Cooling-induced fractionation of mantle Li isotopes from the ultraslow-spreading Gakkel Ridge

Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7 ppm and Ski values of 3 to 5 parts per thousand, which fall within the range of reported normal pristine "MORB mantle" values. Lithium concentrations vary in the order cpx (2.1-4.7 ppm)>opx (0.9-1.7 ppm)olivine (0.4-0.9 ppm), the opposite found in "equilibrated" mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with delta Li-7(olivine) (7.14 parts per thousand-15.09 parts per thousand)>delta Li-7(opx) (1.81 parts per thousand-3.66 parts per thousand)>delta Li-7(cpx) (-2.43 parts per thousand--0.39 parts per thousand). The delta Li-7 values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Delta(olivine-cpx) (delta Li-7(olivine)-delta Li-7(cpx)) and D-ol/cpx (Li-olivine/Li-cpx). Numerical simulations indicate that the observed systematic inter-mineral variations of Li concentrations and isotopic compositions could be explained by a cooling driven diffusive redistribution between minerals in a closed system if there is a temperature dependent partitioning of Li between olivine and clinopyroxene. The studied Gakkel Ridge abyssal peridotites may alternatively have cooled under a variable cooling rate with a rapid cooling before the Li system was closed, which is less likely given the tectonic setting. Our calculations confirm that Li systematics in minerals, especially in coexisting mineral phases could potentially be used as a mantle geospeedometer, even for slowly cooled mantle rocks. Published by Elsevier B.V..Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7 ppm and Ski values of 3 to 5 parts per thousand, which fall within the range of reported normal pristine "MORB mantle" values. Lithium concentrations vary in the order cpx (2.1-4.7 ppm)>opx (0.9-1.7 ppm)olivine (0.4-0.9 ppm), the opposite found in "equilibrated" mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with delta(7)Li(olivine) (7.14 parts per thousand-15.09 parts per thousand)>delta(7)Li(opx) (1.81 parts per thousand-3.66 parts per thousand)>delta(7)Li(cpx) (-2.43 parts per thousand--0.39 parts per thousand). The delta(7)Li values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Delta(olivine-cpx) (delta(7)Li(olivine)-delta(7)Li(cpx)) and (ol/cpx)D (Li(olivine)/Li(cpx))