Mantle geochemistry

mantle_geo_figure 01
Figure 1. A numerical model simulation of whole mantle convection showing hot plumes (red) rising from the core mantle boundary and cold (dark blue) material being subducted back into the mantle. Incorporation of geochemical parameters into this type of model shows that mixing is efficient and that the 670km deep phase change in the mantle will not preserve a chemically layered mantle (From van Keken and Ballentine, 1998, 1999).

Our understanding of the way in which the Earth works has undergone a radical change. For the last 25 years, geochemists have argued successfully that the silicate mantle is geochemically layered and that only the upper 1/3 of the mantle has played any role in continental crust formation or in sourcing volatiles to the oceans and atmosphere. In the last five years, clear evidence from seismic tomography coupled with numerical simulations of mantle convection now show that the phase change at 670km, thought to separate the chemically layered mantle, does not provide any barrier to efficient mixing of the whole mantle system. This now poses a significant problem for geochemists. Where are the reservoirs in the mantle that preserve the primitive volatiles trapped in the mantle since the accretion of the Earth? Also, recycled oceanic crust forms a significant component of oceanic island volcanism. From radiogenic isotopes generated by the parent daughter pairs (U-Pb, Lu-Hf, Sm-Nd, and Rb-Sr) it is estimated that the recycled material has been stored in the mantle for up to 2.0 Ga and is mixed with material containing the primitive volatiles before being returned to the surface. The proportion of recycled/primitive material is poorly known yet may provide a key to understanding the respective size of the different reservoirs and therefore a key constraint in determining their location in the mantle and the mechanism/s for their preservation over geological time. At Manchester we have an internationally recognised track record in using mantle noble gas isotopes to investigate the character and origin of the volatiles in the mantle. With the addition of the new multi-collector ICPMS facility and clean laboratories we now have the ability to investigate the combined information from noble gases and radiogenic isotopes in mantle materials.

References

  • Burgess R., Layzelle E., Turner G. and Harris J.W. (2002) Constraints on the age and halogen composition of mantle fluids in Siberian coated diamonds. Earth Planet. Sci. Lett. 197,193-203.
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