Table 5: Endmember compositions in fluids from the Kulo Lasi and Fatu Kapa vent fields. Kulo Lasi endmembers cannot be extrapolated at Mg = 0. Values presented here for both brine and condensed vapour phases correspond to concentrations in the fluid with the lowest Mg. Elemental compositions in endmember fluids from the various sites of the Fatu Kapa vent field were calculated using the mixing lines (Figure S1) and assuming Mg = 0. Values of the purest fluid were used when linear regression was not possible (). Note that only one sample was available for the site .

ZoneSiteDepthpHNaClClSiSO4BrNaKMgCaLiRbSrFeMnCuZnNa/ClBr/ClNa/KCH4/Mn
°C(wt%)mMmMmMµMmMmMmMmMµMµMµMµMµMµMµM×103

Kulo LasiNaCl poor14753452.242.94978.28.873838818.524.611.6149267.3479686214450.781.48210.007
Kulo LasiNaCl rich14753452.364.373514.66.2113561229.526.510.9238463.4988414162.51750.831.54210.001

Fatu KapaStéphanie15552803.44.57674.70.0156953254.50.098.97081142826552686.6<LOD0.692.05100.76
Fatu KapaCarla16642802.83.55944.30.0113247759.90.031.469110511428753nm4.40.801.9081.37
Fatu Kapa15722703.73.96654.20.0128251866.40.044.37511131602860nm3.40.781.9381.08
Fatu Kapa16692704.64.57714.60.0145858071.00.085.9777nmnmnmnmnmnm0.751.898-
Fatu Kapa 15402654.44.16933.71.0134453364.91.251.1755nmnmnmnmnmnm0.771.948-
Fatu KapaFati Ufu15233003.84.67904.90.0158958048.20.085.4722nmnmnmnmnmnm0.732.0112-
Fatu KapaFati Ufu15032803.34.17004.90.0138053840.00.065.0583nmnmnmnmnmnm0.771.9713-
Fatu KapaTutafi15803154.14.27135.10.0140553552.90.065.1635nmnmnmnmnmnm0.751.9710-

IAPSOStandard sw---3.25460.028.283946810.253.210.3271.390<LOD<LOD<LOD<LOD0.91.546-
Kulo LasiReference sw1150--3.25510.129.08334579.853.210.6254.493<LOD<LOD<LOD<LOD0.831.547-
Fatu KapaReference sw1488--3.35650.028.884148310.454.510.7225.8<LOD<LOD<LOD<LOD<LOD0.851.546-
Fatu KapaReference sw15722-3.35570.028.784147710.454.210.823nmnmnmnmnmnm0.861.546-

Maximum value when linear regression was not possible; only one sample.