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Kalt, Angelika

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Kalt, Angelika
Affiliation principale
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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
    Emplacement of ultramafic rocks into the continental crust monitored by light and other trace elements: An example from the Geisspfad body (Swiss-Italian Alps)
    (2008)
    Pelletier, Laure
    ;
    Müntener, Othmar
    ;
    Kalt, Angelika 
    ;
    Vennemann, Torsten W.
    ;
    Belgya, Tamás
    In order to evaluate the influence of continental crustal rocks on trace element budgets of serpentinized peridotites incorporated into the continental crust, we have analyzed the chemical composition of whole rock samples and minerals of the Geisspfad ultramafic complex (Swiss-Italian Alps). This complex represents a relict oceanic succession composed of serpentinites, ophicarbonates and metabasic rocks, emplaced into crustal gneisses during Alpine collision. Following peak metamorphic amphibolite facies conditions, fluid flow modified some of the trace element contents of ophicarbonates and deformed serpentinites close to the contact with country rocks. The fluid originated from the surrounding continental crustal rocks as documented by the increase of Pb in the serpentinites, and by the strongly negative ∂D values (− 112‰) of some ultramafic rocks close to the contact with surrounding gneisses. Little or no modification of the fluid mobile elements Li, B or U was observed in the serpentinite. In-situ analysis of light elements of serpentinite minerals indicate redistribution of light elements coupled to changes of mineral modes towards the outer 100–150 m of the massif. In the centre of the massif, Li is preferentially concentrated in olivine, while Be and B are hosted by tremolite. In contrast, at the outer rim of the massif, Li and Be are preferentially incorporated into diopside, and B into antigorite. This redistribution of light elements among the different minerals is visible in the serpentinite, at a maximum distance of ~ 100–150 m from the ophicarbonate–metabasite contact. Our results show that interaction of ultramafic rocks and crust-derived fluids can be easily detected by studies of Pb and ∂D in whole rocks. We argue that small ultramafic bodies potentially record an emplacement-related trace element signature, and that crustal light element values in ultramafic rocks are not necessarily derived from a subducting slab.