SUBDUCTION RECYCLES CONTINENTAL CRUST TO THE MANTLE AT A RATE THAT IF APPLIED TO THE PAST BRINGS A CHALLENGING SWEAT TO SUPERCONTINENT RECONSTRUCTION

Recycling of continental crust at ocean margin subduction zones (SZ) is effected by sediment subduction, which conveys the continental-derived debris of ocean floor sediment beneath the bedrock framework of the submerged forearc, and subduction erosion, which thins the crust of the upper plate. We estimate that at least 90% of subducted sediment and eroded continental material is recycled to the mantle via the subduction channel separating the upper and lower plates. For each km of SZ, the globally averaged recycling rate is ~70 km3/Myr. The annual volume flushed to the mantle is ~2.5 km3 (i.e. 2.5 Armstrong Units).

We presume that subduction of oceanic crust has been underway since at least the dawning of the Proterozoic 2.5 billion years ago. Allowing that the present is a key to the past, since 2.5 Ga a volume of continental crust equal to 80-90 % of the Earth existing volume (~7 billion km3) has been recycled to the mantle. The permanent removal of this mass of continental crust has global-scale implications, e.g.,

(1) Since 2.5 Ga the volume of continental material (6 billion km3) returned to the mantle (900 billion km3) is sufficient to have imparted a distinct geochemical aroma of continental crust--which the mantle arguably has.

(2) Since 2.5 Ga the volume of continental material recycled to the mantle is similar to that added back at the currently estimated rate of arc magmatism (2-3 km3/yr). Thus the Earth's volume of continental crust has not significantly changed since the early Proterozoic and perhaps well before.

(3) SZ recycling means that older rocks have had a greater likelihood of being exposed to SZ erosion. Accordingly, with increasing age, reassembly of supercontinents must be based on an age-decreasing amount of original material to work with.

(4) SZ recycling provides the perspective that the areally small exposures of Archean crust is more a measure of what remains preserved than what was originally produced.

(5) Where subduction zones formed along Rodinia's rifted margins, the recycling perspective brings wonderments about what is missing that could have constrained models, e.g., SWEAT or AUSWUS, putting Rodinia back together again, including refitting the subduction-zone ravished western South America margin to Laurentia.