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A Multi-Isotope Approach to Understanding Weathering in Using Either Internal Normalisation (Sr), Sample-Standard the Fluid Sources to the Salado River

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A Multi-Isotope Approach to Understanding Weathering in Using Either Internal Normalisation (Sr), Sample-Standard the Fluid Sources to the Salado River O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 were analysed using a MC ICP-MS at Rutgers University and a spring in the Turi Basin which could represent one of A multi-isotope approach to understanding weathering in using either internal normalisation (Sr), sample-standard the fluid sources to the Salado River. bracketing (Li and B) or a double spike (Mo) techniques. Northern Chile 12 LITHIUM Andes 10 Linda Godfrey1*, Christian Herrerra2 and Carolina Gamboa2 Precordillera 1 8 Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA 7 2 - Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile 6 4 *Contact email: [email protected] Li delta 2 Abstract. Low to zero precipitation in Northern Chile source related to magmatism is the cold temperature 0 means that the dissolution of silicate rocks and the weathering and oxidation of volcanic sulphur phases. In formation of secondary phases occurs largely in the deep contrast, in the Precordillera where magmatic activity -2 subsurface. Although the Li concentration of water can be 0.01 0.1 1 10 ended during the Paleogene, sources of CO2 are limited to high, the very minimal variation in Li isotopes compared to Li (ppm) direct dissolution of atmospheric CO2, or from low levels other regions reflects the limited formation of secondary phases which fractionate Li. There is greater variability in B of biological activity. The study presented here, isotopes due to its volatility in volcanic systems and its pH- investigates the isotopic composition of solutes in two 12 dependent speciation which leads to strong fractionation areas, one in the Western Cordillera and the other in the 10 which is apparent in the Western Cordillera groundwater Precordillera. Three isotope systems were investigated in 8 systems but only in the Precordillera where volcanic rocks, detail, strontium, lithium and boron; a fourth, molybdenum 7 marine limestone and gypsum are weathered. Mo shows a is under way. Strontium isotopes provide information - 6 range in isotope compositions which is similar to temperate about source rocks and groundwater pathways, while Li 4 Andes areas. The differences between the variability in these Li delta Precordillera and B are used to investigate weathering intensity. The 2 isotope systems is related to the occurrence of recent and lighter isotope of both Li and B are adsorbed onto, and for ongoing volcanism in the West Cordillera of the Andes. In 0 the Western Cordillera, there can be large volcanic fluxes Li, is incorporated into the lattice of secondary minerals, -2 of CO2 driving weathering reactions at depth, which are but B exhibits greater variability than Li because its absent in the Precordillera. Water geochemistry in the speciation is affected by pH, and it is much more volatile. -4 -3 -2 -1 0 log (fugacity CO2) Western Arc is more complex as acidity is quickly Lithium and B also differ, because during evaporation – neutralised by volcanic glass and by storage of solutes in and important process in the closed basins of the Atacama evaporitic systems. and Andes, B forms borate minerals, while Li is Figure 2. A (top). Li concentration and isotope composition concentrated into brine and remains in a fluid state. The correlate, except at the highest Li concentration. B (bottom). In Keywords: isotopes, weathering, acidity, lithium, boron, formation of evaporate minerals and brine, is considered to general isotope fractionation from the formation of secondary molybdenum be the dominant factor influencing the geochemistry of minerals is higher when the fugacity of CO2 is lowest, though water draining the Andes (Risacher et al., 2003). This is there are secondary affects apparent. where Mo isotopes may provide different information, 1 Introduction because, like Li, does not form discrete evaporate Figure 1. Map showing sampling areas in the Western 7 minerals, and the variation in its concentration has so far Cordillera (1) and Precordillera (2). Area1 is subdivided into A Samples with high CO2 fugacity have low Li, and there Water and acidity are both important for weathering is overlap between samples from Areas 1A and 2. The silicate rocks. In the Atacama, although water is limited in been much muted compared to Li. and B which is based on previous studies indicating a large volcanic CO flux influencing area B. source of CO2 is most likely magmatic in origin, and the comparison to temperate regions of the world, its 2 resulting depression of pH from CO2 dissolution, but also availability largely limits surface water rather than raised temperatures, enhances the dissolution process. groundwater. The greater effect of limited water supplies 2 Samples and Methods 3 Results and Discussion However, it is rare that low pH water is encountered at the is on the source of acidity. In temperate areas where there surface, and this is due to neutralisation by volcanic glass is abundant vegetation and biological activity in soils, a 2.1 Sampling Areas in the numerous ignimbrites. 3.1 Lithium major and reliable source of protons is from the dissolution Samples were collected using standard trace metal of CO2 generated in the soils and surface water from 3.2 Boron respiration to form carbonic acid. In the Atacama, protocols from the upper Río Loa groundwater basin The total range in Li isotopes is about one third that of systems in temperate and tropical areas (Huh et al., 1998). biological activity occurs at much reduced levels, from (Areas 1A and 1B in figure 1), and the Quebrada Profeta 11 The lowest Li concentrations occur in the Precordillera, Boron isotope compositions ( B) vary between -8 and near zero in the Central Depression due to extremely rare basin whose headwaters are in the Precordillera of the 11 +15 ‰. The lowest B are all associated with Area 1B, precipitation events, or to some fraction of temperate Domeyko Range (Area 2). The CO2 fluxes in the Western but samples from area 1A may have equally low Li content where there is evidence of magmatic CO which enters region levels because for enough precipitation to fall to Cordillera of the Andes is varied, and studies of pH and (Figure 2A). There is a trend where samples with low Li 2 7 6 groundwater. Volcanic rocks in the volcanic front have allow infiltration typically occurs above 4,000 m, where alkalinity have determined that they are substantially larger concentrations also have high Li, indicating loss of Li to 11B between 0 and +5 ‰ (Rosner et al., 2003), so the cold temperatures limit biological activity. in Area 1A than they are in 1B (Aravena and Suzuki, 1990; secondary minerals. At concentrations over 1 ppm, any 7 depletion of 11B is likely due to leaching of clays by acidic Godfrey et al. subm.). The levels of carbonate alkalinity in correlation with Li is lost, and could reflect enhanced Area 2 is similar to Area 1B. dissolution of Li from silicate rocks with little removal of water, or volcanic gases which have cooled and condensed An important source of acidity is derived from the 11 magmatic activity in the arc in the Western Cordillera of Li from solution so that the isotopic composition of the into groundwater. The high B samples occur in the Strontium, Li and Mo were separated in the laboratory rocks being weathered is preserved. These samples Precordillera, and reflects weathering of marine limestones the Andes from volcanic gases such as CO2 and SO2, 11 although these fluxes may be spatially variable. A further using standard ion chromatography, while B was isolated include the Salado River and the Loa downstream of it, which, depending on their time of formation, have B using a micro-sublimation technique. All isotope systems ranging as high as +21 ‰. There is no correlation with B 705 ST 9 MORFOESTRATIGRAFÍA, GEOMORFOLOGÍA, HIDROGEOLOGÍA Y GLACIOLOGÍA were analysed using a MC ICP-MS at Rutgers University and a spring in the Turi Basin which could represent one of using either internal normalisation (Sr), sample-standard the fluid sources to the Salado River. bracketing (Li and B) or a double spike (Mo) techniques. 12 LITHIUM Andes 10 Precordillera 8 7 6 - 4 delta Li delta 2 0 -2 0.01 0.1 1 10 Li (ppm) 12 10 8 7 - 6 4 Andes delta Li delta Precordillera 2 0 -2 -4 -3 -2 -1 0 log (fugacity CO2) Figure 2. A (top). Li concentration and isotope composition correlate, except at the highest Li concentration. B (bottom). In general isotope fractionation from the formation of secondary minerals is higher when the fugacity of CO2 is lowest, though there are secondary affects apparent. Figure 1. Map showing sampling areas in the Western 7 Cordillera (1) and Precordillera (2). Area1 is subdivided into A Samples with high CO2 fugacity have low Li, and there and B which is based on previous studies indicating a large is overlap between samples from Areas 1A and 2. The volcanic CO2 flux influencing area B. source of CO2 is most likely magmatic in origin, and the resulting depression of pH from CO2 dissolution, but also raised temperatures, enhances the dissolution process. 3 Results and Discussion However, it is rare that low pH water is encountered at the surface, and this is due to neutralisation by volcanic glass 3.1 Lithium in the numerous ignimbrites. The total range in Li isotopes is about one third that of 3.2 Boron systems in temperate and tropical areas (Huh et al., 1998). 11 The lowest Li concentrations occur in the Precordillera, Boron isotope compositions ( B) vary between -8 and 11 but samples from area 1A may have equally low Li content +15 ‰.
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