From the lavas to the gabbros: 1.25 km of geochemical characterization of upper oceanic crust at ODP/IODP Site 1256, eastern equatorial Pacific

Here we present trace element and Sr-Nd-Hf-Pb (double spike) isotopic data covering the entire igneous section of oceanic crust drilled at Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) Site 1256 on the Cocos Plate. The penetrated interval extends from the upper lavas through the sheeted dike complex to the gabbroic plutonic rocks, formed during superfast spreading at the mid-Miocene equatorial East Pacific Rise. The data are used to characterize the effects of chemical alteration, resulting from convection of seawater and hydrothermal fluids, on the trace element and isotopic composition of oceanic crust. Compared to normal mid-ocean-ridge basalt, the igneous basement of Site 1256 (Holes 1256C/D) is isotopically slightly enriched but shows only narrow downhole variations in Nd-Hf-Pb isotope ratios: super(143)Nd/ super(144)Nd = 0.513089 plus or minus 0.000028 (2 sigma ), super(176)Hf/ super(177)Hf = 0.283194 plus or minus 0.000033 (2 sigma ), super(206)Pb/ super(204)Pb = 18.61 plus or minus 0.11 (2 sigma ), super(207)Pb/ super(204)Pb = 15.521 plus or minus 0.014 (2 sigma ), super(208)Pb/ super(204)Pb = 38.24 plus or minus 0.15 (2 sigma ). We believe that this minor variability is mainly of primary (magmatic) origin. The Sr isotopic composition shows considerably larger variation and, as expected, serves as sensitive tracer of seawater influence, which is particularly pronounced in the lava-dike transition zone and the sheeted dikes. The seawater influence is most prominent in a highly metal sulfide-enriched breccia layer encountered in the transition zone with super(87)Sr/ super(86)Sr of ~ 0.706, indicating a maximum water-rock mixing ratio of ~ 12. However, compared to the igneous section drilled at Site 504 (Hole 504B), which formed at intermediate, i.e., slower spreading rates at the Galapagos Spreading Center and hosting a much thicker sulfide-rich stockwork zone, the average intensity of water-rock interaction is lower. This is expressed by lesser mobility of base metals, narrower variability of alteration-sensitive incompatible elements, and less radiogenic Sr isotopic compositions on average at Site 1256. The amount of metal sulfide precipitation seems to be positively correlated with the degree of hydrothermal overprint. The less intense alteration of the Site 1256 transition zone, compared to Site 504, most likely reflects the higher rate of spreading, eventually resulting in a shorter period of time of continuous exposure t