Anders Scherstén

Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago.

Earth’s lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium–lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope–time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9–3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5–3.2 Gyr ago from an ancient (3.9–3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

Tungsten isotope evidence that mantle plumes contain no contribution from the Earth's core

Osmium isotope ratios provide important constraints on the sources of ocean-island basalts, but two very different models have been put forward to explain such data. One model interprets 187Os-enrichments in terms of a component of recycled oceanic crust within the source material. The other model infers that interaction of the mantle with the Earths outer core produces the isotope anomalies and, as a result of coupled 186Os–187Os anomalies, put time constraints on inner-core formation. Like osmium, tungsten is a siderophile (‘iron-loving’) element that preferentially partitioned into the Earths core during core formation but is also ‘incompatible’ during mantle melting (it preferentially enters the melt phase), which makes it further depleted in the mantle. Tungsten should therefore be a sensitive tracer of core contributions in the source of mantle melts. Here we present high-precision tungsten isotope data from the same set of Hawaiian rocks used to establish the previously interpreted 186Os–187Os anomalies and on selected South African rocks, which have also been proposed to contain a core contribution. None of the samples that we have analysed have a negative tungsten isotope value, as predicted from the core-contribution model. This rules out a simple core–mantle mixing scenario and suggests that the radiogenic osmium in ocean-island basalts can better be explained by the source of such basalts containing a component of recycled crust.

Zircon U-Pb and Hf - isotopes from the eastern part of the Sveconorwegian Orogen, SW Sweden : Implications for the growth of Fennoscandia

Abstract Current models for the growth of Fennoscandia, including the eastern part of the Sveconorwegian Province, are largely based on U–Pb data and do not discriminate between juvenile and reworked crust. Here we present new combined U–Pb and Hf isotopic data, from the Eastern Segment and the Idefjorden terrane of the Sveconorwegian Province, and suggest a revised model of crustal growth. Most of the crystalline basement in this part of the shield formed by mixing of a 2.1–1.9 Ga juvenile component and Archaean crust. Archaean reworking decreases between 1.9 and 1.7 Ga and a mixed Svecofennian crustal reservoir is generated. Succeeding magmatism between 1.7 and 1.4 Ga indicates reworking of this reservoir with little or no crust generation. At c. 1.2 Ga, an influx of juvenile magma is recorded by granite to quartz-syenite magmatism with mildly depleted (ϵHf 1.18 Ga of c. 3) signatures. The amount of recycled crust in the 1.9–1.7 Ga arc system is in contrast to previously proposed models for the growth of the southwestern part of the Fennoscandian Shield. This model agrees with long-term subduction along the western margin of Fennoscandia, but suggests substantial reworking of existing crust and decreasing amounts of <1.9 Ga crustal growth. Supplementary material: The analytical method, U–Pb SIMS table, U–Pb LA-SF-ICP-MS table and Lu–Hf table are available at www.geolsoc.org.uk/SUP18648

Magmatism during Gondwana break-up: new geochronological data from Westland, New Zealand

Newly determined Late Cretaceous 40Ar/39Ar ages on megacrystic kaersutite from four lamprophyre dikes, and a U–Pb zircon age on a trachyte, from central and north Westland (New Zealand) are presented. These ages suggest that the intrusion of mafic dikes (88–86 and 69 Ma) was not necessarily restricted to the previously established narrow age range of 80–92 Ma. The younger lamprophyre and trachyte dikes (c. 68–70 Ma) imply that tensional stresses in the Western Province were either renewed at this time, or that extension and related magmatism continued during opening of the Tasman Sea. Extension-related magmatism in the region not only preceded Tasman seafloor spreading initiation (starting at c. 83 Ma, lasting to c. 53 Ma), but may have sporadically continued for up to 15 Ma after continental break-up.

Ion microprobe discovery of Archaean and Early Proterozoic zircon xenocrysts in southwest Sweden

Abstract A single zircon grain dated by ion probe with a minimum age of 3432±30 Ma was found in a microgranite dyke, which cuts norite of 916±11 Ma Hakefjorden Complex at Älgön on the Swedish west coast north of Gothenburg. Zircons are a rare accessory mineral in this dyke. They could be classified by morphology as orthocrysts and xenocrysts. Data from four orthocrysts show that the dyke originated between 911±14 and 916±11 Ma, and was probably derived from the Hakefjorden Complex contact migmatite. Eight of the dated grains are xenocrysts, probably derived from the Stora Le-Marstrand Formation. They have 207Pb-206Pb minimum ages from c. 1451 to 3432±6 Ma, and the oldest grain has a probable age of 3457±10 Ma, derived from a discordia regression, with 445±38 Ma lower intercept reflecting Phanerozoic lead loss. This grain is the oldest yet dated in Scandinavia. Together with six >1750 Ma zircon grains in both this sample and a related Stora Le-Marstrand-derived sample, it casts doubt on the prevailing model of incremental crustal growth in Scandinavia, southwestwards from an Archaean core in the northeast of the Baltic Shield. These old ages, together with published Sm-Nd data, also contradict the proposed origin of the Stora Le-Marstrand Formation in an isolated oceanic island arc setting. It is more likely that it formed on the eastern (in present-day terms) margin of an Archaean continent, which did not join Baltica till the Sveconorwegian orogenic cycle.

Documentation of a hydrous ultramafic magma intrusion in the 1.62 Ga crust of southern Sweden

Abstract Field relationships and petrology of a mid Proterozoic (1624±6 Ma) hornblendite from southwestern Sweden demonstrate that it originated as a mafic-ultramafic hydrous melt. Lobes and veins of hornblendite penetrate the surrounding gneiss and small gneiss xenoliths lie in the main body of the hornblendite, evidence that it intruded with relatively low viscosity. The hornblendite has a composition compatible with its derivation from the mantle at 6 GPa and a geochemical subduction signature indicating a subduction zone origin. It is suggested that the hornblendite represents an intrusive equivalent to primitive lavas seen in Phanerozoic subduction zones that form by melting of metasomatised hydrous mantle.

187Re-187Os evidence for crustal lenses of Svecofennian age within the Mylonite Zone, SW Sweden

Abstract Re and Os concentrations and isotopic composition were analysed in four ultramafic cumulates from the Kedum tectonic lens within the Mylonite Zone, SW Sweden. An 1887±36 Ma isochron is obtained for six samples and interpreted to date the Kedum body. This is significantly older than the ∼1600–1700 Ma surrounding crust and the first rocks of Svecofennian age west of the Trans Scandinavian Igneous Belt. The initial gOs of 15.7±4.0 indicates contribution from a radiogenic source such as old crust.

Geochronology and geochemical evidence for a magmatic arc setting for the Ni-Cu mineralised 1.79 Ga Kleva gabbro–diorite intrusive complex, southeast Sweden

Magmatic Ni-Cu sulphide deposits are commonly associated with mafic rocks within the marginal areas of Archean or Proterozoic cratons. The Kleva Ni-Cu sulphide deposit in southeast Sweden is hosted by gabbro–diorite rocks and is sandwiched between two Palaeoproterozoic magmatic arcs: the Transscandinavian Igneous Belt (TIB) and the Oskarshamn–Jönköping Belt (OJB). Major and trace element data corroborate an arc origin for the Kleva intrusive complex. The variety of rocks and geochemical signatures suggest that it formed from several magmatic pulses. Despite the occurrence of supracrustal xenoliths, there is a lack of trace element evidence for significant assimilation. Mafic intrusions are abundant in southeast Sweden and are generally inferred to be synchronous with the TIB. Of these, Kleva is the only known Ni-Cu deposit, and it is unclear if it is associated with the OJB or TIB. Here, we present zircon and baddeleyite U-Pb dates of 1788 ± 4 Ma, 1788 ± 5 Ma and 1792 ± 3 Ma. We interpret a 1790 Ma date to be the best estimate of the crystallisation age of the Kleva intrusive complex, indicating that the emplacement of mafic magma is coeval with the voluminous arc-related TIB magmatism in the area.

An Early Ordovician 40Ar-39Ar age for the ∼50 km Carswell impact structure, Canada

The formation age of the large (∼50 km) Carswell impact structure, Canada, has been a matter of debate ever since its discovery five decades ago, with proposed ages ranging from Mesoproterozoic to Early Cretaceous. Here, we present new 40Ar-39Ar data for aliquots of euhedral adularia, separated from vesicles in an impact melt rock from the central uplift of the structure. The analyses of the adularia yielded a statistically robust Early Ordovician crystallization age of 481.5 ± 0.8 Ma (2σ, mean square of weighted deviates = 1.06, P = 0.30). The most plausible explanation for the formation of vesicle-filling adularia is through low-temperature mineral precipitation during residual hydrothermal circulation that followed the impact, as no other known major intrusive, extrusive, or thermal events have occurred in the Carswell region in the Phanerozoic. The new age of the Carswell impact structure overlaps within uncertainty with the most precise Ar-Ar ages proposed for the L-chondrite parent body breakup event, but not with the age of the stratigraphic sequence from which the meteorites and micrometeorites from this event were recovered. Thus, either the Carswell impact represents a separate, unrelated impact event, or the dynamic evolution of the L-chondrite parent body breakup is more complicated than presently understood, and Carswell represents one of the earliest and largest known impacts of this event on Earth. (Less)