Lithium concentrations and isotope composition of basalt samples from ODP/DSDP Sites from Costa Rica Rift

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?) 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?) 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?, 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.

Carbon chemistry of oceanic crust

Carbonate mineral precipitation in the upper oceanic crust during low-temperature, off-axis, hydrothermal circulation is investigated using new estimates of the bulk CO2 content of seven DSDP/ODP drill cores. In combination with previously published data these new data show: (i) the CO2 content of the upper ~ 300 m of the crust is substantially higher in Cretaceous than in Cenozoic crust and (ii) for any age of crust, there is substantially more CO2 in Atlantic (slow-spreading) than Pacific (intermediate- to fast-spreading) crust. Modelling the Sr-isotopic composition of the carbonates suggests that > 80% of carbonate mineral formation occurs within < 20 Myr of crust formation. This means that the higher CO2 content of Cretaceous crust reflects a secular change in the rate of CO2 uptake by the crust. Oxygen isotope derived estimates of carbonate mineral precipitation temperatures show that the average and minimum temperature of carbonate precipitation was ~10 °C higher temperatures in the Cretaceous than in the Cenozoic. This difference is consistent with previous estimates of secular change in bottom seawater temperature. Higher fluid temperature within the crust will have increased reaction rates potentially liberating more basaltic Ca and hence enhancing carbonate mineral precipitation. Additionally, if crustal fluid pH is controlled by fluid-rock reaction, the higher Ca content of the Cretaceous ocean will also have enhanced carbonate mineral precipitation. New estimates of the rate of CO2 uptake by the upper ocean crust during the Cenozoic are much lower than previous estimates.

Geochemistry of minerals at DSDP Hole 70-504B

Hole 504B, drilled into the 5.9 Ma crust of the southern flank of the Costa Rica Rift, tapped a hydrothermal system in its conductive stage. Three alteration zones were encountered along the 561.5 meters of basement drilled. The upper alteration zone, 274.5 to 584.5 meters below the seafloor (BSF), is characterized by the presence of color zonation in which red halos are located between dark gray inner rock portions and dark gray outer bands. The red halos are characterized by an abundance of iddingsite, and they have higher K2O contents and Fe3+/FeT ratios, but lower SiO2 contents, than the adjacent dark gray inner zones. The dark gray outer bands are characterized by the presence of celadonite-nontronite. Saponite is omnipresent in these three alteration bands. Phillipsite is the only zeolite that occurs in the upper alteration zone. The upper alteration zone is interpreted as being the result of low-temperature alteration, with large amounts of cold oxygenated seawater percolating through the upper ocean crust. In the upper alteration zone, the formation of red halos was both preceded and followed by formation of dark gray outer bands. Then followed formation of dark gray cores. The lower alteration zone (584.5-835.5 m BSF) is characterized by the absence of color zonation, the downward-increasing abundance of pyrite and saponite, and the presence of quartz, talc, and calcite. The chemical changes (downhole MgO enrichment and concomitant CaO depletion) observed in the basalts of the lower alteration zone are thought to result from reactions of oceanic basalts with evolved seawater (i.e., solutions derived from seawater that has already reacted with ocean crust), which is thus depleted in oxygen, potassium, and radiogenic strontium. This alteration process, which was responsible for saponite formation in both the upper and lower alteration zones, was rock dominated, and it took place under suboxic to anoxic conditions during a second stage of alteration. Reaction temperatures could have progressively increased with depth. There is also a zeolitic zone that essentially coincides with the lower part of the upper alteration zone (between 528.5 and 563 m BSF). The host rock adjacent to veins of zeolite exhibits a greenish discoloration due to the intensive replacement of the igneous minerals. The replacement minerals result in significant increases in the bulk rock K2O, MgO, CaO, CO2, and H2O+ contents. The solutions circulating along the newly opened fissures had high Ca activity, and minerals probably precipitated in these fissures at 60°C or 110°C. These hydrothermal solutions circulated later than those responsible for the formation of the minerals that characterize the upper and lower alteration zones.