The role of low-temperature (off-axis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite

摘要

Changes in the global Li-cycle, as recorded in the Li concentration and/or isotopic composition of seawater, have the potential to provide important insight into the controls on the long-term C-cycle. Understanding the magnitude and isotopic composition of the fluxes of Li into and out-of the ocean, and the controls on any variability in these, is necessary if we are to correctly interpret the paleo-record of the Li-cycle. Here the low-temperature (off-axis) hydrothermal sink is investigated using the volcanic section of the exceptionally preserved Troodos ophiolite. Using glass to define the protolith Li content, the uptake flux of Li is determined using bulk-rock analyses from four hydrologically distinct sections through the lava pile of the ophiolite. Differences in paleo-hydrological conditions in the crust appear to have played a significant role in controlling the uptake flux of Li with an `average' uptake flux of equivalent to 14-21 x 10(9) mol yr(-1) -this is considerably larger than generally assumed. Bulk-rock samples that contain a large seawater Li component have delta Li-7 of similar to 10 +/- 2 parts per thousand. Celadonite separates have a delta Li-7 of similar to 6 +/- 1 parts per thousand, considerably lighter than bulk-rock samples with the same Li content. Because celadonite is a significant repository for Li within the Troodos upper crust this means that another phase(s) must have markedly heavier delta Li-7 than the average bulk-rock; i.e. changes in the average mineralogy of altered crust will lead to changes in the bulk isotopic fractionation between the Li added to the upper oceanic crust and seawater (Delta SW-lava). The shallowest samples in three of the four studied sections are isotopically lighter than deeper samples (but do not contain significant celadonite), again indicating that variations in alteration conditions and/or mineralogy can lead to variations in Delta SW-lava. Comparison with other studies of altered upper oceanic crust suggests that changes in alteration conditions lead to significant changes in Delta SW-lava. These changes likely reflect both a temperature dependence of the isotopic fractionation factor and a change in the fractionation factor due to changing mineral assemblage and/or mineral compositions and abundances. A significant portion of the increase in delta Li-7 of seawater over the past 50 Myr may be due to an increase in the bulk fractionation factor between seawater and Li added to the upper oceanic crust due to cooling bottom water. (C) 2017 Elsevier Ltd. All rights reserved.