The lower plate is the dominant agent in modern convergent margins characterized by active subduction, as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight. This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle. As geological and geochemical data seem inconsistent with the existence of modern-style ridges and arcs in the Archaean, a periodically-destabilized stagnant-lid crust system is proposed instead. Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle, perturbing Earth's heat generation/loss balance, eventually triggering mantle overturns. Archaean basalts were derived from fertile mantle in overturn upwelling zones (OUZOs), which were larger and longer-lived than post-Archaean plumes. Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods, allowing basal crustal cannibalism, garnetiferous crustal restite delamination, and coupled development of continental crust and sub-continental lithospheric mantle. Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB (mid-ocean ridge basalt) mantle. Only after the start of true subduction did sequestration of subducted slabs at the core-mantle boundary lead to the development of the depleted MORB mantle source. During Archaean mantle overturns, pre-existing continents located above OUZOs would be strongly reworked; whereas OUZO-distal continents would drift in response to mantle currents. The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion, imbrication, subcretion and anatexis of unsubductable oceanic lithosphere. As Earth cooled and the background oceanic lithosphere became denser and stiffer, there would be an increasing probability that oceanic crustal segments could founder in an organized way, producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga. Plate tectonics today is constituted of: (1) a continental drift system that started in the Early Archaean, driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons; (2) a subduction-driven system that started near the end of the Archaean.

Stagnant lids and mantle overturns: Implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics

Volcano-tectonic and geochemical evolution of an oceanic intra-plate volcano: Tahiti-Nui (French Polynesia)

https://www.africamuseum.be/publication_docs/Bardintzeff%20et%20al%202010%20Madagascar%20volcanic%20provinces.pdf

Madagascar volcanic provinces linked to the Gondwana break-up: Geochemical and isotopic evidences for contrasting mantle sources

[1] Arrays of basalts from the same hotspot usually plot within an elongate tube-like field in 87Sr/86Sr–143Nd/144Nd–206Pb/204Pb space (Hart et al, 1992) Each hotspot array tube (HART) is commonly interpreted as the result of melting multiple basalt sources that are variably proportioned mixtures of the hotspot source components I propose instead that a HART is the isotopic trace of the melt extraction trajectory produced by progressive melting of a heterogeneous source mixture characteristic to that hotspot melt extraction trajectories form when the sources of the individual hotspot basalts differ in the amount of previous melt extraction from the upwelling and melting plume mantle mixture Sequential melts along a melt extraction trajectory trace a one-dimensional path in through isotope space, independent of the number of distinct isotopic components in the initial source mixture Subsequent partial mixing of the melts produced along a melt extraction trajectory would still tends to preserve this tube-like isotope topology The melting physics that leads to a melt extraction trajectory also provides straightforward explanations for the 187Os/188Os contrasts between mid-ocean ridge basalts and their presumed abyssal peridotite source and for the enigmatic trace element and isotopic patterns of pyroxenite veins and peridotite exposed within orogenic lherzolites

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GC000004

Isotope topology of individual hotspot basalt arrays: Mixing curves or melt extraction trajectories?

Enrichment from plume interaction in the generation of Neoproterozoic arc rocks in northern Eritrea: implications for crustal accretion in the southern Arabian–Nubian Shield

The mantle can be sampled directly only very rarely. Geochemists have thus come to rely heavily on mantle-derived magmas to study the composition and evolution of the mantle. Only those compositional features that are unaffected by magmatic processes are useful as tracers of mantle processes. These include radiogenic isotope ratios such as those of He, Sr, Hf, and Os, stable isotope ratios, and ratios of highly incompatible elements or elements of similar incompatibility, such as Ba/Nb or Pb/Ce. The term “incompatible” denotes a preference of the element for a melt over mantle minerals. Highly incompatible elements will partition entirely into the melt under most circumstances, so that the ratio of two such elements in a basalt will be virtually identical to that ratio in its source. This is also true to a lesser degree of ratios such as La/Sm and Zr/Nb, as Zr and Sm are not highly incompatible elements.

William M. White

Papers

Geochemical tracers of mantle processes