Ocean crust recycling rate

Figure 10.1. A generalized diagram for the steady-state rock cycle. Sediments, S, and continental crystalline crust, C, masses are in units of metric tons. Ss, Cs, Sc, Cc, and M are fluxes in units of 109 tons y-l due to erosion of sediments, metamorphism, erosion of crystalline rocks, recycling of crystalline rocks (resetting of ages during tectogenesis), and cycling of oceanic crust, respectively. Total sedimentation rate is 9 x 109 tons y-l. (After Gregor, 1988.)...

Present-day spreading rates allow us to estimate the volume of oceanic crust, which is subducted annually. Estimates are between 18 and 20 km3/ yr, the differences being in part due to different assumptions about the average thickness of the oceanic crust. If this rate of subduction has been constant over geological time, then the Earth s mantle should contain 5 % recycled oceanic crust (Helffrich Wood, 2001), although if this were concentrated in the upper part of the mantle then, of course, the proportion would be considerably greater. 

The pre-human natural rate at which rivers were carrying secondary minerals into the ocean was 1.4 x lO g/y. Assuming that these solids have an average settled density of 1.6g/cm, this input would have filled the ocean basins (volmne= 1.37 x 10 cm from Figure 2.1) within 157 million years. This has not happened because the sediments are recycled through the crust and the upper mantle (Figure 1.2). 

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