| 1. |
- Almqvist, Bjarne, et al.
(författare)
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Magnetic characterisation of magnetite and hematite from the Blötberget apatite-iron-oxide deposits (Bergslagen), south-central Sweden
- 2019
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Ingår i: Canadian journal of earth sciences (Print). - : Canadian Science Publishing. - 0008-4077 .- 1480-3313. ; 56:9, s. 948-957
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Tidskriftsartikel (refereegranskat)abstract
- Rock magnetic measurements were carried out on drill core material and hand specimens from the Blötberget apatite-iron oxide deposit in the Bergslagen ore province, south-central Sweden, to characterise their magnetic properties. Measurements included several kinds of magnetic susceptibility and hysteresis parameters. Petrographic and scanning electron microscopy (SEM) were used to independently identify and quantify the amount and type of magnetite and hematite. Two hematite-rich samples were studied with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to quantify the trace element chemistry in hematite and investigate the potential influence of trace elements on magnetic properties. Three aspects of this study are noteworthy. 1) Hematite-rich samples display strong anisotropy of magnetic susceptibility, which is likely to affect the appearance and modelling of magnetic anomalies. 2) The magnitude-drop in susceptibility across Curie and Néel temperature transitions show significant correlation with the respective weight percent (wt%) of magnetite and hematite. Temperature dependent magnetic susceptibility measurements can therefore be used to infer the amounts of both magnetite and hematite. 3) observations of a strongly depressed Morin transition at ca -60 to -70 C (200 to 210 K) are made during low-temperature susceptibility measurements. This anomalous Morin transition is most likely related to trace amounts of V and Ti that substitute for Fe in the hematite. When taken together, these magnetic observations improve the understanding of the magnetic anomaly signature of the Blötberget apatite-iron oxide deposits and may potentially be utilised in a broader context when assessing similar (Paleoproterozoic) apatite-iron oxide systems.
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| 2. |
- Balashova, Anna, et al.
(författare)
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New tephrostratigraphic data from Lake Emakat (northern Tanzania) : Implications for the eruptive history of the Oldoinyo Lengai volcano
- 2018
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Ingår i: Journal of African Earth Sciences. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1464-343X .- 1879-1956. ; 147, s. 374-382
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Tidskriftsartikel (refereegranskat)abstract
- The northern Tanzanian sector of the Gregory Rift is an area of an active continental rifting, in which sedimentation processes are strongly affected by volcanism. Due to limited stratigraphic exposure, the volcanic record of the region is rather sparse, and assigning volcanic centres for the individual eruptions is difficult. This study presents new data on the tephrostratigraphy of the sedimentary sequence of Lake Emakat, Empakaai Crater, northern Tanzania. Seven volcanic ash layers are identified and described from a 1.1-m core of lake sediments. Geochemical, mineralogical, petrographic and magnetic analyses show that: (1) all ash layers are products of highly explosive eruptions of melilite-bearing magmas; (2) most of the eruptions originate from a complex magmatic system; (3) all ash horizons are very well preserved in the lake environment; and (4) there are significant fluctuations of the bulk magnetic susceptibility of the lacustrine sediments which is related to microtephra from additional eruptions, the result of detritus, washed from the shore during periods of strong lake level fluctuations or periods of high erosion rates, or simply by the contamination by the material from the ash layers. Based on geochemistry and mineralogy of the seven identified ash layers in Lake Emakat, combined with the eruption ages from C-14 datings, we can pinpoint Oldoinyo Lengai volcano as the source of these specific layers. The combination of this new data with existing chronological data from Ryner et al. (2007), retrieved from the same core, provides precise ages of the voluminous highly explosive eruptions in this region of East Africa during the Pleistocene-Holocene transition.
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| 3. |
- Barker, Abigail, et al.
(författare)
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Disequilibrium in historic volcanic rocks from Fogo, Cape Verde traces carbonatite metasomatism of recycled ocean crust
- 2023
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Ingår i: Lithos. - : Elsevier. - 0024-4937 .- 1872-6143. ; , s. 107328-107328
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Tidskriftsartikel (refereegranskat)abstract
- Fogo, Cape Verde, located upon thick oceanic lithosphere, provides a window into processes occurring in the mantle where recycled ocean crust in an upwelling mantle plume interacts with ambient mantle. Our objective is to investigate the nature of the lithologies of the mantle sources involved in the petrogenesis of historic volcanic rocks from Fogo. We observe enclaves and mingling textures in the lavas combined with oxygen isotope disequilibrium between olivine and clinopyroxene phenocrysts. Olivine δ18O values display positive correlations with Zr/Hf and Zr/Y and a negative correlation with U/Th, whereas clinopyroxene δ 18O values correlate positively with Ba/Nb. Heterogeneity between crystal populations and within the groundmass indicates that multiple magma batches are mixed beneath Fogo. In terms of mantle endmembers and source lithologies, a HIMU endmember was generated by melting of carbonated eclogite as indicated by low δ 18O values, Zr/Hf, Ba/Nb and high U/Th ratios. In contrast, we show the EM1 endmember has high δ 18O, Zr/Hf, Ba/Nb and low U/Th ratios, derived from melting of variably carbonated peridotite. Additionally, Ba/Th ratio are high, indicating that carbonatite melts have contributed to alkaline magma compositions at Fogo.
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| 4. |
- 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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| 5. |
- 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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| 6. |
- Bosshard-Stadlin, Sonja A., et al.
(författare)
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Leaching of lava and tephra from the Oldoinyo Lengai volcano (Tanzania) : Remobilization of fluorine and other potentially toxic elements into surface waters of the Gregory Rift
- 2017
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 332, s. 14-25
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Tidskriftsartikel (refereegranskat)abstract
- Volcanic ash leachate studies have been conducted on various volcanoes on Earth, but few have been done on African volcanoes until now. Tephra emissions may affect the environment and the health of people living in this area, and therefore we conducted a first tephra (ash and lapilli sized) leachate study on the Oldoinyo Lengai volcano, situated in northern Tanzania. The recent explosive eruption in 2007-2008 provided us with fresh samples from the first three weeks of the eruption which were used for this study. In addition, we also used a natrocarbonatitic sample from the activity prior to the explosive eruption, as the major activity at Oldoinyo Lengai is natrocarbonatitic. To compare the leaching process affecting the natrocarbonatitic lavas and the tephras from Oldoinyo Lengai, the 2006 natrocarbonatitic lava flow was resampled 5 years after the emplacement and compared to the initial, unaltered composition. Special interest was given to the element fluorine (F), since it is potentially toxic to both humans and animals. A daily intake of fluoride (F-) in drinking water of >1.5 mg/l can lead to dental fluorosis, and higher concentrations lead to skeletal fluorosis. For this reason, a guideline value for fluoride in drinking water was set by the WHO (2011) to 1.5 mg/l. However, surface waters and groundwaters in the Gregory Rift have elevated fluoride levels of up to 9.12 mg/l, and as a consequence, an interim guideline value for Tanzania has been set at 8 mg/l. The total concentration of fluorine in the samples from the natrocarbonatitic lava flow is high (3.2 wt%), whereas we observed a significant decrease of the fluorine concentration (between 1.7 and 0.5 wt%) in the samples collected three days and three weeks after the onset of the explosive 2007-08 eruption. However, the total amount of water-extractable fluoride is lower in the natrocarbonatitic lavas (319 mg/l) than in the nephelinitic tephra (573-895 mg/l). This is due to the solubility of the different F-bearing minerals. In the natrocarbonatites, fluorine exists predominantly in fluorite (CaF2), and in the early tephra as Na-Mg bearing salts such as neighborite (NaMgF3) and sellaite (MgF2). All these three minerals have very low solubility in water (16-130 mg/l). The later nephelinitic tephras contain surface coating of villiaumite (NaF), which is highly soluble (42,200 mg/l) in water and can thus release the fluoride more readily upon contact with water. Although there is still the need for further data and a more precise study on this topic in Tanzania, we can already draw a first conclusion that the intake of water during or directly following the deposition of the tephra is not advisable and should be avoided, whereas the release of fluoride from the lava flow has less influence on the river waters. 2017 Elsevier B.V. All rights reserved.
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| 7. |
- 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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| 8. |
- Delcamp, A, et al.
(författare)
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Control of the stress field and rift structures on the distribution and morphology of explosive volcanic craters in the Manyara and Albertine rifts
- 2019
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Ingår i: Journal of African Earth Sciences. - : Elsevier. - 1464-343X .- 1879-1956. ; 150, s. 566-583
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Tidskriftsartikel (refereegranskat)abstract
- Monogenetic volcanoes are widespread in the East African Rift. Their morphologies are diverse with positive(scoria and tuff cones) and negative reliefs (tuff rings and maars). The Manyara Rift (Tanzania) and AlbertineRift (Uganda) are characterised by a singular abundance of negative volcanic reliefs with about 300 and 350maars and tuff rings, respectively, covering a surface of about ∼3000 km2 in each country. Such high density ofexplosive craters is not observed elsewhere in the East African Rift.We perform spatial distribution and alignment analyses using two different approaches (Two Point Analysisand Point Alignments) to study the link between the distribution of the monogenetic volcanic vents, the riftstructures, and the stress field. Both methods give similar results although the Point Alignments perform betterfor large radius of investigations. Volcanic alignments in the Manyara rift suggest a dominant control of the oldbasement and rift structures on dyke emplacement and distribution of volcanic vents, under an oblique rifting.However, data further suggest that this influence is reducing, and the rift is entering a more mature phase withorthogonal rifting. On the other hand, volcanic alignments in the Albertine rift imply a dominant control of thestress field on dyke emplacement and on the distribution of volcanic vents under orthogonal rifting. This suggeststhat the Albertine rift is a mature rift since at least the past 50 000 yrs.We further tested the influence of rift structures and stress field on maar and tuff ring morphologies. Most ofthe craters are near-circular, or a least not strongly elongated, and show minor complexities in shape which weattribute to lateral migration of explosions. Few maars and tuff rings in Manyara show that lateral migrationhave followed pre-existing faults. In general, we advise not to fit an ellipse to individual maars and tuff rings toassess the orientation of the feeder intrusions.
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| 9. |
- 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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| 10. |
- 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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| 11. |
- 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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| 12. |
- Kervyn, Matthieu, et al.
(författare)
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Voluminous lava flows at Oldoinyo Lengai in 2006 : chronology of events and insights into the shallow magmatic system
- 2008
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 70:9, s. 1069-1086
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Tidskriftsartikel (refereegranskat)abstract
- The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 +/- 3.0x10(5) m(3). The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.
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| 13. |
- Lierenfeld, Matthias Bernhard, et al.
(författare)
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Geochemistry and eruptive behaviour of the Finca la Nava maar volcano (Campo de Calatrava, south-central Spain)
- 2015
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Ingår i: International journal of earth sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 104:7, s. 1795-1817
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Tidskriftsartikel (refereegranskat)abstract
- Here we present a detailed investigation into the geochemistry and the excavational/depositional processes involved in the maar-diatreme forming Finca la Nava (FlN) eruption in south-central Spain. Bulk rock compositions of hand-picked juvenile fragments indicate derivation of the FIN magma from a garnet-bearing mantle source, which has subsequently been overprinted in bulk rock samples by incorporation of a combination of spinel-bearing peridotites and upper-crustal lithics (i.e. quartzites and slates). The dominating phenocryst assemblage with clinopyroxene, olivine, amphibole and phlogopite points to the classification of the juvenile magma as being olivine melilititic in composition. Ascent through the lithosphere was rapid as indicated by the calculations of settling rates of mantle peridotites (similar to 0.8 m s(-1)). The original magma fragmentation level in the conduit was probably relatively shallow carrying mainly juvenile pyroclasts (similar to 60 %) intermixed with accidental crustal lithics (similar to 35 %) and mantle xenoliths (< 5 %) to the surface. The shapes of individual pyroclasts are sub-rounded to rounded and with highly variable vesicularities (5-45 %). This fact, in combination with abundant fine-grained material in the beginning of the eruption, indicates that both magmatic and phreatomagmatic fragmentation processes may have played important roles in forming the FIN maar. A relatively constant increase in quartzitic fragments from similar to 35 to < 60 % with increasing stratigraphic height in the FIN deposits further indicates that the crater area successively widened during the eruption, which resulted in an increased recycling of quartzitic fragments. This eruption scenario, with the formation of a diatreme at depth, is also consistent with the absence of layers dipping inwards into the crater area.
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| 14. |
- Mancini, Andrea, et al.
(författare)
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Origin of the compositional diversity in the basalt-to-dacite series erupted along the Heidarspordur ridge, NE Iceland
- 2015
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 301, s. 116-127
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Tidskriftsartikel (refereegranskat)abstract
- The Heidarspordur ridge, located in the Northern Volcanic Zone of Iceland, was formed approximately 9000 years ago by a volcanic episode known as Ludent Fires. The episode produced a broad spectrum of different magma types, forming approximately 50 small scoria cones and two larger craters (Ludent and Hraunbunga). The bulk compositions cluster in five distinct groups: (1) olivine basalts, (2) Fe-Ti basalts, (3) basaltic icelandites, (4) icelandites, and (5) dacites. Major and trace element trends, together with mineral chemistry and isotopic ratios, suggest that the dominant process involved in generating the evolved magmas was crystal fractionation occurring at variable depth. An origin by polybaric differentiation is confirmed by MELTS modeling. Magma mixing played a dominant role in the formation of the basaltic icelandites. Additionally, the Fe-Ti basalts, which erupted shortly after the dacites and used approximately the same vent area, display unusually high concentrations of Fe, Ti, P, and Sr. Their composition is best explained by some pyroxene-dominated fractionation (prior to Fe-Ti oxide stability), and by entrainment of some crystal cumulate material at shallow depth, mostly left over from the silicic differentiation stage. Textural and chemical features of the minerals (e.g., presence of glomerocrysts, two populations of plagioclase in these basalts) support this interpretation of evolved cumulate remobilization. Fe-Ti basalts with the same field, compositional and textural characteristics have also been erupted in the nearby but magmatically independent Krafla Volcanic System, suggesting that a similar differentiation trend occurs also in this larger central volcano. (C) 2015 Elsevier B.V. All rights reserved.
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| 15. |
- 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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| 16. |
- 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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| 17. |
- 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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| 18. |
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| 19. |
- 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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| 20. |
- Mattsson, Hannes B., et al.
(författare)
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Emplacement and inflation of natrocarbonatitic lava flows during the March-April 2006 eruption of Oldoinyo Lengai, Tanzania
- 2009
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 71:3, s. 301-311
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Tidskriftsartikel (refereegranskat)abstract
- The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March-April 2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in width, 2.5-24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus that formed on the northwestern slope of the volcano (situated similar to 1140 m below the crater floor). The tumulus is roughly circular, measures 17.5 x 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli. All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully) suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust.
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| 21. |
- Mattsson, Hannes B., et al.
(författare)
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Experimental constraints on the crystallization of natrocarbonatitic lava flows
- 2009
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Ingår i: Bulletin of Volcanology. - : Springer Science and Business Media LLC. - 0258-8900 .- 1432-0819. ; 71:10, s. 1179-1193
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Tidskriftsartikel (refereegranskat)abstract
- Natrocarbonatitic magmas are characterized by their extremely low viscosities and fast elemental diffusion, and as a consequence of this, their chemistry and crystallinity can change significantly during residence in shallow reservoirs or even due to cooling during lava flow emplacement. Here, we present the results of a series of crystallization experiments conducted at 1-atm confining pressure and in a temperature range between 630A degrees C and 300A degrees C. The experiments were set up to characterize the chemistry and growth processes of the phenocryst phases present in natrocarbonatites. The results are applicable to (1) processes occurring during residence in shallow magma reservoirs and/or (2) during lava flow emplacement. We show that during crystallization of natrocarbonatites at atmospheric pressure, gregoryite is the first mineral to crystallize at 630A degrees C, followed by nyerereite at 595A degrees C. Crystal size distributions of the gregoryites show that the crystals grow rapidly by textural coarsening (i.e., Ostwald ripening). As the crystallization is a continuous process at this pressure, the composition of the residual melt changes in response to the crystallization. However, the experiments also show that individual crystals completely reequilibrate with the changes in melt composition in as little time as < 11 min. We therefore conclude that crystallization and diffusion are extremely fast processes in the natrocarbonatitic system and that the measured chemical variations in phenocrysts from Oldoinyo Lengai can be explained by different cooling histories. Finally, we model the rheological control on the emplacement of highly crystallized natrocarbonatitic lavas at Oldoinyo Lengai.
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| 22. |
- 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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| 23. |
- 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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| 24. |
- 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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| 25. |
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