Calcium isotope (δ44/40Ca) fractionation along hydrothermal pathways, Logatchev field (Mid-Atlantic Ridge, 14°45′N)
Introduction
Hydrothermal activity on mid-ocean ridges is one of the key controls for the chemical composition of past and present seawater (e.g., Elderfield and Schultz, 1996, Hardie, 1996). Water-rock interactions during convective circulation in hydrothermal systems lead to either addition or removal of chemical species and thus affect oceanic mass balances over space and time (Berner and Berner, 1996). Of particular interest are spatial and temporal variations in the ocean inventory of divalent cations such as Mg, Sr, and Ca, as these reflect variability in the relative roles of continental run-off and ocean floor hydrothermal fluxes and can hence be utilized for paleoceanographic reconstructions (e.g., Zhu and Macdougall, 1998, Henderson, 2002, DePaolo, 2004, Heuser et al., 2005).
The oceanic Ca budget has been discussed in detail by a number of studies showing that continental and oceanic weathering, mid-ocean ridge hydrothermal activity, and carbonate sedimentation are the major factors controlling the marine Ca cycle (e.g., Hart, 1973, Thompson, 1983, Berner and Berner, 1987, Zhu and Macdougall, 1998, Berner, 2004). Besides continental weathering and riverine influx, comprising about 70% of the Ca flux to the ocean, submarine magmatism and associated hydrothermal activity and seafloor weathering are also recognized to be major sources for the marine Ca budget (Berner and Berner, 1996). A number of studies indicated that Ca is released from the oceanic crust during water-rock reactions ranging from high-temperature hydrothermal interactions to seafloor weathering at low temperatures <150 °C (Berner and Berner, 1996). Ca-bearing mineral phases such as aragonite, calcite, anhydrite and subordinately hydrous Ca-silicates (e.g., prehnite, zeolites) are formed during different stages of ocean crust alteration (e.g., Staudigel et al., 1981). Anhydrite precipitation at temperatures of >140 °C also removes most of the seawater sulfate () from fluid circulation in axial hydrothermal systems (Bischoff and Seyfried, 1978). The extent of the chemical and isotopic exchange of Ca between seawater and oceanic crust, however, is still poorly constrained. Generally, the isotopic signature of the calcium influx to the ocean from sub-seafloor alteration is assumed to be similar to that of average ocean crust (Skulan et al., 1997, Zhu and Macdougall, 1998, Fantle and DePaolo, 2005). So far only Schmitt et al. (2003) attempted a closer examination of Ca exchange processes in axial hydrothermal systems by analyzing Ca isotopes in hydrothermal vent fluids. They report an average Ca isotope composition δ44/40Ca of −0.96 ± 0.19‰ relative to seawater for three hydrothermal water samples, one from the Mid-Atlantic Ridge, and two from the East Pacific Rise.
In this study, we present Ca and Sr isotope compositions of hydrothermal fluids and mineral precipitates from the Logatchev Hydrothermal Field as well as for carbonate veins from ODP drill cores from the Mid-Atlantic Ridge 15°20′ fracture zone area. The aim of this study is to constrain the Ca isotope budget in axial hydrothermal convection systems and Ca isotope fractionation processes along the hydrothermal pathway.
Section snippets
Samples and setting
Seventeen hydrothermal vent fluid samples from the active Logatchev Hydrothermal Field at 14°45′N on the Mid-Atlantic Ridge (MAR) were investigated (Fig. 1a). Also examined were the Ca and Sr isotope compositions of two anhydrite samples from the Logatchev field. In addition, eight calcium carbonate samples from ODP Leg 209 drill cores located south of the 15°20′N fracture zone were selected. A list of samples and their locations is given in Table 1, Table 2.
The Logatchev Hydrothermal Field is
Ca isotope ratios
The Ca isotope compositions were determined by thermal ionization mass spectrometry (TIMS) using the double spike technique (Russell et al., 1978, Heuser et al., 2002). The measurements were carried out on a ThermoFinnigan TRITON at the Leibniz-Institut fuer Meereswissenschaften (IFM-GEOMAR), Kiel/Germany, closely following the method of Heuser et al. (2002).
Working splits of the carbonate veins and anhydrite crystals were dissolved in 2.2 N and 6 N HCl, respectively. The carbonate and anhydrite
Results
The results for the fluid samples are summarized in Table 1, Table 4 and those for the mineral precipitates in Table 2. The contribution of seawater to each sample was calculated from Mg concentrations assuming that Mg behaves conservatively during mixing, and assuming zero Mg concentration in the hydrothermal fluid endmember (Table 1). Based on binary mixing and endmember calculations using Mg concentrations, fluid samples represent mixtures of seawater and vent fluids, with fractions of pure
Discussion
During the circulation through mid-ocean ridge basement, seawater is subject to successive chemical modifications (e.g., Von Damm, 1995, Seyfried and Ding, 1995; Fig. 3). Ca and Sr contents and isotope compositions, specifically, are changing along the fluid flow path as a result of exchange reactions with the host-rocks and precipitation of hydrothermal minerals such as anhydrite and calcium carbonates. The actual Ca and Sr contents and isotopic compositions of the hydrothermal fluid endmember
Summary and conclusion
The measurements of Ca and Sr isotopes in hydrothermal solutions and mineral precipitates constrain the processes of hydrothermal fluid flow, mineral precipitation, and temperatures at the Logatchev Hydrothermal Field. As seawater is chemically transformed to a hydrothermal fluid in the recharge and reaction zone of the Logatchev field, [Mg]SW is completely sequestered and [Ca]HydFluid is released from the bedrock to equal molar portions. The assumed initial Ca content of the fluid drops by
Acknowledgments
The work and M. Amini was supported by the priority program 1144 of the “Deutsche Forschungsgemeinschaft, DFG”: “From Mantle to Ocean: Energy-, Material- and Life-cycles at Spreading Axes” (publication No. 25). Furthermore, this study was also supported by the EUROclimate program of the European Science Foundation, ESF in the frame of the CASIOPEIA project (Ei272/21-1). Nico Augustin, Katja Schmidt and Thomas Kuhn are thanked for providing sample material and for fruitful discussions. The help
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2023, LithosCitation Excerpt :Amini et al. (2008) found that calcites directly precipitated from hydrothermal fluids have a Δ44/40Cacalcite-fluid of 0.02 ± 0.21‰, implying negligible Ca isotope fractionation between calcite and hydrothermal fluids. Experimental work by Syverson et al. (2018) found that anhydrite is isotopically lighter than fluids with Δ44/40Caanhydrite fluid from −0.45 to −0.02‰ over a temperature range from 175 to 300 °C, which is in line with observations on hydrothermal anhydrite (Amini et al., 2008; Scheuermann et al., 2018). Similarly, epidotes in ophiolite and AOC from ODP hole 504B (Brown et al., 2020) are isotopically lighter than fluid, with kinetic Δ44/40Caep-fluid from −0.2 to −0.6‰.
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2022, Geochimica et Cosmochimica ActaCitation Excerpt :In addition, the anhydrite data in this study are consistent with the literature values (Amini et al., 2008; Hensley and MacDougall, 2003; Syverson et al., 2018). The 1000lnα between anhydrite and fluid has been determined by experiments (Hensley and MacDougall, 2003; Syverson et al., 2018) and natural sample measurements (Amini et al., 2008). Because equilibrium Ca isotope fractionation between calcite and fluid is near zero (Fantle and DePaolo, 2007), 1000lnαanhydrite-fluid should equal to 1000lnαanhydrite-calcite.
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Present address: Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon SK, Canada S7N 5E2.