Kombinierte U-Pb- und Lu-Hf-Isotopenuntersuchungen an detritischen Einzelzirkonen:Anwendungen für Provenanzstudien und die Untersuchung der frühen Kruste

Ein chemisches Abtrennverfahren zur kombinierten U-Pb Altersbestimmung und Hf Isotopiebestimmung eines Einzelzirkons wird präsentiert. Blei, Lu, U und Hf werden nacheinander mithilfe eines chromatographischen Ein-Säulen-Trennverfahren mit ~100µl Eichrom(r) Ln-Spec Harz von der Zirkonmatrix getrennt. Die kleinsten analysierten Körner sind ~50µm im Durchmesser und ihre Hf Isotopie kann mit einer externen Reproduzierbarkeit von 100ppm bestimmt werden. In einer Fallstudie kann gezeigt werden, dass, basierend auf den paläogeographischen Rekonstruktionen im Kambrium und durch den Vergleich von Hf-Nd-Modellaltern, Liefergebiete für paläozoische Sedimente des heutigen Neuseelands in der heutigen Antarktis und Australien lagen. In einer zweiten Fallstudie wurden Zirkone des Narryer Gneiss Terranes untersucht, um Vorgänge in der frühen Erdkruste zu entschlüsseln. Die Altersverteilung sowie die initiale Hf Isotopie der Zirkone indizieren daß es eine andauernde magmatische Aktivität im Hadaikum und Archaikum gegeben hat, vermutlich in Verbindung mit Subduktionsprozessen

Competing effects of spreading rate, crystal fractionation and source variability on Fe isotope systematics in mid-ocean ridge lavas

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Richter, M., Nebel, O., Schwindinger, M., Nebel-Jacobsen, Y., & Dick, H. J. B. Competing effects of spreading rate, crystal fractionation and source variability on Fe isotope systematics in mid-ocean ridge lavas. Scientific Reports, 11(1), (2021): 4123, https://doi.org/10.1038/s41598-021-83387-7.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 δ57Fe), showing a total range of 0.2 ‰ from δ57Fe =  + 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.This work was supported by an ARC grant FT140101062 to O.N. H.J.B.D was supported by the NSF grants PLR 9912162, PLR 0327591, OCE 0930487 and OCE 1434452

An early cretaceous subduction-modified mantle underneath the ultraslow spreading Gakkel Ridge, Arctic Ocean

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Richter, M., Nebel, O., Maas, R., Mather, B., Nebel-Jacobsen, Y., Capitanio, F. A., Dick, H. J. B., & Cawood, P. A. An early cretaceous subduction-modified mantle underneath the ultraslow spreading Gakkel Ridge, Arctic Ocean. Science Advances, 6(44), (2020): eabb4340, doi:10.1126/sciadv.abb4340.Earth’s upper mantle, as sampled by mid-ocean ridge basalts (MORBs) at oceanic spreading centers, has developed chemical and isotopic heterogeneity over billions of years through focused melt extraction and re-enrichment by recycled crustal components. Chemical and isotopic heterogeneity of MORB is dwarfed by the large compositional spectrum of lavas at convergent margins, identifying subduction zones as the major site for crustal recycling into and modification of the mantle. The fate of subduction-modified mantle and if this heterogeneity transmits into MORB chemistry remains elusive. Here, we investigate the origin of upper mantle chemical heterogeneity underneath the Western Gakkel Ridge region in the Arctic Ocean through MORB geochemistry and tectonic plate reconstruction. We find that seafloor lavas from the Western Gakkel Ridge region mirror geochemical signatures of an Early Cretaceous, paleo-subduction zone, and conclude that the upper mantle can preserve a long-lived, stationary geochemical memory of past geodynamic processes.O.N. was supported by the Australian Research Council (grant FT140101062). P.A.C. was supported by the Australian Research Council (grant FL160100168). H.J.B.D. was supported by the NSF (grants PLR 9912162, PLR 0327591, OCE 0930487, and OCE 1434452). M.R. was supported by a graduate scholarship of Monash University and the SEAE

Spatial and temporal control of Archean tectonomagmatic regimes

Secular trends in plutonic whole-rock geochemistry pose critical, although non-unique, constraints to early Earth tectonics. Here, we present a large whole-rock geochemical (879 collated samples) dataset for granitoids from the Pilbara Craton, Western Australia, applying it to test the link between secular trends and proposed tectonic mechanisms. We show that the spatio-temporal distribution of granitoid trace element geochemistry is constrained within discrete lithotectonic blocks supporting the reconstruction of its tectonomagmatic evolution. Time-sliced geochemical contour mapping of key petrogenetic ratios indicates the craton underwent rifting ∼3.2 Ga (billion years ago), marking a transition from predominantly sodic magmatism to a broader magmatic compositional spectrum. Our results demonstrate that rift-assisted breakup of proto-cratons is a viable craton growth mechanism. We identify a possible evolutionary sequence beginning with drips and upwellings below a Paleoarchean mafic plateau, which is subsequently dismembered by rifting. These plateau fragments form rigid blocks in the Mesoarchean, between which weaker, thinner crust accommodates minor convergence and divergence manifested as short-lived mobile lid-like features before stabilization. We conclude that these features do not require an active lid, plate tectonic regime

The stability of cratons is controlled by lithospheric thickness, as evidenced by Rb-Sr overprint ages in granitoids

The ancient cores of modern continents, cratons, are the oldest blocks of “stable” lithosphere on Earth. Their long-term survival relies on the resistance of their underlying thick, strong, and buoyant mantle keels to subsequent recycling. However, the effect of substantial geographical variations in keel thickness on the post-assembly behaviour and mass movement within these continental cores remains unknown. Here, we demonstrate that the spatial distribution of fluid-reset in-situ Rb-Sr ages for Paleo-Mesoarchean (3.6–2.8 billion years ago; Ga) granitoids of the Pilbara Craton, Australia shows remarkable correlation with independently-constrained lithospheric thickness models. Without craton-wide heating/magmatic events, these anomalously young Rb-Sr ages document episodes of fluid infiltration into granitoid complexes as a response to lithospheric reactivation by far-field stresses. This correlation implies that craton-wide fluid mobilization triggered by extra-cratonic Neoarchean to Mesoproterozoic (2.8–1.0 Ga) tectonic events is facilitated by variations in lithospheric strength and thickness. Compared to areas of older overprints, the two-thirds of the craton comprised of younger reset ages is underlain by comparatively thin lithosphere with higher susceptibility to reactivation-assisted fluid flow. We propose that even the strongest, most pristine cratons are less stable and impermeable than previously thought, as demonstrated by the role of granitoid complexes and cratons as selective lithospheric “sponges” in response to minor tectonic forces. Therefore, variations in lithospheric thickness, likely attained before cratonization, exert a crucial control on billions of years of fluid movement, elemental redistribution and mineralization within ancient continental nuclei

Precambrian sources of Early Paleozoic SE Gondwana sediments as deduced from combined Lu-Hf and U-Pb systematics of detrital zircons, Takaka and Buller terrane, South Island, New Zealand

Combined U-Pb-Lu-Hf isotope investigations on small detrital zircons (30-120 μm) using solution ID-MC-ICPMS and ID-TIMS were carried out on Cambrian-Ordovician sedimentary rocks from the Buller and Takaka terranes of the northeastern South Island of Ne

Lu-Hf isotopic memory of plume-lithosphere interaction in the source of layered mafic intrusions, Windimurra Igneous Complex, Yilgarn Craton, Australia

Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma chambers. These magmas are originally sourced from the mantle, likely via plume activity, but may interact with the overriding lithosphere during ascent and e

Molybdenum isotope systematics in cumulate rock of the 2.8 Windimurra layered intrusion: A test for igneous differentiation and the composition of the Archean mantle

Molybdenum isotopes (reported as δ98Mo relative to NIST-3134) show resolvable isotope differences in igneous rocks with the continental crust being markedly heavier in isotope composition than mid-ocean ridge lavas, lunar basalts or the Earth's mantle. The tholeiitic differentiation series at the intra-plate Hekla volcano (Iceland) shows no resolvable Mo isotope differences from basaltic to rhyolitic compositions. In contrast, convergent margin lavas show a transition from isotopically lighter mantle to heavy continental crust, suggesting that subduction processes drive continental crust towards heavier values. Archean komatiitic lavas, presumed probes of the Archean mantle, have Mo isotope values identical to modern depleted mantle, raising the questions if and how the Mo isotope crust-mantle disparity developed so early in Earth's history. Here we present new Mo isotope data for a set of cumulate rocks from the Upper Zone of late Archean (2.8 Ga) Windimurra Igneous Complex, a mafic/ultramafic layered intrusion. The intrusion is not subduction related and contains no apparent primary hydrous minerals. We tested the effect of crystal fractionation on Mo isotopes in relatively dry melt along a tholeiitic liquid line of descent by using the cumulate effect of normally anhydrous minerals in the layered intrusion. Near mono-mineralic olivine-pyroxene-rich, feldspar-rich and Fe-Ti-rich oxides show small variations (~0.15‰) in Mo isotope signatures. This is consequently to predominantly isotopically light Fe-Ti-oxide-rich and isotopically heavier feldspar-rich rocks, respectively. This is suggesting minor Mo isotope fractionation, even in dry, tholeiitic systems, which however, counterbalance each other and thus potentially remain undetected. On average, the Windimurra mantle source is indistinguishable, or slightly isotopically lighter than the Mo isotope signature of komatiites. This is reinforcing an isotopically light Mo isotope signature of Archean mantle sources of high-degree mantle melts and is extending these signatures to predominantly mafic Archean crust. It remains to be tested if Archean felsic crust resembles modern continental crust in its heavy isotope values and to which extend the mantle was already isotopically depleted in Mo isotopes at Mesoarchean time

A non-zircon Hf isotope record in Archean black shales from the Pilbara craton confirms changing crustal dynamics ca. 3 Ga ago

Plate tectonics and associated subduction are unique to the Earth. Studies of Archean rocks show significant changes in composition and structural style around 3.0 to 2.5 Ga that are related to changing tectonic regime, possibly associated with the onset of subduction. Whole rock Hf isotope systematics of black shales from the Australian Pilbara craton, selected to exclude detrital zircon components, are employed to evaluate the evolution of the Archean crust. This approach avoids limitations of Hf-in-zircon analyses, which only provide input from rocks of sufficient Zr-concentration, and therefore usually represent domains that already underwent a degree of differentiation. In this study, we demonstrate the applicability of this method through analysis of shales that range in age from 3.5 to 2.8 Ga, and serve as representatives of their crustal sources through time. Their Hf isotopic compositions show a trend from strongly positive εHfinitial values for the oldest samples, to strongly negative values for the younger samples, indicating a shift from juvenile to differentiated material. These results confirm a significant change in the character of the source region of the black shales by 3 Ga, consistent with models invoking a change in global dynamics from crustal growth towards crustal reworking around this time

Separation of U, Pb, Lu, and Hf from single zircons for combined U-Pb dating and Hf isotope measurements by TIMS and MC-ICPMS

The U-Pb system has long been used to precisely date zircons because the high U-to-common-Pb ratio in zircon produces very radiogenic Pb isotope compositions over time. In contrast to U/Pb, zircon has very low Lu/Hf and therefore unradiogenic Hf, making this mineral ideally suited for determining the initial Hf composition of its original host rock. A new chemical separation technique presented here enables the determination of both U-Pb age and initial Hf isotope composition of individual zircon grains. The acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies. Zircons are spiked with mixed 176Lu-180Hf and 233U-205Pb tracers and then digested in HF-HNO3. Lead, Lu, U, and Hf are sequentially separated from the zircon matrix on a single ion exchange column filled with Eichrom® Ln Spec resin. Using only ∼100 μl of resin for the separation keeps Pb blanks low (∼5 pg) while achieving better than 90% yields for each of the four elements. Hafnium isotope compositions and Lu concentrations are measured with multiple collector-inductively coupled plasma-mass spectrometry (MC-ICPMS), whereas U and Pb are analyzed by thermal ionization mass spectrometry (TIMS). The minimum grain size that can be processed is dictated by the amounts of Pb and Hf needed for an analysis. The smallest grains we currently analyze, as small as 50 μm (∼12pg of Pb and ∼3ng of Hf), can be analyzed with an external 176Hf/177Hf precision of ∼100 ppm (2 s.d.). The utility of this method is demonstrated with a population of detrital zircons from a Cambrian sediment of the Takaka Terrane, New Zealand. In addition, the technique has been used for 14 analyses of the standard zircon 91500, which yield a mean present-day 176Hf/ 177Hf of 0.282305 ± 12 (2 s.d., i.e., an external reproducibility of 43 ppm). The Hf isotopic compositions and U-Pb ages presented here are in good agreement with those of previous studies (e.g., Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, Trace element and REE analyses. Geostandards Newsletter 19, 1-23.), but our data suggest that the 91500 zircon is heterogeneous with respect to Lu and Hf contents and Lu/Hf