early Archean igneous suites from the Kaapvaal Craton, southern Africa. The mean δ18O values of zircon from TTG (tonalite–trondhjemite–granodiorite) rocks abruptly increase at the Paleo-Mesoarchean boundary (ca. 3230 million years ago; Ma), from mantle zircon values of 5‰–6‰ to approaching 7.1‰, and this increase occurs in ≤3230 Ma rocks with elevated Dy/Yb ratios. The 18O enrichment is a unique signature of low-temperature water–rock interaction on the Earth's surface. Because the later phase was emplaced into the same crustal level as the older one and TTG magmas would derive from melting processes in the garnet stability field (>40 km depth), we suggest that this evident shift in TTG zircon O isotopic compositions records the onset of recycling of the mafic oceanic crust that underwent seawater hydrothermal alteration at low temperature. The onset of the enhanced recycling of supracrustal materials may also have developed elsewhere in other Archean cratons and reflects a significant change in the tectonic realm during craton formation and stabilization, which may be important processes for the operation of plate tectonics on early Earth.

Natural Hexavalent Chromium in the Pollino Massif Groundwater (Southern Apennines, Italy): Occurrence, Geochemistry and Preliminary Remediation Tests by Means of Innovative Adsorbent Nanomaterials.

In this study Cr(tot), Cr(VI), major and trace elements were determined in groundwater of northern sector of the Pollino Massif (southern Italy). The investigated area is characterized by ophiolitic rocks consisting of metabasites, shales and calcschists and fractured serpentinites. Twomain hydro-facies were observed, reflecting lowtemperature water-rock interaction. The Mg-HCO3 hydrofacies is due to the weathering of serpentinites, Ca-HCO3 groundwaters are linked to the interaction with calcschist and metabasites. High Cr(VI) concentrations were detected, exceeding the maximum admissible concentrations by Italian regulation, due to the release of Cr(III) from ophiolitic rocks into water and its oxidation to the hexavalent state. Remediation tests were carried out using two synthetized nanomaterials, Fe(0) and magnetite, characterized by a mean size lower than 50nm. The experiments were conducted at fixed nanoparticles/Cr(VI) molar ratio and according to previous studies. In addition, the kinetic data were interpreted with a suitable mathematical model.

Ediacaran-Cambrian far-field hydrothermal event in the southeast of the São Francisco Craton, Brazil: Evidence from zircon U-Pb SHRIMP, trace elements, Lu-Hf and oxygen isotopes

Iron formation-bearing sequences from the Guanhães Tectonic Block and Southern Espinhaço Belt of the Southeastern São Francisco Craton, were affected by the Ediacaran-Cambrian Brasilian Orogeny, when the Guanhães Block was upthrusted onto Statherian metasedimentary units of the Lower Espinhaço Supergroup. The iron formation contain a number of hydrothermal high-grade iron ore bodies (e.g. Horto-Baratinha, Cuité, Morro Escuro and Morro do Pilar deposits). Three different types of zircons were studied in order to characterize the temporal and geochemical evolution of the late-orogenic, hydrothermal event: detrital zircons (Lower Espinhaço Supergroup – Orosirian-Statherian maximum depositional age), zircons from the gneissic basement (Archean Guanhães Complex), and zircons from metagranitoid (Statherian Borrachudos Suite). All populations include zircons with <5 μm to 50 μm wide areas (mainly rims) that are brighter in BSE images. These zones are much younger than the rest of the grains (U-Pb SHRIMP ages from 492 Ma to 554 Ma) and display higher U contents and lower Th/U ratios. Zircons from pegmatites that crosscut the Paleoproterozoic sequence show similar ages, as well as high U contents and low Th/U ratios. Some of the pegmatite zircons contain inherited cores from the Borrachudos Suite. A general ZrO2, SiO2 and Th depletion and U, Pb, Hf, Mg, Ca, Fe, Yb and P enrichment was observed in the zircon rims in comparison with their cores. Lower εHf(t) values and higher 176Hf/177Hf(t) ratios were obtained in the zircon rims relative to the unaltered cores. Higher δ18O values (above 7‰) were found in the rims of the detrital and pegmatite zircons, being approximately 1‰ higher than the cores, indicating low-temperature water-rock interactions. Uplift and juxtaposition of Neoarchean gneissic rocks of the eastern São Francisco Craton, Statherian granites and metamorphic supracrustal units occurred during the late to mainly post-collisional stages of the Brasilian Orogeny (560–490 Ma). This uplift and decompression promoted partial melting of the Borrachudos Suite, generating pegmatite intrusions and a magmatic-hydrothermal fluid that affected the entire sequence. Zircons from metasedimentary rocks, granite and gneiss underwent recrystallization generally at sub-solidus conditions during interaction with the moderate- to low-temperature hydrothermal fluid. A concentric porous fabric is present in some grains and registers the fluid pathways during crystal alteration and recrystallization. This recrystallization process had a large impact on the chemical and isotopic compositions of the zircons. The pegmatite zircons represent new anatectic, zircon growth or anatectic overgrowth on older cores in the presence of fluid-saturated melts.

Fluid capture during exhumation of subducted lithologies: A fluid inclusion study from Sifnos, Greece

We present phase equilibria modeling, combined with fluid inclusion microthermometry and textural observations, to constrain the composition and relative timing of subduction zone fluid-rock interaction during exhumation of two garnet-bearing blueschists from the northern part of the island of Sifnos, Greece. Water (H2O) from seawater was incorporated into the mafic lithologies by low-temperature water-rock interaction to produce hydrous phases, and was subsequently released during the subduction and exhumation cycle. Coupled phase equilibria modeling and textural observations suggest that fluid release resulted from the breakdown of the hydrous phases epidote and glaucophane, and some of the fluid was trapped as fluid inclusions in quartz. Homogenization temperatures and isochores of fluid inclusions are consistent with petrographic observations that suggest fluid inclusions were trapped during exhumation. Constructed isochores cross the inferred P-T path for Sifnos exhumation at pressures of ~0.3–1.0 GPa (~10–30 km depth) and temperatures of ~440–540 °C, suggesting fluid entrapment during the late stages of exhumation and cooling from peak subduction depths of ~70 km. The fluid inclusions have salinities of ~30–35 wt% NaCl equivalent, likely resulting from a multi-stage process that involves an increase in salinity during formation of hydrous minerals during subduction, leaving high salinity fluids and/or halide salts sequestered in pores. This is followed by the partial release of H2O during subsequent breakdown of high-pressure metamorphic phases, with later incorporation of H2O into newly forming retrograde phases. Portions of this fluid that interacted with earlier-formed saline fluids are now preserved as inclusions trapped in quartz. This interpretation suggests that fluid-rock interaction in the northern part of Sifnos occurred in a closed system, with a low water to rock mass ratio (~0.03 to ~0.05) and little to no infiltration of externally-derived lower salinity fluids. The closed system behavior is consistent with the observation that the blueschist unit is bounded by impermeable marble layers that limited exchange with external fluids during burial and/or exhumation.

The influence of oxygen isotope exchange between CO2 and H2O in natural CO2-rich spring waters: Implications for geothermometry

Oxygen isotope ratio (δ18O) value deviations from the Meteoric Water Line with no significant change in the hydrogen isotope (δ2H) composition have been reported in naturally occurring CO2-rich waters from around the world. Here we review the effects of oxygen isotope exchange with CO2, high temperature equilibration with bedrock minerals and mineral dissolution and precipitation reactions on the CO2-rich water isotopic composition. We present two case studies from Daylesford (Australia) and Pah Tempe (Utah, USA) mineral springs, where we use a numerical geochemical modelling approach to resolve the influence of low temperature water-rock interactions and CO2 equilibration to the oxygen isotope ranges observed in the mineral waters. In both cases, we find that mineral dissolution – precipitation reactions are unlikely to have a significant effect on the groundwater isotopic compositions, and that the observed δ18O values in natural CO2 springs can be simply explained by equilibrium fractionation between water and free phase CO2. Traditionally, the interaction of CO2 and water in a natural CO2-rich groundwater setting has only been associated with water 18O depletion and this is the first study to consider 18O enrichment. We establish that in a natural setting, CO2 and water equilibration can result in water 18O depletion or enrichment, and that the change in the oxygen isotope composition ultimately depends on the initial CO2 and water δ18O values. Our new conceptual model therefore provides a mechanism to explain water 18O enrichment at ambient temperatures. This finding is critical for the use of δ18O in groundwater geothermometry and for the interpretation of natural water circulation depths: we argue that in some cases, natural waters previously interpreted as geothermal based on their oxygen isotope composition may actually have acquired their isotopic signature through interaction with CO2 at ambient temperatures.

An Earth–Moon silicon isotope model to track silicic magma origins

A comparison between terrestrial and lunar igneous rocks reveals that Si isotope compositions become slightly, though significantly enriched in heavy isotopes from basalts to granites and anorthosites as a function of the rock SiO2 concentration and/or tectosilicate content. This is interpreted as the result of a global igneous differentiation process that leads to an increased amount of tectosilicates in the rocks. This relationship of increasing degree of melt polymerization with increasingly heavy silicon isotope composition is particularly apparent in lunar rocks. The terrestrial trend, however, is more scattered. Given the sensitivity of Si isotopes to water–rock interactions, it is likely that the more erratic terrestrial trend reveals the involvement of water that does not occur on the Moon. Hence, Si isotopes appear to reflect the occurrence of low temperature water–rock interaction products, like clay minerals, in the source of peraluminous leucogranites. Conversely, the heavy silicon isotope composition of some andesites possibly trace the input of fluids involved in subduction zones and/or interaction of the oceanic crust with isotopically heavy seawater before subduction.

Oxidative mobilization of cerium and uranium and enhanced release of “immobile” high field strength elements from igneous rocks in the presence of the biogenic siderophore desferrioxamine B

Polyvalent trace elements such as the high field strength elements (HFSE) are commonly considered rather immobile during low-temperature water–rock interaction. Hence, they have become diagnostic tools that are widely applied in geochemical studies. We present results of batch leaching experiments focused on the mobilization of certain HFSE (Y, Zr, Hf, Th, U and rare earth elements) from mafic, intermediate and felsic igneous rocks in the presence and absence, respectively, of the siderophore desferrioxamine B (DFOB). Our data show that DFOB strongly enhances the mobility of these trace elements during low-temperature water–rock interaction.The presence of DFOB produces two distinct features in the Rare Earths and Yttrium (REY) patterns of leaching solutions, regardless of the mineralogical and chemical composition or the texture of the rock type studied. Bulk rock-normalized REY patterns of leaching solutions with DFOB show (i) a very distinct positive Ce anomaly and (ii) depletion of La and other light REY relative to the middle REY, with a concave downward pattern between La and Sm. These features are not observed in experiments with hydrochloric acid, acetic acid or deionized water. In DFOB-bearing leaching solutions Ce and U are decoupled from and selectively enriched relative to light REY and Th, respectively, due to oxidation to Ce(IV) and U(VI). Oxidation of Ce3+ and U4+ is promoted by the significantly higher stability of the Ce(IV) and U(VI) DFOB complexes as compared to the Ce(III) and U(IV) DFOB complexes. This is similar to the relationship between the Ce(IV)- and Ce(III)-pentacarbonate complexes that cause positive Ce anomalies in alkaline lakes. However, while formation of Ce(IV) carbonate complexes is confined to alkaline environments, Ce(IV) DFOB complexes may produce positive Ce anomalies even in mildly acidic and near-neutral natural waters. Siderophore-promoted dissolution processes also significantly enhance mobility of other ‘immobile’ HFSE and may not only cause or modify Ce anomalies and Th–U fractionation, but also mobilization of and fractionation between Zr, Hf, Th and redox-insensitive REY during weathering, pedogenesis, diagenesis and incongruent dissolution of particles in seawater and freshwater.Siderophores may significantly affect the use of HFSE as geochemical tools. Concave downward light REY patterns may be used as a biosignature for water–rock interaction in the presence of siderophores. Enhanced and preferential mobilization of U relative to Th in the presence of siderophores may produce Th–U signals comparable to those indicative of weathering under oxidized conditions, which might constrain the use of U concentrations and Th/U ratios as a paleoredox-proxy. The enhanced mobilization of Zr and especially Hf from igneous rocks in the presence of DFOB might have implications for the use of the latter as a tracer for the impact of continental weathering on seawater chemistry. Because siderophore complexes affect the particle-reactivity of Hf and Zr, they may prevent effective removal of terrigenous Hf and Zr during aggregation/coagulation of riverine particles in estuaries. Siderophore-promoted solubilization and stabilization might hence be an additional way to transport continental Hf and Zr to the oceans. Furthermore, siderophore-enhanced mobilization may also have implications for the remediation techniques employed to immobilize HFSE such as U, Th and REY, at nuclear waste and reprocessing sites and at REY ore processing plants, where soils are commonly contaminated with these (sometimes radioactive) heavy metals.

Mineralogical and δ18O–δD isotopic study of kaolinized micaschists at Penestin, Armorican Massif, France: New constraint in the kaolinization process

Many kaolinite occurrences developed from Paleozoic rocks forming the basement of the Armorican Massif in western France. Their origin, hydrothermal alteration and/or weathering, is still discussed. In order to provide new constraints to this debate, we studied the kaolinite occurrence at Penestin in the southern part of the Armorican Massif using several approaches, including field and petrological observations, mineralogical analyses (XRD) and stable oxygen–hydrogen isotope analyses. An alteration profile was developed from faulted micaschists showing, from the bottom to the top of the profile, a reddish saprolite zone with pure white kaolinite fissures cutting across the schistosity and a white kaolinite zone, 5–10m thick. The detailed study by XRD of the layer silicates enabled us to characterize a mineralogical sequence with increasing alteration intensity, which is coherent with a weathering evolution: mica, chlorite→illite/vermiculite (chlorite) mixed-layer mineral→vermiculite→vermiculite/kaolinite mixed-layer mineral→kaolinite. Moreover, an intense iron leaching process occurs within the kaolinite zone, which is characterized by a bleaching of this zone combined with iron oxyhydroxide precipitation along/within fractures and cracks. The isotopic analyses of kaolinite from the zone and fissures gave δ18O values ranging from 19.64 to 21.21‰, and δD values ranging from −69.4 to −65.8‰. This confirms that kaolinite was formed by low-temperature water–rock interactions in contact with meteoric fluids during weathering processes. Isotopic and stratigraphic data suggest that this weathering occurred before the Messinian (~7Ma) and probably dates back to the Eocene, during which the climate was sub-tropical at this location.

Boron Behavior in the Rivers of Réunion Island, Inferred from Boron Isotope Ratios and Concentrations of Major and Trace Elements

A combined study of boron concentrations and isotopic ratios and of major and trace elements measured in Réunion waters, sampled up to four times between 1995 and 2012, illustrates boron behavior during water-rock interactions in tropical basaltic catchments. Boron isotope ratios measured in Réunion rivers and springs show a large range of variation between 1 and 48‰, that reflect mixed B sources and water-rock interaction processes: rain (δ11B≈40‰), hydrothermalism (δ11B≈0‰), low temperature basalt weathering in steep sided basins (δ11B≈30‰), and cycling within soil and vegetation (δ11B>40‰). Réunion rivers have schematically two types of δ11B signatures for low-temperature water-rock interactions: ≈30‰ for rivers with high weathering rates, and ≈45‰ for small forested catchments with relatively smoother slopes and lower weathering rates. High temperature water-rock interaction produces B enriched waters with a B isotopic signature close to that of the rocks. B behavior in the soil and vegetation cycle is more difficult to characterize but seems to result in soil solutions enriched in 11B, with δ11B≥45‰.

A multi-isotope approach for understanding sources of water, carbon and sulfur in natural springs of the Central Appalachian region

Natural springs have been reliable sources of domestic water and have allowed for the development of recreational facilities and resorts in the Central Appalachians. The structural history of this area is complex and it is unknown whether these natural springs receive significant recharge from modern precipitation or whether they discharge old water recharged over geological times scales. The main objective of this study was to use stable isotopes of water ( $$\delta^{18} {\text{O}}_{{{\text{H}}_{2} {\text{O}}}}$$ and $$\delta^{2} {\text{H}}_{{{\text{H}}_{2} {\text{O}}}}$$ ), dissolved inorganic carbon ( $$\delta^{13} {\text{C}}_{\text{DIC}}$$ ) and dissolved sulfate ( $$\delta^{34} {\text{S}}_{{{\text{SO}}_{4} }}$$ and $$\delta^{18} {\text{O}}_{{{\text{SO}}_{4} }}$$ ) to delineate sources of water, carbon and sulfur in several natural springs of the region. Our preliminary isotope data indicate that all springs are being recharged by modern precipitation. The oxygen isotope composition indicates that waters in thermal springs did not encounter the high temperatures required for O isotope exchange between the water and silicate/carbonate minerals, and/or the residence time of water in the aquifers was short due to high flow rates. The carbon isotopic composition of dissolved inorganic carbon and sulfur/oxygen isotopic composition of dissolved sulfate provide evidence of low-temperature water–rock interactions and various biogeochemical transformations these waters have undergone along their flow path.

Pre-4.0 billion year weathering on Mars constrained by Rb–Sr geochronology on meteorite ALH84001

The timing and nature of aqueous alteration of meteorite ALH84001 has important implications for the history of water on early Mars, the evolution of the Martian atmosphere, and the potential for early Mars habitability. Rubidium–Sr isotope analyses of mineral separates from igneous-textured and carbonate-rich aliquots of Martian meteorite ALH84001 constrain the age of alteration and the source of fluids. The carbonate-rich aliquot defines a precise Rb–Sr isochron between maskelynite, orthopyroxene, and chromite of 3952±22Ma, and this is interpreted to represent a shock resetting event that was broadly coeval with carbonate precipitation. Carbonate, bulk rock, and multi-mineral separates all have high 87Sr/86Sr ratios that can only have been produced by alteration via a fluid derived through interaction with high Rb/Sr phyllosilicates that were produced prior to 3950Ma. These data confirm that the source of Sr in the fluids was previously altered crustal rock, consistent with fluids that underwent low-temperature water–rock interaction (Eiler et al., 2002; Halvey et al., 2011). These results therefore provide evidence for wet, clay-rich conditions on the surface of Mars prior to ∼4.2Ga.

Magnesium isotopic variations in cratonic eclogites: Origins and implications

Cratonic eclogites play an important role in the formation and dynamic evolution of the subcontinental lithospheric mantle. However, their origin, whether as fragments of subducted oceanic crust or high-pressure mantle cumulates, remains controversial. Here, we report Mg isotopic compositions (δ26Mg) for cratonic eclogites from Kaalvallei and Bellsbank kimberlite pipes, South Africa. We find that clinopyroxene is 0.375±0.069 to 0.676±0.075‰ heavier than coexisting garnet, which reflects equilibrium isotope fractionation between these phases, primarily driven by the difference in Mg coordination between clinopyroxene and garnet. Bulk eclogites have strikingly variable Mg isotopic compositions, which range from −0.797±0.075 to −0.139±0.061‰, values that are significantly lighter than the range displayed by global mantle peridotites to date (−0.25±0.07‰, 2SD). As significant Mg isotope fractionation is only known to occur during low-temperature water–rock interaction, our results provide further evidence for the derivation of cratonic eclogites from subducted altered oceanic crust. In addition, the lack of correlation between Δ26Mg and Δ57Fe provides evidence for redox control on equilibrium inter-mineral Fe isotope fractionation.

Geochemistry of arsenic during low-temperature water–rock interaction

Abstract High arsenic water has been a global focus of both scientists and water supply managers because of its serious adverse impact on human health and wide distribution in the world. Processes of redox, sorption, precipitation, and dissolution release arsenic in both natural systems and in environments intensely modified by human activities. In natural systems, groundwater arsenic is controlled by lithologic geochemistry, sedimentation conditions, hydrogeologic setting and groundwater chemistry. However, in the intensely human-affected systems (such as mining and tilling areas), arsenic mobilization is dependent on the composition of the primary materials, treatment methods, storage design, and local climate. Well-designed experimental systems aid in characterizing sorption, precipitation, and redox processes associated with arsenic dynamics during water-rock interaction. Continued investigations of field sites will further refine understanding of the processes favoring arsenic mobility in the range of natural and man-made systems. The combination of field and experimental studies will lead to better understanding of arsenic cycling in all systems and sustainable management of water resources in arsenic-affected areas.

Mass transfer and porosity evolution during low temperature water–rock interaction in gneisses of the simano nappe: Arvigo, Val Calanca, Swiss Alps

Late Alpine fissures and fractures in amphibolite-facies basement gneisses at Arvigo (Val Calanca, Swiss Alps) show distinct cm-sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high-grade gneiss assemblages. The process of selvage or reaction-vein formation occurred in the brittle deformation regime and at temperatures characteristic of, first the prehnite-pumpellyite facies and then later of the zeolite facies. The vein formation occurred during uplift and cooling at very late stages of the Alpine orogeny. The reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space. The central zone of the Arvigo veins contains an early assemblage with epidote, prehnite and chlorite and a late succession sequence of various species of zeolite. The veins of the Arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate Ca–zeolite. This is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of Ca–zeolite species. Vein formation during late uplift and cooling of the Alps occurred at continuously decreasing T and at hydrostatic pressure: (1) coexisting prehnite/epidote records temperatures of 330–380°C, (2) chlorite formation at temperature of 333 ± 32°C and (3) formation of zeolites <250°C. In the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss. In addition to albitisation, chloritisation of biotite is the second important reaction in the alteration process. The reactions release components for the formation of Ca–Al silicates. The water–rock interaction is associated with a depletion of Al, Si, Ca, Fe and K in the altered wall rock. The overall reaction is associated with an increase in porosity of up to 14.2 ± 2.2% in the selvage zone (altered wall rock), caused by the volume decrease during albitisation and the removal of biotite. The propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase alteration reaction in the rock matrix, which releases components for zeolite formation to a CO2-poor aqueous liquid.

Martian Carbon Dioxide

As a primary component of the martian atmosphere and as the primary greenhouse gas for Mars, carbon dioxide has played a role in climate and geological processes during martian history. Niles et al. (p. [1334][1]) present high-precision measurements of the isotopic composition of the martian atmospheric CO2, made in situ by the Mars Phoenix Lander. Atmospheric CO2 on Mars was not enriched in 13C, which implies that volcanic degassing and carbonate formation have been important in the recent past, but it was enriched in 18O, which suggests that low-temperature water-rock interactions have been dominant on Mars. Combined with previous work on martian meteorites, the results suggest that carbonate formation has been active on Mars for the past 200 million years. [1]: /lookup/doi/10.1126/science.1192863

Stable Isotope Measurements of Martian Atmospheric CO 2 at the Phoenix Landing Site

Carbon dioxide is a primary component of the martian atmosphere and reacts readily with water and silicate rocks. Thus, the stable isotopic composition of CO2 can reveal much about the history of volatiles on the planet. The Mars Phoenix spacecraft measurements of carbon isotopes [referenced to the Vienna Pee Dee belemnite (VPDB)] [delta13C(VPDB) = -2.5 +/- 4.3 per mil (per thousand)] and oxygen isotopes [referenced to the Vienna standard mean ocean water (VSMOW)] (delta18O(VSMOW) = 31.0 +/- 5.7 per thousand), reported here, indicate that CO2 is heavily influenced by modern volcanic degassing and equilibration with liquid water. When combined with data from the martian meteorites, a general model can be constructed that constrains the history of water, volcanism, atmospheric evolution, and weathering on Mars. This suggests that low-temperature water-rock interaction has been dominant throughout martian history, carbonate formation is active and ongoing, and recent volcanic degassing has played a substantial role in the composition of the modern atmosphere.

Fluxes of high- versus low-temperature water–rock interactions in aerial volcanic areas: Example from the Kamchatka Peninsula, Russia

Volcanic areas play a key role in the input of elements into the ocean and in the regulation of the geological carbon cycle. The aim of this study is to investigate the budget of silicate weathering in an active volcanic area. We compared the fluxes of the two major weathering regimes occurring at low temperature in soils and at high temperature in the active volcanic arc of Kamchatka, respectively. The volcanic activity, by inducing geothermal circulation and releasing gases to the surface, produces extreme conditions in which intense water–rock interactions occur and may have a strong impact on the weathering budgets. Our results show that the chemical composition of the Kamchatka river water is controlled by surface low-temperature weathering, atmospheric input and, in some limited cases, strongly imprinted by high-temperature water–rock reactions. We have determined the contribution of each source and calculated the rates of CO 2 consumption and chemical weathering resulting from low and high-temperature water/rock interactions. The weathering rates (between 7 and 13.7 t/km 2/yr for cations only) and atmospheric CO 2 consumption rates (∼0.33–0.46 × 10 6 mol/km 2/yr for Kamchatka River) due to rock weathering in soils (low-temperature) are entirely consistent with the previously published global weathering laws relating weathering rates of basalts with runoff and temperature. In the Kamchatka River, CO 2 consumption derived from hydrothermal activity represents about 11% of the total HCO 3 flux exported by the river. The high-temperature weathering process explains 25% of the total cationic weathering rate in the Kamchatka River. Although in the rivers non-affected by hydrothermal activity, the main weathering agent is carbonic acid (reflected in the abundance of HCO 3 - in rivers), in the region most impacted by hydrothermalism, the protons responsible for minerals dissolution are provided not only by carbonic acid, but also by sulphuric and hydrochloric acid. A clear increase of weathering rates in rivers impacted by sulphuric acid can be observed. In the Kamchatka River, 19% of cations are released by hydrothermal acids or the oxidative weathering of sulphur minerals. Our results emphasise the important impact of both low and high-temperature weathering of volcanic rocks on global weathering fluxes to the ocean. Our results also show that besides carbonic acid derived from atmospheric CO 2, hydrochloric acid and especially sulphuric acid are important weathering agents. Clearly, sulphuric acid, with hydrothermal activity, are key parameters that cause first-order increases of the chemical weathering rates in volcanic areas. In these areas, accurate determination of weathering budgets in volcanic area will require to better quantify sulphuric acid impact.

Formation of Nontronite from Oxidative Dissolution of Pyrite Disseminated in Precambrian Felsic Metavolcanics of the Southern Iberian Massif (Spain)

AbstractThis paper describes a rare occurrence of nontronite associated with sulfide-bearing felsic metavolcanics, providing evidence of colloidal deposition in open spaces as result of a low-temperature water-rock interaction. Microbotryoidal masses of green nontronite with impurities of kaolinite, illite, barite, amorphous silica and iron oxyhydroxides are found as vein and cavity fillings in deeply kaolinized rhyolites and rhyolitic tuffs of Precambrian age, at Oliva de Merida in SW Spain. Clay mineral characterization has been carried out by X-ray diffraction, infrared spectroscopy, thermal analysis, analytical electron microscopy and stable isotope (oxygen and hydrogen) analysis. Nontronite was formed under low-temperature alteration conditions, from a continuous sequence of reactions and aqueous solution compositions, involving two basic processes that acted in concert: oxidative dissolution of pyrite and hydrolysis of K-feldspar. After acidity neutralization, dissolved silica released by incongruent dissolution of K-feldspar reacted with ferric sulfate derived from pyrite oxidation to form nontronite under oxidizing conditions, in the presence of relatively warm meteoric water.