Sulfur transfer along a metasomatized serpentinite-metagabbro contact in the Voltri Massif, Italy

Abstract

<p>Subduction zones provide a key link between the surficial biogenic, atmospheric and hydrospheric geochemical cycles with the Earth’s internal reservoirs. Sediment compaction and dehydration of variably altered oceanic lithosphere during subduction release volatile species (containing e.g., S, H, C, N) to the overlying mantle wedge. In particular, sulfur plays a key role in the formation of porphyry ore deposits and has a major control on redox processes in subduction zones, given it occurs in variable oxidation states from oxidized sulfate (S<sup>6+</sup>) to reduced sulfide (S<sup>2-</sup>). Here we studied samples from a contact between serpentinite and partly metasomatized eclogitic metagabbros in the Voltri Massif (Italy). We determined the bulk rock and in situ sulfur isotope composition of pyrite grains and combined this with detailed mineralogic and petrologic investigations. Along the serpentinite-metagabbro contact, the metagabbros are metasomatized to actinolite-chlorite schists and metagabbros rich in epidote and Na- and Na-Ca amphiboles. The serpentinites as well as the actinolite-chlorite schists along the serpentinite-metagabbro contact have very low sulfide contents and provide evidence for the oxidation of sulfides, including formation of Fe-oxides. Sulfur input from the serpentinite-metagabbro contact towards the less metasomatized eclogitic metagabbros is observed. This sulfur input is reflected by bulk rock δ<sup>34</sup>S values that increase from initially around +1.5‰ in samples distant from the contact to +7.3 to +12.5‰ in samples near the contact. This trend correlates with a general increase in the in situ δ<sup>34</sup>S values from core to rim of individual pyrite grains. Distinct Co and Ni growth zones in pyrite and variations in the in situ δ<sup>34</sup>S values indicate multiple phases of pyrite growth during subduction and exhumation of these rocks, with the last stage of pyrite growth clearly related to Mg-metasomatism along the serpentinite-metagabbro contact. Thus, this study provides new insight into processes of sulfur migration during metasomatism of gabbroic rocks within the subducting slab and at the slab–mantle interface.</p>