Investigation of Ge and Ga exchange behaviour and Ge isotopic fractionation during subduction zone metamorphism

Abstract

Germanium (Ge) and gallium (Ga) are slightly to moderately incompatible trace elements during magmatic processes and may be indicators of the nature of the mantle source. Because of their solubility in fluids, Ge and Ga are also geochemical tracers of various hydrothermal processes. Understanding high-pressure/low-temperature (HP–LT) recycling processes of Ge and Ga through fluid-mediated mass transfers in subduction zones can then allow discussing whether they can be source of geochemical heterogeneities in the mantle wedge. We have analysed Ge and Ga abundances and Ge isotope composition of a series of well-characterised HP–LT metabasites of mid-ocean ridge basalt (MORB) affinity and pelitic micaschists from the Ile de Groix (France), to determine Ge and Ga behaviour and Ge isotope fractionation during subduction zone metamorphism (SZM). Metabasites have Ge and Ga contents of 1.2–2.1ppm and 17–22ppm, respectively, typical of tholeiitic basalts. Garnet-bearing blueschists and eclogites have δ74Ge values (+0.42 to +0.65‰) similar within error to that of tholeiitic basalts (+0.55 to +0.57‰; Luais, 2012).Our study shows that during the prograde metamorphism, Ge and Ga abundances and δ74Ge values do not vary from blueschists to eclogites, owing to the large stability field of Ge- and Ga-hosting minerals (epidote, garnet, titanite, amphiboles and omphacite) under varying P–T conditions. During the retrograde metamorphism, the slight decrease of the Ga content of c. 8% in greenschists (19±1ppm; 2σ) compared to HP rocks (21±1ppm; 2σ) suggests Ga loss during retrogression, whereas Ge abundances remain within the same range as blueschists and eclogites. In the garnet-bearing greenschists (early stages of retrogression), the δ74Ge values (+0.38 to +0.49‰) remain similar within error to HP rocks. However, albite- and calcite-bearing greenschists have higher δ74Ge values (+0.84 to +0.98‰), showing evidence of Ge fractionation under intensive fluid-rock interactions (late stage of retrogression). Micaschists have Ge and Ga abundances within the same range as continental crust and schists. Their variations in δ74Ge (+0.29 to +0.70‰) reflect variations in the sedimentary source and protolith composition.The relatively small variation of δ74Ge values in metabasites compared to mantle rocks implies that the subducted oceanic crust cannot trigger Ge isotopic heterogeneities in the mantle through fluid-induced metasomatism or recycling in the deep mantle. However, during the late stages of retrogression, migration of oxidising fluids along the crust-mantle wedge may be responsible for a resolvable increase of the δ74Ge value of the mantle wedge along the subduction channel. Besides, the larger range of Ge concentrations and δ74Ge values in the micaschists suggests that recycling of subducted sediments may generate mantle Ge elemental and isotopic heterogeneities.