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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 Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere
    (2088)
    Vils, Flurin
    ;
    Pelletier, Laure
    ;
    Kalt, Angelika 
    ;
    Müntener, Othmar
    ;
    Ludwig, Thomas
    Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (Fifteen–Twenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44–2.8 μg g−1) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 μg g−1), depleted in Li (most values below 1 μg g−1) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4–65.0 μg g−1), H2O and Cl contents and to lower Li contents (0.07–3.37 μg g−1) of peridotites, implying that—contrary to alteration of oceanic crust—B is fractionated from Li and that the B and Li inventory should depend essentially on rock–water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17–307 kg in a column of 1 m × 1 m × thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25–904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.
  • Publication
    Accès libre
    Li, B and Be Contents of Harzburgites from the Dramala Complex (Pindos Ophiolite, Greece): Evidence for a MOR-type Mantle in a Supra-subduction Zone Environment
    (2009)
    Pelletier, Laure
    ;
    Vils, Flurin
    ;
    Kalt, Angelika 
    ;
    Gméling, Katalin
    The Pindos ophiolite represents oceanic lithosphere obducted during the Jurassic. The Dramala mantle section mainly consists of highly depleted spinel harzburgite and minor plagioclase-bearing harzburgite. Textural observations and major element compositions of minerals indicate that the harzburgites experienced impregnation by a mafic, depleted melt and subsequent high-temperature (high-T) hydration and cooling (>750°C) forming pargasite and edenitic hornblende. During further cooling (from 350–400°C to < 100°C), talc + tremolite ± serpentine ± olivine, serpentine + magnetite, and finally plagioclase alteration phases formed. To test the hypothesis of a supra-subduction zone origin for the Dramala mantle, we measured Li, B and Be contents of minerals by secondary ion mass spectrometry. Whole-rock contents were measured using inductively coupled plasma–mass spectrometry and prompt gamma neutron activation analysis. We observe low Li and B contents of primary minerals (olivine, orthopyroxene, clinopyroxene) consistent with values for unmetasomatized mantle minerals; only Li contents of clinopyroxene (up to 3•7 µg/g) are slightly elevated. The bulk Li contents (0•5–1•1 µg/g) are in the upper range of values for unmetasomatized mantle, whereas B contents (<0•04–1•1 µg/g) are variable and slightly elevated compared with the unmetasomatized mantle as a result of serpentinization. Beryllium abundances in all minerals are very low (<0•005 µg/g), except for pargasite, where a maximum Be content of 0•012 µg/g was measured. The selective addition of Li to clinopyroxene can be related to the interaction with a depleted melt, and/or to partitioning of Li into clinopyroxene upon cooling. During high-T hydration and cooling, the fluid calculated to be in equilibrium with the pargasite or edenitic hornblende (based on Li, Be and B) could have been reaction-modified seawater. Low-T hydration may have led to a very minor increase in bulk B content of most samples and to the formation of serpentine with highly variable B contents (0•1–28 µg/g). Low-T hydration decreased the Li content of orthopyroxene, and Li was probably leached from some samples. The lack of correlation between degree of serpentinization and bulk B contents as well as the presence of high- and low-B serpentine can be explained by low fluid–rock ratios, decreasing T during serpentinization and lack of equilibrium as a result of fast obduction–exhumation. The low light-element contents of primary minerals and whole-rock samples clearly argue against a supra-subduction zone (SSZ) origin of the Dramala mantle section, and against the previous hypothesis of hydrous melting of the Pindos mantle above a subduction zone. We therefore conclude that the Dramala harzburgites represent a mid-ocean ridge (MOR)-type mantle, and not an SSZ-type mantle, juxtaposed with MOR-type and SSZ-type oceanic crust, either in a back-arc or in an intra-oceanic subduction zone setting.
  • 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.
  • Publication
    Accès libre
    Potential input of light elements into subduction zones—Insights from ODP leg 209
    (2006)
    Vils, Flurin
    ;
    Pelletier, Laure
    ;
    Kalt, Angelika 
    ;
    Müntener, Othmar