Abstract

Al2O3/TiO2 and CaO/TiO2 ratios of primitive mid-ocean ridge basalts increase along with degree of partial melting (TiO2 content decreases) until ratios seen in chondrites (about 20 and 17, respectively) are reached at ∼0.8% TiO2. Low-Ti (<0.6%) basalts from ophiolitic complexes (for example, Troodos and Betts Cove), island arcs, and interarc basins are characterized by much higher Al2O3/TiO2 and CaO/TiO2 ratios (up to 60). Such low-Ti ophiolitic basalts could be formed by remelting of a severely depleted source that had experienced a previous episode of magma extraction of the type in mid-ocean ridges. We suggest that these low-Ti basalts are not formed at mid-ocean ridges but are products of a spreading center close to a subduction zone, for example, in an interarc basin or incipient island-arc site. In this model, remelting of the refractory source is induced by the introduction of water from subducted oceanic crust. Geochemical data on high-Mg volcanic rocks from Papua and the Mariana Trench are presented, and an analogy with low-Ti ophiolitic basalts is suggested. “Basaltic komatiites” from ophiolitic complexes are discussed, and it is suggested that there are critical geochemical differences between these and Archean komatiites.

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