Crustal recycling

Understanding predictions of mantle dynamics helps geoscientists predict where subducted crust will end up.

Crustal recycling is a tectonic process by which surface material from the lithosphere is recycled into the mantle during subduction. The subducting slabs carry volatile compounds and water into the mantle, as well as crustal material with an isotopic signature different from that of primitive mantle. Identification of this crustal signature in mantle-derived rocks (such as mid-ocean ridge basalts or kimberlites) is proof of crustal recycling.

Historical and theoretical context

In 1906, seismological data interpreted by R.D. Oldham revealed that the earth had a molten mantle and a solid innermost core^[1]. The development of seismology as a modern tool for imaging the Earth's deep interior occurred during the 1980s^[2], and with it developed two camps of geologists: whole-mantle convection proponents^[3][4] and layered-mantle convection proponents^[5][6].

Layered-mantle convection proponents hold that the mantle's convective activity is layered, separated by densest-packing phase transitions of minerals like olivine, garnet and pyroxene to more dense crystal structures (spinel and then perovskite and post-perovskite). Slabs that are subducted may be negatively buoyant as a result of being cold from their time on the surface and inundation with water, but this negative buoyancy is not enough to move through the 660-km phase transition.

Whole-mantle (simple) convection proponents hold that the mantle's observed density differences (which are inferred to be products of mineral phase transitions) do not restrict convective motion, which moves through the upper and lower mantle as a single convective cell. Subducting slabs are able to move through the 660-km phase transition and collect near the bottom of the mantle in a 'slab graveyard', and may be the driving force for convection in the mantle locally^[7] and on a crustal scale^[2].

The fate of subducted material

The ultimate fate of crustal material is key to understanding geochemical cycling, as well as persistent heterogeneities in the mantle, upwelling and myriad effects on magma composition, melting, plate tectonics, mantle dynamics and heat flow^[8]. Mantle dynamics is likely a mix of the two end-member hypothesis, resulting in a partially layered mantle convection system.

Our current understanding of the structure of the deep Earth is informed mostly by inference from direct and indirect measurements of mantle properties using seismology, petrology, isotope geochemistry and seismic tomography techniques. Seismology in particular is heavily relied upon for information about the deep mantle near the core-mantle boundary.

Evidence from seismic tomography

Although seismic tomography was producing low-quality images^[2] of the Earth's mantle in the 1980s, images published in an editorial article in Science magazine clearly showed a cool slab near the core-mantle boundary, as did work completed in 2005 by Hutko et al., showing a seismic tomography image that may be cold, folded slab material at the core-mantle boundary. ^[9] However, the phase transitions may still play a role in the behavior of slabs at depth. Schellart et al. showed that the 660-km phase transition may serve to deflect downgoing slabs^[10]. The shape of the subduction zone was also key in whether the geometry of the slab could overcome the phase transition boundary^[11].

Mineralogy may also play a role, as locally metastable olivine will form areas of positive buoyancy, even in a cold downgoing slab, and this could cause slabs to 'stall out' at the increased density of the 660-km phase transition^[12]. Slab mineralogy and its evolution at depth^[13] were not initially computed with information about the heating rate of a slab, which could prove essential to helping maintain negative buoyancy long enough to pierce the 660km phase change. Additional work completed by Spasojevic et al.^[14] showed that local minima in the geoid could be accounted for by the processes that occur in and around slab graveyards, as indicated in their models.

Evidence from stable isotopes

Understanding that the differences between Earth's layers are not just rheological, but chemical, is essential to understanding how we can track the movement of crustal material even after it has been subducted. After a rock has moved to the surface of the earth from beneath the crust, that rock can be sampled for its stable isotopic composition. It can then be compared to known crustal and mantle isotopic compositions, as well as that of chondrites, which are understood to represent original material from the formation of the solar system in a largely unaltered state.

One group of researchers was able to estimate that between 5 and 10% of the upper mantle is composed of recycled crustal material ^[15]. Kokfelt et al. completed an isotopic examination of the mantle plume under Iceland^[16] and found that erupted mantle lavas incorporated lower crustal components, confirming crustal recycling at the local level.

Some carbonatite units, which are associated with immiscible volatile-rich magmas^[17] and the mantle indicator mineral diamond, have shown isotopic signals for organic carbon, which could only have been introduced by subducted organic material^[18][19]. The work done on carbonatites by Walter et al.^[17] and others^[20] further develops the magmas at depth as being derived from dewatering slab material.

References

1. ^ doi:10.1144/​GSL.JGS.1906.062.01-04.21
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2. ^ ^{a b c} Kerr, R. A. (1997). "Geophysics: Deep-Sinking Slabs Stir the Mantle". Science 275 (5300): 613. doi:10.1126/science.275.5300.613. 
3. ^ Gurnis, M. (1988). "Large-scale mantle convection and the aggregation and dispersal of supercontinents". Nature 332 (6166): 695. doi:10.1038/332695a0. 
4. ^ Bercovici, D.; Karato, S. I. (2003). "Whole-mantle convection and the transition-zone water filter". Nature 425 (6953): 39–44. doi:10.1038/nature01918. PMID 12955133. 
5. ^ Albarede, F.; Van Der Hilst, R. D. (2002). "Zoned mantle convection". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 360 (1800): 2569. doi:10.1098/rsta.2002.1081. 
6. ^ Ogawa, M. (2003). "Chemical stratification in a two-dimensional convecting mantle with magmatism and moving plates". Journal of Geophysical Research 108. doi:10.1029/2002JB002205. 
7. ^ Forte, A. M.; Mitrovica, J. X.; Moucha, R.; Simmons, N. A.; Grand, S. P. (2007). "Descent of the ancient Farallon slab drives localized mantle flow below the New Madrid seismic zone". Geophysical Research Letters 34 (4). doi:10.1029/2006GL027895.  edit
8. ^ Lay, T. (1994). "The Fate of Descending Slabs". Annual Review of Earth and Planetary Sciences 22: 33–61. doi:10.1146/annurev.ea.22.050194.000341. 
9. ^ Hutko, A. R.; Lay, T.; Garnero, E. J.; Revenaugh, J. (2006). "Seismic detection of folded, subducted lithosphere at the core–mantle boundary". Nature 441 (7091): 333–336. doi:10.1038/nature04757. PMID 16710418. 
10. ^ Schellart, W. P. (2004). "Kinematics of subduction and subduction-induced flow in the upper mantle". Journal of Geophysical Research 109. doi:10.1029/2004JB002970. 
11. ^ Bercovici, D.; Schubert, G.; Tackley, P. J. (1993). "On the penetration of the 660 km phase change by mantle downflows". Geophysical Research Letters 20 (23): 2599. doi:10.1029/93GL02691. 
12. ^ Marton, F. C.; Bina, C. R.; Stein, S.; Rubie, D. C. (1999). "Effects of slab mineralogy on subduction rates". Geophysical Research Letters 26: 119. doi:10.1029/1998GL900230. 
13. ^ Ganguly, J.; Freed, A.; Saxena, S. (2009). "Density profiles of oceanic slabs and surrounding mantle: Integrated thermodynamic and thermal modeling, and implications for the fate of slabs at the 660km discontinuity". Physics of the Earth and Planetary Interiors 172 (3–4): 257. doi:10.1016/j.pepi.2008.10.005. 
14. ^ Spasojevic, S.; Gurnis, M.; Sutherland, R. (2010). "Mantle upwellings above slab graveyards linked to the global geoid lows". Nature Geoscience 3 (6): 435. doi:10.1038/NGEO855. 
15. ^ Cooper, K. M.; Eiler, J. M.; Sims, K. W. W.; Langmuir, C. H. (2009). "Distribution of recycled crust within the upper mantle: Insights from the oxygen isotope composition of MORB from the Australian-Antarctic Discordance". Geochemistry Geophysics Geosystems 10 (12). doi:10.1029/2009GC002728. 
16. ^ Kokfelt, T. F.; Hoernle, K. A. J.; Hauff, F.; Fiebig, J.; Werner, R.; Garbe-Schönberg, D. (2006). "Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume". Journal of Petrology 47 (9): 1705. doi:10.1093/petrology/egl025. 
17. ^ ^{a b} Walter, M. J.; Bulanova, G. P.; Armstrong, L. S.; Keshav, S.; Blundy, J. D.; Gudfinnsson, G.; Lord, O. T.; Lennie, A. R. et al. (2008). "Primary carbonatite melt from deeply subducted oceanic crust". Nature 454 (7204): 622–625. doi:10.1038/nature07132. PMID 18668105. 
18. ^ Riches, A. J. V.; Liu, Y.; Day, J. M. D.; Spetsius, Z. V.; Taylor, L. A. (2010). "Subducted oceanic crust as diamond hosts revealed by garnets of mantle xenoliths from Nyurbinskaya, Siberia". Lithos 120 (3–4): 368. doi:10.1016/j.lithos.2010.09.006. 
19. ^ Shcheka, S. S.; Wiedenbeck, M.; Frost, D. J.; Keppler, H. (2006). "Carbon solubility in mantle minerals". Earth and Planetary Science Letters 245 (3–4): 730. doi:10.1016/j.epsl.2006.03.036. 
20. ^ Bercovici, D.; Karato, S. I. (2003). "Whole-mantle convection and the transition-zone water filter". Nature 425 (6953): 39–44. doi:10.1038/nature01918. PMID 12955133.