A new process for formation of continental crust revealed!

Formation and the origin of continental crust, particularly the lower continental crust, has long been a conundrum. Many researches have argued that delamination (Rudnick, 1995) of arc lower crust is the main mechanism for formation of low continental crust because both arc magma and continental crust are andesitic. However, Kelemen & Behn (2016) suggest a new model for formation of the continental crust which involves “relamination of buoyant magmatic rocks”. The authors argue that this new model is more likely and efficient to form lower continental crust than the delamination model.

It was thought that delamination (Kay and Kay, 1993) of dense materials of lower crust back to mantle well matches with seismological results (Clift et al., 2009) and is thought to be a major process that recycles crustal material back to mantle. On the other hand, relamination is a process that subducting, buoyant arc crustal materials detach from denser materials and rise, accreted to the base of the overlying crust. Previous studies suggested that there are four possible processes for making continental crust (Rudnick, 1995); relamination of subducting crust, delamination of arc crust, chemical weathering of oceanic crust, and ultramafic complement as cumulates within upper mantle. However, authors insist that relamination is solely sufficient to explain characteristics of the lower continental crust.

In this work, authors used regional metamorphic massifs and metamorphic xenoliths to estimate the composition of lower continental crust. Authors point out distinctive differences in incompatible trace element concentrations between arc lower crust and lower continental crust (LCC). The arc lower crust is much more depleted in incompatible elements, especially in Ta, K, La, Ce, and Hf, than the LCC. They also address that although the major element compositions of both reservoirs are similarly andesitic, the isotope composition of juvenile igneous rocks or arcs, considered as origin of continental crust, show that they originated from mantle material, not by recycle of continental crust.

The authors suggest that in order to explain the discrepancy between two reservoirs, arc lower crust and lower continental crust, relamination is necessary. Analysis of density sorting of materials and mass balance from two possibilities, relamination and delamination, more strongly supports on the side of relamination. Relamination of buoyant magmatic rocks has several pathways to form the continental crust (Hacker et al., 2011). Also, this conclusion is in agree with geophysical constrains in terms of heat flow and seismic data. Considering rate of global arc production, continental crust could have been produced by arc magmatism followed by subduction and relamination during entire Earth history. However, this article does not mean to forget delamination as continental crust forming mechanism. Relamination and delamination should be simultaneously considered, neither can be excluded, but difference in contribution exists.

Material recycling process in convergent margin is called “subduction factory”. Understanding the mechanisms producing the continental crusts is important to know how elements are distributed within the Earth. Further research of same kind of this work matters for finding more processes how continental materials recycle into the mantle and measure each contributions as well as measure the contribution among the processes such as relamination and delamination and so on.

Figure 1 Possible pathways of relamination that has significant affect in forming the LCC.

References

Clift, P. D., Vannucchi, P., and Morgan, J. P. 2009, Crustal redistribution, crust-mantle recycling and Phanerozoic evolution of the continental crust, Earth-Science Reviews, 97, 80-104.

Hacker, B. R., Kelemen, P. B., and Behn, M. D., 2011, Differentiation of continental crust by relamination, Earth and Planetary Science Letters, 307, 501-516.

Kay, R.W., Kay, S.M., 1993, Delamination and delamination magmatism. Tectonophysics, 219 (1–3), 177–189.

Kelemen, P. B. and Behn, M. D.,2016, Formation of lower continental crust by relamination of buoyant arc lavas and plutons, nature Geoscience 09, 197-205.

Rudnick, R. L., 1995, Making continental crust, NATURE, vol. 378, 571-578.

Tatsumi, Y., 2005, The subduction factory: How it operates in the evolving Earth, GSA Today, v. 15, no. 7.