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Supplementary files for Gernon et al. (2023) Kimberlite ascent by rift-driven disruption of cratonic mantle keels
No full textSupplementary files for Gernon et al. (2023) Kimberlite ascent by rift-driven disruption of cratonic mantle keels
This directory contains datasets, GIS files, Uninet model files and R scripts used to generate the figures in Gernon et al. (2023) Kimberlite ascent by rift-driven disruption of cratonic mantle keels.</span
Segregation of particles in a tapered fluidized bed
No full textTapered fluidized beds are widely used in industrial operations to fluidize a wide range of particle sizes, and are thought to induce relatively strong particle mixing. Like their straight-sided counterparts, tapered fluidized beds are often considered as a homogeneous emulsion phase through which bubbles propagate. However, it has been shown that gas flow through the dense-phase of tapered beds is heterogeneous, generating a central fluidized core and unfluidized peripheral regions. When polydisperse particle mixtures are fluidized in a tapered bed, the structure becomes much more complex once the particles reach the minimum fluidization velocity (Umf), and a variety of segregation structures are generated. The aim of this study is to investigate how the two different types of structure (i.e. flow structure owing to the tapered shape of the bed and the segregation structures) interact and affect each other. Experiments were performed in a tapered (? = 15°) planar bed using bidisperse mixtures of ballotini. The growth and extent of flow and segregation structures were measured, as well as the fabrics observed under different conditions. Under most conditions, the structure of the tapered bed is unaffected by the bed composition and segregation structures that form. An exception is at flow-rates just in excess of Umf when vertical columns of particles form, completely displacing larger-scale flow structures.The time scale for particle turnover in the central fluidized region is much shorter than that of particles captured within the peripheral regions. However, at sufficiently high gas flow-rates, uniform mixing can take place across the entire width of the bed
Eruptive history of an alkali basaltic diatreme from Elie Ness, Fife, Scotland
No full textThe Elie Ness diatreme (Fife, Scotland) is an ideal place to study the internal architecture and emplacement processes of diatremes. Elie Ness is one of approximately 100 alkali basaltic diatremes and intrusions in the East Fife area, emplaced during Upper Carboniferous to Early Permian times into an extensive rift system in the northern Variscan foreland. Within the diatreme, seven lithofacies and three lithofacies associations (LFAs 1-3) are recognised. Field, petrographic and geochemical studies demonstrate that the diatreme experienced a protracted history of eruption and infill, initially driven by volatile expansion and later by magma-water interaction. Massive lapilli tuffs of LFA 1 contain abundant highly vesicular juvenile scoria and magma-coated clasts, which are best explained by a magmatic origin for the early explosive eruptions. On a large-scale, the tuffs are well mixed and locally exhibit small-scale degassing structures attributed to fluidisation processes occurring within the diatreme fill. The occurrence of abundant volcaniclastic autoliths and megablocks within LFA 1 can be explained by subsidence of volcaniclastic strata from the maar crater and upper diatreme during emplacement. Pyroclastic density current deposits of LFA 2 form a series of continuous sheets across the diatreme, some of which may have originated from phreatomagmatic explosions in a neighbouring vent. We attribute the overall bedding pattern to a combination of primary volcanic processes and post-depositional folding related to movement along an adjacent fault. Minor steeply inclined breccias and tuffs of LFA 3 cross-cut the LFA 2 succession and are interpreted as late-stage volcaniclastic dykes and conduits, signalling the final phase of eruptive activity at Elie Ness. The study offers new insights into the volcanic evolution of diatremes fed by low viscosity, alkali-rich magmas
The role of gas-fluidisation in the formation of massive volcaniclastic kimberlite
No full textMassive volcaniclastic kimberlite (MVK) deposits are a volumetrically important constituent in most kimberlite diatremes. MVKs are typically characterised by their uniform texture and homogeneous composition, and previous studies suggest that they exhibit little appreciable internal variation. However, new field results from the Venetia K1 diatreme (South Africa) show that MVK displays significant changes over metre-scales. The textural characteristics and large- and small-scale structure of the deposits are described and interpreted as being due to fluidisation. Laboratory experiments of tapered fluidised beds demonstrate how they are heterogeneous, with fluidisation and mixing limited to the centre of the bed. Marginal wedge-shaped regions remain unfluidised. The unfluidised regions are internally laminated and slip downwards when a high proportion of the bed is fluidised. The observations also demonstrate how fluctuations in gas velocity can produce steep internal boundaries between laminated and well-mixed regions. Data consistent with gas-fluidisation in MVK deposits includes the massive structure, uniform distribution of lithic clasts, presence of steep internal contacts, dominantly sub-vertical crystal fabrics across the vent, occurrence of surface-derived material along the tapered diatreme walls, and the presence of degassing structures. We propose that the overall structure of the Venetia K1 diatreme-fill is consistent with a multi-stage fluidised emplacement, which involved (1) strong initial fluidisation that caused thorough mixing of juvenile and lithic material; (2) a gradual step-wise decrease in gas flux, that produced nested pipe-like sequences; and (3) migration of the narrow fluidised column towards the steep eastern diatreme wall. The occurrence of (4) degassing structures in MVK deposits across the vent is consistent with an overall rapid transition from high to low degrees of fluidisation towards the end of the eruption.Massive volcaniclastic kimberlite (MVK) deposits are a volumetrically important constituent in most kimberlite diatremes. MVKs are typically characterised by their uniform texture and homogeneous composition, and previous studies suggest that they exhibit little appreciable internal variation. However, new field results from the Venetia K1 diatreme (South Africa) show that MVK displays significant changes over metre-scales. The textural characteristics and large- and small-scale structure of the deposits are described and interpreted as being due to fluidisation. Laboratory experiments of tapered fluidised beds demonstrate how they are heterogeneous, with fluidisation and mixing limited to the centre of the bed. Marginal wedge-shaped regions remain unfluidised. The unfluidised regions are internally laminated and slip downwards when a high proportion of the bed is fluidised. The observations also demonstrate how fluctuations in gas velocity can produce steep internal boundaries between laminated and well-mixed regions. Data consistent with gas-fluidisation in MVK deposits includes the massive structure, uniform distribution of lithic clasts, presence of steep internal contacts, dominantly sub-vertical crystal fabrics across the vent, occurrence of surface-derived material along the tapered diatreme walls, and the presence of degassing structures. We propose that the overall structure of the Venetia K1 diatreme-fill is consistent with a multi-stage fluidised emplacement, which involved (1) strong initial fluidisation that caused thorough mixing of juvenile and lithic material; (2) a gradual step-wise decrease in gas flux, that produced nested pipe-like sequences; and (3) migration of the narrow fluidised column towards the steep eastern diatreme wall. The occurrence of (4) degassing structures in MVK deposits across the vent is consistent with an overall rapid transition from high to low degrees of fluidisation towards the end of the eruption
Geology of the Snap Lake kimberlite intrusion, Northwest Territories, Canada: field observations and their interpretation
Full text linkThe Cambrian (523 Ma) Snap Lake hypabyssal kimberlite intrusion, Northwest Territories, Canada, is a complex segmented diamond-bearing ore-body. Detailed geological investigations suggest that the kimberlite is a multi-phase intrusion with at least four magmatic lithofacies. In particular, olivine-rich (ORK) and olivine-poor (OPK) varieties of hypabyssal kimberlite have been identified. Key observations are that the olivine-rich lithofacieshas a strong tendency to be located where the intrusion is thickest and that there is a good correlation between intrusion thickness, olivine crystal size and crystal content. Heterogeneities in the lithofacies are attributed to variations in intrusion thickness and structural complexities. The geometry and distribution of lithofacies points to magmaticco-intrusion, and flow segregation driven by fundamental rheological differences between the two phases. We envisage that the low-viscosity OPK magma acted as a lubricant for the highly viscous ORK magma. The presenceof such low-viscosity, crystal-poor magmas may explain how crystal-laden kimberlite magmas (>60 vol.%) are able to reach the surface during kimberlite eruptions. We also document the absence of crystal settling and the development of an unusual subvertical fabric of elongate olivine crystals, which are explained by rapid degassing-induced quench crystallization of the magmas during and after intrusio
The thermal regime around buried submarine high voltage cables
Full text linkThe expansion of offshore renewable energy infrastructure and the need for trans-continental shelf power transmission require the use of submarine High Voltage (HV) cables. These cables have maximum operating surface temperatures of up to 70°C and are typically buried 1–2 m beneath the seabed, within the wide range of substrates found on the continental shelf. However, the heat flow pattern and potential effects on the sedimentary environments around such anomalously high heat sources in the near surface sediments are poorly understood. We present temperature measurements from a 2D laboratory experiment representing a buried submarine HV cable, and identify the thermal regimes generated within typical unconsolidated shelf sediments—coarse silt, fine sand and very coarse sand. We used a large (2 × 2.5 m) tank filled with water-saturated spherical glass beads (ballotini) and instrumented with a buried heat source and 120 thermocouples, to measure the time-dependent 2D temperature distributions. The observed and corresponding Finite Element Method (FEM) simulations of the steady state heat flow regimes, and normalised radial temperature distributions were assessed. Our results show that the heat transfer and thus temperature fields generated from submarine HV cables buried within a range of sediments are highly variable. Coarse silts are shown to be purely conductive, producing temperature increases of >10°C up to 40 cm from the source of 60°C above ambient; fine sands demonstrate a transition from conductive to convective heat transfer between c. 20°C and 36°C above ambient, with >10°C heat increases occurring over a metre from the source of 55°C above ambient; and very coarse sands exhibit dominantly convective heat transfer even at very low (c. 7°C) operating temperatures and reaching temperatures of up to 18°C above ambient at a metre from the source at surface temperatures of only 18°C. These findings are important for the surrounding near surface environments experiencing such high temperatures and may have significant implications for chemical and physical processes operating at the grain and sub-grain scale; biological activity at both micro-faunal and macro-faunal levels; and indeed the operational performance of the cables themselves, as convective heat transport would increase cable current ratings, something neglected in existing standards
Basaltic maar-diatreme volcanism in the Lower Carboniferous of the Limerick Basin (SW Ireland)
Full text linkLead-zinc exploration drilling within the Limerick Basin (SW Ireland) has revealed the deep internal architecture and extra-crater deposits of five alkali-basaltic maar-diatremes. These were emplaced as part of a regional north-east south-west tectonomagmatic trend during the Lower Carboniferous Period. Field relationships and textural observations suggest that the diatremes erupted into a shallow submarine environment. Limerick trace element data indicates a genetic relationship between the diatremes and extra-crater successions of the Knockroe Formation, which records multiple diatreme filling and emptying cycles. Deposition was controlled largely by bathymetry defined by the surrounding Waulsortian carbonate mounds. An initial non-diatreme forming eruption stage occurred at the water-sediment interface, with magma-water interaction prevented by high magma ascent rates. This was followed by seawater incursion and the onset of phreatomagmatic activity. Magma-water interaction generated poorly vesicular blocky clasts, although the co-occurrence of plastically deformed and highly vesicular clasts indicate that phreatomagmatic and magmatic processes were not mutually exclusive. At a later stage, the diatreme filled with a slurry of juvenile lapilli and country rock lithic clasts, homogenised by the action of debris jets. The resulting extra-crater deposits eventually emerged above sea level, so that water ingress significantly declined, and late-stage magmatic processes became dominant. These deposits, largely confined to the deep vents, incorporate high concentrations of partially sintered globular and large ‘raggy’ lapilli showing evidence for heat retention. Our study provides new insights into the dynamics and evolution of basaltic diatremes erupting into a shallow water (20–120 m) submarine environment
Reconciling the apparent discrepancy between Cenozoic deep-sea temperatures from proxies and from benthic oxygen isotope deconvolution
No full textUnderstanding past deep-sea temperature and sea-water oxygen isotope ratios is fundamental to environmental Earth science. For example, it provides crucial insight into past ice-volume variations, an important climate system feedback. Moreover, deep-sea temperature is important to deep-sea ecology and biogeochemical cycling. Here we compare deep-sea temperature and sea-water oxygen isotope ratios from model-based deconvolution of benthic foraminiferal carbonate δ18O with clumped isotope-based deep-sea temperature data in 1000-year timesteps over the Cenozoic. To assess wider implications of the observed differences, we quantitatively evaluate a range of potential explanatory hypotheses—such as diagenetic overprints, carbonate ion effects, ice-sheet morphology changes, and warm saline deep-water admixture—but find that, individually, none can explain the observed differences satisfactorily. We then evaluate the implications of possible combined effects and recent advances in clumped isotope temperature calibration. We find that combined consideration of a recently proposed cool clumped isotope calibration and possible carbonate ion or pH influences can provide results that approximate deep-sea temperature reconstructions based on conventional δ18Oc deconvolution. The match can be further improved if modest warm saline deep-water contributions are considered during past warm periods. This contrasts with ice-volume and ice-sheet morphology changes, which appear unrealistic or insignificant, respectively. Our quantitative comparison offers a means toward formulation of a comprehensive and internally consistent understanding of Cenozoic variability in sea level (ice volume), GIA-corrected ice-sheet heights and mean ice δ18O, sea-water δ18O, sea-water δ18Ow, deep-sea temperature, and deep-sea [CO32−] variations
Enriched mantle generated through persistent convective erosion of continental roots
No full textThe origin of geochemically enriched mantle in the asthenosphere is important to understanding the physical, thermal, and chemical evolution of Earth’s interior. While subduction of oceanic sediments and deep mantle plumes have been implicated in this enrichment, they cannot fully explain the observed geochemical trends. Here we use geodynamic models to show that enriched mantle can be liberated from the roots of the sub-continental lithospheric mantle by highly organised convective erosion, a process tied to continental rifting and breakup. We demonstrate that this ‘chain’ of convective instabilities sweeps enriched lithospheric material into the sub-oceanic asthenosphere, in a predictable and quantifiable manner, over tens of millions of years—potentially faster for denser, removed keels. We test this model using geochemical data from the Indian Ocean Seamount Province, a near-continent site of enriched volcanism with minimal deep mantle plume influence. This region shows a peak in enriched mantle volcanism within 50 million years of breakup followed by a steady decline in enrichment, consistent with model predictions. We propose that persistent and long-distance lateral transport of locally metasomatised, removed keel can explain the billion-year-old enrichments in seamounts and ocean island volcanoes located off fragmented continents. Continental breakup causes a reorganisation of shallow mantle dynamics that persists long after rifting, disturbing the geosphere and deep carbon cycle
Multi-stage collapse events in the South Soufrière Hills, Montserrat, as recorded in marine sediment cores
No full textWe present new evidence for sector collapses of the South Soufrière Hills (SSH) edifice, Montserrat during the mid-Pleistocene. High-resolution geophysical data provide evidence for sector collapse, producing an approximately 1 km3 submarine collapse deposit to the south of SSH. Sedimentological and geochemical analyses of submarine deposits sampled by sediment cores suggest that they were formed by large multi-stage flank failures of the subaerial SSH edifice into the sea. This work identifies two distinct geochemical suites within the SSH succession on the basis of trace-element and Pb-isotope compositions. Volcaniclastic turbidites in the cores preserve these chemically heterogeneous rock suites. However, the subaerial chemostratigraphy is reversed within the submarine sediment cores. Sedimentological analysis suggests that the edifice failures produced high-concentration turbidites and that the collapses occurred in multiple stages, with an interval of at least 2 ka between the first and second failure. Detailed field and petrographical observations, coupled with SEM image analysis, shows that the SSH volcanic products preserve a complex record of magmatic activity. This activity consisted of episodic explosive eruptions of andesitic pumice, probably triggered by mafic magmatic pulses and followed by eruptions of poorly vesiculated basaltic scoria, and basaltic lava flows
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