Gabbro recovered from the deep drill hole of the Michigan Basin has been altered to secondary minerals indicative of the lower greenchist facies. A metagabbro from the East Pacific Rise (EPR) and two greenstones from the Mid Atlantic Ridge (MAR) have similar secondary minerals that resulted from submarine hydrothermal alteration. The albitization of the plagioclase in the Michigan sample (MDH) provided Al and Ca for chlorite, tremolite and epidote formation. The clinopyroxene shows minimal alteration. By comparison the plagioclase in the MAR greenstones is altered to albite and epidote or calcite with the primary pyroxenes showing variable alteration. The predominant alteration phase for both the Michigan gabbro and the MAR greenstones is an aluminous IIb chlorite polytype with no associated expandable clays. The EPR metagabbro is similarly altered to chlorite and actinolite, but saponite, vermiculite, and talc are also present. Temperatures of alteration for MDH and MAR can be estimated at 200¿-300¿C. The absence of smectite indicates that both MAR and MDH were altered under low water-rock ratios. The preservation of the primary pyroxene and the preponderance of chlorite in MDH suggest lower PCO2 and lower fO2 for this sample than for any of the oceanic samples. The mineralogical changes in MDH appear to be accompanied by a loss of Si and Ca, with an increase in Mg observed in the bulk rock composition, similar to seawater hydrothermal alteration.