Micro-zircon: origin and evolution during metamorphism

The size and distribution of zircon within a garnet-mica-schist from the Scottish Highlands were assessed using scanning electron microscopy. The study reveals that abundant 0.2–3.0 μm sized zircon is preferentially concentrated within garnet and biotite porphyroblasts. The micro-zircon has grown during regional metamorphism and represents >90% of the total number of zircon in the schist. It is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution, and more rarely, small metamorphic outgrowths. The sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified. Zircon is reactive and soluble, and responds to medium-grade metamorphism in a series of dissolution and crystallization events, linked to possible changes in fluid composition. Deformation also has a significant influence on the distribution of zircon, allowing inclusions previously trapped within biotite to react. About 8 × 10⁶ micro-zircon occur as inclusions within a typical individual 5-mm garnet porphyroblast and their presence must be considered prior to trace-element or isotopic analysis of such metamorphic phases.     

Systematic palaeontology and biostratigraphy of two Early Cretaceous condensed sections from the Barents Sea

Bivalve, brachiopod and cirripede faunas from the latest Jurassic and Early Cretaceous Barents Sea boreholes 7320/3-U-l and 7425/9-U-l are systematically described and illustrated. Microfossils have also been studied and the cores arc dated on the basis of the fossil recovery. The bivalve Buchia whose zonal sequence has been used for correlation of boreal marine sections is the most important biostratigraphic marker group in the condensed Boreal Berriasian-Hauterivian intervals of these cores. A new species of cirripede Zeugmatolepas? borealis Collins sp. nov. and dinoflagellate cyst Muderongia aequicornus Århus sp. nov. are described.
The Late Jurassic fine-grained elastics of core 7320/3-U-l are overlain by about 3 m of grey dolomitic limestone of Valanginian and Hauterivian age. The lowermost part of 7425/9-U-l is represented by a latest Volgian-earliest Berriasian fossiliferous greyish green marl. It is followed by a reddish brown fossilifcrous claystone of Berriasian and perhaps partly Valanginian age. Core 7425/9-U-l also contains a mainly Valanginian greyish green marly limestone which changes into a dark grey to black limestone of Early Barremian age in its upper part. The sedimentological change from condensation to dark grey clay deposition took place in the middle Barremian H. rude-fissicostatum ammonite Zone in 7425/9-U-l and probably slightly earlier in 7320/3-U-l. This was commenced at about the same time as deposition of the inaccurately and only indirectly dated fluvio-deltaic Festningen Sandstone Member on Spitsbergen. The dark claystone may thus be a distal equivalent to this sandstone unit.     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

Implications of garnet resorption for the Lu–Hf garnet geochronometer: an example from the contact aureole of the Makhavinekh Lake Pluton,Labrador

In the contact aureole of the Makhavinekh Lake Pluton (MLP), Labrador, garnet resorption caused redistribution of Lu and loss of Hf, creating spuriously young Lu–Hf garnet ages. Garnet grew during granulite facies regional metamorphism at 1860–1850 Ma. At 1322 Ma, garnet rims were replaced by coronas of cordierite and orthopyroxene during contact metamorphism. Garnet–ilmenite Lu–Hf geochronology using bulk‐garnet separates yields apparent ages that young from 1876 ± 21 Ma at 4025 m from the contact to 1396 ± 8 Ma at 450 m from the contact. Toward the contact, garnet crystals are progressively more resorbed. Concentrations of Lu measured by LA‐ICP‐MS along radial traverses on central sections through relict garnet decrease gently away from the cores but rise steeply within 50–200 μm of the edges of the relicts. Enrichments of Lu in rims of relict garnet demonstrates strong partitioning of Lu into garnet during resorption and modest intracrystalline diffusion. Hafnium distributions could not be measured, but considering the strong incompatibility of Hf with garnet, it is likely that nearly all Hf in resorbed portions of the garnet was lost from the crystals. Lu–Hf ages in the aureole are thus controlled predominantly by this retention of Lu and loss of Hf during garnet resorption. This deduction was tested with a simple numerical model in which the partial retention of Lu and loss of Hf is tracked as a population of garnet is resorbed. Assuming a spherical geometry for garnet porphyroblasts, Rayleigh fractionation is used to approximate initial Lu zoning profiles ranging from flat to steeply decreasing toward garnet rims. The model simulates: (i) Lu–Hf decay for a specified period before resorption; (ii) instantaneous resorption with retention of Lu and loss of Hf from the resorbed portion of the crystal and (iii) Lu–Hf decay during a specified period after resorption. Several parameters influence the modelled age, but garnet resorption and Lu retention are the primary factors. When all other parameters are held constant, larger amounts of resorption and higher degrees of Lu retention produce younger apparent ages (false ages). Similarly, flatter initial Lu profiles yield younger apparent ages as a consequence of the larger proportion of Lu and Hf that resides in the outer portions of the porphyroblast. The difference between the apparent and actual ages is greater if the duration of the pre‐resorption decay period is large relative to the post‐resorption decay period. Larger crystals in a Gaussian crystal‐size distribution (CSD) generally dominate the Lu–Hf budget and produce an older apparent age relative to the age of the mean crystal size. Compared to a symmetrical Gaussian CSD, positively skewed CSDs result in reduced resorption of large crystals and produce an older apparent age. Application of the model to the MLP aureole, positing growth at 1850 Ma and resorption at 1320 Ma, yields model ages that young from 1850 to 1374 Ma toward the contact, in good agreement with the apparent ages determined from geochronology.     

The Eemian and Weichselian stratigraphy of the Langelandselv area, Jameson Land, East Greenland

Coastal Jameson Land is characterized by thick Quaternary deposits from the last interglacial/glacial cycle. The successions at the mouth of Langelandselv exhibit a key stratigraphy where sediments from the Langelandselv interglaciation (Eemian) are overlain by three till units interbedded with glacimarine and deltaic interstadial successions. Immediately after the retreat of glaciers after the extensive Scoresby Sund glaciation (Saalian). advection of warm Atlantic surface water surpassed what is known from the Holocene. The two lowermost Weichselian tills, deposited during the Aucellaelv and Jyllandselv stades (Early Weichselian), reflect short-lasting readvances of fjord glaciers. Luminescence dates and correlation with adjacent areas suggest ages of 110–80 ka and 70–60 ka for the Hugin Sø and the Møselv interstades, respectively.     

Context and contingency: the coffee crisis for conventional small-scale coffee farmers in Brazil

Brazil is the largest economy in Latin America and the world's leading producer and exporter of coffee. Smallholders are the predominant producers of Brazilian coffee and have been significantly impacted by post-ISI (import substitution industrialisation) neoliberal reforms that swept the nation's agricultural sector beginning in the 1980s. These reforms stimulated increased coffee production while simultaneously diminishing state interventions, including agricultural subsidies, farmer credits, research and development programmes, extension services, and rural development initiatives. These changes were closely followed by major restructuring of the global coffee market and the elimination of international coffee export quotas. Our research joins a growing body of recent literature exploring how changes related to market liberalisation and globalisation affect localities and regions where producers supply coffee for conventional markets. Through a case study, we describe the impacts of neoliberal globalisation on the lives, livelihoods, and environment of small-scale coffee producers in Minas Gerais, Brazil. We explore how neoliberal reform, reflected in the specific historical, geographical, and environmental context of Brazilian coffee production, has resulted in the increased marginalisation of small-scale farmers, the degradation of soils, and overall rural decline.     

Orthopyroxene-Corundum in Mg-Al-rich Granulites from the Oygarden Islands, East Antarctica

High-Mg–Al, silica-undersaturated metapelites from theOygarden Group of islands, East Antarctica, preserve clear evidencefor the stable coexistence of the assemblage orthopyroxene +corundum in natural rocks. The quartz-absent metapelite occursas pods and isolated layers within a high-strain zone relatedto deformation during the c. 0·93 Ga Rayner StructuralEpisode. Assemblages that include orthopyroxene, corundum, sapphirine,sillimanite, cordierite, garnet and kornerupine are developedacross a pre-existing compositional zoning, leading to contrastingmineral Fe–Mg ratios. The assemblage orthopyroxene–corundumis shown to exist in only a very restricted range of bulk compositionsand P–T histories. Simplified qualitative FMAS grids havebeen constructed for kornerupine-absent and -present systems,illustrating MAS terminations and divariant equilibria thathelp to describe the mineral assemblage and reaction history.Reaction textures that include coronas of sapphirine and sillimaniteseparating orthopyroxene and corundum, and symplectites of orthopyroxene+ sapphirine ± cordierite/plagioclase, orthopyroxene+ sillimanite ± cordierite/plagioclase and orthopyroxene+ sapphirine + sillimanite embaying garnet, imply a clockwiseP–T–t evolution. Conditions of P > 9–10kbar and T     

Chronology of deglaciation based on 10Be dates of glacial erosional features in the Grimsel Pass region, central Swiss Alps

Surface exposure dating, using in situ produced cosmogenic ¹⁰Be, is applied to determine the time since deglaciation of bedrock surfaces in the Grimsel Pass region. Nine ¹⁰Be dates from bedrock surfaces corrected for cover by snow are minimum ages for deglaciation of the pass. Four ¹⁰Be dates from surfaces below 2500 meters above sea level (m a.s.l.) on Nägelisgrätli, east of Grimsel Pass, yield ages that range from about 14 000 to 11 300 years. Three ¹⁰Be dates from locations above 2600 m a.s.l. on Nägelisgrätli are between about 11 700 and 10 400 years. Two ¹⁰Be dates from locations at 2560 m a.s.l. below Juchlistock are about 12 100 and 11 000 years. The geographical distribution of ¹⁰Be dates on Nägelisgrätli either may show the timing of progressive deglaciation of Grimsel Pass or may reflect differences in subglacial erosion of bedrock in the pass region. All dates are discussed in the context of deglaciation of the late Würmian Alpine ice cap and deglaciation from Last Glacial Maximum (LGM) ice extents in other regions.