New data for vent-fluid compositions at Kamaʻehuakanaloa including CO2 and CH4 concentrations Persistent vent-fluid composition for three decades Geochemical modeling of water-rock-gas reactions that produce the 20-50 ºC vent fluids Low-temperature basalt alteration results in high Fe/low H2S vent fluids Key role of active intraplate volcanoes on ocean Fe-cycle Abstract The contribution of venting fluids at mid-ocean ridges to global ocean biogeochemical cycles is well recognized. Less is known about the role of magmatically-active intra-plate volcanoes. In this study, new compositional fluid data were acquired from 20-50 ºC vent fluids at Kamaʻehuakanaloa (previously known as Lōʻihi) seamount (Hawai'ian archipelago) and used to model the wide diversity of reaction conditions capable of producing the Fe-, Si-and CO2-rich vent fluids observed. Our conceptual model includes a first step where seawater reacts with increasing proportions of basalt and gas as the temperature increases, and a second step where the resulting hydrothermal fluid mixes with unaltered seawater while continuing to react with basalt until the fluid mixture reaches 20 ºC. A series of reaction paths were chosen to vary: the maximum temperature during Step 1 (50 to 400 ºC) and the proportions of basalt and gas reacting; the degree, F, of low-temperature basalt alteration during Step 2, which corresponds to the extent to which the hot fluid generated during Step 1 continues to react with more basalt as it ascends to the seafloor. Our model shows that the 20-50 ºC vent fluids are greatly dependent on the degree of low-temperature basalt alteration during fluid upwelling. Indeed, the compositions of Kamaʻehuakanaloa vent fluids cannot be reconciled with a general model of subsurface mechanical mixing of high-temperature end-member vent fluid and seawater alone. Instead, they require both subsurface equilibrium mixing between a ≥350 ºC hydrothermal fluid end-member and seawater and further basalt alteration that must occur as the fluid mixture rises to the seafloor. Although it involves only ~4% of the amount of basalt having reacted during Step 1, this lowtemperature basalt alteration during Step 2 leads to the characteristic enrichments in Fe observed in the Kamaʻehuakanaloa vent fluids and a concomitant depletion in H2S. We hypothesize that low-temperature basalt alteration during an extended path of fluid upwelling through the subseafloor might arise as a direct consequence of the height and steep-sloped topography of Kamaʻehuakanaloa seamount. If correct, this suggests a more general case -that input from magmatically-active intraplate volcanoes, which have been relatively overlooked throughout the history of submarine vent investigations to date, could differ significantly from global mid-ocean ridge fluxes and contribute more substantially than previously recognized to the global ocean Fe cycle.