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.