High-Ca vent fluids discharged from the Lutao arc volcanic hydrothermal system are associated with albitization and recycling of marine carbonate

Abstract

The chemical and isotopic characteristics of calcium (Ca) in subduction zones are closely related to the budget of Ca and carbon cycles. Here we investigate the ultra-high Ca concentrations that characterize the hydrothermal fluids discharged from two types of vents, named the Zhudanqu brine vent (ZDQ) and the Huwaichi vapor spring (HWC), in the Lutao hydrothermal system at the north Luzon arc. The Ca concentrations of up to 159 mM and Ca/Cl ratio of up to 0.26 in the ZDQ vent fluids are possibly the highest ever reported for Ca enrichment in global seawater-circulated hydrothermal/geothermal systems. The differences in chemical compositions between the ZDQ and the HWC vent fluids are primary controlled by subcritical phase separation. The brine phase constitutes the ZDQ vent fluids, while the HWC vent fluids represent mixtures of the vapor phase and seawater. Both the vapor and the brine phases exhibit similar δ^44/40Ca values (0.72 ± 0.05‰), suggesting no significant Ca isotope fractionation has occurred during phase separation.

The hydrothermal endmember before phase separation (the “Lutao endmember”) presents depletions of 213 ± 15 mM of Na, 24.4 ± 0.4 mM of SO₄²⁻, and 10.2 mM of K, and enrichment of 130.2 ± 5.5 mM of Ca with respect to the percolated seawater. The total gained Ca is 154.6 ± 5.9 mM with a δ^44/40Ca value of 0.67‰ – 0.77‰ (0.72 ± 0.05‰), considering anhydrite precipitation during hydrothermal circulation. The Holocene raised coral reef is unlikely to contribute substantial Ca into the Lutao system. Much of the gained Ca (111.6 ± 7.5 mM) is produced by high-degree albitization of the Lutao host rock, which is promoted by the low water/rock ratio (~ 2), slightly alkaline conditions, and relatively lower temperature of the Lutao system with respect to most mid-ocean ridge hydrothermal systems. Ca derived from this process inherits the Ca isotopes of plagioclase in the Lutao host rocks (δ^44/40Ca = 0.82 ± 0.06‰). According to mass and isotopic balances, the recycled marine carbonate is proposed to contribute 43 ± 13.4 mM Ca with a δ^44/40Ca value of 0.46_−0.63^+0.35‰ into the Lutao system. Such isotopically lighter Ca is derived from either pore fluids expulsed from underlying Philippine Sea sediments, or more probably, carbonate-bearing subduction fluids from the subducting South China Sea sediments and slab. The carbonate solubility in the subduction fluids could maintain at 600 mM near the reaction zone. The carbonate-rich fluids were subsequently migrated into the Lutao reaction zone and released an additional 43 ± 13.4 mM Ca via dolomitization. A small amount (~ 9%) addition of carbonate-rich fluids would not significantly change the budgets of Na, Mg, and Cl but could generate substantial Ca enrichment and Ca isotopic variations.

Introduction

Seafloor hydrothermal activity, which offers pathways to exchange heat and matter between Earth's interior and surface, has been extensively surveyed after its first discovery at the Galapagos spreading center (Corliss et al., 1979). Vent fluids are a result of hydrothermal circulation, their geochemical characteristics reflect the processes occurring during recharge, fluid-rock interaction, and discharge stages (Foustoukos and Seyfried, 2007; Zeng, 2011; Beermann et al., 2017). While most fully-reacted hydrothermal fluids are typically absent of dissolved magnesium and sulfate, venting fluids could have dissolved Mg and sulfate as a consequence of low temperature subseafloor mixing of hydrothermal fluids and seawater (Von Damm et al., 1985; German and Von Damm, 2006; Schmidt et al., 2007; Chen et al., 2018). In addition, the chemical compositions of hydrothermal endmembers usually cover broad ranges due to fluid-rock/sediment interactions, phase separation, and secondary precipitation of minerals (German and Von Damm, 2006; Zeng, 2011).

The Ca geochemistry in hydrothermal systems is relatively complicated. Ca could be either enriched or depleted in vent fluids with respect to seawater, depending on the processes such as albitization, phase separation, and/or anhydrite precipitation and dissolution during hydrothermal circulation (Hannington et al., 2001; Foustoukos and Seyfried, 2007; Schmidt et al., 2017). Continental weathering, hydrothermal input, and Ca replacement by Mg during dolomitization are major sources of Ca in the ocean (Nielsen et al., 2012). The major sinks for Ca are calcium carbonate precipitation on the seafloor and anhydrite precipitation during hydrothermal circulation (German and Von Damm, 2006; Scheuermann et al., 2018). Both processes preferentially remove isotopically lighter Ca from seawater/hydrothermal fluids and, consequently, produce Ca isotopic fractionations (Fantle and DePaolo, 2005; Scheuermann et al., 2018; Syverson et al., 2018). The source and sink of Ca are particularly important for carbon cycle because Ca and the carbon cycle are coupled during processes like continental weathering, marine carbonate precipitation, decarbonation and carbonate dissolution during plate subduction (De La Rocha and DePaolo, 2000; Frezzotti et al., 2011; Kelemen and Manning, 2015). Therefore, it is essential to study the geochemical cycle of Ca, especially at subduction zones.

The Lutao hydrothermal system is located at the southeastern corner of the Lutao volcanic island, offshore southeast Taiwan. Tectonically, Lutao belongs to the Northern Luzon arc, which is generated by the subduction of the South China Sea (SCS) Plate beneath the Philippine Sea (PPS) Plate (Bowin et al., 1978; Yang, 1992). Previous studies suggest the vent fluids sampled from two types of vents, the Zhudanqu (ZDQ) brine vent and the Huwaichi (HWC) vapor spring, are a result of low-degree subcritical phase separation. The separated brine phase accounts for the ZDQ vent fluids, while the HWC vent fluids are mainly supplied by the vapor phase and seawater (Chen et al., 2020b). The measured Ca contents of the ZDQ and HWC fluids were 144–181 mM and 45–55 mM, respectively, much higher than seawater values (Chen et al., 2020b). The mechanism for such high Ca contents, however, is still unknown. Because Lutao hydrothermal field is related to the subduction of the SCS Plate underneath the PPS Plate, deciphering the source and behavior of Ca in the Lutao hydrothermal circulation could help better understanding the Ca and carbon cycles in subduction zones. Furthermore, studying the Ca isotopic characteristics of the Lutao vent fluids could complement the Ca isotopic evolution during hydrothermal circulation (Scheuermann et al., 2018; Syverson et al., 2018).

In this study, we determined the Ca and Sr isotopes and trace elemental concentrations of the Lutao vent fluids. Combined with our previously reported data on major ions and chemical parameters (pH, salinity, temperature), we investigate the Ca-related geochemical processes and try to unveil the mechanisms producing the high Ca contents in the Lutao vent fluids. We further propose a potential model to illustrate the source and evolution of Ca in the arc volcanic Lutao hydrothermal system.