Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
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.