Abstract
The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. Basalts from back-arc basins, with intermediate compositions, show that such a slab flux can be transported behind the volcanic front of the arc and incorporated into mantle flow. Hence it is puzzling why melts of subduction-modified mantle have rarely been recognized in mid-ocean ridge basalts. Here we report the first mid-ocean ridge basalt samples with distinct arc signatures, akin to back-arc basin basalts, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence in this region. This influence can also be identified in Atlantic and Indian mid-ocean ridge basalts but is nearly absent in Pacific mid-ocean ridge basalts. Such a hemispheric-scale upper mantle heterogeneity reflects subduction modification of the asthenospheric mantle which is incorporated into mantle flow, and whose geographical distribution is controlled dominantly by a “subduction shield” that has surrounded the Pacific Ocean for 180 Myr. Simple modeling suggests that a slab flux equivalent to ~13% of the output at arcs is incorporated into the convecting upper mantle.
Highlights
The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab
A classic paradigm of mantle heterogeneity has been that oceanic plates subduct at trenches, a water-rich flux comes off the slab to create arc volcanics, and mantle heterogeneity is produced by the processing of residual crust and sediment circulating in the mantle[1,2,3]
Might some of this subduction-modified mantle be incorporated into mantle flow? And where back-arc basins are not present, should not the subduction-modified mantle that would be sampled if there were a back-arc basin contribute to the convecting asthenosphere, where it should occasionally be later sampled by open ocean ridges? To explore these questions requires a careful examination of MORB data to see if a signal akin to back-arc basin basalts (BABB) might be more widespread than currently realized, and to determine whether this signal is asthenosphere modified by a slab flux, rather than dispersed sediment from recycled plates[12]
Summary
The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts and arc volcanics, with the latter enriched in elements such as Ba, Rb, Th, Sr and Pb and depleted in Nb owing to the water-rich flux from the subducted slab. For the global ocean ridge system, of the 4353 samples with at least three of the ratios available to apply the BABB filter, 550 samples pass the filter, leading to the conclusion that 13 percent of global MORB, and over 17% of MORB outside the Pacific basin, contain an observable subduction influence.
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