Page 368 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 351

11.3.3 The formation of itic magma never reached the surface or other processes
must have contributed to root formation. One of these
Archean lithosphere processes is an efficient sorting mechanism that concen-

trated the unusual components of the Archean mantle
The distinctive composition and physical properties of lithosphere at the expense of all others (Arndt et al.,
the stiff, buoyant mantle roots beneath the cratons 2002). The most likely driving force of the sorting is the
(Section 11.3.1) result from the chemical depletion and buoyancy and high viscosity of high-Mg olivine and
extraction of melts from the primitive mantle. These orthopyroxene, although exactly how this happened is
two processes lowered the density and increased the uncertain. The density and viscosity of these minerals
viscosity of the residue left over from partial melting depends upon their Mg–Fe ratios and water content,
and resulted in a keel that consists mostly of high-Mg respectively; which are lower in Archean mantle litho-
olivine and high-Mg orthopyroxene (O’Reilly et al., sphere compared to normal asthenospheric mantle.
2001; Arndt et al., 2002). Both of these components are Arndt et al. (2002) considered three processes that could
absent in fertile (undepleted) mantle peridotite and are have resulted in the mechanical segregation and accu-
rare in the residues of normal mantle melting, such as mulation of a layer of buoyant, viscous mantle near the
that which produces modern oceanic crust and oceanic Earth’s surface during Archean times. First, upwelling
islands. Consequently, most workers have concluded buoyant residue in the core of a mantle plume could
that the distinctive composition is related to unusually have separated from the cooler, denser exterior and
high degrees (30–40%) of mantle melting over a range accumulated during ascent (Fig. 11.2a). In this model,
(4–10 GPa) of mantle pressures (Pearson et al., 2002; melting begins at high pressure (∼200 km depth) and
Arndt et al., 2002). High-degree partial melting of continues to shallow depths, by which point melt
mantle peridotite produces magma of komatiitic volumes are high and the dense residue of early, low-
(Section 11.3.2) composition and a solid residue that is degree melting is swept away by mantle fl ow. Second,
very similar to the composition of Archean lithospheric buoyant residue could have segregated slowly as mate-
mantle (Herzberg, 1999; Arndt et al., 2002). rial was transported down subduction zones and recy-
One radiometric system that has been of consider- cled through the mantle in convection cells (Fig. 11.2b).
able use in determining when melt extraction and Third, some subcontinental lithosphere could be the
Archean root formation occurred involves the decay of remnants of an initial crust that crystallized in an
187 188
Re to Os (Walker et al., 1989; Carlson et al., 2005). Archean magma ocean that formed during the fi nal
The key feature of this isotopic system is that Os is stages of Earth accretion (Fig. 11.2c). In all three cases,
compatible during mantle melting and Re is moderately buoyant, viscous material rises and is separated from
incompatible. Consequently, any residue left behind higher density residue during mantle fl ow. Whether
after melt extraction will have a lower Re and a higher some combination of these or other processes helped
Os concentration than in either the mantle melts or the to form the cratonic keels is highly speculative. Never-
fertile mantle. This characteristic allows Re-Os isotope theless, they illustrate how several different mechanisms
analyses of mantle xenoliths to yield information on the could have concentrated part of the residue of mantle
age of melt extraction. The data from mantle xenoliths melting into a near-surface layer.
show that the oldest melting events are Early–Middle In addition to high-degree partial melting and effi -
Archean in age. Significant amounts of lithospheric cient sorting, most authors also have concluded that the

mantle also formed in Late Archean times and are asso- formation and evolution of mantle lithosphere involved
ciated with voluminous mafic magmatism (Pearson a multi-stage history involving many tectonic and mag-

et al., 2002). matic events (James & Fouch, 2002). However, opinions
Although high-degree partial melting undoubtedly are divided over whether root construction preferen-
occurred, this process alone cannot explain the origin tially involved the underthrusting and stacking of sub-
of the Archean mantle lithosphere. The main reason for ducted oceanic slabs (Carlson et al., 2005), the accretion
this conclusion is that the abundance of komatiite and thickening of arc material (Lee, 2006), or the extrac-
found in the Archean crustal record is much too low to tion of melt from hot mantle plumes (Wyman &
balance the amount of highly depleted peridotite found Kerrich, 2002). By applying a range of criteria some
in the cratonic mantle (Carlson et al., 2005). This imbal- geologic studies have made compelling cases that
ance suggests that either a large proportion of komati- ancient mantle plumes played a key role in the