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

Abstract

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 2 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.

The Surtsey volcano geothermal system: An analogue for seawater-oceanic crust interaction with implications for the elemental budget of the oceanic crust

“…Thus, seawater dominated geothermal systems in Iceland, such as Reykjanes and Svartsengi, serve as valuable analogues for hydrothermal vent systems on the seafloor, in terms of authigenic mineralization, geothermal solutions and primary lithologies (Hannington et al, 2005;Hardardóttir et al, 2009;Hardardóttir et al, 2013;Kadko et al, 2007;Kleine et al, 2020;Kleine et al, 2018;Marks et al, 2011;Marks et al, 2015;Michard et al, 1984;Mottl and Holland, 1978). Surtsey volcano is located in the southern offshore extension of the Icelandic eastern rift zone (Einarsson, 2008;Jakobsson et al, 2009;Thorarinsson et al, 1964) (Fig.…”

Characterizing Hydration of the Ocean Crust Using Shortwave Infrared Microimaging Spectroscopy of ICDP Oman Drilling Project Cores

“…(2020) estimated H 2 O in altered basalts from the on‐land section of the Mid‐Atlantic Ridge ranging from 0.5 – 9.1 wt%. Using δD values measured in previous work (Agrinier, Hékinian, et al., 1995; Agrinier, Lavern & Tartarotti, 1995; Coogon et al., 2016; Kawahata et al., 1987; Kusakabe et al., 1989; Shilobreeva et al., 2011), H 2 O has been estimated to be 1.6 ± 1.3 wt% in the volcanic ocean crust, 1.6 ± 0.6 wt% in the sheeted dikes, and 0.8 ± 0.1% in the gabbros (Kleine et al., 2020). Their average H 2 O estimate in the sheeted dikes is similar to our study (2.1 wt% for GT3A).…”

“…Our results fall in the range measured in the Troodos Massif gabbros, where H 2 O values ranged between 0 -7.5 wt% H 2 O (Moores & Vine, 1971). More recently, Kleine et al (2020) estimated H 2 O in altered basalts from the on-land section of the Mid-Atlantic Ridge ranging from 0.5 -9.1 wt%. Using δD values measured in previous work (Agrinier, Hékinian, et al, 1995;Agrinier, Lavern & Tartarotti, 1995;Coogon et al, 2016;Kawahata et al, 1987;Kusakabe et al, 1989;Shilobreeva et al, 2011), H 2 O has been estimated to be 1.6 ± 1.3 wt% in the volcanic ocean crust, 1.6 ± 0.6 wt% in the sheeted dikes, and 0.8 ± 0.1% in the gabbros (Kleine et al, 2020).…”

“…Water stored within minerals is the dominant reservoir of water on Earth, and the quantification of water in minerals and rocks illuminates the processes of hydration and alteration in the crust and, through subduction, impact the water budgets of the mantle (e.g., Bell & Rossman, 1992). Previous studies have measured bulk water contents of the ocean crust sampled by scientific ocean drilling ranging from 1 to 8 wt% H 2 O in the upper oceanic crust and from 0.1 to 0.5 wt% H 2 O in the lower gabbros (Agrinier, Hékinian, et al., 1995; Agrinier, Lavern & Tartarotti, 1995; Alt et al., 1996; Coggan et al., 2016; Jarrad, 2003; Godard et al., 2009; Kawahata et al., 1987; Kleine et al., 2020; Kusakabe et al., 1989; Shilobreeva et al., 2011). However, core recovery is low for the lavas and dikes of the upper ocean crust (Alt et al., 1996), and cumulate gabbros have not been reached in intact ocean crust (Teagle et al., 2012).…”

Sulfate (re-)cycling in the oceanic crust: Effects of seawater-rock interaction, sulfur reduction and temperature on the abundance and isotope composition of anhydrite

“…To simulate the fate of anhydrite upon cooling of a hydrothermal system within the oceanic crust, we expanded our isotope geochemical modelling following a similar approach as recently introduced for the assessment of water contents in the altered oceanic crust (Kleine et al, 2020a). In brief, mineral modes and d 34 S values of anhydrite were calculated for varying degrees of alteration of the oceanic crust along simplified geotherms representing young (<10 Ma) and old (<100 Ma) oceanic crust, respectively (Table A8, Fig.…”