Magmatic degassing of ore-metals at the Menez Gwen: Input from the Azores plume into an active Mid-Atlantic Ridge seafloor hydrothermal system

Menez Gwen is an active seafloor hydrothermal system located on the Mid-Atlantic Ridge at 37°50′N under the Azores plume influence and within enriched MORB type rocks. Two distinct highly plagioclase phyric (HPP) lavas occur at Menez Gwen, type I and type II. Type I HPP lavas are vesicular and highly enriched in incompatible trace elements; type II HPP lavas are less vesicular and less enriched. Melt inclusions in plagioclase and olivine phenocrysts in these HPP basalt lavas were studied by laser-ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The LA-ICP-MS data support previous findings that mixing of melts occurred below Menez Gwen. Olivine crystals enclosing melt inclusions having compositions of both type I and type II magmas occur in type I HPP basalts. All plagioclase hosted melt inclusions have type II signatures regardless of their host rock character. The melt inclusions of the less incompatible element enriched type II lavas are more enriched in Cu than type I, an observation that is relevant to the origin of metals characteristic of these particular basalt-hosted seafloor hydrothermal systems. Furthermore, petrographic and analytical observations of melt inclusions indicate exsolution from the magma of an immiscible volatile phase carrying Cu and S. Subsequent injection into the magma chamber of the more enriched type I melt coupled with extensive pre-eruptive degassing of CO2 and S likely triggered the mobilization of the exsolved ore-metals. High volatile contents and extensive magmatic degassing in a shallow magma chamber and mixing of enriched melts were contributors to the seafloor hydrothermal activity at Menez Gwen.

► Two distinct plagioclase-phyric basalts occur at the Menez Gwen.
► LA-ICP-MS data provides new evidence for mixing of different melts.
► Melt inclusions show pre-eruptive degassing of an ore-metal rich volatile phase.
► Melt less enriched in incompatible trace elements relates to higher Cu contents.