Re-evaluation and extension of the Marine Isotope Stage 5 tephrostratigraphy of the Faroe Islands region: The cryptotephra record

PMA, SMD, WENA and NJGP are supported by NERC through the SMART project (NE/F020600/1, NE/F02116X/1, NE/F021445/1). The research leading to the results for the MIS 4 and 5a tephra horizons has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n° [259253]. PMA, SMD and NJGP acknowledge the support of the Climate Change Consortium of Wales (C3W). JB is funded by the Research Council of Norway through the INTERACT project (project no. 221999).Abstract Previous studies of marine sequences from the Faroe Islands region have identified a series of coarse-grained tephra horizons deposited during Marine Isotope Stage (MIS) 5. Here we reassess the MIS 5 tephrostratigraphy of the Faroe Islands region and focus on the cryptotephra deposits preserved within the fine-grained fraction of marine core LINK 16. We also extend the record to encompass the late MIS 6 and early MIS 4 periods. A density separation technique, commonly used for tephra investigations in lacustrine settings but rarely applied to marine sediments, is utilised to explore the fine-grained material and EPMA and LA-ICP-MS are employed to determine the major and trace element composition of individual tephra shards. In total, 3 basaltic and 3 rhyolitic Icelandic cryptotephra deposits with homogeneous geochemical compositions are identified — all of which have the potential to act as isochronous tie-lines. Geochemical results highlight that the Grímsvötn volcanic system of Iceland is the predominant source of the basaltic horizons and the Öraefajökull or Torfajökull systems are the likely sources of the rhyolitic deposits. Three of the horizons have been previously recognised in Faroe Islands region marine sequences, with two of these deposits traceable into a Norwegian Sea sequence. An early MIS 4 rhyolitic horizon is the most widespread deposit as it can be traced into the Norwegian Sea and to the south into a record from the Rockall Trough. Basaltic and rhyolitic horizons deposited during late MIS 6 have not been recognised in other sequences and represent new additions to the regional tephrostratigraphy.Publisher PDFPeer reviewe

Last Glacial Period Cryptotephra Deposits in an Eastern North Atlantic Marine Sequence: Exploring Linkages to the Greenland Ice-Cores

The establishment of a tephra framework for the Greenland ice-cores spanning the last glacial period, particularly between 25 and 45 ka b2k, provides strong potential for precisely correlating other palaeoclimatic records to these key archives. Tephra-based synchronisation allows the relative timing of past climatic changes recorded within different depositional environments and potential causal mechanisms to be assessed. Recent studies of North Atlantic marine records have demonstrated the potential of tracing cryptotephra horizons in these sequences and the development of protocols now allows a careful assessment of the isochronous nature of such horizons. Here we report on tephrochronological investigations of a marine sequence retrieved from the Goban Spur, Eastern North Atlantic, covering ?25–60 ka b2k. Density and magnetic separation techniques and an assessment of potential transport and depositional mechanisms have identified three previously unknown isochronous tephra horizons along with deposits of the widespread North Atlantic Ash Zone II and Faroe Marine Ash Zone III. Correlations between the new horizons and the Greenland ice-core tephra framework are explored and despite no tie-lines being identified the key roles that high-resolution climatostratigraphy and shard-specific trace element analysis can play within the assessment of correlations is demonstrated. The previously unknown horizons are new additions to the overall North Atlantic tephra framework for the last glacial period and could be key horizons for future correlations

The petrology and geochemistry of the Igaliko Dyke Swarm, South Greenland

2 volsAvailable from British Library Document Supply Centre- DSC:D76895 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Correlating weathered, microphenocryst-rich, intermediate tephra: An approach combining bulk and single shard analyses from the Lepu\ue9 Tephra, Chile and Argentina

Chemical correlation of intermediate tephra deposits using microanalytical data is problematic because (i) the phenocryst content of their component glass shards affects major and trace element analyses (ii) bulk chemistry can be affected by variations in mineral/lithic components across the fall-out, and (iii) weathering readily alters their composition. All of these problems affect the Lepu\ue9 Tephra, a prominent marker horizon extensively distributed across the Los Lagos Region of Chile and the Chile-Argentina frontier in north-western Patagonia, which was erupted from Volc\ue1n Michinmahuida at c. 11000 cal a BP. Weathering of terrestrial cover-bed deposits in this hyper-humid depositional environment leaves only a few occurrences of the tephra which contain fresh glass shards for microbeam analysis, but their highly phenocrystic nature makes data interpretation difficult. Equally, leaching of mobile elements during weathering causes considerable compositional changes across the fall-out region and is evident in bulk sample analyses. Elements such as the REE and Y, generally regarded as immobile, show marked mobility. Within the REE, the development of \u201cM-type\u201d tetrad effects and positive Ce-anomalies result from a combination of dissolution/leaching of the REE from the bulk sample and retention by co-precipitation of Ce4+ on Fe-oxyhydroxides in this high-rainfall, hyper-humid, oxic environment. Chemical correlation of the Lepu\ue9 Tephra is thus not straightforward. However, by careful consideration of the data for a limited range of elements, chemical correlation can be achieved using elements which (i) are incompatible in magmatic systems (and thus their ratios are unaffected by the presence of phenocrysts in single glass shard microbeam analysis) and (ii) are not mobilised in these weathering conditions. These elements are Zr, Hf, Nb, Ta and Th. Their ratios (i) allow for the comparison of single grain and bulk sample analyses, extending the geographic range over which data can be compared for the Lepu\ue9 Tephra, (ii) provide a robust chemical correlation of this weathered, intermediate tephra deposit, enabling correlation even where elements traditionally considered immobile (REE, Y, and U) have been significantly mobilised, and (iii) allow the Lepu\ue9 Tephra to be distinguished from other local tephra deposits. This combined analytical approach enables tephras that have been variably weathered to become useful marker beds over much wider geographical areas than previously feasible, thereby enhancing their tephrochronological application in Quaternary research

The origins of carbonatites and related rocks from the Gr\uf8nnedal-\ucdka Nepheline Syenite complex, South Greenland: C-O-Sr isotope evidence

The Gr\uf8nnedal-\ucdka ring complex (1299 \ub1 17 Ma) in the Gardar province, South Greenland is composed of a range of layered nepheline syenites which were intruded at a late stage by xenolithic syenite and a plug of carbonatite. The complex was subsequently intruded by a variety of basic dykes, including olivine dolentes, kersantites, vogesites, spessartites, camptonites and an aln\uf6ite, and then extensively faulted. The nepheline syenite magmas, produced by fractional crystallisation of basic magmas, show a range in \u3b413C (-3.86 to -7.57\u2030) and \u3b418O (8.27 to 15.12\u2030), distinctly different to the carbonatites which form a tight group with average \u3b413C= -4.31 \ub1 0.22 \u2030, (1 s.d.) and average \u3b418O= 7.18 \ub1 0.41\u2030(1 s.d.). Initial 87Sr/86Sr isotope ratios (typically 0.703) suggest the syenites and carbonatites have not assimilated crustal rocks, and therefore the C and O isotope variation within each group is a result of isotopic evolution during fractional crystallisation. A suite of lamprophyre dykes (\u3b413C -3.86 to -7.86\u2030 and \u3b418O 9.12 to 10.81\u2030) form a coherent group whose stable isotope compositions overlap part of the syenite field, and again are distinctly different from the carbonatites. A single aln\uf6ite has \u3b413C = -3.32\u2030 and \u3b418C= 12.34\u2030. C and O isotope ratios are consistent with origins of syenitic and lamprophyric magmas from a similar source. Despite geochemical evidence which suggests a genetic link between nepheline syenites and carbonatites, C and O isotopic evidence shows that they are not related directly by liquid immiscibility. Comparisons are made between similar rock types from Gr\uf8nnedal-\ucdka and from the Gardar Igaliko Dyke Swarm. The possible role of F in controlling \u3b413C and \u3b418O during crystallisation of calcite from carbonatite magmas is discussed

Trace-element microanalysis by LA-ICP-MS: The quest for comprehensive chemical characterisation of single, sub-10 \u3bcm volcanic glass shards

Recent developments in laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) have enabled improvements in spatial resolution and analytical detection limits. Here, the analysis of individual glass shards from tephra deposits using a 193 nm Excimer laser (producing ablation craters as small as 4 \u3bcm diameter), coupled to a magnetic sector ICP-MS, is described. Analyses of individual glass shards with crater diameters of 20 \u3bcm and 10 \u3bcm is essentially routine, and when element fractionation is corrected for, good accuracy is achieved. Analytical precision is good, being around \ub115-30% at 1 ppm and around \ub12-3% at 500 ppm from 10 \u3bcm diameter ablation craters, and lower limits of detection (LLD) are <1 ppm for most elements from 10 \u3bcm craters, when 25-28 trace elements are determined in a 3c20 s analysis. Neither 44Ca nor 43Ca can be used reliably as the internal standard for the analysis of rhyolites from 10 \u3bcm ablation craters, because CaO is close to the lower limit of quantitation (LLQ); thus 29Si must be used, although either could be used in the analysis of basaltic glasses at 10 \u3bcm. With analyses at 6 \u3bcm or 4 \u3bcm, many trace elements in rhyolites (e.g. Zr, Ba, LREE, Y, Rb, U, Th) remain above the LLQ, but at this resolution, only Si can be used as an internal standard for glass analysis. Element fractionation is an issue for all analyses <20 \u3bcm in diameter, resulting from the formation of a thin melt film on the ablation crater walls. This melt film becomes an increasingly larger proportion of the volume of ablated material as crater diameters become smaller, because the surface area/volume ratio increases. Element retention or volatility from this melt film appears to cause much of the fractionation. For larger craters (20 \u3bcm) this fractionation appears to affect all compositions similarly. For many elements determined from smaller craters (10 \u3bcm-4 \u3bcm) there is a systematic variation in the degree of fractionation with the glass (and thus melt film) composition. This relates to a change in the degree of polymerisation of the glass, with, for example, the REE being the most fractionated in polymerised rhyolitic samples at the smallest crater diameters (4 \u3bcm). This systematic behaviour, however, offers some hope for the analysis of a selection of abundant trace elements in individual shards of glass using ablation craters of 6 \u3bcm and 4 \u3bcm in diameter. \ua9 2011 Elsevier Ltd and INQUA

Age determination using feldspar: Evaluating fading-correction model performance

The recent introduction of post-IR IRSL measurement protocols has prompted a resurgence in luminescence applications using feldspar, some of which are affected by anomalous fading related signal loss. Many fading-corrected feldspar ages are reported in the literature, however few of those ages have been corrected using the model of Huntley (2006) [Huntley, D.J., 2006. An explanation of the power-law decay of luminescence. Journal of Physics: Condensed Matter 18(4), 1359\u20131365]. Here we present a new R function that calculates fading-corrected ages using the model of Huntley (2006), implemented with either a single-saturating exponential (1EXP) or general-order kinetic (GOK) fit. We evaluate the performance of the model through (i) contrasting measured and modelled field saturation values for a suite of 41 published saturated samples, and (ii) through using the model to fading-correct feldspar ages of samples with independent age control. Our results indicate that when implemented with 1EXP this model has an accuracy of 10% for predicting sample saturation, but that independent ages may be overestimated when the model is used to fading-correct samples across a range of timescales. In contrast, providing that the dose response curve has been characterised beyond 600 Gy, implementing the Huntley (2006) model with a GOK fit yields accurate age estimations. Modelled age overestimation for 1EXP is associated with dose response curve deviation from a single-saturating exponential. Finally we contrast the laboratory measured light levels of a suite of 50 saturated samples with their corresponding fading rates. We show that these saturated samples may yield De values below 2D0, and thus that 2D0 is not an effective screening criterion for sample saturation where no anomalous fading correction is made

Multiple melt bodies fed the AD 2011 eruption of Puyehue-Cord\uf3n Caulle, Chile

Within the volcanological community there is a growing awareness that many large- to small-scale, point-source eruptive events can be fed by multiple melt bodies rather than from a single magma reservoir. In this study, glass shard major- and trace-element compositions were determined from tephra systematically sampled from the outset of the Puyehue-Cord\uf3n Caulle (PCC) eruption ( 3c1 km3) in southern Chile which commenced on June 4th, 2011. Three distinct but cogenetic magma bodies were simultaneously tapped during the paroxysmal phase of this eruption. These are readily identified by clear compositional gaps in CaO, and by Sr/Zr and Sr/Y ratios, resulting from dominantly plagioclase extraction at slightly different pressures, with incompatible elements controlled by zircon crystallisation. Our results clearly demonstrate the utility of glass shard major- and trace-element data in defining the contribution of multiple magma bodies to an explosive eruption. The complex spatial association of the PCC fissure zone with the Liqui\uf1e-Ofqui Fault zone was likely an influential factor that impeded the ascent of the parent magma and allowed the formation of discrete melt bodies within the sub-volcanic system that continued to independently fractionate