Geochemistry of hydrothermally altered oceanic crust; DSDP/ODP Hole 504B; implications for seawater-crust exchange budgets and Sr- and Pb-isotopic evolution of the mantle

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doi: 10.1029/2002GC000419
Author(s): Bach, Wolfgang; Peucker-Ehrenbrink, Bernhard; Hart, Stanley R.; Blusztajn, Jerzy S.
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Marine Chemistry and Geochemistry, Woods Hole, MA, United States
Volume Title: Geochemistry, Geophysics, Geosystems - G<sup>3</sup>
Source: Geochemistry, Geophysics, Geosystems - G>3`, 4(3). Publisher: American Geophysical Union and The Geochemical Society, United States. ISSN: 1525-2027
Note: In English. Accessed on July 24, 2003; 29 p.. 91 refs.; illus., incl. 6 tables, sketch map
Summary: This paper presents petrographic, chemical, and isotopic (Sr, S) analyses of whole rock samples from a 1.8 km section of upper ocean crust (DSDP/ODP Hole 504B). The samples were selected to cover all lithologies (pillows, flows, breccias, dikes) and alteration/mineralization styles. The chemical and petrographic data were used to calculate weighted averages for upper crustal composition, based on which seawater-ocean crust exchange fluxes were calculated. These results confirm earlier estimates that identify the upper crust as a significant sink for K and Mg and a source of Ca and Si to the oceans. Changes in trace element geochemistry implies that the upper ocean crust in 504B is a sink for CO2, Rb, Cs, and U, although the flux rates are an order of magnitude smaller than suggests by previous estimates for DSDP Sites 417 and 418 in 118 Ma Atlantic crust. Fluxes of these components are similar, within a factor of four, to flux rates estimated for the Juan de Fuca Ridge flank, which may relate to similarities in the thermal and hydrogeological evolution at both sites that is controlled by rapid termination of fluid circulation and conductive reheating of the upper crust. The contrast between the fluxes of trace elements derived for those settings and the open-ocean sites 417/418 likely reflects prolonged fluid-rock interaction at the latter location. If the Mg uptake and Sr exchange reconstructed from 504B core is representative, ridge flank hydrothermal alteration cannot account for the imbalance in the Mg and Sr budgets of the oceans. Up to 10% of the crustal Pb resides in the mineralized parts of the transition zone between the volcanic section and the sheeted dike complex. Combined, the Pb mobilized in the deepest parts of the hydrothermal systems (probably not penetrated in 504B) and hosted in metalliferous sediments and mineralized stockwork may account for the Pb surplus of the continental crust and the evolution of Ce/Pb of the mantle. Hydrothermal alteration results in net increases of Rb/Sr and U/Pb, in particular in the uppermost 600 m of crust, but the increases are not large enough to make altered upper ocean crust a plausible precursor for the HIMU mantle component. Moreover, the fractionation between Th and Pb, if any, is insufficient to account for the development of highly radiogenic 208Pb/204Pb in a HIMU mantle source. Potential HIMU precursors can be derived from altered ocean crust after 1-2 Ga, if on the order of 80-90% Pb, 40-55% Rb, 40% Sr, and 35-40%U are removed during partial dehydration in subduction zones.
Year of Publication: 2003
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 05 Petrology, Igneous and Metamorphic; Alkaline earth metals; Basalts; Crust; DSDP Site 504; Deep Sea Drilling Project; Dehydration; HIMU; Hydrothermal alteration; Hydrothermal conditions; IPOD; Igneous rocks; Isotope ratios; Isotopes; Lead; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Mass transfer; Metals; Metasomatism; Mid-ocean ridge basalts; Ocean Drilling Program; Oceanic crust; Pb-206/Pb-204; Pb-208/Pb-204; Radioactive isotopes; Rare earths; Rb/Sr; S-34/S-32; Sea water; Sr-87/Sr-86; Stable isotopes; Strontium; Subduction zones; Sulfur; Transport; Volatiles; Volcanic rocks
Coordinates: S020000 N080000 W0800000 W0920000
Record ID: 2004021448
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