Geological archive of the onset of plate tectonics Cawood, Peter A.; Hawkesworth, Chris J.; Pisarevsky, Sergei A. ...
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
10/2018, Volume:
376, Issue:
2132
Journal Article
Peer reviewed
Open access
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'.
Combined in situ U-Pb and Hf-O isotope analyses for zircon are often used to date igneous events precisely and to gain insights into the origin of the magma from which the zircon crystallized. In ...conjunction with its resistance to weathering, zircon can therefore be considered a unique crystal toolbox and an ideal crustal archive. This concept, however, relies on the basic assumption that each zircon crystal is in Hf isotope equilibrium with its host magma. Here we test this hypothesis for zircon crystals from mafic-ultramafic layered intrusions and show that this assumption may not always be correct. We find Hf isotope disequilibrium between zircon crystals and their host-rocks in three Neoproterozoic mafic-ultramafic layered intrusions from the northwestern margin of the Yangtze Block, Central China, formed as part of convergent margin magmatism along the Hannan-Panxi subduction zone. Zircon crystals separated from diorite samples from these three intrusions confirm prolonged magmatism for over 90 Myr for the Beiba (869 plus or minus 5Ma), Wangjiangshan (822 plus or minus 4Ma) and Bijigou (785 plus or minus 5Ma) intrusions, with a chronologically progressive decrease in delta super(18)O values from 7.4ppt to 6.3ppt and 6.0ppt, respectively. We interpret the transition from an isotopically evolved (high delta super(18)O) towards a progressively more primitive mantle source (lower delta super(18)O) as the fading influence of subducted sediment-derived melts in a subduction zone, consistent with a reconstructed change in subducting plate motions from the northern to the western margin of the Yangtze Block. Unlike the coherent O isotopes, the epsilon Hf sub((t)) values of zircon populations from each intrusion show a range of several epsilon Hf units (Beiba: -1.0 to+3.0; Wangjiangshan: +2.7 to +8.3; Bijigou: +2.3 to +7.8), outside analytical uncertainty and inconsistent with an origin from a single magma batch. Whole-rock Hf isotope analyses obtained by high-pressure dissolution indicate that the diorite samples from the Beiba, Wangjiangshan and Bijigou intrusions have epsilon Hf sub((t)) of +8.2, +7.5 and +9.3, respectively. In contrast, table-top dissolutions for the same samples yield epsilon Hf sub((t)) of +9.7, +10.0 and +11.7, respectively. The apparent systemic offset in epsilon Hf sub((t)) values towards more crustal compositions in high-pressure dissolutions is interpreted here to reflect mixing of zircon-hosted Hf isotopes with less evolved Hf isotopes in associated mineral phases. The more crustal character of in situ Hf isotope determinations in zircons and their range of several epsilon Hf units are interpreted here to reflect progressive crustal contamination in magma chambers at the time of zircon saturation. This implies that Hf isotope compositions of zircon crystals can be biased towards crustal signatures, particularly in mafic-ultramafic intrusions that are more susceptible to crustal contamination. In such cases, source interpretation as well as Hf model ages calculated from these isotopic mixing pools are geologically meaningless. Inevitably, contaminated igneous suites, mafic-ultramafic complexes in particular, and detrital zircon populations derived therefrom may have a complex Hf isotopic history that cannot be resolved by fast in situ analyses of Hf isotopes alone. This history may be revealed only by an atypical range of Hf isotope compositions within single magmatic suites and, if unidentified, can lead to biased geological interpretations.
Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone ...processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both lighter and heavier Mo isotopic values when compared to the chondritic average (δ98/95MoNIST3134 −0.16 ± 0.02‰), with a concentration weighted mean isotopic composition (δ98/95MoNIST3134 +0.03‰) distinctly heavier than that of the mantle (δ98/95MoNIST3134~−0.21 to −0.16‰). The absence of isotopic fractionation during tholeiitic igneous differentiation indicates that heavy Mo isotope signatures in convergent margin rocks are intrinsic to subduction zone processes, caused either by recycling of subducted components, by Mo isotope fractionation during magmatic differentiation, or by a combination thereof. In order to gain a more detailed understanding of the Mo isotope variability in arc lavas, we have studied Mo isotopes and concentrations in calc-alkaline lavas sampled along the Banda Arc in Indonesia, an intra-oceanic subduction zone well known for variable contributions of subducting continental meterial.
Banda Arc lavas exhibit Mo isotopic compositions covering a large range from −0.48 to +0.24‰ in δ98/95MoNIST3134. We find that, combined with published data from other subduction zones, the majority of arc data are isotopically heavier compared to its presumed mantle wedge source or mid-ocean-ridge basalts. Furthermore, arc lava Mo isotope signatures show an apparent covariation between indices of amphibole-clinopyroxene fractionation from the melt. Generally, heavier Mo isotopic signatures are associated with higher degrees of REE fractionation expressed as λ1, λ2, and Dy*/Dy, high Ca/Al, and higher Sc contents. These observations can be best explained with an open system magma reservoir where fractional crystallisation, recharge and eruption are tightly linked. Notable exceptions in the Banda region are four samples with isotopically light Mo. Three of these isotopically light lavas indicate the assimilation of an isotopically light Mo reservoir with heavy δ18O and radiogenic 87Sr/86Sr-206Pb/204Pb, tentatively identified as lower arc crust. Together, our data demonstrate, in line with previous findings, that magmatic processes modify the Mo isotopic composition in arc lavas, thus adding complexity to the Mo isotope systematics inherent to subduction zones.
Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the ...mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO
) and dissolved SO
in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe
/∑Fe in spinel record a S
-Fe
redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ
S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.
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 HNO3, an anion exchange chromatography procedure using AG1‐×8 (200–400 mesh) and 0.4 × 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 δ57Fe, δ65Cu and δ66Zn, respectively.
Les rapports isotopiques des éléments lourds varient au niveau de la quatrième décimale dans les matériaux de haute température, fournissant ainsi un accès aux processus ayant participé à la formation de ces derniers. S'assurer que le rapport isotopique mesuré est précis et exact dépend de l'emploi d'une technique de purification chimique efficace et de protocoles analytiques optimisés. L'exploitation de la spéciation différentielle de Cu, Fe et Zn dans l'HCl et le HNO3 a permis le développement d'une procédure de chromatographie échangeuse d'anions utilisant de la résine anionique AG1‐X8 (200–400 mesh) et des colonnes en téflon (0.4 x 7 cm) pour séparer ces éléments à la fois les uns des autres mais aussi de ceux de la matrice dans des roches felsiques, des basaltes, des péridotites et des météorites. Il n'a fallu qu'un seul passage dans la résine pour produire un isolat mono élémentaire ultra pur, en minimisant le temps de préparation, la consommation de réactif et le blanc analytique total. En utilisant un MC‐ICP‐MS ThermoFinnigan Neptune plus, une alternance standard‐échantillon avec étalon externe a servi à corriger le biais de masse. Le Nickel était l'élément externe dans les mesures de Cu et de Fe alors que le Cu a été utilisé pour corriger les isotopes du Zn. Ces corrections ont été apportées en supposant que le biais de masse pour le spike et l'élément analysé était identique, et il est démontré que cela n'a pas introduit de biais artificiel. La reproductibilité des mesures a été de ± 0.03‰, ± 0.04‰ et ± 0.06‰ (2s) pour respectivement δ57Fe, δ65Cu et δ66Zn.
Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Basalts along these margins display a chemical diversity, which is consequent to ...a complex interplay of partial mantle melting in the upper mantle and magmatic differentiation processes in lower crustal levels. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts. This variability is reflected in iron isotope systematics (expressed as δ
Fe), showing a total range of 0.2 ‰ from δ
Fe = + 0.05 to + 0.25 ‰. Respective contributions of source heterogeneity and magma differentiation leading to this diversity, however, remain elusive. This study investigates the iron isotope systematics in basalts from the ultraslow spreading Gakkel Ridge in the Arctic Ocean and compares them to existing data from the fast spreading East Pacific Rise ridge. Results indicate that Gakkel lavas are driven to heavier iron isotope compositions through partial melting processes, whereas effects of igneous differentiation are minor. This is in stark contrast to fast spreading ridges showing reversed effects of near negligible partial melting effects followed by large isotope fractionation along the liquid line of descent. Gakkel lavas further reveal mantle heterogeneity that is superimposed on the igneous differentiation effects, showing that upper mantle Fe isotope heterogeneity can be transmitted into erupting basalts in the absence of homogenisation processes in sub-oceanic magma chambers.
The Rb–Sr decay system is one of the most widely used geochronometers for obtaining ages and cooling rates of terrestrial magmatic, metamorphic, and hydrothermal events. It has also been extensively ...applied to date extraterrestrial, early solar system events. The accuracy of Rb–Sr ages, however, strongly depends on the accuracy of the 87Rb decay constant (λ87Rb). We determined λ87Rb relative to the decay constants of 235U and 238U by comparing Rb–Sr ages of minerals with U–Pb ages obtained from the same intrusion. Comparison of U–Pb emplacement ages with high-precision Rb–Sr mineral ages from three rapidly cooled igneous rocks covering an age range of ca. 2.5Ga yields an unweighted mean λ87Rb of 1.393±0.004×10−11yr−1 (i.e., ±0.3%), corresponding to a half-life of 49.76×109years. Because this decay constant is 2% lower than the presently recommended one, many previously published ages are 2% too young and the resulting geologic interpretations may need revision.
► Age comparison experiments suggest that the 87Rb decay constant is 2% lower than the currently accepted value. ► Our value of 1.394 ±0.004 ×10-11yr-1 agrees with the results of the most recent age comparisons and the latest beta counting experiment. ► Ages calculated with the currently accepted value need revision and will become older by 2%.
Abstract
The formation of stable, evolved (silica-rich) crust was essential in constructing Earth’s first cratons, the ancient nuclei of continents. Eoarchaean (4000–3600 million years ago, Ma) ...evolved crust occurs on most continents, yet evidence for older, Hadean evolved crust is mostly limited to rare Hadean zircons recycled into younger rocks. Resolving why the preserved volume of evolved crust increased in the Eoarchaean is key to understanding how the first cratons stabilised. Here we report new zircon uranium-lead and hafnium isotope data from the Yilgarn Craton, Australia, which provides an extensive record of Hadean–Eoarchaean evolved magmatism. These data reveal that the first stable, evolved rocks in the Yilgarn Craton formed during an influx of juvenile (recently extracted from the mantle) magmatic source material into the craton. The concurrent shift to juvenile sources and onset of crustal preservation links craton stabilisation to the accumulation of enduring rafts of buoyant, melt-depleted mantle.
Abstract
Unlike many Archean diorites and granitoids that arguably formed in different geodynamic settings, their post-Archean counterparts are commonly regarded to have formed at convergent margins, ...although in detail their petrogenesis remains contentious. Here we present new whole-rock data and zircon Hf–O isotope analyses from dioritic (750–730 Ma), granitic (810–790 Ma) and tonalite–trondhjemite–granodiorite (TTG)-like intrusions (800–740 Ma) from the Panxi and Hannan regions, which form part of an extensive Neoproterozoic convergent margin exposed in South China. The dioritic rocks from the Panxi region exhibit high zircon εHf(t) (+10.1 to +13.1) and sub-mantle to mantle-like δ18O (3.1–6.3 ‰) values, whereas those from the Hannan region preserve low εHf(t) (+4.1 to +8.1) and high δ18O values (5.9–6.6 ‰), indicating that the dioritic melts were derived from subduction-modified lithospheric mantle sources and experienced variable degrees of lower crustal contamination. Zircons within granite and TTG from the Panxi region show a narrow range of Hf isotopic compositions generally spanning 2–4 εHf units (+3.1 to +7.9 for most felsic intrusions). By contrast, those from the Hannan region show a much wider range of zircon εHf(t) values spanning almost 10 εHf units (+1.1 to +10.9). Based on their O–Hf–Nd isotopic signatures, we propose that the granite and TTG from both areas were derived through partial melting of hydrated basaltic rocks in the arc root, and that the isotopic variability between the intrusions mirrors spatial and temporal chemical variations in these deep crustal source rocks. In both regions, the granites, along with mantle-derived mafic–ultramafic and intermediate rocks, show a coupled evolution associated with increasing εNd(t) and εHf(t) and decreasing δ18O with decreasing ages, whereas the TTGs formed during late-stage arc magmatism and preserve relatively homogeneous Nd–Hf isotopes and mantle-like δ18O values. Combined, these data record continuous crustal thickening through underplating of juvenile magmas and a progressive increase in the depth of melting, along with a decrease in the degree of interaction between the melts and basement rocks within the arc root. Our results suggest that slab melting was not required to produce post-Archean TTG signatures. Further, we suggest that the variability in the Hf–O–Nd isotopic compositions of metaluminous (I-type) granites mostly does not reflect a heterogeneity in upper mantle signatures, and that there is no conclusive evidence for the involvement of partial melts of subducted sediment based on Hf–O isotope signatures in zircon.
When and how Earth's earliest continents-the cratons-first emerged above the oceans (i.e., emersion) remain uncertain. Here, we analyze a craton-wide record of Paleo-to-Mesoarchean granitoid ...magmatism and terrestrial to shallow-marine sedimentation preserved in the Singhbhum Craton (India) and combine the results with isostatic modeling to examine the timing and mechanism of one of the earliest episodes of large-scale continental emersion on Earth. Detrital zircon U-Pb(-Hf) data constrain the timing of terrestrial to shallow-marine sedimentation on the Singhbhum Craton, which resolves the timing of craton-wide emersion. Time-integrated petrogenetic modeling of the granitoids quantifies the progressive changes in the cratonic crustal thickness and composition and the pressure-temperature conditions of granitoid magmatism, which elucidates the underlying mechanism and tectonic setting of emersion. The results show that the entire Singhbhum Craton became subaerial ∼3.3 to 3.2 billion years ago (Ga) due to progressive crustal maturation and thickening driven by voluminous granitoid magmatism within a plateau-like setting. A similar sedimentary-magmatic evolution also accompanied the early (>3 Ga) emersion of other cratons (e.g., Kaapvaal Craton). Therefore, we propose that the emersion of Earth's earliest continents began during the late Paleoarchean to early Mesoarchean and was driven by the isostatic rise of their magmatically thickened (∼50 km thick), buoyant, silica-rich crust. The inferred plateau-like tectonic settings suggest that subduction collision-driven compressional orogenesis was not essential in driving continental emersion, at least before the Neoarchean. We further surmise that this early emersion of cratons could be responsible for the transient and localized episodes of atmospheric-oceanic oxygenation (O
-whiffs) and glaciation on Archean Earth.
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