Genesis of the Cratonic Upper Mantle
Genesis of the Cratonic Upper Mantle (last updated 2000)
• It has long been known that "low temperature" mantle xenoliths from the Kaapvaal and Siberian cratons have very high Mg# (~93%) and high modal orthopyroxene contents (~30% or more). The reasons for this have been debated.
• Discovery of high Mg#, low orthopyroxene, Archean mantle xenoliths from the Greenland craton at Wiedemann Fjord in East Greenland, together with similar lithologies in the Slave craton (and renewed appreciation for similar xenoliths reported from West Greenland in 1982), supports the theory that the high Mg# of Archean mantle xenoliths is due to high extents of partial melting (~40%) at an average pressure of about 4 GPa. Residues of this process had very low orthopyroxene contents.
• Correlation of Ni in olivine with modal orthopyroxene in cratonic mantle xenoliths worldwide is best explained as the result of reactions with SiO2-rich melt or fluid that consumed olivine and produced orthopyroxene. We modeled this in terms of reaction between partial melts of subducting eclogite and overlying, highly depleted mantle peridotite, which may have been common during hot Archean subduction. Such reactions may also have been essential in forming the continental crust. If so, this hypothesis provides a genetic link between the cratonic upper mantle and the continental crust.
• Cratonic, garnet peridotites formed as garnet-free residues of partial melting, at pressures less than 4 GPa. Garnet in these rocks formed during subsolidus re-equilibration. Those that record pressures > 4 GPa must have undergone pressure increase due to tectonic processes such as imbrication, compression (pure shear), or convection.
• ONGOING RESEARCH includes phase equilibrium calculations to constrain the hypothesis that cratonic mantle peridotites are generally positively buoyant with respect to the adiabatically convecting mantle (some other lithology must balance this buoyancy), trace element analyses of orthopyroxene in a variety of cratonic mantle samples to constrain their origin, and study of xenoliths from a new locality that we discovered in East Greenland in 2000.