Li-isotope exchange during low-temperature alteration of the upper oceanic crust at DSDP Sites 417 and 418

Abstract

The record of the Li-isotopic composition of ancient-seawater has the potential to provide important insight into the Earth system. However, we still have an incomplete understanding of the controls on the magnitude and isotopic compositions of the fluxes of Li into and out of the ocean. An important sink for Li from seawater is through low-temperature hydrothermal alteration of the upper oceanic crust. Here we present a detailed study of three adjacent drill cores in 120 Myr old Atlantic crust (Holes 417A, 417D and 418A) that had very different sedimentation histories. Samples from Hole 417A, drilled on a topographic high and exposed to seawater for >20 Myr before sediment started to accumulate, have much higher Li contents (up to ∼70 ppm) than samples from the other sites where sediment began accumulating immediately after crustal accretion (up to ∼20 ppm). Despite this difference in Li content, the δ7Li of the altered basalts in all cores increases with depth over the upper ∼40 m from around −1‰ at the lava-sediment interface to 6 ± 2‰ at ∼40 m depth, remaining roughly constant with depth after this. If the uppermost lavas equilibrated with seawater, with a bulk fractionation factor of ∼0.980 ± 0.003, then the isotopic composition of seawater 120 Myr ago was ∼19 ± 3‰, within the range of other estimates for Cretaceous seawater. Fluid-rock reaction models, with the hydrological regime varying as a function of depth and sediment accumulation history, can explain the observed rock compositions. However, modelling shows that complexities related to the time-varying hydrological regime, along with the possibility of time-varying δ7Li of seawater, mean that the Li-isotopic composition of altered oceanic lavas cannot be uniquely interpreted. The bulk Li content and isotopic composition of the well-sampled 120 Myr crust in Holes 417A, 417D and 418A suggest that Cretaceous upper oceanic crust was a sink for ∼4×109 mol yr−1 of Li. Surprisingly, the bulk Li-isotopic composition of the lavas sampled at these sites (∼6‰) is not much higher than that of fresh MORB, and within uncertainty of that of high-temperature hydrothermal vent fluids; i.e., high- and low-temperature alteration processes approximately balanced one another for Li-isotopes in the late Cretaceous. This could be fortuitous; alternatively, if the ocean was at steady state the main source of Li to the ocean at this time may have had a δ7Li similar to that of high-temperature hydrothermal fluids.