ULTRAFAST SUBDUCTION - THE KEY TO SLAB RECYCLING EFFICIENCY AND MANTLE DIFFERENTIATION

Recycling of altered oceanic crust and sediment at subduction zones, probably the most important process leading to the development of mantle heterogeneities, is still extremely poorly understood. In an attempt to study some of the controls of volatile recycling efficiency, we evaluated the sensitivity of slab thermal structure to changing subduction velocity. Slab temperatures at a given depth of subduction decrease non-linearly with increasing slab velocity. The largest change in slab temperature occurs in the range of currently observed plate velocities (i.e., 3-12 cm/a). Above this range the rate of changes in slab temperature decreases with increasing slab velocity. Ultrafast plate velocities up to 20 cm/a are likely for the Cretaceous, for long periods in the Paleozoic and possibly in the early Earth. Colder slab temperatures during fast subduction velocities support the coherent transport of slab material deeper into the mantle. At a 30-degrees subduction angle and subduction velocity of 3 cm/a the 600-degrees-C isotherm only penetrates to about 140 km depth and at 15 cm/a to 250 km. Increasing the slab angle changes the depth of penetration of a given isotherm, whereby the results along the slab trajectory remain essentially unchanged. Deeper penetration of brittle slab material into the mantle may alter the physical process of subduction and enhances downward circulation of slab and mantle materials. At ultrafast plate velocities (> 12 cm/a), the temperatures in the lower crustal portion of the subducted slab are sufficiently low that hydrous magnesian silicates and other hydrous silicates may remain continuously stable well past the depth of slab penetration above which island arc volcanoes are formed. Therefore, fast and ultrafast subduction facilitates water subduction and, by virtue of the enhanced material transport into the mantle, allows for large quantities of water to be subducted during specific times of very fast subduction. Therefore, slab recycling is not linearly related to the volume of slab material that is subducted, but rather is highly accelerated during periods of fast spreading. It is suggested here that short periods of fast and ultrafast spreading, such as occurred during the Cretaceous, may catastrophically produce localized mantle chemical anomalies such as the DUPAL anomaly [1] or SOPITA [2]. Furthermore. enhanced volatile recycling during prolonged periods of rapid plate motion in the early history of Earth may have effectively influenced development of the early atmosphere and delayed mantle degassing and formation of the present-day ocean water masses.