Description: Highlights • The vent fluids discharged from the Lutao hydrothermal field experienced low-degree subcritical phase separation. • The temperature and chemical compositions of the vent fluids were modulated by tides. • The time delay between tides and the response of hydrothermal system was about 3 h. • The typhoon “Fung-wong” cooled the reaction zone and decreased the degree of phase separation. • The hydrothermal system began to recover after the typhoon passed by. Abstract The Lutao hydrothermal field is an intertidal arc-volcanic system located offshore southeast Taiwan, hosting a Zhudanqu (ZDQ) vent and a Huwaichi (HWC) spring with strongly contrasting fluid chemistry. Low Mg, moderately enriched Cl, and H+ with respect to seawater indicate that the ZDQ endmember was derived from the brine phase that was formed during low-degree subcritical phase separation. In contrast, the endmember for the HWC vent fluids is related to the vapor phase. Temperature and pressure of the phase separation were estimated as ~150 °C and ~7 bar, respectively. The water/rock ratio was roughly calculated as about 2. The Lutao hydrothermal system was slightly affected by semi-diurnal tides, by some combination of tidal loading and tidal currents. The time delay between tides and the response of the hydrothermal system was about 3 h. While freshwater was almost absent in the HWC vent fluids at normal conditions, the typhoon “Fung-wong” on Sep 21st, 2014, led to intrusions of freshwater into the vent fluids with a percentage of ~16%. Both the ZDQ and the HWC endmember compositions showed some changes after the typhoon event, suggesting a cooling of the reaction zone. After the typhoon passed by, the hydrothermal system began to recover, evidenced by increasing percentages of the HWC endmember and decreasing freshwater contributions. The flux of the HWC endmember was estimated as 460–560 L h−1 based on these observations. This study, for the first time, reports a shallow-depth tidal-influenced hydrothermal system that was temporarily cooled by a tropical storm.

Description: 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 δ44/40Ca 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 δ44/40Ca 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 (δ44/40Ca = 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 δ44/40Ca 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.