Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of lithospheric formation and interactions range from the Hadean to the Neoproterozoic. In this paper, we review sedimentary, igneous and metamorphic proxies along with palaeomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archaean, between 3.2 Ga and 2.5 Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of sea-floor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780–2710 Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700–2440 Ma suggest significant relative movements. We consider these changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics.This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.

The terrestrial planets are depleted in volatile elements with respect to chondritic meteorites, their possible building blocks. However, the timing, extent and origin of volatile depletion is debated. Zinc is a moderately volatile element (MVE), whose stable isotopic composition can distinguish when and where this depletion took place. Here, we report data for 40 ultramafic rocks comprising pristine upper mantle peridotites from the Balmuccia orogenic lhezolite massif and Archean komatiites that together define the Zn isotope composition of the Earth's primitive mantle. Peridotites and komatiites are shown to have indistinguishable Zn isotopic compositions of δ 66 Zn = +0.16±0.06‰ (2SD), (with δ 66 Zn the per mille deviation of 66 Zn/ 64 Zn from the JMC-Lyon standard), implying a constant Zn isotope composition for the silicate Earth since 3.5 Ga. After accounting for Zn sequestration during core formation, the Earth falls on the volatile-depleted end of a carbonaceous chondrite array, implying Earth avoided modification of its MVE budgets during late accretion (e.g., during a giant impact), in contrast to the Moon. The Moon deviates from the chondritic array in a manner consistent with evaporative loss of Zn, where its δ 66 Zn co-varies with Mn/Na, implying post-nebular volatile loss is more pronounced on smaller bodies. Should the giant impact deliver the Earth's volatile complement of Pb and Ag, it cannot account for the budget of lithophile MVEs (e.g., Zn, Rb, Mn), whose abundances reflect those of Earth's nebular building blocks. The Earth initially accreted from material that experienced chemical-and mass-dependent isotopic fractionation akin to carbonaceous chondrites, though volatile depletion was more pronounced on Earth.

No abstract

Isotope ratios of heavy elements vary on the 1/10000 level in high temperature materials, providing a fingerprint of the processes behind their origin. Ensuring that the measured isotope ratio is precise and accurate depends on employing an efficient chemical purification technique and optimised analytical protocols. Exploiting the disparate speciation of Cu, Fe and Zn in HCl and HNO 3 , an anion exchange chromatography procedure using AG1-38 (200-400 mesh) and 0.4 3 7 cm Teflon columns was developed to separate them from each other and matrix elements in felsic rocks, basalts, peridotites and meteorites. It required only one pass through the resin to produce a quantitative and pure isolate, minimising preparation time, reagent consumption and total analytical blanks. A ThermoFinnigan Neptune Plus MC-ICP-MS with calibrator-sample bracketing and an external element spike was used to correct for mass bias. Nickel was the external element in Cu and Fe measurements, while Cu corrected Zn isotopes. These corrections were made assuming that the mass bias for the spike and analyte element was identical, and it is shown that this did not introduce any artificial bias. Measurement reproducibilities were ± 0.03‰, ± 0.04‰ and ± 0.06‰ (2s) for d 57 Fe, d 65 Cu and d 66 Zn, respectively.

No abstract