Seawater signatures in the supracrustal Lewisian Complex, Scotland

J. Bowie, C. Brolly, J. Armstrong and J. Johnston provided skilled technical support. Electron microscopy was performed with the help of J. Still in the ACEMAC Facility at the University of Aberdeen. Scapolite was analysed on a sample from the Hunterian Museum, Glasgow (no. 134720), loaned courtesy of J. Faithfull. The work was supported in part by UK Natural Environment Research Council grant NE/M010953/1. Careful review by A.A. Cabral helped to improve the manuscript.Peer reviewedPublisher PD

Seawater signatures in the supracrustal Lewisian Complex, Scotland

Marble in the supracrustal rocks of the Lewisian Complex, Tiree, includes chlorine-bearing amphiboles, chlorine-rich apatite, sulphur-rich scapolite, albite and phlogopite, all of which are regarded as evidence for evaporites in other metamorphosed sequences. Titanite yields U-Pb ages of ∼1.6 Ga, i.e. late Laxfordian, which excludes a younger imprint of sodium metasomatism. Traces of anhydrite, and isotopically heavy pyrite, also indicate deposition from seawater. Elsewhere in the Hebrides, tourmaline in Lewisian Complex marbles may represent seafloor exhalative deposits. Combined, the evidence suggests Lewisian Complex supracrustal marbles formed in an evaporative environment, like other Palaeoproterozoic successions across the North Atlantic region

Formation of Archean continental crust constrained by boron isotopes

The continental crust grew and matured compositionally during the Palaeo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11B/10B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicate differentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The B- and 11B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga

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 temperature1, 2. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time3, 4, 5, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon3, 5. 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 dynamics6. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago7, 8, 9, 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 orogens10, 11, 12. 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