Lithium Behaviour and Isotope Fractionation During Fluid–Rock Interactions in Variscan Oceanic Suture Zones: Limousin Ophiolite and Ile de Groix High-pressure Terrane (France)

Abstract

AbstractOphiolites and high-pressure/low-temperature (HP–LT) terranes are important sites for the study of geochemical cycling in ancient oceanic lithosphere. We have analysed Li abundances and isotope composition in a series of ultrabasic and basic rocks from the Variscan Limousin ophiolite, as well as in basic and pelitic rocks from the Ile de Groix HP–LT terrane. Both bulk and in situ analyses are employed to evaluate Li mobility and isotope fractionation in the oceanic lithosphere during fluid–rock interactions related to seafloor and sub-seafloor hydrothermal alteration, subduction and exhumation processes. In the Limousin ophiolite, early stages of high-temperature (high-T) hydrothermal alteration of oceanic ultrabasic rocks produced serpentine with low Li abundances (0·9–4·6 ppm) and low δ7Li (–8·9‰). The δ7Li increase from –2·2 to +4·2‰ in the following generations of serpentine during late-stage hydrothermal alteration results from changes in the fluid composition and temperature conditions. Therefore, even if dehydrating subducted serpentinites generate high amounts of fluids during subduction, abyssal serpentinites do not constitute an important source of Li for Li-rich metabasic rocks. In the associated amphibolites, hornblende displays typical Li contents (3·1–8·2 ppm) and isotopic compositions (+3·5 to +12·5‰) similar to hydrothermally altered sheeted dykes and gabbros. In contrast, the low Li abundances and extremely high δ7Li values recorded by omphacite and pargasitic amphibole in the ultrahigh-pressure (UHP) zoisite-eclogite from the Limousin probably reflect interaction with a heavy-Li sediment-derived fluid. The HP–LT metabasites of the Ile de Groix record different Li behaviour, with high Li abundances and low δ7Li. They contain Li abundances significantly higher than fresh mid-ocean ridge basalts (MORB) (16–124 ppm), indicating a metasomatic overprint by fluids derived from the neighbouring Li-rich mica-schist (15–52 ppm) in addition to seawater during the early stages of subduction. Lithium is mainly hosted by (1) glaucophane and omphacite in blueschists and eclogites, (2) chlorite and albite in retrograde greenschists, and (3) phengite and chlorite in mica-schists. The metabasites have δ7Li values of –4·8 to +3·2‰ that are generally lower than those of fresh and altered MORB. The intercalated mica-schists display δ7Li values ranging from –1·7 to +0·2‰ that are typical of subducted sediments. The δ7Li decrease from blueschists to eclogites from +1·8 to –4·8‰, as well as the rimward δ7Li decrease in glaucophane from MORB-like δ7Li values to negative values in blueschists (core: –2·4 to +8·8‰; rims: –7·1 to +2·2‰), reveals that significant fluid-induced Li isotope fractionation occurred at the transition from the lawsonite-blueschist facies to the epidote-blueschist facies, and may be triggered by prograde lawsonite breakdown. In eclogites, the low δ7Li measured in whole-rocks (–4·8 to –2·5‰), omphacite (–22·4 to +3·3‰) and glaucophane (–6·9 to +1·4‰) indicates that Li isotope kinetic fractionation had stronger effects under eclogite-facies conditions. The δ7Li increase toward positive values in the most retrogressed greenschist samples suggests Li mineral–fluid isotopic exchange during rehydration reactions and interaction with a Li-heavy fluid that is probably derived from the dehydrating metabasites. Thus, lithium isotope fractionation in the HP–LT rocks of the Ile de Groix highlights migration of heavy-Li fluids along the oceanic crust–mantle interface in the subduction zone.