Behavior of rare earth elements and high-field strength elements during peridotite-seawater interaction

Serpentinized abyssal peridotites are evidence for active communication between the Earth's hydrosphere and the upper mantle, where exchange and retention of both major and trace elements occur. Rare earth elements (REE) and high-field strength elements (HFSE: Ti, Zr, Hf, Nb, Ta) are generally thought of as immobile during serpentinization. In partially serpentinized abyssal peridotites, clinopyroxenes typically retain radiogenic Nd and Hf isotopic signatures that have been used to infer the composition and high temperature processes in the upper mantle. However, bulk rock Nd isotopes in some serpentinized abyssal peridotites less radiogenic values that approach that of seawater, which has been previously explained as elemental exchange during seawater -- rock interactions. This discrepancy between the clinopyroxene and their bulk rock abyssal peridotite Nd isotope values question the presumed immobile nature of REE during serpentinization. Yet the relationship between the clinopyroxene and bulk rock Nd isotopes and REE concentrations and whether REE are added or exchanged with peridotite, has not been explicitly tested. In addition, to date no bulk rock Hf isotopes have been reported in abyssal peridotites, so it is unclear if HFSE in peridotites are being affected by serpentinization. To answer these questions this dissertation reports detailed bulk rock trace element, Sr, Nd, Hf, Pb isotopes as well as trace and major element concentrations of primary and secondary mineral phases of serpentinized peridotites recovered from the ocean floor. A subset of these rocks was analyzed by sequential leaching experiments. These data shows that LREE are added to peridotites during serpentinization in varying amounts and are being confined to secondary mineral phases. The extent of addition also depends on the water / rock mass ratio, a proxy for the extent of serpentinization, and the relative concentrations of REE in peridotite protolith prior to alteration. Additionally, bulk rock Hf isotopes match with clinopyroxene Hf isotopes within 1 epsilonHf unit while bulk rock Nd isotopes extend to much less radiogenic values than their clinopyroxene counterparts. This data suggests that the bulk rock peridotite Hf isotopes are more resilient during serpentinization that Nd isotopes, and can be used to trace the magmatic history of the rock. Additional experiments were performed between artificial seawater and fresh mineral separates from peridotites at a range of low temperatures <100°C at atmospheric pressures, intermediate temperatures of ~170°C at 8 bars pressure and high temperatures at 300°C and 400°C at 500 bars pressure to constrain some of the driving mechanisms for the observed REE behavior during serpentinization in the natural samples. Experiments at <100°C at atmospheric pressures show a preferential uptake of HREE over that of LREE to the olivine surface. The kinetic rate constants of REE adsorption to olivine where calculated and are found to vary with the electron configuration of REE, rather than their radius. Experiments at 170°C and 8 bars, this same trend but resulted in near complete removal of all REE in solution. Experiments between clinopyroxene and artificial seawater doped with isotopically enriched REE showed both uptake of REE from solution, but also exchange of REE between the mineral and seawater. This implies that both uptake and exchange are operational during seawater -- rock exchange, but the uptake dominates, consistent with the natural samples data above. Experiments at 300°C and 400°C at 500 bars confirm the high reactivity of REE on to the peridotite even at these temperature. In summary this dissertation shows that light REE are dominantly added to peridotites during serpentinization, and that serpentinization may be a sink for REE. The bulk rock Hf isotopes are far less sensitive to serpentinization than Nd, possibly due to lower concentrations found in seawater relative to REE. These data have implications on the isotopic evolution of serpentinized recycled lithosphere and the development of isotope heterogeneities in the mantle, the use of Nd isotopes to calculate water / rock ratios during serpentinization, and possibly the flux of LREE from the serpentinized lithosphere to the mantle wedge and arc volcanism at convergent boundaries.