Faculté des sciences

The oceanic mantle as an important repository for the light elements Li, Be and B

Pelletier, Laure ; Kalt, Angelika (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2008 ; Th.1988.

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... Plus

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    Summary
    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.