There were no large volumes of felsic continental crust in the early Earth

New model growth curves for the continental crust based upon Hf-isotopes in zircon suggest that large volumes of felsic continental crust were present in the Hadean and early Archaean. These models sit uncomfortably with estimates of the volume of ancient crust preserved today and imply that the large volumes of crust that were created early in Earth history are now lost. However, this paper argues that there is no evidence from modern mantle geochemistry that very large volumes of early continental crust have been recycled into the mantle. In contrast significant volumes of Archaean crust may have been reworked into younger crust, although there is no evidence that this process took place in the early Archaean and Hadean. Geological evidence from the detrital zircon record does not show evidence for large volumes of very early felsic crust, rather, geochemical proxies for Eo-Archaean and Hadean crust strongly suggest that the earliest crusts on Earth, some of which may have been subaerial, were mafic. A lack of very early felsic crust on Earth calls for a re-evaluation of current crustal growth curves and geodynamic models for the start of plate tectonics, the role of supercontinents in early continent formation and the role of the subcontinental lithosphere in continent preservation. The earliest felsic rocks on Earth may have taken the form of oceanic plagiogranites or ocean-island potassic granites as found in the modern.University of Derby URS

The Archcan Komatiite-Related Inyala Chromitite, Southern Zimbabwe

Abstract The chromitites at the Invala mine, Zimbabwe, form part of a layered intrusion enclosed •vithin the Archcan granulite facies gneisses o(the north marginal zone (NMZ) of the Limpopo belt. Chromitites occur within an olivine-orthopyroxene-ehromite eumulate sequence which was derived from a komatiitie magma. An electron microprobe study and a comparison with the mineralogy of unaltered komatiites from the Belingwe greenstone belt, Zimbabwe, strongly suggest that some of the original olivine, chromite, and orthopyroxene mineral compositions are preserved. Olivine-ehromite mineral pairs continued to equilibrate doxvntemperature until about 680øC. Calculations based on the diffusion of Fe and Mg in olivine suggest that the cooling was rapid (several 1,000øC/Ma), hence it is reasonable to expect igneous mineral compositions to be preserved. Calculations based on mineral-melt, Fe-Mg partitioning for olivine and orthopyroxene indicate that the parent komatiitie liquids varied in composition from 26 to 16 vet percent MgO. Thus some of the ehromites crystallized from melts which are among the most magnesian komatiites known and which would have had an eruption temperature of ca. 1,520øC. Orthopyroxenite eumulates, which are unusual in komatiitie sequences probably formed in response to assimilation of adjacent supracrustal rocks, possibly banded iron-formation. Oxygen activity calculations suggest that assimilation may have led to an increase in the oxygen activity in the melt which may be responsible for the crystallization of both chromite and orthopyroxene

The eastern French Pyrenees: from mountain belt to foreland basin

The Pyrenees is a young mountain belt formed as part of the larger Alpine collision zone. This excursion explores the development of the Pyrenean Mountain Belt in southern France, from its early extensional phase in the mid‐Cretaceous and subsequent collisional phase, through its uplift and erosion in the Late Cretaceous and again in the Eocene, which led to the development of the Aquitaine‐Languedoc foreland basin. One of the complexities of the Pyrenean Belt is that thrusting, uplift and erosion during the Pyrenean orogeny exposed older Variscan basement rocks in the central core of the mountains, rocks which were metamorphosed during an earlier event in the late Carboniferous. Thus, this orogenic belt also tells the story of an earlier collision between Laurussia in the north and Gondwana in the south at c. 300 Ma, prior to the onset of the Pyrenean events at c. 100 Ma. Here we seek to unravel these two separate orogenic stories.N/

Archaean crustal evolution in West Africa: A new synthesis of the Archaean geology in Sierra Leone, Liberia, Guinea and Ivory Coast

A new synthesis of the geology and geochronology of the little-known Archaean rocks in Sierra Leone, Liberia, Guinea and Ivory Coast is presented in order to better understand the processes of Archaean crustal evolution in this region, and to attempt to interpret these data in the light of our current understanding of Archaean crustal evolution. In addition, this study seeks to identify those aspects of Archaean crustal evolution which are currently not known in this area and which need to become the subject of future studies, given the economic importance of this region in terms of the mineral deposits hosted in the Archaean rocks. These include greenstone-belt hosted iron ore, lode gold, chromite and columbite-tantalite and younger diamondiferous kimberlites intrusive into Archaean felsic gneisses. The new results show that this cratonic nucleus comprises of four main geological units: 1. The oldest crust is made up of 3.5-3.6 Ga TTG (tonalite-trondjemite-granodiorite) gneisses. These only outcrop in the east of the craton in Guinea but their presence is indicated elsewhere in the central part of the craton though xenocrystic zircon cores in younger rocks. 2. The major rock type found throughout the craton is 3.26-2.85 Ga TTG gneiss. In detail these magmas are thought to have formed in two episodes one between 3.05-3.26 Ga and the other between 2.85-2.96 Ga. The presence of inherited zircons in the younger suite indicate that this event represents the partial reworking of the older gneisses. 3.4 Ga eclogite xenoliths in kimberlite derived from the sub-continental lithospheric mantle are thought to be the restite after the partial melting of a basaltic protolith in the production of the TTG magmas. 3. Supracrustal rocks form linear belts infolded into the TTG gneisses and metamorphosed to amphibolite and granulite grade. They are of different sizes, contain a variety of lithological sequences and may be of several different ages. The larger supracrustal belts in Sierra Leone contain a thick basalt-komatiite sequence derived by the partial melting of two different mantle sources, unconformably overlain by a sedimentary formation. They are seen as an important resource for gold, iron-ore, chromite and columbite-tantalite. 4. A suite of late Archaean granitoids formed by the partial melting of the TTG gneisses in a craton wide deformation-metamorphic-partial melting event at 2800 +/- 20 Ma. This thermal event is thought to be responsible for the stabilisation of the craton. This new synthesis highlights major geological and geochronological similarities between the Archaean rocks of Sierra Leone, Liberia, Guinea and Ivory Coast and those in the Reguibat Shield in the northern part of the West African Craton suggesting that the two regions were once more closely related.UoD URS

Archaean chromitites show constant Fe 3+ /ΣFe in Earth's asthenospheric mantle since 3.8 Ga

Theoretical and planetary studies show that the Earth’s upper mantle is more oxidised than it should be. The mechanism by which this took place and the timing of the oxidation is contested. Here we present new Mössbauer spectroscopy measurements of the ionic ratio Fe3+/(Fe3++Fe2+) in the mineral chromite hosted in mantle-derived melts to show that there is no change in mantle Fe3+/(Fe3++Fe2+) ratio before and after the oxidation of the Earth’s atmosphere at ca. 2.4 Ga and over Earth history from 3.8 Ga to 95 Ma. Our finding supports the view that the oxidation of the asthenospheric mantle was very early and that the oxygenation of the Earth’s atmosphere was not directly coupled to mantle processes.Uo

Lymnaea schirazensis, an Overlooked Snail Distorting Fascioliasis Data: Genotype, Phenotype, Ecology, Worldwide Spread, Susceptibility, Applicability

BACKGROUND: Lymnaeid snails transmit medical and veterinary important trematodiases, mainly fascioliasis. Vector specificity of fasciolid parasites defines disease distribution and characteristics. Different lymnaeid species appear linked to different transmission and epidemiological patterns. Pronounced susceptibility differences to absolute resistance have been described among lymnaeid populations. When assessing disease characteristics in different endemic areas, unexpected results were obtained in studies on lymnaeid susceptibility to Fasciola. We undertook studies to understand this disease transmission heterogeneity. METHODOLOGY/PRINCIPAL FINDINGS: A ten-year study in Iran, Egypt, Spain, the Dominican Republic, Mexico, Venezuela, Ecuador and Peru, demonstrated that such heterogeneity is not due to susceptibility differences, but to a hitherto overlooked cryptic species, Lymnaea schirazensis, confused with the main vector Galba truncatula and/or other Galba/Fossaria vectors. Nuclear rDNA and mtDNA sequences and phylogenetic reconstruction highlighted an old evolutionary divergence from other Galba/Fossaria species, and a low intraspecific variability suggesting a recent spread from one geographical source. Morphometry, anatomy and egg cluster analyses allowed for phenotypic differentiation. Selfing, egg laying, and habitat characteristics indicated a migration capacity by passive transport. Studies showed that it is not a vector species (n = 8572 field collected, 20 populations): snail finding and penetration by F. hepatica miracidium occur but never lead to cercarial production (n = 338 experimentally infected). CONCLUSIONS/SIGNIFICANCE: This species has been distorting fasciolid specificity/susceptibility and fascioliasis geographical distribution data. Hence, a large body of literature on G. truncatula should be revised. Its existence has henceforth to be considered in research. Genetic data on livestock, archeology and history along the 10,000-year post-domestication period explain its wide spread from the Neolithic Fertile Crescent. It is an efficient biomarker for the follow-up of livestock movements, a crucial aspect in fascioliasis emergence. It offers an outstanding laboratory model for genetic studies on susceptibility/resistance in F. hepatica/lymnaeid interaction, a field of applied research with disease control perspectives

Geochemical Studies on the Scourian Complex, N. W. Scotland

The gabbro-tonalite-trondhjemite-granodiorite-granite suite of the Scourian complex of northwest Scotland was studied at Scourie, Gruinard Bay and Torridon and represents highly deformed and metamorphosed plutonic igneous rocks, whose formation ca. 2900 Ma. ago represents the generation of new continental crust. Amphibolites, found predominantly in amphibolite facies gneisses at Gruinard Bay may be metavolcanic and are chemically similar to present-day island arc tholeiites; they are complementary to the layered cumulitic gabbro-anorthosite complexes in granulite facies gneisses and together they represent the remnants of an early basic volcanic-plutonic suite. Tonalite was generated by the partial melting of amphibolite, similar in chemistry to that found at Gruinard Bay. Trondhjemite, granodiorite and granite evolved from tonalite by the fractional crystallisation of plagioclase and hornblende at depths of less than 60 km. Granulite facies metamorphism at Scourie depleted originally wet tonalitic and trondhjemitic magmas in LIL elements but not potassium. Relatively dry granitic magmas were not depleted during granulite facies metamorphism. There is a change in the principal magmatype from Scourie in the north (tonalite) through Gruinard Bay (trondhjemite) to Torridon in the south (granodiorite) reflecting a southerly increase in K20 across the complex. Granulite facies grieisses at Scourie show a continuum of rock types between ultramafic and granitic, but amphibolite facies gneisses show a distinct bimodality. Igneous temperatures of 1000°C to 1150°C were obtained from feldspar and ilmenite-magnetite thermometry. The peak of the granulite facies metamorphism was at 820°C and 10 kb. and was determined using plagioclase-scapolite and garnet-pyroxene equilibria. Many mineral pairs equilibrated at lower temperatures and reflect the cooling history of the complex. Water was introduced into granite sheets during cooling at about 600°C. The Archaean crust at Scourie was originally 60 km thick and the thermal history of the area suggests that the crust was magmatically thickened. The plutonic igneous and metamorphic history of these rocks invite comparison with modern Cordilleran batholiths and suggest that the Scourian complex formed in an analogous tectonic setting

Comment on ‘Podiform chromitites do form beneath mid-ocean ridges’ by Arai, S. and Miura, M.

N