Pre-eruption history of enriched MORB from the Menez Gwen (37°50′N) and Lucky Strike (37°17′N) hydrothermal systems, Mid-Atlantic Ridge

Silicate melt inclusions, hosted in plagioclase phenocrysts, exhibit small metallic precipitates of Fe + Ni + Cu ± Zn (± Co ± As) ± S ± Cl within the vapour bubbles. The heterogeneous trapping of distinct coexisting phases during plagioclase growth indicates that a high temperature ore-metal rich immiscible fluid must have been exsolved from the magma prior to eruption. On the other hand, lava vesicularities are remarkably different between the group I and group II lavas. They indicate ubiquitous degassing of magmatic volatiles, particularly in group I lavas that underwent significant pre-eruptive degassing. The presence of an ore-metal-rich fluid being exsolved from the magma chamber in conjunction with pre-eruptive degassing is a strong indicator of direct magmatic contribution of metals and volatiles to the seafloor hydrothermal systems. At the Menez Gwen site, timing of degassing and the enriched chemistry of lavas proved to be crucial factors contributing to the exsolution of a metal-rich fluid prior to magma eruption.