Magma mixing at oceanic transform fracture zones

International Symposium on the Activity of Oceanic Volcanoes. Ponta Delgada, 4-9 August 1980.Ocean ridge eruptive chemistry reflects isochronic compositional «gradients», often attributed to the interaction of lithophile element-rich mantle plumes with the relatively depleted asthenosphere. Inspection of Atlantic and Pacific axial gradients shows lithophile-rich eruptions do not correspond exclusively to proposed loci for mantle upwelling. In contrast, transform fracture zones appear to mark the majority of lithophile-enrichment peaks, suggesting relatively superficial tectonic phenomena influence magma chemistry. A general association of lithophile and radiogenic isotope enrichment with increasingly frequent offset spreading culminates at major fracture zone ridge axis intersections (e.g., Galapagos, Iceland, Azores, Jan Mayen, and Siqueiros, Oceanographer and Romanche fracture zones). Major element and normative compositions show that enrichment also coincides with increased depth of melt segregation in the mantle. […].