Foreword: Magma generation and evolution in the Earth

ON THE OCCASION OF HIS 70TH BIRTHDAY The 20th century was eventful in all areas of Earth Science. Continental drift and sea-floor spreading became embodied in the theory of plate tectonics, isotopically heterogeneous mantle was recognized as a by-product of plate tectonics, large igneous provinces were identified as possibly originating from mantle plumes the list goes on. One thing these revolutions have in common is the process of scientific debate which Mike O’Hara has stimulated vigorously in the field of the petrology and geochemistry of igneous and metamorphic rocks on the Earth and its Moon in over 150 scientific publications since 1960. Part of this body of work is now acknowledged as fundamental truth, whilst some aspects remain controversial [e.g., O’Hara (2000), Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar Petrogenesis Revisited, Journal of Petrology 41, 1545-1651]. Mike’s discoveries in igneous petrology and geochemistry have fundamentally altered the way geoscientists study igneous rocks. In his famous 1968 paper ‘‘Are ocean floor basalts primary magmas?’’ [O’Hara (1968) Nature 220, 683–686], he demonstrated for the first time that erupted basalts from the mid-ocean ridges cannot be primary magmas from the mantle, a minority point of view at the time. This conclusion is now universally accepted as true and this classic paper still continues to be cited. In another paper that same year ‘‘The bearing of phase equilibria studies in synthetic and natural systems on the origin of basic and ultrabasic rocks’’ [O’Hara (1968) Earth Science Reviews 4, 69– 133], Mike showed how liquidus crystallization phase diagrams can be used as a tool to distinguish the products of partial melting from the products of partial crystallization in igneous petrogenesis. He invented the CMAS projection technique for analysis and description of crystallization and melting paths, and for the representation of the geochemistry of basaltic rocks. This projection method has been used ever since in various modified forms. He showed that crystallization of magmas in nature follows phase equilibrium systematics that can be approximated by phase diagrams in simple 4-component systems; this also holds, to a lesser extent, for partial melting. In this way, O’Hara became peerlessly the principal successor to N.L. Bowen, and was recognized for this achievement with the Bowen Award of the American Geophysical Union in 1984. One of the major tasks in volcanology-petrologygeochemistry is to distinguish between the effects of partial melting and partial crystallization in the geochemistry of erupted magmas. Although Mike first recognized it 35 years ago and despite recent advances in computational procedures for simulating crystallization and melting, this problem persists. Indeed, it is so complex that petrologists continue to debate interpretations about the P-T conditions of mantle melting based on basalt chemistry which, quite often, reflects the effects of fractional crystallization and mantle source compositional variability! From 1977 onwards, Mike continued to investigate the processes of partial melting and partial crystallization of magmas through theoretical modelling of mineralmelt trace element partitioning. This body of work forms a sequel to the seminal papers of Gast and Shaw. He explored the consequences of magma mixing, crustal contamination and fractional crystallization in