Enriched oceanic crust can make juvenile continental crust
The outermost layer of the solid Earth is divided in two types: oceanic crust and continental crust. Oceanic crust consists of basaltic rocks while continental crust is andesitic. Most other terrestrial planets have basaltic crusts, which are similar to Earth’s oceanic crust, and the andesitic continental crust is a unique characteristic of Earth, only in our Solar System. Consequently, it is necessary to understand formation process of the continental crust to understand Earth’s history. Previous studies suggested that subducting oceanic crust is required for the formation of juvenile continental crust, but the incompatible elements of typical intra-oceanic volcanic arc systems do not match with the continental crusts. Gazel et al. investigated geochemical evolution of arc lava in Central American Land Bridge (CALB), also developed CI (Continentality Index) that show relation between P-wave velocities (Vp) and geochemical composition. They applied their method for various arc systems including CALB, and concluded that subducting plate needs to be enriched to construct continental crust in arc systems.
According to geophysical data, Vp of CALB is close to that of continental crust while most modern oceanic arcs have Vp 0.5~2.0 km/s higher than average continental crust. Also moho reflections at the base of the crust in central and northern Costa Rica show that crustal thickness of CALB is 40-45km, which is greater than most modern oceanic arcs. These two evidence indicate that CALB probably evolved as juvenile continental crust. Furthermore, Gazel et al. proposed that CALB evolved to be a continental crust from a geochemical perspective. They found the two peaks in the La/Yb, Sr/Y ratios and Pb-isotope ratios during the past 70Myr. The peak records correspond with the subduction of Galápagos tracks, so they suggested there was an interaction with Galápagos tracks twice (older in 40-50Ma, last since 10Ma). For making continental crust, there is a problem dense residues are produced when the light melts are formed. In CALB, magmas older than 12Myr have normal arc compositions that low SiO2 and K2O contents, but magmas younger than 10Myr have compositions that are similar to continental crust. They suggest there was a partial melting of subduction slab occurred after 10Ma in CALB.
They excluded arcs located within continental crust from all the volcanic arc systems on the Earth to remove geochemical affects from pre-existing continental material. Although silicic magmas are common in arc systems, only few arc systems including the CALB show compositions similar to that of continental crust. Others have bimodal distributions of basaltic and silicic compositions. They developed CI to show how well composition of an arc system matches with the continental crust value in a quantitative way. Only magmas from Costa Rica have CI<50, close to continental crust values of CI~18. Some arc systems show CI values of 50-100. All of these are influenced by seamounts, sediments or ridge. Meanwhile, most of Izu-bonin, Marianas, South Scotia and Tonga arcs show the least coincidence to continental crust, with a CI of >100. Their subducting plates are old and there was no influence of intraplate volcanism. Also, their magmas derived from slab-derived fluids without melting of subducting slab. They also found strong correlation between CI and Vp. The correlation and the lack of a significant component derived from recycled, continentally derived sediment suggest formation of juvenile continental crust is in progress in the CALB and also possibly in Iwo-Jima and W. Aleutians.
Geologists have tried to solve the problem of continental crust formation process in arc systems. Gazel et al. brought new answer to this long-lasting debate that subducting slab should be enriched and melt to make continental crusts. Still, there are problems with the process after melting of slab, and we need to make a further study.
Figure 1. In most arc systems, subduction of an oceanic crust creates volcanic arc island(a). When subducting oceanic crust is enriched by seamounts, hot spot tracks or ridge, it can make juvnile continental crust(b).
