MODES OF ACCRETION AT SLOWER SPREADING OCEAN RIDGES

Seafloor mapping and drilling at oceanic core complexes exposing the sub-volcanic basement at slow and ultraslow spreading ocean ridges has documented an enormous diversity of ocean crust structure. Locally, large regions of ‘crust’ are generated much as originally envisaged by Harry Hess, with altered mantle rock pulled directly to the seafloor with only scattered volcanism, elsewhere thick gabbro massifs in excess of 1.5 km are generated with an overlying volcanic carapace centered on magmatic ridge segments, and a diversity of crustal architectures form between these extremes with local magmatic centers formed within mantle rock. This diversity goes a long way to explaining that seen in ophiolite complexes.

While a full understanding of the diversity of the ocean crust, its composition, and the mass flux it represents from the Earth’s interior to the lithosphere, oceans and atmosphere will require substantial future mapping and sampling, it would appear that we can identify the principle processes involved in its construction. These include pooling of melts in the mantle at the base of the crust, where melt-rock reaction with the mantle forms dunite, troctolite and gabbro, and direct intrusion of small bodies of melt to higher levels up to the base of the sheeted dikes. Mode two occurs by vertical melt infiltration due to compaction of partially molten rock, resulting in a significant mass addition to higher levels of the lower crust, with accompanying excess enrichments in trace elements. Finally, mode three occurs by upward rafting of olivine-rich troctolite and gabbro from the crust-mantle transition zone, and local intrusions in the overlying section to higher levels. This process essentially represents an extension of corner flow, normally associated with the mantle beneath the crust to the base of the sheeted dikes. Evidence for this process is seen in mantle-derived olivine-rich troctolites drilled within the 1.4 km gabbro section at the Atlantis Massif on the MAR (Drouin et al, 2007, Geoph. Res. Abs., 2009, Chem Geol.; Suhr et al., 2008, G3), intercalation of sheeted dikes with amphibolitized and ductily deformed gabbros, (Dick et al., 2008, G3), and the very formation of oceanic core complexes. One major implication of these modes of accretion is that MORB evolves quite differently in the lower crust than previously supposed, no longer representing simple fractional crystallization of a primary melt.