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°N and 16°N are highly serpentinized (50-100%), but still preserve relics of primary phases (olivine>=orthopyroxeneNNclinopyroxene). We determined whole-rock B and Li isotope compositions in order toconstrain the effect of serpentinization on delta11B and delta7Li. 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 delta11B (+29.6 to+40.52?) and lightest delta7Li (-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 delta11B of +21.52? and delta7Li 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 delta11B and delta7Li of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with delta11B and delta7Li of ODP Leg 209 samples, we obtain delta11B and delta7Li values of +50 to +60?, -2 to +12?, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high delta11B have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the delta11B and delta7Li evolution in the fluid, shows that modification of delta7Li during serpentinization requires higher water/rock ratios than modification of delta11B. Extremely heavy delta11B 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 delta7Li is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions.