Boron, lithium and strontium isotopes as tracers of seawater–serpentinite interaction at Mid-Atlantic ridge, ODP Leg 209

Abstract

Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14°N and 16°N are highly serpentinized (50–100%), but still preserve relics of primary phases (olivine ≥ orthopyroxene >> clinopyroxene). We determined whole-rock B and Li isotope compositions in order to constrain the effect of serpentinization on δ 11B and δ 7Li. Our data indicate that during serpentinization Li is leached from the rock, while B is added. The samples from ODP Leg 209 show the heaviest δ 11B (+ 29.6 to + 40.52‰) and lightest δ 7Li (− 28.46 to + 7.17‰) found so far in oceanic mantle. High 87Sr/ 86Sr ratios (0.708536 to 0.709130) indicate moderate water/rock ratios (3 to 273, on the average 39), in line with the high degree of serpentinization observed. Applying the known fractionation factors for 11B/ 10B and 7Li/ 6Li between seawater and silicates, serpentinized peridotite in equilibrium with seawater at conditions corresponding to those of the studied drill holes (pH: 8.2; temperature: 200 °C) should have δ 11B of + 21.52‰ and δ 7Li of + 9.7‰. As the data from ODP Leg 209 are clearly not in line with this, we modelled a process of seawater–rock interaction where δ 11B and δ 7Li of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with δ 11B and δ 7Li of ODP Leg 209 samples, we obtain δ 11B and δ 7Li values of + 50 to + 60‰, − 2 to + 12‰, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high δ 11B have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the δ 7Li and δ 11B evolution in the fluid, shows that modification of δ 7Li during serpentinization requires higher water/rock ratios than modification of δ 11B. Extremely heavy δ 11B in serpentinized oceanic mantle can potentially be transported into subduction zones, as the B budget of the oceanic plate is dominated by serpentinites. Extremely light δ 7Li is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions.