Impact of fluid-rock interaction on water uptake of the Icelandic crust: Implications for the hydration of the oceanic crust and the subducted water flux

Pre-print (óritrýnt handrit)Oceanic crust is a major transport medium of water into the mantle wedge and the convecting mantle. Yet, the water content of the oceanic crust remains uncertain. Active geothermal systems situated at on-land spreading centers provide a unique opportunity to study the hydration of the oceanic crust, with well constrained systems and boreholes reaching depths of >4 km. Here, we present hydrogen isotope data of geothermal fluids and altered basalt for three Icelandic geothermal systems: the meteoric water fed system at Krafla and the seawater fed systems at Reykjanes and Surtsey. The bulk rock δD values of altered and hydrated basalts from these localities, which exhibit significantly higher water contents (up to 8.9 wt.%) than magmatic (non-hydrated) basalts, vary greatly from −125 to −96 at Krafla, from −80 to −46 at Reykjanes and from −78 to −46 at Surtsey. The corresponding fluids have δD values of −84.1 to −81.1 at Krafla, −23.1 to −14.9 at Reykjanes and +2.1 to +4.3 at Surtsey. Comparison of isotope modeling results to the natural data reveals that hydration of the Icelandic crust and corresponding hydrogen isotopic characteristics are controlled by (1) the isotope composition of the source fluid, (2) isotope fractionation between the aqueous geothermal fluids and the alteration minerals formed, and (3) the type and quantity of alteration minerals formed. These factors in turn depend on the extent of fluid-rock interaction and temperature. Using the same modeling approach and expanding it to datasets available for the oceanic crust, we assessed the hydration state and δD values of the oceanic crust as a function of depth. We show that 1400 to 1650 Tg H2O/yr is added to the igneous oceanic crust upon alteration by seawater and that the upper part (<2 km) of oceanic crust hosts almost 50% of the added water. The corresponding hydrogen isotope composition of the hydrated crust was calculated to an average of −55 ±6 . Upon subduction and subsequent dehydration, 80–90% of water with δD values of −35 to −10 will be released to the crustal forearc and mantle wedge. The remaining dehydrated slab with δD values of ∼−160 to −85 is expected to be transported to deeper levels modifying the mantle’s water budget and isotopic composition.This project was financially supported by NordVulk, the International Continental Scientific Drilling Program (ICDP) through a grant to the SUSTAIN project, and the Icelandic Research Fund (project number: 163083-051). SAH acknowledges support from the Icelandic Research Fund (project number: 196139-051). HS Orka and Landsvirkjun kindly provided access to the drill cuttings. J. Cullen, T. Larson, R. Ólafsdóttir and Á.E. Sveinbjörnsdóttir are thanked for assistance during sample preparation and data acquisition. BIK is particularly grateful of being part of this project as without the project-related lab work she would have never met her future husband E.W. Marshall IV. We thank four anonymous reviewers for their constructive comments and suggestions to an earlier version of this manuscript. Louis Derry is thanked for careful editorial handling of this study

Noble gas (He-Ne-Ar) and stable isotope (C-N-O) constraints on volatile sources at convergent and divergent plate boundaries with examples from Indonesia, Iceland and the East African Rift System

Studies of volatile elements and their isotopes have provided fundamental constraints on the formation and evolutionary history of the many diverse chemical reservoirs that together form the primary building blocks of planet Earth. This largely stems from significant differences in volatile abundances and isotopic characteristics of Earth's dynamically evolving reservoirs, which are prone to modifications following the transfer of volatiles between them. In this dissertation, I discuss how different volatile sources in the solid Earth can be resolved by means of a combined noble gas and stable isotope approach. I investigate different volatile source regions in the silicate Earth, and the transfer of volatiles from source regions towards the exterior of Earth in various tectonic settings. These settings include the western Sunda arc, Indonesia, as an example of a divergent plate boundary, in addition to two prominent plume-influenced divergent plate boundaries; the East African Rift System (EARS) and Iceland.Chapter I is intended to provide a general introduction to the use of noble gas and stable isotopes as geochemical tracers, with particular emphasis on their utilization in the elucidation of processes, timescales and source-specific features in the solid Earth. Following this overview, we discuss the main objectives of the various studies of which this dissertation consists. Chapter II examines the combined He-CO2-N2 abundance and isotope systematics of geothermal fluids and gases from volcanic centers along the western Sunda arc, Indonesia. We assess controls on volatile provenance in this major subduction system by resolving volatiles associated with the sub-arc mantle, that includes the subducting slab and mantle wedge, from inputs derived from the over-riding arc crust.Chapter III describes a combined He-Ne-Ar isotope study of mantle-derived xenoliths and lavas from different segments of the EARS. This coupled approach provides a powerful tool with which to identify volatile provenance from the deep mantle versus shallow lithospheric sources, which allows for a unique investigation of the number of plume sources located in the East African mantle, and the ultimate source of melts being supplied to the different segments of the EARS.Chapter IV is a detailed study of noble gases and the stable isotopes of carbon and nitrogen trapped in fluid inclusions of mantle-derived xenoliths from the EARS. In considering their coupled systematics, we evaluate a number of mantle/crustal features controlling their elemental and isotope characteristics, and separate the various sources contributing to the volatile components trapped in the fluid inclusions.Chapter V focuses on the N2 isotope and abundance systematics of subglacially formed basalts from the Iceland hotspot. After characterization of the nitrogen elemental and isotopic signals of the Iceland mantle plume, we investigate relationships with other isotope and relative abundance systematics in order to discern nitrogen characteristics of the mantle source underlying one of Earth's most prominent hotspots.Finally, Chapter VI provides concluding remarks on these studies and discusses some future prospects and ways of addressing outstanding questions by, for example, further chemical and isotope characterization of the same sample suite discussed in this dissertation

Spatial variations in gas and stable isotope compositions of thermal fluids around Lake Van: Implications for crust-mantle dynamics in eastern Turkey

We investigate the helium (He-3/He-4) and carbon (delta C-13) isotope compositions and relative abundance ratios (CO2/He-3) of gas samples together with the stable isotope compositions of dissolved carbon and sulfur and the oxygen and hydrogen isotopic compositions of the associated water phase from a number of geothermal fields located around Lake Van in eastern Anatolia, Turkey. The mantle-derived helium component, which is likely transferred to the crust beneath eastern Turkey by recent magmatism, is found to constitute up to 96% (e.g. Nemrut Caldera) of the total He content in fluids. As regards the spatial distribution of He, samples collected from areas of Pliocene-Quaternary volcanics are characterized by a wide and generally higher range of R/R-A ratios (0.93 to 7.76 R-A) compared to those of non-volcanic regions ((1.85 to 1.0 R-A). CO2/He-3 ratios vary over a wide range (2.4 x 10(5)-3.8 x 10(13)) but are mostly higher than that of the nominal upper mantle (similar to 2 x 10(9)). Oxygen-hydrogen isotope values of the waters are conformable with the Global Meteoric Water Line and indicate a local meteoric origin. Sulfate in waters is most probably derived from dissolution of marine carbonates and terrestrial evaporite units. Temperatures calculated by SO4-H2O isotope geothermometry lie between 40 and 199 degrees C, and are in poor agreement with reservoir temperatures estimated from silica geothermometers. Discordant temperatures may be due to either the relatively slow rate of isotopic equilibrium between water and sulfate or mixing of geothermal water with sulfate-bearing shallow waters which may modify the delta O-18 value. The delta C-13 (CO2) values of gas samples are consistently lower than those of their water counterparts, consistent with loss of CO2 from waters by degassing. Mixing between mantle and various crustal C-sources appears to be the main control on the C-isotope composition. The principal origin of CO2 in all samples is crustal lithologies, mainly limestone (similar to 85 to 98% of the total carbon inventory): thus, the crustal carbon flux is at least 10 times that from the mantle. There is a broad correlation between high He-3/He-4 values and thinner crust in the western part of the Lake Van region, where several historically-active volcanoes are located. This observation indicates that localized volcanic and magmatic activity exerts the primary control on the balance between mantle and crustally-derived volatiles in the region

High He-3/He-4 in central Panama reveals a distal connection to the Galapagos plume

It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high He-3/He-4 (up to 8.9R(A)) in low-temperature (10.3R(A) (and potentially up to 26R(A), similar to Galapagos hotspot lavas) markedly greater than the upper mantle range (8 +/- 1R(A)). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high He-3/He-4 values in central Panama are likely derived from the infiltration of a Galapagos plume-like mantle through a slabwindowthat opened similar to 8 Mya. Two potential transport mechanisms can explain the connection between the Galapagos plume and the slab window: 1) sublithospheric transport of Galapagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galapagos plume material blown by a "mantle wind" toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galapagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits