Paper No. 3
Presentation Time: 8:30 AM
RECYCLING OF CONTINENTAL CRUST AT MODERN SUBDUCTION ZONES, IMPLICATIONS FOR PRECAMBRIAN CRUSTAL GROWTH, SUPERCONTINENT CONSTRUCTIONS, AND LITTERING THE MANTLE WITH CONTINENTAL DEBRIS
INTRO: At ocean margin subduction zones (SZ) recycling of continental crust is effected by sediment subduction, which conveys ocean basin and trench sediment of the lower plate beneath the forearc, and subduction erosion, which strips crustal material from the upper plate. The combined debris is flushed toward the mantle in the subduction channel topping the underthrusting lower plate. VOLUMES: Geophysical and drilling data back estimates that for each km of margin the average solid-volume flux of subducted sediment is ~30 cubic km/Myr and that of eroded material is ~40 cubic km /Myr. We estimate that at least 90% of this material avoids a short-path return to the upper plate by underplating or arc-magmatism and is recycled to deep mantle circulation. The long-term recycling volume of ~70 cubic km/Myr/km is supplied by a ~1-km-thick subduction channel supplying a global volume of ~2.5 cubic km/yr. The corresponding landward migration (i.e., forearc truncation) of the trench is 2-3 km/Myr. IMPLICATIONS: If the present is a key to the past, then since 2.5 Ga a volume of continental crust equal to 70-80 % of the existing volume (~7 billion cubic km) has been recycled to the mantle. (Supercontinents): With increasing crustal age the greater is the likelihood that rock of this age has been exposed to SZ recycling. For example, at a margin-truncation rate of 2-3 km/Myr, during just the past 250 Ma alone a ~6000-km-long and ~600-km-wide strip of western South American craton has been recycled to the mantle. Supercontinents must thus be reassembled based on the prospect of an age-progressing decrease in remaining reconstructive material. (Aroma): Since the early Proterozoic the volume of continental returned to the mantle is small compared to its volume (~0.4 %), but it is sufficient to geochemically impart a distinct continental aroma, which the mantle has. (Growth): The estimated rate of delivery of continental material to the mantle is similar to the rate of addition of new arc magmatic crust (2-3 km/yr). The similarity implies that the Earth's mass of continental crust has not significantly changed since at least 2.5 Ga and probably somewhat before. If the greater volume of continental crust is made and lost at SZs, then the extant volume of ancient crust also represents what has been preserved rather than just that originally produced.