Cooling-induced fractionation of mantle Li isotopes from the ultraslow-spreading Gakkel Ridge |
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| Authors: | Yongjun Gao Jonathan E. Snow John F. Casey Junbao Yu |
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| Affiliation: | 1. Dipartimento di Scienze della Terra e dell’Ambiente, Universita’ degli Studi di Pavia, Via Ferrata 1, 27100, Pavia, Italy;2. Kanazawa University, Kakuma, Kanazawa 920-1192, Japan;3. Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan;4. University of Tokyo, Tokyo 164-8639, Japan;1. Department of Earth and Planetary Sciences, Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Macquarie University, Sydney, NSW 2109, Australia;2. Resources Division, Geoscience Australia, Canberra, ACT 2601, Australia;1. GeoZentrum Nordbayern, University of Erlangen-Nürnberg, Schloßgarten 5a, 91054 Erlangen, Germany;2. Institute of Mineralogy and Petrography, University of Innsbruck, Innrain 52f, 6020 Innsbruck, Austria |
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| Abstract: | 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 δ7Li values of 3 to 5‰, 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 δ7Liolivine (7.14‰–15.09‰) > δ7Liopx (1.81‰–3.66‰) > δ7Licpx (?2.43‰ ? ?0.39‰). The δ7Li 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 Δolivine–cpx (δ7Liolivine ? δ7Licpx) and ol/cpxD (Liolivine/Licpx).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. |
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