Aspects of magma genesis in arcs

Abstract

Geochemical analysis of magmas developed above subduction zones in combination with chemical, phase equilibria and physical modeling of petrogenetic processes have shown that the mantle wedge overlying the subducted lithosphere is volumetrically the most significant source of the majority of island arc magmas. Locally and at distinct episodes in the history of some arcs, direct melts of the subducted lithosphere may be erupted essentially unmodified or become involved as components of wedge-derived magmas. A flux of hydrous fluid from the subducted lithosphere is the trigger for partial melting of the overlying wedge, a source of oxidation of the magmas produced, and a transporting agent for some trace elements that are typically enriched relative to elements of equal crystal-melt incompatibity but non-hydrophilic character, in arc magmas compared with ocean ridge settings. For some hydrophilic trace elements (e.g., B, Be), the subducted lithosphere is volumetrically the largest supplier in arc magmas. In addition however, leaching of some of these elements (e.g., Sr) from the wedge by fluid infiltration prior to melting significantly augments the eventual magmatic abundances. Wedge compositional variability can be demonstrated on a global basis. A number of processes have probably led to this variability including concurrent and/or past magma extraction in backarcs, localised past input of hot-spot/ocean island magmas, global variability of sediment input, and incorporation of ancient subcontinental mantle domains. In the case of the Bonin arc, a remarkably stable environment of magma generation has perisisted for the past ∼45 million years since arc inception, with no evidence of secular increases in alkalinity. Similarly, a consistent trend of increasing alkalinity with evolution of the Mariana arc is absent. In both examples, extension of the lithosphere concurrent with magmatism may have maintained vertically extensive melting domains between arc lithosphere and the subducted slab. There is no convincing support for production of greater arc crustal thicknesses than backarc crust for either the Bonins or Marianas, indicative of partial melting percentages in arcs⩽backarcs. For most estimates of the percentages of melting beneath arcs, mass balance of Fe 2O 3/FeO in the mantle wedge requires addition of oxygen (via H 2O). Although the melting relations of amphibole are critical in arc magma genesis, it is likely that this phase does not persist to the mantle wedge domains where the majority of high-Mg island arc basalts with eruption temperatures ⩾ 1200°C are formed.