| 1. |
- Weis, Franz A., et al.
(författare)
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Magmatic water contents determined through clinopyroxene : Examples from the Western Canary Islands, Spain
- 2015
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 16:7, s. 2127-2146
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Tidskriftsartikel (refereegranskat)abstract
- Water is a key parameter in magma genesis, magma evolution, and resulting eruption styles, because it controls the density, the viscosity, as well as the melting and crystallization behavior of a melt. The parental water content of a magma is usually measured through melt inclusions in minerals such as olivine, a method which may be hampered, however, by the lack of melt inclusions suitable for analysis, or postentrapment changes in their water content. An alternative way to reconstruct the water content of a magma is to use nominally anhydrous minerals (NAMs), such as pyroxene, which take up low concentrations of hydrogen as a function of the magma's water content. During magma degassing and eruption, however, NAMs may dehydrate. We therefore tested a method to reconstruct the water contents of dehydrated clinopyroxene phenocrysts from the Western Canary islands (n=28) through rehydration experiments followed by infrared and Mossbauer spectroscopy. Employing currently available crystal/melt partitioning data, the results of the experiments were used to calculate parental water contents of 0.710.07 to 1.490.15 wt % H2O for Western Canary magmas during clinopyroxene crystallization at upper mantle conditions. This H2O range is in agreement with calculated water contents using plagioclase-liquid-hygrometry, and with previously published data for mafic lavas from the Canary Islands and comparable ocean island systems elsewhere. Utilizing NAMs in combination with hydrogen treatment can therefore serve as a proxy for pre-eruptive H2O contents, which we anticipate becoming a useful method applicable to mafic rocks where pyroxene is the main phenocryst phase.
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| 2. |
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| 3. |
- Sahlström, Fredrik, et al.
(författare)
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Iron isotopes constrain sub-seafloor hydrothermal processes at the Trans-Atlantic Geotraverse (TAG) active sulfide mound
- 2022
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Ingår i: Communications Earth & Environment. - : Springer Science and Business Media LLC. - 2662-4435. ; 3:1
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Tidskriftsartikel (refereegranskat)abstract
- Sub-seafloor hydrothermal processes along volcanically active plate boundaries are integral to the formation of seafloor massive sulfide deposits and to oceanic iron cycling, yet the nature of their relationship is poorly understood. Here we apply iron isotope analysis to sulfide minerals from the Trans-Atlantic Geotraverse (TAG) mound and underlying stockwork, 26°N Mid-Atlantic Ridge, to trace hydrothermal processes inside an actively-forming sulfide deposit in a sediment-free mid-ocean ridge setting. We show that data for recently formed chalcopyrite imply hydrothermal fluid–mound interactions cause small negative shifts (<−0.1‰) to the δ56Fe signature of dissolved iron released from TAG into the North Atlantic Ocean. Texturally distinct types of pyrite, in turn, preserve a δ56Fe range from −1.27 to +0.56‰ that reflects contrasting precipitation mechanisms (hydrothermal fluid–seawater mixing vs. conductive cooling) and variable degrees of progressive hydrothermal maturation during the >20 kyr evolution of the TAG complex. The identified processes may explain iron isotope variations found in fossil onshore sulfide deposits.
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| 4. |
- Mathieu, Lucie, et al.
(författare)
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Geological constraints on the dynamic emplacement of cone-sheets - The Ardnamurchan cone-sheet swarm, NW Scotland
- 2015
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Ingår i: Journal of Structural Geology. - : Elsevier BV. - 0191-8141 .- 1873-1201. ; 80, s. 133-141
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Tidskriftsartikel (refereegranskat)abstract
- Cone-sheets are a significant constituent of many central volcanoes, where they contribute to volcano growth by intrusion and through flank eruptions, although the exact emplacement mechanisms are still controversially discussed. In particular, it is not yet fully resolved whether cone-sheets propagate as magma-driven, opening-mode fractures or as shear fractures, and to what extent pre-existing host-rock structures and different stress fields influence cone-sheet emplacement. To shed further light on the role of these parameters in cone-sheet emplacement, we use detailed field and remote sensing data of the classic Ardnamurchan cone-sheet swarm in NW-Scotland, and we show that the cone-sheets primarily propagated as opening-mode fractures in the sigma(1)-sigma(2) plane of the volcanic stress field. In addition, more than one third of the Ardnamurchan cone-sheet segments are parallel to lineaments that form a conjugate set of NNW and WNW striking fractures and probably reflect the regional NW SE orientation of sigma(1) during emplacement in the Palaeogene. Cone-sheets exploit these lineaments within the NE and SW sectors of the Ardnamurchan central complex, which indicates that the local volcanic stress field dominated during sheet propagation and only allowed exploitation of host-rock discontinuities that were approximately parallel to the sheet propagation path. In addition, outcrop-scale deflections of cone-sheets into sills and back into cone-sheets (also referred to as "staircase" geometry) are explained by the interaction of stresses at the propagating sheet tip with variations in host-rock strength, as well as the influence of sheet-induced strain. As a consequence, cone-sheets associated with sill-like segments propagate as mixed-mode I/II fractures. Hence, cone-sheet emplacement requires a dynamic model that takes into account stress fields at various scales and the way propagating magma interacts with the host rock and its inherent variations in rock strength.
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| 5. |
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| 6. |
- Radu, Ioana-Bogdana, et al.
(författare)
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Water in clinopyroxene from the 2021 Geldingadalir eruption of the Fagradalsfjall Fires, SW-Iceland
- 2023
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Ingår i: Bulletin of Volcanology. - : Springer. - 0258-8900 .- 1432-0819. ; 85:5
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Tidskriftsartikel (refereegranskat)abstract
- Water content plays a significant role in magma genesis, ascent rate, and, ultimately, in the style and intensity of volcanic eruptions, due to its control on the density, viscosity and melting behaviour of silicate melts. A reliable method for determining the pre-eruptive magmatic water content is to use phenocrysts of nominally anhydrous minerals (NAMs) which can preserve water as hydrogen configurations in structural defects. The advantage of this method is that eruptive changes such as water loss during magma degassing may be experimentally reconstructed and analysed by infrared spectroscopy. Applying this to clinopyroxene crystals (n=17) from lava samples (n=7) from April 2021 of the Geldingadalir eruption, SW-Iceland, reveals parental water contents of 0.69 ± 0.07 to 0.86 ± 0.09 wt. % H2O. These values are higher than those expected for typical mid-ocean ridge basalts (MORB 0.3–0.5 wt. % on average) indicating a significant plume (OIB) contribution to the magma source. Moreover, such water concentrations would imply that water saturation in the ascending Geldingadalir magmas was attained only at very shallow levels within the plumbing system. This could explain the at times pulsating behaviour within the uppermost conduit system as being the result of shallow episodic water vapour exsolution in addition to the deep-sourced CO2 flux.
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| 7. |
- 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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| 8. |
- Blythe, Lara, et al.
(författare)
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CO2 bubble generation and migration during magma–carbonate interaction
- 2015
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 169:4
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Tidskriftsartikel (refereegranskat)abstract
- We conducted quantitative textural analysis of vesicles in high temperature and pressure carbonate assimilation experiments (1200 °C, 0.5 GPa) to investigate CO2 generation and subsequent bubble migration from carbonate into magma. We employed Mt. Merapi (Indonesia) and Mt. Vesuvius (Italy) compositions as magmatic starting materials and present three experimental series using (1) a dry basaltic-andesite, (2) a hydrous basaltic-andesite (2 wt% H2O), and (3) a hydrous shoshonite (2 wt% H2O). The duration of the experiments was varied from 0 to 300 s, and carbonate assimilation produced a CO2-rich fluid and CaO-enriched melts in all cases. The rate of carbonate assimilation, however, changed as a function of melt viscosity, which affected the 2D vesicle number, vesicle volume, and vesicle size distribution within each experiment. Relatively low-viscosity melts (i.e. Vesuvius experiments) facilitated efficient removal of bubbles from the reaction site. This allowed carbonate assimilation to continue unhindered and large volumes of CO2 to be liberated, a scenario thought to fuel sustained CO2-driven eruptions at the surface. Conversely, at higher viscosity (i.e. Merapi experiments), bubble migration became progressively inhibited and bubble concentration at the reaction site caused localised volatile over-pressure that can eventually trigger short-lived explosive outbursts. Melt viscosity therefore exerts a fundamental control on carbonate assimilation rates and, by consequence, the style of CO2-fuelled eruptions.
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| 9. |
- 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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| 10. |
- Geiger, Harri, et al.
(författare)
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Magma plumbing for the 2014-2015 Holuhraun eruption, Iceland
- 2016
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 17:8, s. 2953-2968
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Tidskriftsartikel (refereegranskat)abstract
- The 2014-2015 Holuhraun eruption on Iceland was located within the Askja fissure swarm but was accompanied by caldera subsidence in the Baroarbunga central volcano 45 km to the southwest. Geophysical monitoring of the eruption identified a seismic swarm that migrated from Baroarbunga to the Holuhraun eruption site over the course of two weeks. In order to better understand this lateral connection between Baroarbunga and Holuhraun, we present mineral textures and compositions, mineral-melt-equilibrium calculations, whole rock and trace element data, and oxygen isotope ratios for selected Holuhraun samples. The Holuhraun lavas are compositionally similar to recorded historical eruptions from the Baroarbunga volcanic system but are distinct from the historical eruption products of the nearby Askja system. Thermobarometry calculations indicate a polybaric magma plumbing system for the Holuhraun eruption, wherein clinopyroxene and plagioclase crystallized at average depths of approximate to 17 km and approximate to 5 km, respectively. Crystal resorption textures and oxygen isotope variations imply that this multilevel plumbing system facilitated magma mixing and assimilation of low-O-18 Icelandic crust prior to eruption. In conjunction with the existing geophysical evidence for lateral migration, our results support a model of initial vertical magma ascent within the Baroarbunga plumbing system followed by lateral transport of aggregated magma batches within the upper crust to the Holuhraun eruption site.
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