| 1. |
- Berghuijs, Jaap F., et al.
(författare)
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Magma ascent, fragmentation and depositional characteristics of “dry” maar volcanoes : Similarities with vent-facies kimberlite deposits
- 2013
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 252, s. 53-72
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Tidskriftsartikel (refereegranskat)abstract
- Several maar craters within the Lake Natron-Engaruka monogenetic volcanic field (LNE-MVF) of northern Tanzania show compelling evidence for magmatic fragmentation and dry deposition. This is in contradiction of the common belief that most maars are formed through the explosive interaction between ascending magma and ground- or surface water. We here present a detailed study on the eruptive and depositional characteristics of the Loolmurwak and Eledoi maar volcanoes, two of the largest craters in the LNE-MVF, focusing on high-resolution stratigraphy, sedimentology, grain size distribution, pyroclast textures and morphologies, bulk geochemistry and mineral chemistry. At both maars, ejected material has been emplaced by a combination of pyroclastic surges and fallout. Indicators of phreatomagmatic fragmentation and wet deposition, such as impact sags, accretionary lapilli, vesiculated tuffs and plastering against obstacles, are absent in the deposits. juvenile material predominantly occurs as fluidal-shaped vesicular melt droplets and contains no glass shards produced by the breakage of bubble walls. The Eledoi deposits comprise a large amount of inversely graded beds and lenses, which result from grain flow in a dry depositional environment. Preferential deposition of fine material toward the northern side of its crater can be related to effective wind winnowing in a dry eruption plume. This large variety of observations testifies to the dominance of magmatic fragmentation as well as dry deposition at the Loolmurwak and Eledoi maars, which is in line with what has been found for other structures in the LNE-MVF but contrasts with current ideas on maar formation. We infer that a volatile-rich, olivine melilitic magma was formed by small amounts of partial melting at upper mantle depths. With minimum average ascent rates of 5.3 m s(-1) for Loolmurwak and 26.2 m s(-1) for Eledoi, this magma rapidly moved toward the surface and exsolved a substantial amount of volatiles, sufficiently large to drive magmatic fragmentation. Both eruptions were pulsating in intensity and relatively short-lived, with estimated durations of 23 and 10 h for Loolmurwak and Eledoi, respectively. The depositional characteristics of these maars, including the abundant occurrence of mantle xenoliths in the deposits, as well as their envisaged mode of emplacement show a strong similarity to the often poorly preserved vent-facies of kimberlitic diatremes. Therefore, future research on well-preserved melilititic maar-diatreme deposits may provide valuable insights into kimberlite emplacement processes. (C) 2012 Elsevier B.V. All rights reserved.
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| 2. |
- Bosshard, Sonja A., et al.
(författare)
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Origin of internal flow structures in columnar-jointed basalt from Hrepphlar, Iceland : I. Textural and geochemical characterization
- 2012
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 74:7, s. 1645-1666
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Tidskriftsartikel (refereegranskat)abstract
- Basalt columns from Hrepphlar (Iceland) show distinct internal structures produced by alternating brighter and darker bands through the column, locally exhibiting viscous fingering features. Here, we present geochemical and petrographic data retrieved from analyses of major and trace elements and mineral chemistry from a cross section of a single basaltic column. This is combined with petrographic descriptions and data on crystal size distributions of plagioclase. We use our data from Hrepphlar to test four existing models that have been proposed to explain banded structures inside columns: (1) deuteric alteration, (2) double-diffusive convection, (3) constitutional supercooling, and (4) crystallization-induced melt migration. We find that the internal structures at Hrepphlar represent primary magmatic features, because approximately 20 % of the observed structures crosscut the column-bounding fracture for each meter along the main axis of the column. These features must thus have been formed before the column-delimiting crack advanced. Major and trace element analyses show small but significant variations across the column and strong correlation between oxides like FeO and TiO2, as well as K2O and P2O5. The geochemical variations correlate with the presence of darker/brighter bands visible on a polished surface and can be explained by a variation in the modal proportions of the main phenocryst phases (specifically variable plagioclase and titanomagnetite content). This banding enhances the internal structures apparent in the polished cross section from columnar joints at Hrepphlar. The measured variations in major and trace element geochemistry, as well as mineral chemistry, are too small to distinguish between the proposed band-forming models. Plagioclase crystal size distributions, however, display a systematic change across the column that is consistent with late-stage migration of melt inside the column (i.e., the crystallization induced melt migration hypothesis). The central part of the columns have plagioclases indicative of slow cooling and these are also more steeply oriented (i.e., subparallel to the column axis) compared with plagioclases present in the more rapidly cooled edges. This redistribution of melt within individual columns may significantly affect the cooling rate of columnar-jointed lava flows and intrusions.
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| 3. |
- Bosshard-Stadlin, Sonja A., et al.
(författare)
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Magma mixing and forced exsolution of CO2 during the explosive 2007-2008 eruption of Oldoinyo Lengai (Tanzania)
- 2014
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 285, s. 229-246
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Tidskriftsartikel (refereegranskat)abstract
- Oldoinyo Lengai is probably most famous for being the only active volcano on Earth which is erupting natrocarbonatitic magma. However, the mildly explosive natrocarbonatitic activity is alternating with highly explosive, nephelinitic eruptions of which the most recent episode occurred in September 2007 (and lasted until April 2008). Here we present petrographic observations, mineral chemistry as well as major- and trace element analyses of samples covering the evolution of the eruption with time. In the early phases of the eruption, the phenocryst assemblages are dominated by the carbonate minerals nyerereite and gregoryite, but as the eruption progresses the mineralogy becomes dominated by silicate minerals like nepheline, pyroxene, garnet, alumoakermanite, combeite and wollastonite. The observed major- and trace element variations during the 2007-2008 eruption indicate mixing between a natrocarbonatitic magma and a combeite-wollastonite-bearing nephelinitic magma (CWN), with higher portions of natrocarbonatite in the early stages of the eruption. Euhedral and uncorroded clinopyroxene crystals are abundant in the late 2007 deposits but quickly start to break-down and corrode as the eruption continues, indicating that the natrocarbonatite and the CWN are not in fact conjugate magmas derived from a single magma reservoir, but must have evolved separately in the crust from the point of immiscibility. When these magmas interact beneath the volcano, a hybrid silicate magma forms (where clinopyroxene is no longer stable) and the composition of this hybrid causes the overall solubility of CO2 in the system to decrease drastically. This results in rapid exsolution of CO2 (g) which is allowed to expand during ascent, and we conclude that this is most likely the reason behind the unexpected vigor in the explosive eruptions of Oldoinyo Lengai. This massive release of CO2 during ascent may also explain the petrographic features of the pyroclasts as these are dominated by near-spherical droplets with moderate vesicularities, indicative of being transported in a hot gas-stream/jet in the upper conduit and forming an aerosol-type spray. (C) 2014 Elsevier B.V. All rights reserved.
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| 4. |
- Hetenyi, Gyoergy, et al.
(författare)
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Scales of columnar jointing in igneous rocks : field measurements and controlling factors
- 2012
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 74:2, s. 457-482
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Tidskriftsartikel (refereegranskat)abstract
- Columnar jointing is a common feature of solidified lavas, sills and dikes, but the factors controlling the characteristic stoutness of columns remain debated, and quantitative field observations are few in number. In this paper, we provide quantitative measurements on sizing of columnar joint sets and our assessment of the principal factors controlling it. We focus on (1) chemistry, as it is the major determinant of the physical (mechanical and thermal) properties of the lava, and (2) geology, as it influences the style of emplacement and lava geometry, setting boundary conditions for the cooling process and the rate of heat loss. In our analysis, we cover lavas with a broad range of chemical compositions (from basanite to phonolite, for six of which we provide new geochemical analyses) and of geological settings. Our field measurements cover 50 columnar jointing sites in three countries. We provide reliable, manually digitized data on the size of individual columns and focus the mathematical analysis on their geometry (23,889 data on side length, of which 17,312 are from full column sections and 3,033 data on cross-sectional area and order of polygonality). The geometrical observations show that the variation in characteristic size of columns between different sites exceeds one order of magnitude (side length ranging from 8 to 338 cm) and that the column-bounding polygons’ average order is less than 6. The network of fractures is found to be longer than required by a minimum-energy hexagonal configuration, indicating a non-equilibrium, geologically quick process. In terms of the development and characteristic sizing of columnar joint sets, our observations suggest that columns are the result of an interplay between the geological setting of emplacement and magma chemistry. When the geological setting constrains the geometry of the emplaced body, it exerts a stronger control on characteristic column stoutness. At unconstrained geometries (e.g. unconfined lava flows), chemistry plays the major role, resulting in stouter columns in felsic lavas and slenderer columns in mafic lavas.
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| 5. |
- Keller, Joerg, et al.
(författare)
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Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania
- 2010
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 72:8, s. 893-912
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Tidskriftsartikel (refereegranskat)abstract
- With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917,1940-1941, and 1966-1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007-2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-(a) over circle kermanite, Ti-andradite, resorbed pyroxene and Fe-Ti oxides, and a Na-Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite- wollastonite-melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25-30 wt.% SiO2, with 7-11 wt.% CO2, high alkalies (Na2O 15-19%, K2O 4-5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite-wollastonite nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.
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| 6. |
- Kervyn, Matthieu, et al.
(författare)
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Fundamental changes in the activity of the natrocarbonatite volcano Oldoinyo Lengai, Tanzania
- 2010
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 72:8, s. 913-931
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Tidskriftsartikel (refereegranskat)abstract
- On September 4,2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2-15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a similar to 400 m wide, similar to 75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007-2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.
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| 7. |
- Martin, Lukas H. J., et al.
(författare)
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Element Partitioning between Immiscible Carbonatite and Silicate Melts for Dry and H2O-bearing Systems at 1-3 GPa
- 2013
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Ingår i: Journal of Petrology. - : Oxford University Press (OUP). - 0022-3530 .- 1460-2415. ; 54:11, s. 2301-2338
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Tidskriftsartikel (refereegranskat)abstract
- Carbonatite and silicate rocks occurring within a single magmatic complex may originate through liquid immiscibility. We thus experimentally determined carbonatite/silicate melt partition coefficients (D-carbonate melt/silicate melt, hereafter D) for 45 elements to understand their systematics as a function of melt composition and to provide a tool for identifying the possible conjugate nature of silicate and carbonatite magmas. Static and, when necessary, centrifuging piston cylinder experiments were performed at 1-3 GPa, 1150-1260 degrees C such that two well-separated melts resulted. Bulk compositions had Na >> K, Na similar to K, and Na << K; for the latter we also varied bulk H2O (0-4 wt%) and SiO2 contents. Oxygen fugacities were between iron-wustite and slightly below hematite-magnetite and were not found to exert significant control on partitioning. Under dry conditions alkali and alkaline earth elements partition into the carbonatite melt, as did Mo and P (D-Mo >8, D-P = 1.6-3.3). High field strength elements (HFSE) prefer the silicate melt, most strongly Hf (D-Hf = 0.04). The REE have partition coefficients around unity with D-La/Lu = 1.6-2.3. Transition metals have D51 except for Cu and V (D-Cu similar to 1.3, D-V = 0.95-2). The small variability of the partition coefficients in all dry experiments can be explained by a comparable width of the miscibility gap, which appears to be flat-topped in our dry bulk compositions. For all carbonatite and silicate melts, Nb/Ta and Zr/Hf fractionate by factors of 1.3-3.0, in most cases much more strongly than in silicate-oxide systems. With the exception of the alkalis, partition coefficients for the H2O-bearing systems are similar to those for the anhydrous ones, but are shifted in favour of the carbonatite melt by up to an order of magnitude. An increase of bulk silica and thus SiO2 in the silicate melt (from 35 to 69 wt %) has a similar effect. Two types of trace element partitioning with changing melt composition can be observed. The magnitude of the partition coefficients increases for the alkalis and alkaline earths with the width of the miscibility gap, whereas partition coefficients for the REE shift by almost two orders of magnitude from partitioning into the silicate melt (D-La = 0.47) to strongly partitioning into the carbonatite melt (D-La = 38), whereas D-La/D-Lu varies by only a factor of three. The partitioning behavior can be rationalized as a function of ionic potential (Z/r). Alkali and alkaline earth elements follow a trend, the slope of which depends on the K/Na ratio and H2O content. Contrasting the sodic and potassic systems, alkalis have a positive correlation in D vs Z/r space in the potassic case and Cs to K partition into the silicate melt in the presence of H2O. For the divalent third row transition metals on the one hand and for the tri-and tetravalent REE and HFSE on the other, two trends of negative correlation of D vs Z/r can be defined. Nevertheless, the highest ionic strength network-modifying cations (V, Nb, Ta, Ti and Mo) do not follow any trend; understanding their behavior would require knowledge of their bonding environment in the carbonatite melt. Strong partitioning of REE into the carbonatite melt (D-REE = 5.8-38.0) occurs only in H2O-rich compositions for which carbonatites unmix from evolved alkaline melts with the conjugate silicate melt being siliceous. We thus speculate that upon hydrous carbonatite crystallization, the consequent saturation in fluids may lead to hydrothermal systems concentrating REE in secondary deposits.
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| 8. |
- Martin, Lukas H. J., et al.
(författare)
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Element partitioning between immiscible carbonatite-kamafugite melts with application to the Italian ultrapotassic suite
- 2012
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 320, s. 96-112
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Tidskriftsartikel (refereegranskat)abstract
- Trace element partition coefficients between conjugate carbonatite-silicate melts are a suitable tool to test whether such natural magmas originated by liquid immiscibility. In the Intra-Apennine Magmatic Province, Central Italy, highly silica-undersaturated ultrapotassic kamafugites coexist with carbonate-rich rocks of controversial nature: a mantle origin and magmatic nature of the carbonatites are indicated by the primitive nature of some kamafugites and by mantle-debris occurring within the kamafugite-carbonate/carbonatite deposits. An alternative hypothesis suggests that the CO2 originates from crustal carbonate assimilation by a basaltic magma, resulting in silica undersaturation of the kamafugite rocks. This experimental study investigates whether carbonatite and kamafugite melts such as the Italian ones can be immiscible at upper mantle conditions. Partition coefficients between conjugate melts determined in this study are compared with calculated element partitioning between natural carbonatite/kamafugite pairs to test a possible origin of the carbonatites by liquid immiscibility. The experiments demonstrate that kamafugite and alkali-rich carbonatite melts are immiscible at 1.7 GPa, 1220 degrees C. Trace element partition coefficients obtained from static- and centrifuging piston cylinder experiments are within factor 5 from unity and deviate less than a factor of 2 from unity for 26 out of 37 trace elements investigated. The alkali- and earth alkali-elements have D-i >= 1, indicating that carbonatite melts in equilibrium with alkaline silicate melts should be at least as alkali-rich as the silicate melts. HFSE partition preferentially into the silicate melt. A comparison of the experimental partition coefficients with natural carbonatite-kamafugite pairs from the Intra Apennine Province reveals good agreement (except for the alkalis), supporting that liquid immiscibility could indeed explain the observed rock suites. Original carbonatite compositions are not well preserved due to secondary leaching that almost completely removed the alkalis leading to significant uncertainties and compositional variations and potentially modified trace element concentrations too. Based on the observed trace element concentrations, large volume assimilation of sedimentary calcite is unlikely. We thus propose that CO2 was already present in the parental primitive magmas forming the kamafugites. The carbonatites formed by unmixing from the silicate magmas and the related CO2-saturation were most probably responsible and triggered the explosive eruptions in the Intra Apennine Province. (C) 2012 Elsevier B.V. All rights reserved.
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| 9. |
- Mattsson, Hannes B., et al.
(författare)
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Contemporaneous phreatomagmatic and effusive activity along the Hverfjall eruptive fissure, north Iceland : Eruption chronology and resulting deposits
- 2011
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 201:1-4, SI, s. 241-252
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Tidskriftsartikel (refereegranskat)abstract
- The 2500 BP Hverfjall eruption in northern Iceland produced pyroclastic and effusive deposits of widely different characteristics along the length of the eruptive fissure. The southern half of the fissure was located in a shallow lake, whereas the northern part of the fissure extended onto dry land. This specific setting, with overlapping periods of activity at the different vents, resulted in various mingling features between fine-grained phreatomagmatic deposits and lava flows. Here we reconstruct the course of events during this eruption based on field observations and granulometric analyses of the resulting deposits. The eruption can be divided into three main phases depending on the main depositional characteristics and vent locations. The initial phase is dominated by phreatomagmatic fall deposits which are attributed to an overall high eruption rate in a shallow lacustrine setting. The second phases involved opening of two new vents on dry ground, and deposition of scoria and lava flows. The third and final phase of the eruption is associated with a lowering of the eruption rate in the southernmost vent, with a shift in the activity from continuous uprush and fall deposits to discrete explosions and emplacement of base surges. These surges display features consistent with drying up with increasing distance from the vent, suggesting that their dynamics changed during emplacement. Most wet surges were channelized within a preexisting graben structure close to the vent, but some more dilute (i.e., dry) surges were able to flow over this obstacle and continued to flow for more than 5 km away from the vent and 100 m uphill before stopping. (C) 2010 Elsevier B.V. All rights reserved.
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| 10. |
- Mattsson, Hannes B., et al.
(författare)
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Depositional characteristics and volcanic landforms in the Lake Natron-Engaruka monogenetic field, northern Tanzania
- 2011
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 203:1-2, s. 23-34
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Tidskriftsartikel (refereegranskat)abstract
- The Lake Natron-Engaruka monogenetic volcanic field (LNE-MVF) is situated in the East African Rift of northern Tanzania, where it comprises approximately 200 vents scattered over an area of 2500 km(2). Similar to most other monogenetic volcanic fields in the world, the landforms of the LNE-MVF are characterized by a wide array of morphologies, such as maar-diatreme volcanoes, tuff cones and tuff rings, scoria cones and spatter cones. However, in contrast to most other MVFs (which are basaltic in composition) the magmas erupted within the LNE-MVF are predominantly of olivine-melilititic to nephelinitic compositions. Here we show by field observations, granulometric analyses and morphological studies of particle shapes from a large selection of different landforms from the LNE-MVF that there are some crucial differences compared to the more common basaltic equivalents. These differences are reflected in both eruption dynamics and fragmentation mechanisms as well as the overall characteristics of the deposits. Landforms within the LNE-MVF that resemble those produced by phreatomagmatic eruptions in morphology, display strong evidence for dry fragmentation and also dry deposition. Therefore, to reconcile the observed depositional characteristics with eruptive processes within the LNE-MVF we propose a hypothetical eruption scenario. Partial melting of a carbonate-bearing mantle source produces small volume volatile-rich melilititic melts (rich in phlogopite and amphibole). These volatile-rich, and mantle xenolith-bearing, magmas ascend rapidly from the mantle to the surface without being subject to significant degassing. The volatile-rich nature of these melilitite magmas, in combination with a significant exsolution of CO2 during decompression (ascent) can explain the dry characteristics of these apparent “phreatomagmatic” landforms. (C) 2011 Elsevier B.V. All rights reserved.
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| 11. |
- Mattsson, Hannes B., et al.
(författare)
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Mineralogical and geochemical characterization of ashes from an early phase of the explosive September 2007 eruption of Oldoinyo Lengai (Tanzania)
- 2010
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Ingår i: Journal of African Earth Sciences. - : Elsevier BV. - 1464-343X .- 1879-1956. ; 58:5, SI, s. 752-763
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Tidskriftsartikel (refereegranskat)abstract
- Ashes from Oldoinyo Lengai were collected four days after the onset of the recent explosive episode (i.e., on September 7th 2007). The ash is composed of poorly-vesicular natrocarbonatite droplets, vesicular microcrystalline nephelinite shards, and a mixed variety containing both silicate and carbonate minerals in variable proportions. Simple mixing calculations show that the whole-rock composition of the ashes can be explained by mixing natrocarbonatite and nephelinite magmas with a ratio of 4:1. The dominant silicate minerals are clinopyroxene, nepheline, Ti-andradite, wollastonite and alumoakermanite. Ti-magnetite is the most common oxide mineral. This mineral assemblage is similar to that present in the 1966 eruption products. In contrast to the 1966-1967 explosive eruption where clinopyroxene is resorbed and corroded, the ashfall from September 7th contains a large amount of euhedral clinopyroxene crystals, suggesting that magma mixing was heterogeneous and incomplete in this initial stage of the eruption. This is also supported by the petrography of the ashes. The composition of the dominant carbonate minerals (i.e., gregoryite and nyerereite) and the fluidal textures of the natrocarbonatite droplets suggest mixing of higher-viscosity nephelinite and low-viscosity natrocarbonatite magmas. Characteristic carbonate minerals produced by alteration cannot be found in the ashes. This suggests limited interaction with the older, pre-existing, natrocarbonatites inside the summit crater of the volcano. The carbonate minerals show textural evidence of being partially resorbed into the hotter nephelinitic magma. At least part of this decomposition of carbonate phases (releasing CO(2) and contributing to increased explosivity) must have occurred within the volcanic edifice such that the released gas is allowed to expand during decompression. (C) 2010 Elsevier Ltd. All rights reserved.
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| 12. |
- Mattsson, Hannes B., et al.
(författare)
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Petrogenesis of the melilititic and nephelinitic rock suites in the Lake Natron-Engaruka monogenetic volcanic field, northern Tanzania
- 2013
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 179, s. 175-192
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Tidskriftsartikel (refereegranskat)abstract
- The Lake Natron-Engaruka Monogenetic Volcanic Field (LNE-MVF) in northern Tanzania consists of more than 150 vents of Upper Pleistocene to Holocene age that are scattered over an area of 2500 km(2). Here we describe the petrological characteristics of these eruptions in detail and link the magma chemistry to eruptive behavior when the magmas reach the surface. Erupted magmas are predominantly of melilititic or nephelinitic compositions (70 and 25%, respectively), together with minor amounts of basanites (5%). The melilititic magmas form by small degrees (1-2%) of partial melting of a metasomatized upper-mantle source (containing 1-4% gamet together with both amphibole and phlogopite). The melilitites ascend very rapidly through the lithosphere prior to eruption minimizing the effect of fractional crystallization and/or crustal contamination. These eruptions also frequently carry relatively large amounts of mantle debris to the surface which is also reflected in their bulk-rock compositions. The nephelinitic rock suite, on the other hand, forms by larger degrees of melting (2-4%) at higher levels of the sub-continental lithosphere containing less garnet (<< 1%). The scarcity of mantle debris in the nephelinitic eruption deposits, combined with the more evolved magma chemistry, indicates ponding in crustal reservoirs en-route to the surface. For many of the nephelinitic magmas this ponding resulted in fractional crystallization of predominantly olivine, which is also one of the principal phenocryst phases in these rocks. However, these periods of residence in the crust must have been short as none of the investigated rocks show any clear evidence of being affected by crustal contamination. Within the LNE-MVF a rough correlation between magma chemistry and resulting volcanic landforms is recognized. Large maar volcanoes and tuff cones/rings are predominantly of melilititic composition, whereas the nephelinites typically form scoria cones. This is attributed to the fact that melilititic magmas can hold more CO2 dissolved in the liquid compared to nephelinites, in combination with a rapid ascent from the upper mantle to the surface for the melilitites (<1-2 days). We interpret the violent exsolution of CO2 (in response to rapid decompression) to be responsible for the higher explosivity of the melilititic eruptions compared to the nephelinitic magmas within the LNE-MVF. (C) 2013 Elsevier B.V. All rights reserved.
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| 13. |
- Mattsson, Hannes B.
(författare)
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Rapid magma ascent and short eruption durations in the Lake Natron-Engaruka monogenetic volcanic field (Tanzania) : A case study of the olivine melilititic Pello Hill scoria cone
- 2012
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 247, s. 16-25
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Tidskriftsartikel (refereegranskat)abstract
- The Pello Hill scoria cone displays widely different characteristics dependent on which side of the cone is studied. This follows as a direct result of a strong depositional asymmetry (i.e., the crater rim varies between 14 and 111 m in height), preferentially depositing material to the northwestern side of the construct. This is interpreted to reflect sedimentation from a sustained eruption plume (with prevailing winds at the time of the eruption coinciding with the direction of maximum deposition). The scoria deposits on this side of the cone form relatively fine-grained and well-sorted deposits which are laterally continuous over distances of 10’s of meters. To all other sides of the vent, deposits are characterized by coarse-grained lenticular deposits rich in mantle xenoliths, occasionally showing inverse grading (consistent with an origin as ballistic ejecta and slight reworking down-slope as grain-flows). The pyroclastic textures are dominated by a moderate vesicularity (similar to 40 vol. %) in combination with smooth, fluidal, outer surfaces. Two other features that stands out in comparison with “normal” scoria cones, these are defined by the absence of: (i) inward dipping layers into the crater area, and (ii) no agglutination/welding features can be found in any of the exposed outcrops. Calculated magma ascent rates yields values between 8.5 and 36.0 m s(-1), which is similar to that previously reported for kimberlitic magmas. Building on these ascent rates, and the volume of the pyroclastic construct (similar to 5 x 10(6) m(3) DRE), the eruption duration is estimated to less than 6 hours (for any vent area larger than 28 m(2)). Therefore, it may not have taken longer time than 9 hours from the time the magma started to ascend from upper-mantle depth (90 km) and the point at which the eruption halted. Overall, the olivine melilititic Pello Hill scoria cone displays many characteristics that can also be found in the vent-facies deposits of kimberlite eruptions. Therefore, further detailed studies of well-preserved, CO2-rich, olivine melilitite eruptions (such as the provided by the landforms within the LNE-MVF and Pello Hill) could provide a tool to understand the emplacement dynamics of the vent-facies of kimberlitic eruptions. (C) 2012 Elsevier B.V. All rights reserved.
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| 14. |
- Mattsson, Hannes B.
(författare)
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Textural variation in juvenile pyroclasts from an emergent, Surtseyan-type, volcanic eruption : The Capelas tuff cone, Sao Miguel (Azores)
- 2010
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 189:1-2, s. 81-91
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Tidskriftsartikel (refereegranskat)abstract
- Here I present textural data (i.e., vesicularity, vesicle size distributions (VSD), plagioclase crystallinity, crystal size distributions (CSD), combined with fractal analyses of particle outlines) from a natural succession of alternating fall and surge deposits in the emergent Capelas tuff cone (Azores). The textural variation in the Capelas succession is surprisingly small considering the wide variety of fragmentation processes, vent activity and emplacement mechanisms that are characteristic of emergent eruptions. The plagioclase crystal content varies between 24 and 33 vol.% CSD analyses of plagioclase show near-linear trends with a slight increase in time for the smallest crystal sizes (with surge deposits having more groundmass plagioclase when compared with fall deposits). This is consistent with crystallization induced by degassing and decompression at lower eruption rates. The vesicularities of the Capelas pyroclasts are more variable (18 to 59 vol.%), with VSDs displaying kinked trends characteristic of coalescence. This is especially evident in the fall deposits, and consistent with being formed in continuous uprush (jetting) with an overall shallow fragmentation level within the conduit. Bubble coalescence can also be identified in the surge deposits, although to a much lesser extent. The amount of bubble coalescence is negatively correlated with the amount of groundmass crystallization (i.e., plagioclase) in the Capelas deposits. A relatively broad range of fractal dimensions (with average D-box = 1.744 and sigma = 0.032) for the outlines of pyroclastic fragments emplaced by fall or as surges indicate that there is little difference in the fragmentation process itself at Capelas. In addition to this, the fact that the fractal dimensions for both the fall and surge end-members completely overlap suggests that shape modification due to abrasion and chipping of grain edges was minor during emplacement of base surges. These results are consistent with emergent eruptions, building tuff cones, to be a relatively low-energy phreatomagmatic landform (e.g., at least when compared with more energetic phreatomagmatic eruptions producing tuff rings and maar volcanoes). (C) 2009 Elsevier B.V. All rights reserved.
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| 15. |
- Weidendorfer, Daniel, et al.
(författare)
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Dynamics of Magma Mixing in Partially Crystallized Magma Chambers : lextural and Petrological Constraints from the Basal Complex of the Austurhorn Intrusion (SE Iceland)
- 2014
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Ingår i: Journal of Petrology. - : Oxford University Press (OUP). - 0022-3530 .- 1460-2415. ; 55:9, s. 1865-1903
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Tidskriftsartikel (refereegranskat)abstract
- The Tertiary Austurhorn intrusize complex in SE Iceland represents an exhumed magma chamber that has recorded an extensive history of magma mixing and mingling. The basal part of the intrusion consists predominantly of granophyres that have been intensively and repeatedly intruded by more mafic magma. This association of granophyres, basic and hybrid rocks at Austurhorn is referred to in the literature as a ‘net-veined” complex, but field relations suggest a much more complex emplacement history. Here we present petrological and physical constraints on the various processes that resulted in magma mixing and mingling and the le rmation of different generations of hybrid rocks at Austurhorn. The complexity of the mixing and mingling processes increases towards the inferred centre of the intrusion, where chaotic hybrid rocks dominate the exposed lithology. Complex cross-cutting relations between different hybrid generations strongly suggest multiple magma injection and reheating events in the basal part of the shallow magma chambey: Model calculations employing distribution coefficients based on rare earth element concentrations reveal that early stage hybrid magma generations firmed by pure era/member mixing between.felsic and mulic magma with about 102 n fraction in the hybrids. With repeated injections of mufic magma into the base of the magma chamber, the intruding magma interacted to a greater extent with, pre-existing hybrids. 7his led to the formation of hybrid magma compositions that are shifted towards the malic em/member oVel time, with up to.7022 of the maficje action in the hybrids. These mixing processes are recorded in the zonation patterns oflinopyroxene and plagioclase phenocrysts; the latter have been divided ttolo- ur main groups by cross-correlation analysis. Melt z’iscosity calculations were,pojo rmed to constrain the possible conditions of and the results indicate that the interaction of the contrasting magmas most probably occurred at temperatures of approximately 1000 degrees C up to 1.12 degrees C. This suggests that the initiation of efjective magma mixing requires local superheating of the felsic magmas, thereby confining the process to areas of localized, substantial ma ic magma injection.
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