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

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doi: 10.1016/j.lithos.2014.08.013
Author(s): Höfig, Tobias W.; Geldmacher, Jörg; Hoernle, Kaj; Hauff, Folkmar; Duggen, Svend; Garbe-Schönberg, Dieter
Author Affiliation(s): Primary:
Helmholtz Centre for Ocean Research, Kiel, Germany
Christian Albrechts University, Germany
Volume Title: Lithos (Oslo)
Source: Lithos (Oslo), Vol.210-211, p.289-312. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0024-4937 CODEN: LITHAN
Note: In English. Includes appendices. 48 refs.; illus., incl. 2 tables, sketch map
Summary: 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: 143Nd/144Nd = 0.513089 ± 0.000028 (2σ), 176Hf/177Hf = 0.283194 ± 0.000033 (2σ), 206Pb/204Pb = 18.61 ± 0.11 (2σ), 207Pb/204Pb = 15.521 ± 0.014 (2σ), 208Pb/204Pb = 38.24 ± 0.15 (2σ). 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 87Sr/86Sr 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 to hydrothermal convection at the ridge crest. The observed seafloor alteration, leading to modified radiogenic parent/daughter ratios in the Site 1256 rocks, is ultimately not sufficient to develop time-integrative high 206Pb/204Pb and moderate 87Sr/86Sr ratios, as being characteristic of the HIMU (high µ = high 238U/204Pb) mantle signature proposed to originate from hydrothermally altered, subducted oceanic crust. Therefore, additional modification during the subduction process must be taken into account. Abstract Copyright (2014) Elsevier, B.V.
Year of Publication: 2014
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Absolute age; Alkaline earth metals; Basalts; Correlation; Crust; DSDP Site 504; Dates; East Pacific; Equatorial Pacific; Gabbros; HIMU; Hafnium; Hydrothermal alteration; ICP mass spectra; Igneous rocks; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Jurassic; Lava; Lead; Mass spectra; Mesozoic; Metals; Metasomatism; Mid-ocean ridge basalts; Neodymium; North Pacific; Northeast Pacific; ODP Site 1256; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Plate tectonics; Plutonic rocks; Rare earths; Sea-floor spreading; Spectra; Spreading centers; Sr-87/Sr-86; Stable isotopes; Strontium; U/Pb; Upper crust; Volcanic rocks
Coordinates: N064400 N064400 W0915600 W0915600
Record ID: 2015031960
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands