| 1. |
- Henriksson, Jens, et al.
(författare)
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Fe-O isotope systematics and magnetite chemistry of the Malmberget iron-oxide apatite deposit, Sweden
- 2023
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Ingår i: Proceedings of the 17th SGA Biennial Meeting. - : The Society for Geology Applied to Mineral Deposits (SGA). - 9782839940443 ; , s. 354-357
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Konferensbidrag (refereegranskat)abstract
- Fe-O isotopes from drill core samples and whole rock chemistry from massive magnetite in LKAB's geochemical database have been used to unravel the origin, affinity, and stratigraphic position of the ore bodies in the Malmberget iron-oxide apatite deposit. Fe-O isotopes suggest a magmatic to magmatic-hydrothermal origin of the Malmberget deposit. Seven distinct magnetite populations have been identified using a novel Fe/V versus V/Ti magnetite discrimination diagram. Six of the magnetite populations form distinct spatio-geochemical clusters, whereas the massive magnetite data from the Fabian-Kapten ore body are scattered. Aggregated, this can be explained by a system where separate events form chemically distinct sill-type apatite-iron oxide intrusions within the volcano-sedimentary package, and repeated eruptions form a chemically scattered ore package on top.
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| 2. |
- Barker, Abigail K., et al.
(författare)
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Magmatic evolution of the Cadamosto Seamount, Cape Verde : Beyond the spatial extent of EM1
- 2012
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 163:6, s. 949-965
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Tidskriftsartikel (refereegranskat)abstract
- The Cadamosto Seamount is an unusual volcanic centre from Cape Verde, characterised by dominantly evolved volcanics, in contrast to the typically mafic volcanic centres at Cape Verde that exhibit only minor volumes of evolved volcanics. The magmatic evolution of Cadamosto Seamount is investigated to quantify the role of magma-crust interaction and thus provide a perspective on evolved end-member volcanism of Cape Verde. The preservation of mantle source signatures by Nd-Pb isotopes despite extensive magmatic differentiation provides new insights into the spatial distribution of mantle heterogeneity in the Cape Verde archipelago. Magmatic differentiation from nephelinite to phonolite involves fractional crystallisation of clinopyroxene, titanite, apatite, biotite and feldspathoids, with extensive feldspathoid accumulation being recorded in some evolved samples. Clinopyroxene crystallisation pressures of 0.38-0.17 GPa for the nephelinites constrain this extensive fractional crystallisation to the oceanic lithosphere, where no crustal assimilants or rafts of subcontinental lithospheric mantle are available. In turn, magma-crust interaction has influenced the Sr, O and S isotopes of the groundmass and late crystallising feldspathoids, which formed at shallow crustal depths reflecting the availability of oceanic sediments and anhydrite precipitated in the ocean crust. The Nd-Pb isotopes have not been affected by these processes of magma-crust interaction and hence preserve the mantle source signature. The Cadamosto Seamount samples have high Pb-206/Pb-204 (> 19.5), high epsilon Nd (+6 to +7) and negative Delta 8/4Pb, showing affinity with the northern Cape Verde islands as opposed to the adjacent southern islands. Hence, the Cadamosto Seamount in the west is located spatially beyond the EM1-like component found further east. This heterogeneity is not encountered in the oceanic lithosphere beneath the Cadamosto Seamount despite greater extents of fractional crystallisation at oceanic lithospheric depths than the islands of Fogo and Santiago. Our data provide new evidence for the complex geometry of the chemically zoned Cape Verde mantle source.
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| 3. |
- Day, James M. D., et al.
(författare)
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Mantle source characteristics and magmatic processes during the 2021 La Palma eruption
- 2022
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Ingår i: Earth and Planetary Science Letters. - : Elsevier. - 0012-821X .- 1385-013X. ; 597
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Tidskriftsartikel (refereegranskat)abstract
- The 2021 eruption of La Palma (September 19-December 13) was the first subaerial eruption in the Canary Islands in 50 years. Approximately 0.2 km3 of lava erupted from a newly formed, broadly basaltic composite volcanic edifice on the northwestern flank of the Cumbre Vieja volcanic ridge. Comprehensive sampling of the olivine-and clinopyroxene-phyric lavas over the eruption period reveals temporal changes in mineralogy and bulk rock geochemistry from tephrite to basanite. Initial tephrite lavas have low MgO (-6 wt.%) and elevated TiO2 (-4 wt.%) and contain amphibole crystals and gabbroic micro -xenoliths. In contrast, lavas with progressively more mafic compositions erupted to approximately day 20 of the eruption and thereafter remained as basanite (-8 wt.% MgO; 3.7 wt.% TiO2) until eruption termination. Temporal changes in lava chemistry reflect initial eruption of fractionated magmas that crystallized 5-10% olivine and clinopyroxene, as well as minor spinel, sulfide, and magnetite, followed by later eruption of deeper-sourced and more primitive magma. Vanadium-in-olivine oxybarometry indicates parental magmas were oxidized (fO2 = +1.5 to +2 FMQ) with 8.2 +/- 0.8 wt.% MgO and were generated from between 2.5-3% partial melting of a mantle source potentially containing a pyroxenite component (Xpx = 0.31 +/- 0.12). Day 1-20 tephrites have more radiogenic 187Os/188Os (0.143-0.148) and lower Pd, Pt, Ir and Os contents than post day 20 basanites (187Os/188Os = 0.141-0.145). Combined with available seismic data, the lavas provide a high-resolution record of eruptive evolution. Initial fractionated tephrite magma was stored in the upper lithosphere up to four years prior to eruption, consistent with pre-cursor seismicity and the presence of partially reacted amphibole and micro-xenoliths. The later lavas of the eruption were fed by more primitive basanitic parental magmas that were likely sourced from the deeper portion of the magma storage system that is underplating the island. Precursor events to the 2021 La Palma eruption involved seismicity and magma emplacement, storage and differentiation, which was followed by mobilisation, eruption, and eventual exhaustion of stored magma and partial melts. This magmatic progression is similar to that documented from the 1949 and 1971 Cumbre Vieja eruptions. Ocean islands with limited basaltic magma supply show similarities to the magmatic evolution observed in large silicic systems, where initial magma emplacement and differentiation is followed by later magma remobilisation that induces volcanic activity.
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| 4. |
- Byrne, Paul K., et al.
(författare)
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The geometry of volcano flank terraces on Mars
- 2009
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 281:1-2, s. 1-13
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Tidskriftsartikel (refereegranskat)abstract
- Flank terraces are subtle, expansive structures on the slopes of many large Martian shield volcanoes. Several terrace formation hypotheses - including self-loading, lithospheric flexure, magma chamber tumescence, volcano spreading, and shallow gravitational slumping - have been suggested. Terraces are not readily visible on photogeological data; consequently, terrace geometry has not yet been comprehensively described. Terrace provenance, therefore, is poorly understood. We used three-dimensional Mars Orbiter Laser Altimeter (MOLA) data to characterise the geometry of these elusive structures, with a view to understanding better the role that flank terraces play in the tectonic evolution of volcanoes on Mars. Terraces have a broad, convex-upward profile in section, and a systematic "fish scale" imbricate stacking pattern in plan. They are visible at all elevations, on at least nine disparate Martian volcanoes. Terrace-like features also occur on three shield volcanoes on Earth, an observation not recorded before. Analysis of a suite of morphometric parameters for flank terraces showed that they are scale-invariant. with similar proportions to thrust faults on Earth. We compared predicted formation geometries to our terrace observations, and found that only lithospheric flexure can fully account for the morphology, distribution, and timing of terraces. As a volcano flexes into the lithosphere beneath it, its upper surface will experience a net reduction in area, resulting in the formation of outward verging thrusts. We conclude, therefore, that flank terraces are fundamental volcanotectonic structures, that they are the surface expressions of thrust faults, probably formed by lithospheric flexure. and that they are not restricted to Mars.
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| 5. |
- Carracedo, J.C., et al.
(författare)
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The NE Rift of Tenerife: towards a model on the origin and evolution of ocean island rifts
- 2009
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Ingår i: Estudios Geologicos. - : Editorial CSIC. - 1988-3250 .- 0367-0449. ; 65:1, s. 5-47
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Tidskriftsartikel (refereegranskat)abstract
- The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evidence of the origin and dynamics of these major volcanic features. The rift developed in three successive, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quiescence or reduced activity: A Miocene stage (7266 +/- 156 ka), apparently extending the central Miocene shield of Tenerife towards the Anaga massif; an Upper Pliocene stage (2710 +/- 58 ka) and the latest stage, with the main eruptive phase in the Pleistocene. Detailed geological (GIS) mapping, geomagnetic reversal mapping and stratigraphic correlation, and radioisotopic (K/Ar) dating of volcanic formations allowed the reconstruction of the latest period of rift activity. In the early phases of this stage the majority of the eruptions grouped tightly along the axis of the rift and show reverse polarity (corresponding to the Matuyama chron). Dykes are of normal and reverse polarities. In the final phase of activity, eruptions are more disperse and lavas and dykes are consistently of normal polarity (Brunhes chron). Volcanic units of normal polarity crossed by dykes of normal and reverse polarities yield ages apparently compatible with normal subchrons (M-B Precursor and Jaramillo) in the Upper Matuyama chron. Three lateral collapses successively mass-wasted the rift: The Micheque collapse, completely concealed by subsequent nested volcanism, and the Guimar and La Orotava collapses, that are only partially filled. Time occurrence of collapses in the NE rift apparently coincides with glacial stages, suggesting that giant landslides may be finally triggered by sea level changes during glaciations. Pre-collapse and nested volcanism is predominantly basaltic, except in the Micheque collapse, where magmas evolved towards intermediate and felsic (trachytic) compositions. Rifts in the Canary Islands are long-lasting, recurrent features, probably related to primordial, plume-related fractures acting throughout the entire growth of the islands. Basaltic volcanism forms the bulk of the islands and rift zones. However, collapses of the flanks of the rifts disrupt their established fissural feeding system, frequently favouring magma accumulation and residence at shallow emplacements, leading to differentiation of magmas, and intermediate to felsic nested eruptions. Rifts and their collapse may therefore act as an important factor in providing petrological variability to oceanic volcanoes. Conversely, the possibility exists that the presence of important felsic volcanism may indicate lateral collapses in oceanic shields and ridge-like volcanoes, even if they are concealed by post-collapse volcanism or partially mass-wasted by erosion.
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| 6. |
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| 7. |
- Emeleus, C. H., et al.
(författare)
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The Rum Igneous Centre, Scotland
- 2014
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Ingår i: Mineralogical magazine. - : Mineralogical Society. - 0026-461X .- 1471-8022. ; 78:4, s. 805-839
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Tidskriftsartikel (refereegranskat)abstract
- The publication of the British Geological Survey memoir on Rum and the Small Isles in 1997 was followed by a period of intense petrological and mineralogical research, leading to some 40 papers, books and other publications. The research progress since then is reviewed here and integrated with the information previously available to provide an overview of the current status of understanding of the centre. New data on the acidic and mixed acid/basic magmas of the early Rum caldera demonstrate that frequent mafic replenishments were the main driver for magmatic activity at Rum right from its initial stages. The caldera is bound by the Main Ring Fault, a structure which probably also exercised an influence on the emplacement of the subsequent basic and ultrabasic intrusions. The later emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the crustal isotope signatures of the early intra-caldera ignimbrite magmas and the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Rare picritic dykes provide an indication of the possible parent magma for the mafic and ultrabasic rocks, but these, as with most other magmatic rocks on Rum, have undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite-type crust but, notably, no Moine metasedimentary compositions as is the case at the nearby Ardnamurchan centre. Detailed textural studies on the gabbroic and ultrabasic rocks allow a distinction between intrusive peridotites and peridotite that forms part of the classic layered cumulate units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movement of trapped magma and magma derived from later intrusions, may produce textures regarded previously as of primary cumulate origin. Sulfides in the chromitite seams and ultrabasic rocks, in turn, show possible influences from assimilated Mesozoic sediments. Igneous activity on Rum was short-lived, possibly only between 0.5 and 1 m.y. in duration and commenced at similar to 60.5 Ma. The Rum Central Complex was extinct by the time the main activity at the nearby Skye Central Complex commenced (similar to 59 Ma). From recent apatite fission-track studies it seems probable that Rum, in common with other Palaeogene centres, underwent a brief, but significantly later heating event (similar to 45 Ma).
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| 8. |
- Jaxybulatov, Kairly, et al.
(författare)
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Evidence for high fluid/melt content beneath Krakatau volcano (Indonesia) from local earthquake tomography
- 2011
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 206:3-4, s. 96-105
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Tidskriftsartikel (refereegranskat)abstract
- Within the KRAKMON project for multiparameter monitoring of Anak Krakatau volcano (Indonesia), a network of temporary stations was installed on the islands of the Krakatau complex as well as in the surrounding areas of the Sunda Strait, Sumatra and Java. The network was operated from June 2005 until January 2006. More than 700 local events were recorded during this experiment, and travel times from these events were used to perform a tomographic inversion for P and S velocities and for the Vp/Vs ratio. In this study, special attention was paid to the validation of the computed model based on different tests, such as inversion of independent data subsets and synthetic modeling. Although the network configuration and the distribution of the events are not favorable for high-quality tomographic imaging, we have obtained some important and robust features which give information about sources of volcanic activity in the Krakatau complex. The most interesting feature of this study is a zone of high Vp/Vs ratio beneath the Krakatau complex. At depths down to 4 km depth we observe anticorrelation of higher P- and lower S-velocities that leads to Vp/Vs ratio higher than 2. This is a probable indicator of the presence of partially molten and/or with high fluid content material with a composition corresponding to deeper layers. It is important that the anomaly of high Vp/Vs ratio beneath the Krakatau complex appears to be separated in two parts at a depth of 5-6 km. This fits to results of geobarometric analysis that presume the existence of several levels of magma chambers beneath Anak Krakatau.
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| 9. |
- Manconi, Andrea, et al.
(författare)
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The effects of flank collapses on volcano plumbing systems
- 2009
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 37:12, s. 1099-1102
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Tidskriftsartikel (refereegranskat)abstract
- The growth of large volcanoes is commonly interrupted by episodes of flank collapse that may be accompanied by catastrophic debris avalanches, explosive eruptions, and tsunamis. El Hierro, the youngest island of the Canary Archipelago, has been repeatedly affected by such mass-wasting events in the last 1 Ma. Our field observations and petrological data suggest that the largest and most recent of these flank collapses—the El Golfo landslide—likely influenced the magma plumbing system of the island, leading to the eruption of higher proportions of denser and less evolved magmas. The results of our numerical simulations indicate that the El Golfo landslide generated pressure changes exceeding 1 MPa down to upper-mantle depths, with local amplification in the surroundings and within the modeled magma plumbing system. Stress perturbations of that order might drastically alter feeding system processes, such as degassing, transport, differentiation, and mixing of magma batches.
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| 10. |
- Mathieu, Lucie, et al.
(författare)
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The interaction between volcanoes and strike-slip, transtensional and transpressional fault zones : Analogue models and natural examples
- 2011
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Ingår i: Journal of Structural Geology. - : Elsevier BV. - 0191-8141 .- 1873-1201. ; 33:5, s. 898-906
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Tidskriftsartikel (refereegranskat)abstract
- Regional strike-slip faulting can control magma movements, deform volcanoes and may destabilise their flanks. The aim of this study is to address this problem by comparing two natural examples, Basse Terre Island volcanoes, Lesser Antilles and Maderas volcano, Nicaragua, with analogue experiments. The field and remote sensing analyses of their structures reveal that Guadeloupe volcanoes, which developed in a 145 degrees-striking sinistral transtensional fault zone, are dominantly fractured in a 090 degrees-120 degrees direction, which is parallel to the maximum principal horizontal stress and to the elongation direction of the summit graben of analogue models. This graben is bordered by the Sigmoid-I fault, or Y shear structure, and has facilitated the formation of the Beaugendre and Vieux-Habitants valleys by faulting, erosion or collapse. This structure has also influenced the injection of dykes and the transport of hydrothermal fluids. The comparison of Maderas volcano with the analogue models confirms that the volcano has developed parallel to a 135 degrees-striking dextral transtensional fault zone and is also gravitationally spreading over a weak substratum. This study illustrates how regional strike-slip faulting and gravitational loading combine to produce a clear set of structures within volcanic edifices, which control the location of intrusive zones, hydrothermal activity and collapse directions.
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