Diffuse porous melt flow and melt-rock reaction in the mantle lithosphere at a slow-spreading ridge: A structural petrology and LA-ICP-MS study of the Othris Peridotite Massif (Greece)

Harzburgites and plagioclase-peridotites from the Othris Peridotite Massif in Central Greece preserve microstructural and petrological evidence for interaction with a melt which became saturated in orthopyroxene while migrating by km-scale diffuse porous flow through the thermal boundary layer (TBL) and the base of the thermal lithosphere. The melt precipitated orthopyroxene, and eventually also plagioclase and clinopyroxene within the peridotites. Major and trace element geochemistry suggests that the melt was a depleted melt, i.e., a melt fraction from the melting column underneath a spreading centre produced by shallow melting of refractory peridotites. We see no evidence for the presence of boninitic melts. We argue that the melts in Othris migrated by diffuse porous flow as they crystallised orthopyroxenes and were therefore inherently unable to create their own high-permeability melt channels. We propose that depleted melt fractions can remain isolated from deeper melt fractions, possibly already aggregated into a MORB-like magma, because they migrate by different mechanisms through the TBL and the lithosphere.