Lithium isotopic systematics of hydrothermal vent fluids at the Main Endeavour Field, Northern Juan de Fuca Ridge

Vent fluids issuing from the Main Endeavour Field (MEF), Juan de Fuca Ridge, were analyzed for δ7Li to help constrain subseafloor hydrothermal alteration and phase separation processes. Magmatic activity prior to sampling of the fluids in 1999 enhanced heat and mass transfer, as indicated by the large scale, but temporary, changes in vent fluid chemistry. In particular, dissolved chloride concentrations indicate formation of supercritical Cl-poor vapors, which affected alteration throughout the MEF system. δ7Li of fluids, however, ranges from +7.2 to +8.9‰ and reveals no significant correlation with dissolved chloride, being consistent with results of hydrothermal experiments that show no lithium isotope fractionation during supercritical phase separation. On a chloride-normalized basis, Li concentration data indicate relatively short residence times or high fluid/rock mass ratios of vent fluids most impacted by phase separation effects. Reaction path models involving Li isotope data also show elevated fluid/rock mass ratios. Boron data, in contrast, suggest direct input from degassing magma. Enhanced heat flow associated with magmatic injection at depth inhibits penetration of seawater-derived hydrothermal fluid into fresh basalt, particularly in those systems where magmatic volatile input is most active. The inverse correlation between Li/Cl and B/Cl in vapor-rich vent fluids may be a useful indicator of recent subseafloor magmatic activity

An experimental study of alteration of oceanic crust and terrigenous sediments at moderate temperatures (51 to 350°C): insights as to chemical processes in near-shore ridge-flank hydrothermal systems

Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 15 × 106 mol U yr?1 from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 106 mol yr?1), which concords with arguments that the ?element/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement

Evolutionary ecology during the rise of dioxygen in the Earth's atmosphere

Pre-photosynthetic niches were meagre with a productivity of much less than 10−4 of modern photosynthesis. Serpentinization, arc volcanism and ridge-axis volcanism reliably provided H2. Methanogens and acetogens reacted CO2 with H2 to obtain energy and make organic matter. These skills pre-adapted a bacterium for anoxygenic photosynthesis, probably starting with H2 in lieu of an oxygen ‘acceptor’. Use of ferrous iron and sulphide followed as abundant oxygen acceptors, allowing productivity to approach modern levels. The ‘photobacterium’ proliferated rooting much of the bacterial tree. Land photosynthetic microbes faced a dearth of oxygen acceptors and nutrients. A consortium of photosynthetic and soil bacteria aided weathering and access to ferrous iron. Biologically enhanced weathering led to the formation of shales and, ultimately, to granitic rocks. Already oxidized iron-poor sedimentary rocks and low-iron granites provided scant oxygen acceptors, as did freshwater in their drainages. Cyanobacteria evolved dioxygen production that relieved them of these vicissitudes. They did not immediately dominate the planet. Eventually, anoxygenic and oxygenic photosynthesis oxidized much of the Earth's crust and supplied sulphate to the ocean. Anoxygenic photosynthesis remained important until there was enough O2 in downwelling seawater to quantitatively oxidize massive sulphides at mid-ocean ridge axes