ISOTOPIC AND GEOPHYSICAL PERSPECTIVES ON DEEP MANTLE PROCESSES FOR CONSTRUCTING CRATONS

Combinations of isotopic data on deep mantle samples (peridotites, eclogites, and diamonds+their inclusions) can be combined with geophysical techniques (tomography, anisotropy, receiver functions) to examine the construction and modification of cratons. Peridotites record mantle depletion associated with initial mantle keel formation while eclogites record slab incorporation into the keel during collision. Diamonds can potentially be related these processes and additionally carry a record of multiple external exogenous fluid sources related to accretion at craton margins or sublithospheric magmatism. Geophysical techniques sample the craton at high enough resolution to image present lithospheric structure that can be connected to documented construction/modification processes. Studies of the Kaapvaal, Slave, North China, Siberian and Mongolian cratons and the Kimberley block each have exemplified unique aspects of craton construction. The techniques used for their study can be applied to the growth and modification of Laurentia. Laurentia has the longest record of construction (~4 Ga to ~1.4 Ga) -one that spans putative major changes in Earth’s tectonic regimes. Not surpisingly, Laurentia’s growth records diverse tectonic styles including clear evidence for terrain accretion, plume activity, mantle heterogeneity, and in-situ crustal reworking and growth. If, for example, there was a step-wise change in Earth’s tectonic style from rapid mantle convection, small plates, shallow subduction, and localized recycling >3.2 Ga, followed by large plates, steep subduction, and full upper mantle recycling <3.0 Ga, how can this be reconciled with the record of Laurentia’s growth which shows very mixed evidence for this change? A major research goal is to explain this diversity of Laurentia’s growth processes with realistic geological models that can be related to potential changes in Earth’s geodynamic processes.