87Sr/86Sr Ratios and Atmospheric Noble Gases in Theistareykir Geothermal Fluids: A Record of Glacial Water

Daniele Luigi Pinti,Maria Clara Castro,André Poirier,Ásgerður K Sigurðardóttir,Kim Berlo,Bjarni Gautason,Marion Saby,Marie Haut-Labourdette,Vincent J Van Hinsberg

doi:10.3390/geosciences12030119

Abstract

The determination of the current and past recharge sources, as well as the reconstruction of the timing of the recharge in geothermal reservoirs, is required in order to correctly assess the resource potential of these systems. Theistareykir is a newly developed geothermal field close to the well-known exploited fields of Krafla and Námafjall in NE Iceland. In this study, the 87Sr/86Sr ratios measured in deep geothermal fluids are presented and, together with the Cl and noble gas signatures, are used to place constraints on the fluid sources. The Cl/Sr and 87Sr/86Sr ratios show a peculiar and unique composition among Icelandic geothermal fluids. The 87Sr/86Sr ratios range from 0.70355 to 0.70671, suggesting the presence of a significant seawater component—possibly marine aerosols added to rain or snowfall—as well as an additional source of Sr leached from local basalts. Moreover, a correlation between the atmospheric noble gas (ANGs) elemental ratios Ne/Ar, Kr/Ar and Xe/Ar, and the 87Sr/86Sr ratios is observed. The latter results from the mixing of meteoric water with Sr leached from local basalts, meteoric water containing unrelated Sr from seawater, and recharge water with ANGs derived from trapped air bubbles in snow. We suggest that the combined ANGs and Sr seawater signatures are representative of a glacial water source derived from the melting of compacting snow.

Highlights

The radiogenic Sr isotope ratio (87 Sr/86 Sr) is a robust tracer of surface water, groundwater and geothermal fluid sources [1–7]
The isotopic signatures are those of the atmosphere, but their elemental ratios (e.g., Ne/Ar, Kr/Ar, Xe/Ar) are those expected for the ASW, Byrne et al [16] have recently suggested that atmospheric noble gas (ANGs) could be fractionated by boiling processes taking place in the reservoir
The Ne/Ar ratios show a clear correlation with the 87 Sr/86 Sr ratios, a pattern that has not been previously observed in other geothermal areas studied using these two families of fluid tracers

Summary

Introduction

The radiogenic Sr isotope ratio (87 Sr/86 Sr) is a robust tracer of surface water, groundwater and geothermal fluid sources [1–7]. Noble gases—He, Ne, Ar, Kr and Xe—are excellent tracers of geothermal fluid sources. They are inert, and have distinct isotopic signatures among the terrestrial reservoirs: the mantle, the crust, and the atmosphere [8–13]. Meteoric waters, which are generally assumed to be the most abundant fluid in a magmatic–hydrothermal system [14], contain atmospherically derived noble gases (or ANGs hereafter). The isotopic signatures are those of the atmosphere, but their elemental ratios (e.g., Ne/Ar, Kr/Ar, Xe/Ar) are those expected for the ASW, Byrne et al [16] have recently suggested that ANGs could be fractionated by boiling processes taking place in the reservoir. Magmatic fluids acquire mantle-derived noble gases such as helium, which shows an enrichment in solar 3 He over

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