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

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

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

Age, gender, and cancer but not neurodegenerative and cardiovascular diseases strongly modulate systemic effect of the Apolipoprotein E4 allele on lifespan

Enduring interest in the Apolipoprotein E (ApoE) polymorphism is ensured by its evolutionary-driven uniqueness in humans and its prominent role in geriatrics and gerontology. We use large samples of longitudinally followed populations from the Framingham Heart Study (FHS) original and offspring cohorts and the Long Life Family Study (LLFS) to investigate gender-specific effects of the ApoE4 allele on human survival in a wide range of ages from midlife to extreme old ages, and the sensitivity of these effects to cardiovascular disease (CVD), cancer, and neurodegenerative disorders (ND). The analyses show that women's lifespan is more sensitive to the e4 allele than men's in all these populations. A highly significant adverse effect of the e4 allele is limited to women with moderate lifespan of about 70 to 95 years in two FHS cohorts and the LLFS with relative risk of death RR = 1.48 (p = 3.6×10(−6)) in the FHS cohorts. Major human diseases including CVD, ND, and cancer, whose risks can be sensitive to the e4 allele, do not mediate the association of this allele with lifespan in large FHS samples. Non-skin cancer non-additively increases mortality of the FHS women with moderate lifespans increasing the risks of death of the e4 carriers with cancer two-fold compared to the non-e4 carriers, i.e., RR = 2.07 (p = 5.0×10(−7)). The results suggest a pivotal role of non-sex-specific cancer as a nonlinear modulator of survival in this sample that increases the risk of death of the ApoE4 carriers by 150% (p = 5.3×10(−8)) compared to the non-carriers. This risk explains the 4.2 year shorter life expectancy of the e4 carriers compared to the non-carriers in this sample. The analyses suggest the existence of age- and gender-sensitive systemic mechanisms linking the e4 allele to lifespan which can non-additively interfere with cancer-related mechanisms

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 mu m) 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 New Zealand. The sediments in these two terranes are considered correlatives of voluminous sedimentary belts in the Lachlan and Adelaidian fold belts of southeastern Australia and in the Wilson and Bowers terranes of Antarctica (SE Gondwana). In order to constrain the sources of these sediments, we use combined U-Pb and Lu-Hf data from detrital zircons, enabling the calculation of Lu-Hf model ages that date the crustal residence time of the zircon protoliths. The Lu-Hf age information is combined with Nd model ages for the bulk sediments. U-Pb systematics of the detrital zircons show age maxima at 0.5-0.6 Ga, 0.8 Ga, and 1.0-1.2 Ga, with initial epsilon Hf values for the New Zealand detrital zircons ranging from +14.9 to -27.6. These zircon age populations broadly agree with age patterns for sediments found elsewhere in SE Gondwana, confirming a link between the Paleozoic belts of New Zealand and Australia-Antarctica. Lu-Hf model ages obtained from the detrital zircon populations range from 0.9 to 3.7 Ga with an age maximum at similar to 1.5 Ga. A comparison with average crustal residence ages of potential source regions indicates that the Ross-Delamerian orogen of present day Eastern Australia and Antarctica, and crustal provinces in Western Australia are likely sources. The presence of sources as distant as Western Australia suggests long transport pathways for the zircons, possibly via large river systems. A comparison of model age distributions for sediments from Australia, Antarctica, and New Zealand indicate that the Australian sediments have been fed by an additional Archean source (similar to 2.8-3.1 Ga), in contrast to sedimentary rocks from New Zealand and Antarctica that lack an Archean model age peak. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved

Initial Hf isotope compositions in magmatic zircon from early Proterozoic rocks from the Gawler Craton, Australia: a test for zircon model ages

We evaluate the validity of age information obtained from two-stage Hf model ages in zircon. Multiple isotope investigations of minerals from the Kiana Granite of the Archean Sleafordian Complex from the southern Gawler Craton, Australia, reveal a complex geologic evolution of the suite. Rubidium-Sr mineral ages define a tectono-thermal event at 1673.4 ± 4.2 Ma, associated with the Kimban orogeny. ID-TIMS U-Pb ages from single zircons yield an upper concordia intercept at 2348 ± 21 Ma, which marks the time of magmatic emplacement. Whole-rock Nd model ages yield crustal residence ages of ∼ 2.8 Ga. For each single zircon grain, the Lu-Hf isotope systematic was determined by solution MC-ICPMS. Two-stage Hf model ages for the magmatic zircons vary by as much as 500 Myrs, depending on the choice of Lu/Hf for the source of the host rock, putting further emphasis on the need to constrain the source Lu/Hf of a rock used for model age calculations. An average Lu/Hf for mafic crust was assumed for the source reservoir of the rocks to calculate its Hf isotope evolution through time, which yields zircon model ages of ∼ 3.5 Ga that coincide with the initial growth episode postulated for the Gondwana supercontinent. It is demonstrated that two-stage zircon model ages deduced from initial Hf isotopes can provide important age information for the source of igneous rocks, and consequently on important crustal formation events, even when conventional geochronologic investigations fail, e.g., due to thermo-tectonic overprints or mixing