Beschreibung: Ocean Drilling Program (ODP) Hole 504B near the Costa Rica Rift is the deepest hole drilled in the ocean crust, penetrating a volcanic section, a transition zone and a sheeted dike complex. The distribution of Li and its isotopes through this 1.8-km section of oceanic crust reflects the varying conditions of seawater alteration with depth. The upper volcanic rocks, altered at low temperatures, are enriched in Li (5.6-27.3 ppm) and have heavier isotopic compositions (delta7Li=6.6-20.8 per mil ) relative to fresh mid-ocean ridge basalt (MORB) due to uptake of seawater Li into alteration clays. The Li content and isotopic compositions of the deeper volcanic rocks are similar to MORB, reflecting restricted seawater circulation in this section. The transition zone is a region of mixing of seawater with upwelling hydrothermal fluids and sulfide mineralization. Li enrichment in this zone is accompanied by relatively light isotopic compositions (-0.8-2.1 per mil ) which signify influence of basalt-derived Li during mineralization and alteration. Li decreases with depth to 0.6 ppm in the sheeted dike complex as a result of increasing hydrothermal extraction in the high-temperature reaction zone. Rocks in the dike complex have variable isotopic values that range from -1.7 to 7.9 per mil , depending on the extent of hydrothermal recrystallization and off-axis low-temperature alteration. Hydrothermally altered rocks are isotopically light because 6Li is preferentially retained in greenschist and amphibolite facies minerals. The delta7Li values of the highly altered rocks of the dike complex are complementary to those of high-temperature mid-ocean ridge vent fluids and compatible to equilibrium control by the alteration mineral assemblage. The inventory of Li in basement rocks permits a reevaluation of the role of oceanic crust in the budget of Li in the ocean. On balance, the upper 1.8 km of oceanic crusts remains a sink for oceanic Li. The observations at 504B and an estimated flux from the underlying 0.5 km of gabbro suggest that the global hydrothermal flux is at most 8*10**9 mol/yr, compatible with geophysical thermal models. This work defines the distribution of Li and its isotopes in the upper ocean crust and provides a basis to interpret the contribution of subducted lithosphere to arc magmas and cycling of crustal material in the deep mantle.

Beschreibung: Skiff Bank is a bathymetric and gravimetric high located ~350 km southwest of the Kerguelen Islands in the northern portion of the Kerguelen Igneous Province (KIP). Ocean Drilling Program Site 1139 was drilled at a water depth of 1415 m on Skiff Bank's western edge. Hole 1139A penetrated to a depth of 695 meters below seafloor (mbsf) with sediment recovered from the upper 462 m and igneous basement recovered from the lower 233 m. A total of 19 basement units were identified, including variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks (Units 1-5) and 14 lava flows with intermediate to mafic compositions (Units 6-19) (Shipboard Scientific Party, 2000, doi:10.2973/odp.proc.ir.183.101.2000). To better assess the mass transfer of major elements associated with these unique alteration patterns, we have conducted a detailed chemical analysis of both major and trace elements in samples from the basement units of Hole 1139A. These data are reported here, along with whole-rock analyses for carbon, sulfur, and ferrous/ferric ratios (Table T1). Downhole trends for SiO2, C, Fe3+/Fe total, and loss on ignition (LOI) are shown. The trends serve to quantify the distinct alteration observed in some basement units within Hole 1139A. For example, carbon increases dramatically below 570 mbsf and shows large enrichments in basement Units 5-19. The data also show that iron becomes more reducing with depth overall, with Fe3+/Fe total ratios dropping to as low as 0.13 in basement Unit 18. The large variations in this ratio with depth probably reflect alteration zones with variable amounts of hematite and siderite. These unique alteration assemblages and whole-rock compositions were likely produced by CO2-rich hydrothermal fluids interacting with intermediate to felsic volcanic rocks at high water-to-rock ratios. The fluids involved were probably unlike hydrothermal fluids that affected other parts of the KIP.

Beschreibung: Leg 168 of the Ocean Drilling Program (ODP) investigated the heat flow, fluid chemistry and crustal alteration associated with ridge flank hydrothermal systems. Ten sites were drilled on the eastern flank of the Juan de Fuca Ridge, along an 80 km transect, between 20 and 100 km east of the spreading centre. Recovered cores consisted of 100-500 m of sediment with shallow penetration (1.7-48.1 m) into the underlying igneous basement (0.8-3.6Ma). Here we use the composition of calcium carbonate minerals, from veins within the upper basement, to reconstruct the evolving chemistry of hydrothermal fluids with increasing crustal age and sediment cover thickness. We show for the first time a clear link between the alteration of the basement rocks as recorded by secondary minerals, and the nearbasement sedimentary pore fluids, which are often assumed to be representative of the basement fluids responsible for low temperature alteration of the upper crust. Carbonates precipitated from basement fluids that ranged in strontium isotopic composition from near-modern seawater (87Sr/86Sr =~ 0.70918) to the near-basement pore fluid values at any one site. 87Sr/86Sr ratios are independent of mineralogy with both aragonite and calcite precipitating from variably evolved fluids with the range in carbonate 87Sr/86Sr increasing with crustal age. A parallel geochemical evolution of basement fluids and sediment porewaters is shown since 87Sr/86Sr ratios of near-basement pore fluids decrease from 0.709013 to 0.707108 away from the ridge axis. A correlation exists between 87Sr/86Sr ratios and d18O-calculated fluid temperatures, with more geochemically evolved carbonates having precipitated from warmer fluids. Basement fluid compositions, calculated from carbonate Sr, Mg, Fe and Mn concentrations combined with suitable partition coefficients, are also temperature-dependent. Given an observed increase in basement temperature with age, from 16°C to 64°C along the transect, a progressive chemical development of basement fluid is demonstrated. Carbonate veins in volcanic basement from ODP Holes 504B and 896A, on the Costa Rica Rift, record the same temperature compositional evolution of basement fluid as those from the Juan de Fuca Ridge flank. Although these locations have different thermal histories and therefore must have experienced different temporal geochemical evolution of basement fluid, basement temperature appears to be the dominant control on basement fluid composition.