Middle Valley is a sediment-covered rift valley located near the northern Juan de Fuca Ridge. The hemipelagic, pelagic, and turbiditic sediments that fill the valley have been hydrothermally altered to form a variety of diagenetic products. The diagenetic history of such spreading centers is significant to the understanding of the formation of geologically important massive sulfide and petroleum ore bodies as well as hydrologic circulation patterns associated with hydrothermal systems. Leg 139 of the Ocean Drilling Program cored the hydrothermally altered strata of Middle Valley. Diagenetic minerals present in cores from Hole 858A of that leg were mapped and characterized using computer-aided image analysis of half-core X-radiographs. Carbonate nodules, burrows, cements, and bulk sediments were analyzed for stable C and O isotopes, elemental concentrations of Fe, Mn, Mg, and Sr, and organic carbon and total nitrogen content. Temperatures derived from delta (super 18) O are too high to be explained by the present day geothermal gradient of 1.4 degrees C/m, suggesting a paleotemperature gradient as high as 13.6 degrees C/m. Within the upper 15 meters below sea floor, delta (super 13) C values of -20 to -40 per mil indicate sulfate reduction by oxidation of organic matter and thermogenic methane. Below 15 mbsf, delta (super 13) C values of -5 to -20 pe mil indicate the production of CO (sub 2) during thermocatalytic decarboxylation and acetate fermentation. The elemental concentrations of the carbonates are a direct result of changes to the sediments and pore waters during diagenesis. Changes in iron concentration reflect the presence of dissolved sulfide within the pore waters. Manganese values ranging from .00174 to .0288 mol Mn/Ca indicate lateral circulation and advection of hydrothermal fluids. The lack of correlation between strontium (<.0065 mol Sr/Ca) and magnesium (<.08 mol Mg/Ca) concentrations suggests that the strontium concentrations are a result of precipitation rate and temperature rather than the magnesium concentrations in the carbonates. Organic carbon content (.6% to .3%) decreases with depth, reflecting biogenic and abiotic, thermal degradation of organic matter. The abundant and diverse ichnofauna within muddier sections of the turbidites represent distinct depositional environments and sedimentation rates. These extensive burrow systems create conduits for hydrothermal flow as evidenced by the diagenetic infilling and coalescing nature of the burrows. A thin, typically vertical and winding burrow less than 1 mm in diameter is believed to be produced by bacteria.