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Kalt, Angelika

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Kalt, Angelika
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https://libra.unine.ch/handle/123456789/160

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  • Publication
    Accès libre
    The oceanic mantle as an important repository for the light elements Li, Be and B
    (2008)
    Pelletier, Laure
    ;
    Kalt, Angelika 
    It is important to quantify the Li, Be and B content of oceanic peridotites, in order to evaluate their contributions to the light element input in subduction zones (compared to oceanic crust). In previous studies, the input related to oceanic mantle was usually neglected, because no strong data are available for the light element contents (minerals, whole rock samples). The objective of this thesis is to provide a dataset of Li, Be and B contents of minerals and whole rock samples from fresh and serpentinized oceanic mantle, and to determine processes which can potentially modify the light element signature of the oceanic mantle. The Li, Be and B content of the oceanic mantle can be modified during processes acting close to mid-ocean ridges, like mafic melt percolation/impregnation and serpentinization. The Li, Be and B content can also be changed during emplacement of oceanic mantle into the continental crust. In order to study these processes, oceanic mantle from various tectonic settings was studied: (i) Pindos ophiolite (Greece) for melt-related processes, (ii) Pindos and Vourinos ophiolites (Greece), Mid-Atlantic ridge (MAR) ODP Leg 209 for serpentinization, (iii) Geisspfad ultramafic body (Alps) for the effect of the emplacement into the continental crust. The study of the Dramala harzburgites (Pindos), recording high degree of partial melting prior to melt percolation, shows that there is a Li enrichment of the depleted harzburgite during the crystallization of clinopyroxene cumulate (Li in Cpx ≤ 3.7 µg/g), related to percolation of N-MORB melt. Subsequent impregnation by ultra-depleted melt did not change the Li, Be and B content of the harzburgites. Light element contents of the fresh Dramala harzburgite after melt-related processes are low (Li: 0.9-1.0 µg/g, Be: <0.003 µg/g, B: <0.03 µg/g). These low contents are certainly due to the high degree partial melting, while melt impregnation and/or percolation does not strongly modify the light element content of whole rock samples. During serpentinization, there is a B enrichment in whole rock samples (no Li or Be enrichment), while Li, Be and B contents of the primary mantle phases stay constant. The major B carrier phase is serpentine (≤ 28 µg/g). The quantity of B incorporated into serpentinized harzburgite probably depends on the nature of serpentinization (temperature, pH, water/rock ratio). B contents in serpentine/serpentinites from Dramala serpentinized harzburgites are low compared to serpentinites from the MAR. Samples from Dramala show low whole rock B contents in highly serpentinized harzburgites (up to 1.1 µg/g) and heterogeneous B content in serpentine (0.1-28 µg/g). It probably reflects serpentinization occurring at high temperature and low water/rock ratio. In contrast, serpentinization in the MAR samples led to high B content in serpentine (≤ 200 µg/g) and serpentinites (10-65 µg/g), probably related to low temperatures and high water/rock ratio. The Geisspfad serpentinites showed that Li, Be and B contents of oceanic serpentinites are modified during emplacement into the continental crust by fluids related to retrograde metamorphism (evident from Li, Be and B contents in minerals/whole rock samples). These fluids can penetrate ultramafic bodies or travel along the contact between the ultramafics and the surrounding crustal rocks. It shows that only large ultramafic bodies can potentially maintain their prograde light element systematics in the core. In conclusion, light element content of the fresh oceanic mantle is low, except for Li (can be enriched during N-MORB melt impregnation). The oceanic mantle is variably enriched in B during serpentinization, depending on temperature, pH and water/rock ratio. Due to its big volume compared to the oceanic crust, the oceanic mantle could strongly contribute to the Li and B input into subduction zones.
  • Publication
    Accès libre
    Modélisation 3D de l'arc alpin
    (2006)
    Vouillamoz, Naomi
    ;
    Kalt, Angelika 
    L’exploitation du logiciel Éditeur Géologique© développé par le BRGM1 permet la création d’un modèle 3D des unités crustales de l’arc alpin. Ce modèle apporte une première vue d’ensemble, à grande échelle, de la géométrie tridimensionnelle des structures de la chaîne. La zone d’étude englobe l’arc des Alpes occidentales, de la mer Ligure au Sud, jusqu’au dôme lépontin et au granite du Bergell au Nord-Est. Au Nord, le modèle s’arrête à l’arc du Jura et il est délimité de manière naturelle au Sud par le bassin ligurien. Verticalement, le modèle est défini entre la surface topographique et l’interface du Moho. La chaîne alpine étant non cylindrique dans la zone cible justifie le développement d’un tel modèle structural 3D. Les interactions entre le socle et les structures du Graben du Rhin au Nord du Jura ne sont pas prises en compte. De même, au Sud du modèle, la connexion avec les Appenins ainsi que les problématiques liées au massif des Maures ne sont pas modélisées. La constitution du modèle est basée d’une part sur le MNT des Alpes et sur la carte du Moho qui définissent l’interface supérieure et inférieure du modèle et d’autre part, sur la carte géologique simplifiée ainsi que sur les coupes crustales ECORS-CROP et NFP-20 qui permettent d’établir les limites entre les différentes formations à modéliser, à la surface et à l’intérieur du modèle. Du reste, des coupes complémentaires ciblées contribuent à mieux contraindre le modèle. D’un point de vue technique, le travail comporte deux parties, basées respectivement sur l’exploitation des logiciels principaux suivants : les logiciels SIG (Systèmes d’Information Géographique) et l’Éditeur Géologique©. Les logiciels ESRI ArcGIS© permettent tout d’abord de préparer les bases de données servant à la modélisation 3D. Une carte tectonique numérique des Alpes, un MNT de la zone cible ainsi qu’un modèle numérique du Moho sont établis. Les systèmes de références spatiales de ces objets sont ensuite homogénéisés. Ces données sont finalement rendues compatibles avec l’outil de modélisation 3D. Dans un second temps, l’exploitation du logiciel Éditeur Géologique© rend possible la création du modèle 3D des unités crustales de l’arc alpin. L’Éditeur Géologique© permet en effet la réalisation de modèles géologiques tridimensionnels, à partir de données communément utilisées en géologie, comme des cartes, des coupes ou des MNT. Le modèle se veut évolutif, en ce sens qu’il peut être amélioré, soit localement, soit globalement. De plus, plusieurs perspectives de développement de ce travail sont envisageables. En effet, le modèle apporte une vision sans laquelle un certain nombre de problèmes tectoniques ou géodynamiques alpins ne peuvent à priori être abordés.
  • Publication
    Accès libre
    Mantle xenoliths from the Marsabit volcanic field: a case study on the evolution of the lithospheric mantle in a continental rift environment
    (2006)
    Kaeser, Benjamin
    ;
    Kalt, Angelika 
    Mantle xenoliths, rock fragments sampled by magmas during their ascent from depth to the surface, provide direct information on the nature and composition of the Earth’s mantle. This thesis is the result of a petrographic, geochemical and petrological case study on mantle xenoliths hosted by Quaternary basanitic and alkali basaltic scoriae of the Marsabit volcanic field (northern Kenya). Magmatic activity is related to the development of the East African rift system. Results from previous seismic, geological and petrological studies show that continental rifting in East Africa is strongly controlled by pre-existing structures in the lithosphere. Further, the nature of the lithosphere has been shown to play a crucial role for the locus and composition of volcanic rocks, as magmas partly derive from, or at least interacted with the lithospheric mantle. The xenoliths from Marsabit provide a direct window in the mantle and allow constraining the nature of the East African Rift lithosphere. The xenoliths comprise several groups of ultramafic (peridotite) and mafic (pyroxenite and gabbro) rocks. Peridotite includes porphyroclastic or statically recrystallised, formerly garnet-bearing lherzolite (Group I and II, respectively), porphyroclastic spinel harzburgite and dunite (Group III) and mylonitic spl harzburgite and lherzolite (Group IV). Mafic rocks comprise garnet-bearing and garnet-free pyroxenite (Group V and VI, respectively) and gabbro (Group VII). The integration of textural and compositional data, together with results from thermobarometry and evaluation of mineral zoning indicate a complex evolution of the lithospheric mantle. The possibly oldest features are preserved in the formerly garnet-bearing lherzolites (Group I and II) and in the garnet pyroxenites. These rocks provide evidence of an earlier high-pressure / high temperature stage (~970-1100°C at depths around 60-90 km), similar to non-rifted sub-continental lithospheric mantle such as actually present underneath southern Kenya. This stage most likely corresponds to the lithospheric conditions prior to continental rifting which started during Mesozoic times with the formation of the Anza Graben (an older rift perpendicular to the present-day East African rift). The garnet pyroxenites formed prior to rifting as well. It is suggested that the garnet pyroxenites represent the products of high-pressure crystallisation of opx-saturated melts, possibly formed during Pan-African (Neoproterozoic-Paleozoic) orogenesis. Crustal rocks issued from this time period make up most of the present-day crystalline basement of the Marsabit area. All peridotite types, as well as the garnet pyroxenites were subjected to later cooling, decompression and pervasive deformation (to very low mantle P-T conditions of ~700-800°C at depths ~30-40 km). These features are best explained by continental rifting during Mesozoic-Paleogene times that led to the formation of the Anza Graben Subsequently magmatism and metasomatism related to the development of the Tertiary-Quaternary East African rift obliterated features related to Mesozoic-Paleogene rifting. Evidence for this comes, for example, from the statically recrystallised lherzolites, where textural annealing is associated with a young heating event (up to 1100°C). Heating was accompanied by cryptic metasomatism (i.e., enrichment of clinopyroxene in Fe-Ti and incompatible trace elements). The metasomatising melts were compositionally similar (and possibly genetically related) to the Quaternary basanites erupted at the surface of Marsabit. Probably in the same period, garnet-free pyroxenites (Group VI xenoliths) crystallised from alkaline melts, presumably in dykes within the shallow mantle or at the mantle-crust boundary (between ~30-60 km depths). Also these alkaline melts were most likely related to the lavas erupted at the surface. Further evidence for Tertiary-Quaternary metasomatism can be found in the porphyroclastic Group III peridotite xenoliths (Group III), which show a textural transition from virtually non-metasomatised spl harzburgite to modally metasomatised amphibole dunite. The latter contain rather unusual mantle minerals such as apatite, graphite, Na-rich phlogopite and katophorite (amphibole). The phase assemblage, as well as major and trace element characteristics indicate that this type of metasomatism resulted from the infiltration of volatile (H2O, CO2)-rich silicic melt and/or fluid in a pre-existing heterogeneous and probably reduced mantle. Such melts may have evolved from previous melt-rock reaction processes. In a very late stage (i.e., shortly before the xenoliths were transported to the surface in their host magma), the metasomatic minerals partially melted. This led to the formation of patches consisting of newly formed minerals (microlites) and glass (quenched melt). The metasomatised harzburgites and dunites are further strongly enriched in the low-atomic mass elements (light elements) Lithium, Beryllium and Boron. Therefore, these elements were investigated with special emphasis. The light elements are now widely used to trace recycled components in mantle and volcanic rocks in subduction zone settings. In the case of Marsabit, the light element systematics could potentially be interpreted as reflecting such components in the mantle, added by ancient, pre-rift subduction events. The detailed investigation of Li, Be and B systematics in minerals from the Marsabit xenoliths, however, clearly points to young disequilibrium features and to modification of light element budgets during very late-stage melting events (i.e. the formation of melt pockets). These results highlight that the application of light element systematics to trace subduction-related components is not un-problematic. This applies in particular to xenoliths where a careful quantification of late-stage metasomatic events with respect to the light elements is necessary.
  • Publication
    Accès libre
    Formation and metamorphism of aluminous upper mantle and lower crustal rocks: a case study on websterite and granulite xenoliths from basanites of the Chyulu Hills volcanic field, Kenya
    (2005)
    Ulianov, Alexey
    ;
    Kalt, Angelika 
    The Chyulu Hills volcanic field is located on the eastern flank of the Kenya rift some 150 km to the east of the Kenya Rift Valley. It lies on the Pan-African crystalline basement of the Mozambique mobile belt. Quaternary basanites of the Chyulu Hills contain peridotite, pyroxenite and granulite xenoliths entrained from the underlying upper mantle and lower crust. This study focuses on a suite of garnet-spinel olivine websterite, Mg-Al sapphirine-bearing and Ca-Al hibonite-bearing granulite xenoliths from several volcanic cones of the Chyulu Hills. In terms of protoliths, they form an igneous suite linked by fractionation. High bulk rock Mg#’s and very low concentrations of most incompatible elements indicate that the rocks represent a sequence of cumulates rather than crystallized melts. The websterites are the most magnesian part of this sequence, while the granulites are less magnesian and more rich in aluminum, calcium and alkalis. Along with constraints from postmagmatic P-T evolution, low HSFE abundances, LILE enrichment and fractionation of LREE over HREE in whole rock samples suggest that the protoliths of the xenoliths may represent arc cumulates, which implies that they should be of Pan-African age. The normative compositions of the granulites are dominated by plagioclase and olivine. This, as well as the elevated abundances of Ni, low concentrations of Cr and HFSE and positive Eu anomalies in REE patterns suggests that the protoliths of the granulites were troctolitic cumulates. The original mineral assemblages were almost completely transformed by subsolidus processes. Mg-Al granulites contain the minerals spinel, sapphirine, sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxene and garnet, while Ca-Al granulites are characterized by hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase, quartz, clinopyroxene, corundum, and garnet. In the Mg-Al granulites, the first generation of orthopyroxene and some spinel may be of igneous origin. In the Ca-Al granulites, hibonite (and eventually some spinel) are the earliest mineral in the crystallization sequence and may represent igneous relicts. Most pyroxene and spinel as well as corundum in both Mg-Al and Ca-Al granulites have formed by subsolidus reactions. The qualitative P-T path derived from metamorphic reactions corresponds to subsolidus cooling accompanied or followed by compression. Final equilibration was achieved at T ≈ 600-740 oC and P < 8 kbar, the pressure range being constrained by the stability of sillimanite. The early coexistence of corundum and pyroxenes (± spinel) as well as the association of sillimanite and sapphirine with clinopyroxene and the occurrence of hibonite makes both types of the studied granulite lithologies rare. The Ca-Al hibonite-bearing granulites are unique. Both types enlarge the spectrum of known Ca-Al-Mg-rich granulites worldwide. Hibonite found in two xenoliths of the Ca-Al granulites occurs as small grains in the inner parts of complex corona textures where it forms intergrowths with spinel and sapphirine and shows reaction relationships with later mullite and sillimanite. Chemically, the analyzed hibonite is close to the idealized formula Ca(Al,Cr,Ti,Si,Mg,Fe2+)12O19 and does not contain other major components. It is similar to terrestrial hibonite in its (Fe+Mg) contents but shows the elevated Al and Ca abundances as well as the relative depletion in Ti and REE typical of meteoritic hibonite. Silica contents are high and exceed those in any other terrestrial and meteoritic hibonite. In order to evaluate the possibility of magmatic crystallization of hibonite in the igneous protoliths of the studied rocks, we compare some of the measured element abundances with those expected from element partitioning data for hibonite and Ca,Al-rich silicate melt. Based on this comparison, formation of low-Ti hibonite that is relatively rich in LREE appears consistent with magmatic crystallization, whereas hibonite with elevated Ti contents, low in LREE, is obviously the result of diffusion re-equilibration in the course of subsolidus cooling. For the websterite xenoliths, there is an apparent contradiction between the textural-mineralogical data and results of P-T calculations that appear to suggest high-P igneous crystallization of pyroxenes under upper mantle conditions on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can in part be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith at near-igneous temperatures to mantle depth where it obtained its ultramafic phase assemblage. This would require foundering of dense lower crustal material into the mantle towards the end of Pan-African subduction and accretion, a process that is possible in areas with high geothermal gradients such as arcs. The subsequent P-T evolution of the websterites as well as of most other types of upper mantle xenoliths from the Chyulu Hills is defined by a long period of cooling that may in some specimens be followed by (a) recent heating event(s). The chemical zoning patterns of orthopyroxene are either flat (suggesting complete subsolidus diffusion reequilibration) or show a core-to-rim decrease of Al and Ca indicating cooling. In both cases, Al and Ca may show a rimwards increase in the outermost zones of the orthopyroxene grains that is consistent with heating. Results from diffusion modeling for pyroxenes, albeit highly approximate, suggest that the rimwards increase in Al and Ca associated with heating is temporally related to the young volcanic activity of the Chyulu Hills, whereas the duration of cooling is much longer and may correspond to the age of the Pan-African orogeny (ca. 600 Ma). Pressure-temperature data on equilibrated xenoliths suggest a geotherm beneath the Chyulu Hills similar to a ~ 60–70 mW m-2 steady-state model geotherm. The lithospheric thickness is constrained to ca. 115 km. The late heating affected xenoliths derived from depths of ca. 80-40 km. It represents a local feature most probably related to small magma intrusions in the lithospheric mantle.