The role of sedimentation history and lithology on fluid flow and reactions in off-axis hydrothermal systems: A perspective from the Troodos ophiolite

Abstract

Off-axis hydrothermal systems are thought to carry globally significant chemical fluxes but different ocean crust sections show widely differing extents of alteration making the quantification of these fluxes complex. With the aim of better understanding the origin of this diversity in alteration extent we have studied two sections of the lava pile in the Troodos ophiolite with distinct sedimentation history and volcanic architecture. The Akaki section is dominated by pillow lavas and the oldest sediments overlying the crust are ~20Myr younger than the ophiolite. Here there is a 300m thick zone at the top of the lavas that is enriched in CO2 and alkali elements, and has high 87Sr/86Sr and δ7Li. These features indicate extensive chemical exchange with seawater. In contrast to the Akaki area, the Onophrious section is dominated by sheet flows and the oldest sediments are of the same age as the ophiolite. Here the CO2 and alkali element enriched zone is much thinner (<100m), is less enriched in these elements (e.g. by a factor of three for CO2), and has lower 87Sr/86Sr and δ7Li. The O-isotopic compositions of calcites from these CO2- and alkali-enriched zones were precipitated from fluids with bottom water temperatures (~10°C). Maintaining such low temperatures to 300m depth in the crust in the Akaki area suggests that this was a region of recharge. Below these CO2- and alkali-enriched zones temperatures increase with depth such that calcite precipitation in the Onophrious area occurred at ~10°C higher temperature, at any given depth, than in the Akaki area. The increase in precipitation temperature with depth indicates poor thermal mixing within the crustal aquifer, likely due to laterally continuous sheet flows restricting the permeability. The chemical and thermal constraints suggest that both timing of the onset of sedimentation and volcanic architecture play important roles in controlling fluid and chemical fluxes. The same signal can be seen in drill core data from the modern ocean basins, with higher sedimentation rates leading to lower fluid fluxes and higher temperatures in the crustal aquifer.