Distribution of boron and lithium in serpentinites using laser ablation ICP-MS

Boron and lithium are fluid-mobile elements that can be used as tracers for seawater and slab-derived fluids in subduction zones. Hydrothermally altered oceanic lithospheric mantle (in the form of serpentinite) may be the major sink for seawater-derived B and Li. In this study, we investigate how these elements are distributed among the primary and secondary minerals in variably serpentinized peridotites, helping us understand how B and Li may be released in slab-derived fluids during subduction. Serpentinite sections from the Feather River Ophiolite in California were mounted in epoxy and then hand-polished. They were then analyzed for major elements and certain trace elements, including Li and B, using a ThermoFinnigan Element II laser ablation ICP-MS. These measurements complement bulk rock measurements done by ICP-AES and presented in another paper. All samples were shown previously to have substantial bulk-rock enrichment of B and Li, indicating an external contribution from fluids, perhaps seawater. We examined two lithologies on the micron scale: partially serpentinized harzburgites and serpentinite veins. Relatively fresh olivine has 5-10 ppm B, which is much higher than that expected for the mantle, but still lower than the bulk rock B. B contents increase up to 50 ppm with serpentinization of olivine. Some serpentinized pyroxenes also show B contents up to 50 ppm, while others are not enriched, having about bulk rock B. The behavior of Li is more complicated. Fresh olivine was found to be enriched in Li, containing from 0.5 up to 2.5 ppm. Serpentinized olivines lost Li and mostly contained less than 0.5 ppm, generally close to or slightly less than bulk rock. Serpentinized pyroxenes are highly enriched in Li, containing up to 10 ppm. Collectively, our data confirm the notion that serpentinites are sinks for B and Li. Our data suggest, however, that much of the bulk rock enrichment in B and Li is stored in the unaltered olivines as well as in the serpentine. These observations suggest that significant amounts of B and Li (particularly Li) may be retained in the subducting slab, even after serpentine breakdown. Thus, some B and Li will be liberated into the mantle wedge and transferred to arc magmas, but a non-negligible fraction will not be liberated and hence will be subducted deep into the interior of the Earth.