Formation of Archaean continental lithosphere and its diamonds: the root of the problem

Abstract

Cratonic lithospheric mantle plays an integral role in defining the physical behaviour of ancient continents and their mineral potential. Bulk compositional data show that modern-day melting residues from a variety of tectonic settings can be as depleted in Al and Ca as cratonic peridotites. Cratonic peridotites are strongly affected by secondary introduction of pyroxenes and garnet such that the extent and depth of melting cannot be reliably determined. Olivine compositions are probably the most reliable tracer of the original melting process and indicate that typical cratonic peridotites have experienced 40% or more melt extraction. Homogeneous levels of depletion indicated by olivine compositions, combined with mildly incompatible trace element evidence, indicate that melting took place at shallow depths, dominantly in the spinel stability field. Consideration of melt production models shows that shallow (,3 GPa) anhydrous melting is not capable of producing residues dominated by large degrees of melt extraction. Instead, a critical role for water is indicated, implicating the formation of cratonic peridotites within Archaean subduction zones. This melting occurred in the Neoarchaean in some cratonic blocks, initially forming dunitic residues that are still evident in the xenolith inventory of some cratons. Release and migration up-section of siliceous melt produced during orthopyroxene breakdown metasomatizes the proto-lithospheric via re-enrichment in orthopyroxene crystallizing from this hydrous Si-rich melt, forming the variably orthopyroxene-rich refractory harzburgites typical of most cratonic roots. Melting in Archaean subduction zones is followed by subduction stacking to form the cratonic root. Gravitational forces may then be responsible for the loss of imbricated mafic crust during periods of transient thermal and physical disturbances prior to final long-term tectonic stability. Most diamonds form in the base of these cratonic roots during pulses of thermal or tectonic activity, initially during root construction and subsequently associated with large-scale regional lithospheric events that may be correlated to pulses in global mantle dynamic evolution.

Hydrous origin of the continental cratonic mantle

“…The cratonic mantle peridotites are characterized by a large amount of SiO 2 in combination with a high Mg/(Mg + Fe) ratio. Despite the considerable number of studies conducted to explore the origin of the unusual cratonic peridotites, their origin remains controversial (e.g., Pearson and Wittig 2013). Based on the melting experiments of the pyrolite-H 2 O system at high pressure, we demonstrate the possibility that the Si-and Mg-rich cratonic peridotites originated as simple melting residues under hydrous conditions at depths of approximately 100 to 200 km.…”

Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

“…In the second case, we may speculate that it might represent an interface marking a change in the geodynamical process responsible for the formation and thickening of the mantle root. A possible geodynamic scenario allowing for the development of 45 • dipping foliations and lineations within the cratonic lithosphere would be the formation of the Kaapvaal craton keel by subduction stacking, as suggested by several geochemical and petrological studies (Shirey et al, 2001;Simon et al, 2007;Pearson and Wittig, 2008). However, in this case, fast SKS polarization directions normal to the main sutures should be expected, rather than the parallel to slightly oblique directions measured in the Kaapvaal craton (Silver et al, 2001).…”

Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure–temperature evolution of the lithospheric mantle

“…In this context, it is important to understand how bulk composition affects mineral phase assemblage and density in the uppermost mantle, which is known to be heterogeneous in both chemical and mineralogical composition, mainly due to localized metasomatism and melt extraction processes (Menzies and Hawkesworth 1987;Carlson et al 2005;Pearson and Wittig 2008). Petrological studies of peridotite massifs and mantle xenoliths, integrated with geophysical observations and thermodynamic constraints, may help to understand how chemical composition, thermal state and density of the uppermost mantle are related to each other in a variety of geological settings.…”