Lithium Isotopic Systematics in Minerals from Fresh Abyssal Peridotite at Gakkel Ridge: Subsolidus Diffusive Fractionation Versus Melt-Rock Interaction

We report lithium elemental and isotopic compositions of the mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of fresh Gakkel Ridge peridotites. These rocks are absolutely fresh, with <0.1% modal serpentine. Bulk rocks have lithium contents of 1.6 to 2.7 ppm and d7Li values of 3 to 5‰, which fall within the range of reported normal “MORB mantle” values. Lithium concentrations vary on the order of cpx (2.1–4.7 ppm) > opx (0.9 – 1.7 ppm) ≥ olivine (0.4 – 0.9 ppm), which reveals an inverse order of those found in “equilibrated” mantle peridotites (Seitz and Woodland, 2000, Chemical Geology, 166, 47-64). The lithium isotopic compositions indicate a systematic mineral variation with d7Liolivine (7.14‰ – 15.09‰) > d7Liopx (1.81‰ – 3.66‰) > d7Licpx (-2.43‰ – -0.39‰). The d7Li values of cpx are negatively correlated with their lithium concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Δolivine-cpx (d7Liolivine- d7Licpx) and ol/cpxD (Liolivine/Licpx). Collectively, these observations suggest that these inter-mineral fractionations may be related to a re-distribution of Li between minerals as a result of sub-solidus cooling driven by temperature or pressure sensitive inter-mineral partitioning. Alternatively, Li exchange via melt-rock reaction may have occured. However, the systematic correlation between inter-mineral lithium partitioning and inter-mineral isotopic fractionation together with the unaltered nature of the bulk rock d7Li values argues against the presence of significant melt-rock reaction, unless the reaction did not change the lithium system. The inter-mineral elemental and isotopic fractionation of lithium were further investigated using numerical diffusion modeling and its significance in deriving cooling rates of upper mantle beneath mid-ocean ridges was evaluated.