Subcontinental lithospheric mantle
The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere.
The modern understanding of the Earth's upper mantle is that there are two distinct components - the lithospheric part and the asthenosphere. The lithosphere, which includes the continental plates, acts as a brittle solid whereas the asthenosphere is hotter and weaker due to mantle convection. The boundary between these two layers is rheologically based and is not necessarily a strict function of depth. Specifically, oceanic lithosphere (lithosphere underneath the oceanic plates) and subcontinental lithosphere, is defined as a mechanical boundary layer that heats via conduction and the asthenosphere is a convecting adiabatic layer. In contrast to oceanic lithosphere, which experiences quicker rates of recycling, subcontinental lithosphere is chemically distinct, cold, and older. This translated into the differences between the SCLM and the oceanic lithospheric mantle.
There are two different types of subcontinental lithosphere that formed at different times in Earth's history: Archean and Phanerozoic subcontinental mantle.
Archean lithosphere is strongly depleted in fertile melt indicators such as CaO and Al2O3. This depletion in major-elements should then be consequence of the Archean lithosphere's formation. Trace-elements are abundant in Archean lithosphere relative to MORB (which samples modern upper mantle) and have been sampled by Re-Os isotope dating of peridotites and ophiolites. The trace element composition of these xenoliths suggest mixing between the two different layers of subcontinental mantle. Particularly, the theory for the removal of Archean subcontinental lithosphere below Archean continental crust via delamination helps to explain mantle-peridotites found the extinct Sierra Nevada arc. Though there is evidence for the preservation of the Archean lithosphere, there is controversy over the preservation of the Archean mantle, for which the Archean lithosphere would have been derived.
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