Global Importance of Mafic Magma with Low TiO2

Abstract:

I discuss the distribution of very low-TiO₂ basaltic lava in the ocean basins, which petrologic and geologic evidence suggests originated from refractory mantle that was emplaced during continental rifting. Glass compositions have TiO₂ ~0.3-0.8%, Na₂O <2% and MgO ~8-9%, similar to some lava (e.g., boninite) in island arcs and ophiolites. Not well known is that it is a widespread component or actual eruptive at spreading ridges, some large igneous provinces (LIPs), and at volcanic rifted margins. It is an end component of the global MORB array. Although at high MgO it is rare, differentiates with higher TiO₂ are regionally important. The most typical occurrence in usual MORB is as melt inclusions in calcic plagioclase phenocrysts (>An₈₈), where its influence can also be inferred from low-TiO₂ clinopyroxene. The crystals are incorporated into more typical MORB by magma mixing. In some cases, most of the global array can be inferred from crystallization histories of single samples. At ridges, low-TiO₂ basalts approach calcic boninite in composition, and have similar mineralogy including presence of both low-Ca and high-Ca pyroxene. Type localities are basalt from DSDP Site 236 in the Indian Ocean and a dredge haul from the Danger Island Trough at Manihiki Plateau, a fragment of a large igneous province (LIP) in the SW Pacific. A third location is Padloping Island in the Labrador Sea, a part of the North Atlantic Igneous Province, where mixing relations in picrites entail a low-TiO₂ component similar to boninite. This component is likely the source of forsteritic olivine (>Fo₉₁) in these rocks and did not require either high eruptive or potential temperatures when such olivine crystallized. As with boninite, low-TiO₂ magma in ridge settings is likely derived from a refractory (harzburgitic) and probably somewhat hydrous mantle source by extents of melting and temperatures comparable to those of typical MORB extracted from more fertile peridotite. Refractory mantle in oceanic settings probably resulted from incorporation of ancient mantle that was originally beneath island arcs or continental crust, but which was added to oceanic mantle by delamination or major stoping that occurred while continents were rifted. That mantle has geochemical attributes reflective of ancient melting events in the history of the planet.