The geochemistry of epidote and chlorite-dominated hydrothermal assemblages in the Iceland Research Drilling Project drill core has been studied by electron microprobe. The association of epidote, andradite, and Cu--Fe sulphides suggests maximum basalt alteration temperatures of 350¿--300¿C. Near-pure andradite implies a fluid XCO2 300¿C to 300¿C during burial of the lower part of the IRDP sequence before vigorous fluid circulation, producing rapid cooling and intense alteration, was established. Transition metals (Fe, Mn, Cu) were released from primary assemblages but were incorporated in epidote, chlorite, and sulphide. Epidote contains average SrO=0.23%, but intersample variations in trace element contents of secondary minerals are complex, and there are no correlations with depth. It is concluded that the effects on whole rock chemistry (including Sr) and secondary mineral chemistry were largely controlled by kinetic factors such as (1) differing reponses to alteration by primary assemblages and (2) permeability variations.