2019 | Adakitic Magmatism – The role of fluids and redox conditions in magma genesis
Adakites represent an important type of magmatism in subduction zones. It is believed that ada- kitic magmas are produced by direct melting of the subducted slab. The melting and subsequent interaction with the overlying mantle impose characteristic geochemical and isotopic signatures on adakitic rocks. Despite numerous hypotheses and models on slab melting, the exact mechanism of adakitic magma generation remains elusive. In particular, the effects of slab fluids and redox condi- tions on the melting process are not well understood. We have collected unique adakite samples by dredging the outcrops at 3000-4000 m water depth at the Western Aleutians (ship cruise SO249 to north Pacific). The preliminary analyses of the rocks revealed highly oxidizing conditions during magma eruption on the surface (presence of anhydrite) and very reducing conditions at depth of magma genesis (stability of sulfide phases), which is sur- prising. The mechanism of such a redox change (by several orders of magnitude) is unknown but crucial for the understanding of adakite generation and evolution from the slab to the surface. We have conducted a series of pilot experiments with basaltic and adakitic magmas at pressures of 0.5 to 1.5. GPa, i.e., at the conditions close to that of adakite genesis and evolution. We have found that pressure has a dramatic effect on sulfur speciation, although the extent of this effect towards the slab depths is not yet known (we would need pressures up to 4-5 GPa). Furthermore, results of another pilot experiment suggest that supercritical fluid conditions affect the melting of basaltic sub- strate and shift the melt composition to adakite. If true, these experiments will have crucial implica- tions for the understanding and reconstruction of natural adakitic systems. In this project we will focus on detailed investigations of the unique samples and on the additional experimental studies. In particular, the amphiboles from natural samples contain zircon inclusions, which will provide key information about sources, geochemical processes and dynamics of adakite magmatism. The experiments will help to reveal and quantify the effects of fluids and redox condi- tions on the compositions of adakitic magmas. The results will be combined in a general model of adakite genesis and can be used in future geodynamic models.