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Measurements of U–Th–Pb, Lu–Hf and oxygen isotopes as well as selected trace and rare earth elements were carried out on zircon grains from high-grade metasedimentary rocks from the Albany–Fraser Orogen in southwest Australia. Oxygen isotopes from detrital zircon domains yield δ18O (VSMOW) values ranging from 5.8 to 8.0‰ and exhibit coupled Hf- and O-isotope compositions. In contrast, metamorphic zircon domains show considerably less variability in O-isotope compositions with a median δ18O of 5.6 ± 0.5‰ and exhibit decoupled Hf- and O-isotope compositions. 176Hf/177Hf isotope ratios of metamorphic zircon grains are within the evolutionary trend defined by the detrital grains implying negligible input from external sources during their growth. In addition, the relatively low δ18O value of the metamorphic zircons implies crystallisation from a relatively 18O depleted crustal melt. Such δ18O values are lower than those typically observed in most metamorphic zircon, including those preserved in partial melts of metasediments (10–12‰) and suggest that the sedimentary protolith was hydrothermally altered by meteoric fluids prior to high-temperature metamorphism. We suggest that alteration of the sedimentary protolith is related to the proximity to the Fraser Shear Zone, a major crustal scale structure within the orogen, which may represent an old reactivated structure. Occurrences of low δ18O metamorphic zircon, and potentially also low δ18O igneous rocks, in ancient collisional settings may, therefore, delineate long-lived fluid pathways within the crust. [ABSTRACT FROM AUTHOR]

The Oceanic Anoxic Event 2 (OAE 2; ca. ∼94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO 2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (△ ε Hfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (∼62°S) during the late Cretaceous. The reliability of △ ε Hfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low △ ε Hfclay values (−5.9 ± 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in △ ε Hfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced △ ε Hfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest δ 13 C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO 2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments. • Combined Nd-Hf isotopes as a proxy for continental weathering during OAE 2. • Negligible influence of reverse weathering on Hf-Nd isotopes in marine clays. • Intensifying weathering and river discharge in SW Australia during peak OAE 2 warmth. • Southward shift of westerlies drove enhanced weatherability in southern high-latitude regions. [ABSTRACT FROM AUTHOR]

Abstract: 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 emplacement in the crust. The magma injections lead to the establishment of different layers and zones with complex macroscopic, microscopic and cryptic compositional layering through magmatic differentiation and associated cumulate formation, sometimes accompanied by crustal assimilation. These complex mineralogical and petrological processes obscure the nature of the mantle sources of LMI, and typically have limited the degree to which parental liquids can be fully characterised. Here, we present Lu–Hf isotope data for samples from distinct layers of the Upper Zone of the Windimurra Igneous Complex (WIC), an immense late-Archean LMI in the West Australian Yilgarn Craton. Lu–Hf isotope systematics of whole rocks are well correlated ( , ) with an age of Ga and initial . This age, however, is older than whole rock Sm–Nd and zircon U–Pb ages of the intrusion, both of which are ca. 2.8 Ga. Stratigraphically-controlled initial Hf isotope variations (associated with multiple episodes of emplacement at ca. 2.8 Ga) indicate isotope mixing between a near-chondritic and an ultra-radiogenic component, the latter with . This Hf isotope mixing creates a pseudochron-relationship at the time of intrusion of ∼250 Myr that is superimposed on subsequent radiogenic ingrowth after crystallisation, generating an age that predates the actual emplacement event. Mixing between late-stage crystallisation products (melt + crystals) from the Middle Zone and replenishing, plume-derived liquids was followed by crystal accumulation in a chemically evolving magma chamber. The ultra-radiogenic Hf isotope endmember in the WIC mantle source requires parent–daughter ratios consistent with very early formation in Earth history, akin to early Archean komatiitic plume sources. We propose that plume-derived melts that formed the Windimurra LMI reacted with ancient refractory lithospheric keels already underpinning ancient cratons, creating a melt with extremely high . Melting a refractory component with super-chondritic, time-integrated high Lu/Hf, in this case by plume–lithosphere interaction, simultaneously accounts for the extreme Hf isotope signals, Hf–Nd isotope decoupling, and difference in radiometric Lu–Hf and Sm–Nd ages. [Copyright &y& Elsevier]

Abstract: Geochemical and Hf–Nd isotope studies of Neoproterozoic sedimentary and volcaniclastic rocks in the Nanhua Rift Basin, South China, demonstrate that their source provenances contained large proportions of mafic rocks and various amounts of granites. A significant proportion of the studied Neoproterozoic rift sedimentary and volcaniclastic rocks have initial εNd(t) values higher than those of Neoproterozoic granites, but fall within the range of ca. 825–750Ma basaltic rocks. Their initial εNd(t) values correlate with ratios of La/Sc, La/Cr, La/V, La/Co and La/Ni. The Nd isotope and trace element data, in combination with existing in-situ U–Pb and hafnium–oxygen isotope data from the detrital zircon grains indicate a dominant ca. 825–800Ma mafic provenance. Furthermore, the studied samples formed a linear array and plot within the field defined by the remanent of ca. 825–810Ma continental flood basalts in the Hf–Nd isotopic space. Thus, the inferred large proportions of mafic rocks in the source provenance of the Neoproterozoic rift sedimentary and volcaniclastic rocks likely signify an eroded continental flood basalt province, similar to that reported for the Neoproterozoic sedimentary rocks in Australia. This work thus provides further evidence for the possible once existence of a common large igneous province between South China and eastern Australia as adjacent parts of the supercontinent Rodinia. [Copyright &y& Elsevier]

Abstract: Zircon Hf isotopic data from six granite supersuites recognised in the New England Batholith of eastern Australia indicate that magma mixing is present in all plutons. Apart from the S-type supersuites, inherited zircons are rare to absent, suggesting that initial melt temperatures were above zircon saturation for the M-, I- and A-type granites. In all supersuites, the range of zircon Hf model ages calculated relative to arc mantle varies greatly, with younger model ages somewhat less than the Permian and Triassic age of crystallisation, and older model ages indicating derivation from source rocks that may be as old as Neoproterozoic. The older model ages are consistent with late Precambrian rocks in the lower crust that may be elements of the Lachlan Fold Belt underlying the New England Fold Belt. Whole-rock δ18O and εNd values, 87Sr/86Sr initial ratios (Sri) and oxidation state (Fe2O3/FeO) are summarised for the five named supersuites, and for a previously unnamed group of Triassic plutons situated east of the main Batholith for which we propose the name Carrai supersuite. Supersuites with higher δ18O, higher Sri, lower δ7Li and lower Fe2O3/FeO ratios indicate a metasedimentary component input. The pattern of zircon Hf isotopic variation reflects the other isotopic and geochemical indicators closely, the additional value being that it preserves a magmatic crystallisation record of Hf isotopic variation, that is, a measure of magma mixing from magma generation to final crystallisation. Using the above criteria, at least three distinct crustal components are considered necessary to explain the compositional and isotopic diversity within the Batholith. The crustal components indicated are: 1) a metasedimentary component of Carboniferous and Devonian age for the Hillgrove, Bundarra and Uralla supersuites; 2) a lower crustal K-rich (and Sr-rich) I-type component for the Moonbi, Carrai and possibly Uralla supersuites that could be as old as Proterozoic; and 3) a lower crustal K-poor I-type component of uncertain age for the Clarence River supersuite. In all supersuites, there is strong evidence of an M-type component based on the presence of zircons with high +εHf values. [Copyright &y& Elsevier]

Abstract: Several lines of isotopic evidence – the most direct of which is from Hadean Jack Hills zircons – suggest a very early history of crust formation on Earth that began by about 4.5Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial 176Hf/177Hf ratios and δ18O systematics for <4Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic 176Hf/177Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5Ga, Jack Hills zircons younger than ca. 3.6Ga are more radiogenic than –13ε (CHUR) at 3.4Ga in contrast to projected values at 3.4Ga of –20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu–Hf systematics together with a narrow range of mostly mantle-like δ18O values among the <3.6Ga zircons (in contrast to the spread towards sedimentary δ18O among Hadean samples) suggests a period of transition between 3.6 and 4Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution. [Copyright &y& Elsevier]

Provenance data from Paleoproterozoic and possible Archean sedimentary units in the central eastern Gawler Craton in southern Australia form part of a growing dataset suggesting that the Gawler Craton shares important basin formation and tectonic time lines with the adjacent Curnamona Province and the Isan Inlier in northern Australia. U-Pb dating of detrital zircons from the Eba Formation, previously mapped as the Paleoproterozoic Tarcoola Formation, yields exclusively Archean ages (ca 3300-2530 Ma), which are consistent with evolved whole-rock Nd and zircon Hf isotopic data. The absence of Paleoproterozoic detrital grains in a number of sequences (including the Eba Formation), despite the proximity of voluminous Paleoproterozoic rock units, suggests that the Eba Formation may be part of a Neoarchean or early Paleoproterozoic cover sequence derived from erosion of a multi-aged Archean source region. The ca 1715 Ma Labyrinth Formation, unconformably overlying the Eba Formation, shares similar depositional timing with other basin systems in the Gawler Craton and the adjacent Curnamona Province. Detrital zircon ages in the Labyrinth Formation range from Neoarchean to Paleoproterozoic, and are consistent with derivation from >1715 Ma components of the Gawler Craton. Zircon Hf and whole-rock Nd isotopic data also suggest a source region with a mixed crustal evolution (εNd -6 to -4.5), consistent with what is known about the Gawler Craton. Compared with the lower Willyama Supergroup in the adjacent Curnamona Province, the Labyrinth Formation has a source more obviously reconcilable with the Gawler Craton. Stratigraphically overlying the Eba and Labyrinth Formations is the 1656 Ma Tarcoola Formation. Zircon Hf and whole-rock Nd isotopic data indicate that the Tarcoola Formation was sourced from comparatively juvenile rocks (εNd -4.1 to + 0.5). The timing of Tarcoola Formation deposition is similar to the juvenile upper Willyama Supergroup, further strengthening the stratigraphic links between the Gawler and Curnamona domains. Additionally, the Tarcoola Formation is similar in age to extensive units in the Mt Isa and Georgetown regions in northern Australia, also shown to be isotopically juvenile. These juvenile sedimentary rocks contrast with the evolved underlying sequences and hint at the existence of a large-scale ca 1650 Ma juvenile basin system in eastern Proterozoic Australia. [ABSTRACT FROM AUTHOR]

Abstract: Discoveries of >4Ga old zircon grains in the northwest Yilgarn of Western Australia led to the conclusion that evolved crust formed on the Earth within the first few 100Ma after accretion. Little is known, however, about the fate of the first crust that shaped early Earth''s surface. Here we report combined solution and laser-ablation Lu–Hf–U–Pb isotope analyses of early Archean and Hadean detrital zircon grains from different rocks of the Narryer Gneiss Complex (NGC), Yilgarn Craton, Western Australia. The zircons show two distinct groups with separate evolutionary trends in their Hf isotopes. The majority of the zircon grains point to separation from a depleted mantle reservoir at ∼3.8–3.9Ga. The second Hf isotope trend implies reworking of older Hadean zircon grains. The major trend starting at 3.8–3.9Ga defined by the Hf isotopes corresponds to a Lu/Hf that is characteristic for felsic crust and consequently, the primary sources for these zircons presumably had a chemical composition characteristic of continental crust. Reworked Hadean crust appears to have evolved with a similar low Lu/Hf, such that the early crust was probably evolved with respect to Lu–Hf distributions. The co-variation of Hf isotopes vs. age in zircon grains from Mt. Narryer and Jack Hills zircon grains implies a similar crustal source for both sediments in a single, major crustal domain. Age spectra and associated Hf isotopes in the zircon grains strongly argue for ongoing magmatic reworking over hundreds of millions of years of the felsic crustal domain in which the zircon grains formed. Late-stage metamorphic zircon grains from the Meeberrie Gneiss unit yield a mean U–Pb age of 3294.5±3.2Ma with initial Hf isotopes that correspond to the evolutionary trend defined by older NGC zircon grains and overlap with other detrital zircon grains, proving their genetic relationship. This ‘Meeberrie event’ is interpret here as the last reworking event in the precursor domain before final deposition. The continuous magmatic activity in one crustal domain during the Archean is recorded by the U–Pb ages and Hf isotope systematics of zircon grains and implies reworking of existing crust. We suspect that the most likely driving force for such reworking of crustal material is ongoing crustal collision and subduction. A comparison of Hf isotope signatures of zircon grains from other Archean terranes shows that similar trends are recognised within all sampled Archean domains. This implies either a global trend in crustal growth and reworking, or a genetic connection of Archean terranes in close paleo-proximity to each other. Notably, the Archean Acasta gneiss (Canada) shows a similar reworking patterns to the Yilgarn Craton of Hadean samples implying either a common Hadean source or amalgamation at the Hadean–Archean transition. [Copyright &y& Elsevier]

Abstract: The trace element signature of Earth''s continental crust resembles that of arc lavas, but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra-subduction zone magmatism. We explore the role of coupled arc–back-arc accretionary processes in crust generation by considering the tectonic context of whole rock Nd isotope and zircon Hf–O isotope data from igneous rocks of the Australian Tasmanides (515–230 Ma), which is thought to have evolved by the repeated opening and closure of sediment-filled back-arc basins behind a long-lived subduction zone. The significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective. Granitic rocks in this area define striking secular ε Nd–ε Hf–δ18O trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity. These trends reveal that juvenile magmatic input was enhanced during extensional, back-arc rifting episodes that immediately followed crustal thickening, suggesting a relationship between slab rollback and continental growth. Interaction between juvenile magma and sedimentary units deposited during a preceding back-arc rifting cycle was integral to the formation of stable continental material. This highlights the importance of back-arc environments for both the generation and differentiation of continental crust. The juvenile component within the Tasmanide igneous rocks increased from the Cambrian to the Triassic, consistent with a diminished input from craton-derived metasedimentary material as the subduction zone migrated outboard of the Gondwana margin. Subduction zone retreat formed large tracts of new crust in eastern Australia at comparable rates to crust generation at modern island arcs, providing a mechanism for rapid continental growth at convergent margins. Using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation. [Copyright &y& Elsevier]

Rifting between eastern India and western Australia in the Early Cretaceous (∼140–133 Ma) resulted in the formation of the eastern Indian Ocean. Tholeiitic flood basalts erupted on the southwestern margin of Australia (Bunbury basalts at ∼132 and ∼123 Ma) and the eastern margin of India (Rajmahal Traps at ∼118 Ma) shortly after this time, while the Kerguelen Plateau began to form in the nascent ocean basin (∼119 Ma). We studied the Hf and Os isotopic systematics of the continental basalts in order to better understand the relationship between these volcanic events and the Kerguelen plume. The objective of this work was to determine the mantle sources of the continental tholeiites and the nature and quantity of continental contamination. Both the Hf and Os isotopic compositions of these basalts vary with indicators of continental contamination (SiO2, (Th/Ta)PM, (La/Nb)PM). There is no obvious correlation of γOs(T) with Sr or Nd isotopic ratios, but rough correlations are present between Os and Hf isotopes and 208Pb/204Pb; in contrast, #x03B5;Hf(T) shows strong correlations with all isotopic systems and indicators of continental crust assimilation. We demonstrate that the mantle sources of these continental tholeiites cannot originate purely from the depleted asthenosphere. An enriched mantle source is required for both of these basalt suites and the geochemical characteristics of this source are broadly consistent with those inferred to be present in the Kerguelen plume during Early Cretaceous time. The Os isotopic results suggest that at least some continental contamination is present in all samples from both the Bunbury basalts and the Rajmahal Traps. In the Rajmahal Traps, this contaminant appears to be derived from the continental crust and there is little to no evidence for involvement of subcontinental lithospheric mantle (SCLM). The Bunbury basalts have consistently unradiogenic Os isotopic compositions and high #x03B5;Hf(T) for a given #x03B5;Nd(T), characteristics ascribed to the SCLM. Previous investigations of these basalt suites have concentrated on major and trace elements and Sr, Nd and Pb isotopes; none of these results suggested the involvement of SCLM in the Bunbury basalts. Here, we emphasize that the study of Hf and Os isotopes provide additional, useful tools for deciphering mantle sources and continental contaminants involved in the petrogenesis of continental tholeiites. [Copyright &y& Elsevier]

• Processes that formed the earliest continental crust are hotly contested. • The oldest zircon-bearing rocks are 4.02 Ga, while the oldest zircons are 4.36 Ga. • We compare zircon trace-element geochemistry between these two samples. • Zircons from each location are chemically identical, indicating a similar process. • The oldest crust we know of was formed by a transition from shallow-to-deep melting. The composition and origin of the earliest continental crust is intensely debated. The understanding of its composition relies heavily on the oldest vestiges of continental crust, which includes detrital zircon grains that are up to 4.36 billion-years-old (Ga) and <4.03 Ga zircon-bearing rocks. However, the interpretation of these two sample suites has thus far remained largely separate. Here, we demonstrate that the trace-element compositions of magmatic zircons from the Acasta Gneiss Complex compare favorably with those of ancient detrital zircons from western Australia. We combine these new data with existing oxygen and hafnium isotope datasets to show that the petrological processes that formed the Acasta Gneiss Complex, namely partial melting of mafic crust at various depths, are an appropriate analogue for the formation of the Hadean crust parental to the Jack Hills zircon grains. We also suggest that a transition from shallow-, to deep-crustal melting occurred in the JH source region ca. 3.8 Ga. [ABSTRACT FROM AUTHOR]