Abstract Breccias recovered from the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK) and the East Pacific Rise at Hess Deep record distinct alteration signatures associated with the upwelling of hydrothermal fluids in the shallow oceanic crust. Two breccia types have been defined based on the mineralogy of the matrix. Type I breccias have a matrix characterized by Fe-rich chlorite, quartz, pyrite, and anatase. Type II breccias exhibit a matrix consisting of Mg-rich chlorite, epidote, quartz, pyrite, and titanite. Fluid inclusion analyses indicate a similar range in homogenization temperatures for the two breccia types (150–375°C) but distinct ranges in salinity. Type I breccias have salinities greater than that of seawater (3.3–10 wt% NaCl) whereas type II breccias display much lower salinities (< 0.1–5.6 wt% NaCl). Supercritical phase separation at temperatures from 450–500°C and 550–625°C in the lower crust at Hess Deep and MARK, respectively, is consistent with the observed salinity variations. Mineral stability relationships coupled with the fluid inclusion data suggest that the type I breccias were produced by hydrothermal fluids relatively enriched in Cl and depleted in H 2S whereas the type II breccias were associated with hydrothermal fluids relatively depleted in Cl and enriched in H 2S. These chemical trends are consistent with those displayed by hydrothermal fluids actively venting on the seafloor. Thus, we propose that the observed variability in the chemistry of modern submarine vent fluids is paralleled by distinct alteration assemblages in the upflow zones below the seafloor, as defined by the type I and type II breccia suites. Based on these results, we predict that the relatively H 2S-depleted vent systems at TAG, southern Juan de Fuca, and 11–13°N along the East Pacific Rise should be characterized by upflow zones with a mineralogy similar to the type I breccias. Conversely, the relatively H 2S enriched vent systems at 21°N and 11–13°N along the East Pacific Rise are likely associated with upflow zones that have a mineralogy similar to the type II breccias. As more fossil upflow zones in the oceanic crust are identified, these results could be used to define temporal variability of upwelling fluid chemistry at specific ridge sites on a geologic time scale.