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

Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14 degrees N and 16 degrees 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 delta B-11 and delta Li-7. 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 delta B-11 (+ 29.6 to + 40.52 parts per thousand) and lightest delta Li-7 (-28.46 to + 7.17%parts per thousand) found so far in oceanic mantle. High Sr-87/Sr-86 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 B-11/B-10 and Li-7/Li-6 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 degrees C) should have delta B-11 of + 21.52 parts per thousand and delta Li-7 of + 9.7 parts per thousand. As the data from ODP Leg 209 are clearly not in line with this, we modelled a process of seawater-rock interaction where delta B-11 and delta Li-7 of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with delta B-11 and delta Li-7 of ODP Leg 209 samples, we obtain delta B-11 and delta Li-7 values of + 50 to + 60 parts per thousand, -2 to + 12 parts per thousand, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high delta B-11 have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the delta Li-7 and delta B-11 evolution in the fluid, shows that modification of delta Li-7 during serpentinization requires higher water/rock ratios than modification of delta B-11. Extremely heavy delta B-11 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 delta Li-7 is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions. (C) 2009 Elsevier B.V. All rights reserved.