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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
    Boron, lithium and strontium isotopes as tracers of seawater–serpentinite interaction at Mid-Atlantic ridge, ODP Leg 209
    (2009)
    Vils, Flurin
    ;
    Tonarini, Sonia
    ;
    Kalt, Angelika 
    ;
    Seitz, Hans-Michael
    Spinel harzburgites from ODP Leg 209 (Sites 1272A, 1274A) drilled at the Mid-Atlantic ridge between 14°N and 16°N are highly serpentinized (50–100%), but still preserve relics of primary phases (olivine ≥ orthopyroxene >> clinopyroxene). We determined whole-rock B and Li isotope compositions in order to constrain the effect of serpentinization on δ11B and δ7Li. Our data indicate that during serpentinization Li is leached from the rock, while B is added. The samples from ODP Leg 209 show the heaviest δ11B (+ 29.6 to + 40.52‰) and lightest δ7Li (− 28.46 to + 7.17‰) found so far in oceanic mantle. High 87Sr/86Sr ratios (0.708536 to 0.709130) indicate moderate water/rock ratios (3 to 273, on the average 39), in line with the high degree of serpentinization observed.
    Applying the known fractionation factors for 11B/10B and 7Li/6Li between seawater and silicates, serpentinized peridotite in equilibrium with seawater at conditions corresponding to those of the studied drill holes (pH: 8.2; temperature: 200 °C) should have δ11B of + 21.52‰ and δ7Li of + 9.7‰. As the data from ODP Leg 209 are clearly not in line with this, we modelled a process of seawater–rock interaction where δ11B and δ7Li of seawater evolve during penetration into the oceanic plate. Assuming chemical equilibrium between fluid and a rock with δ11B and δ7Li of ODP Leg 209 samples, we obtain δ11B and δ7Li values of + 50 to + 60‰, − 2 to + 12‰, respectively, for the coexisting fluid. In the oceanic domain, no hydrothermal fluids with such high δ11B have yet been found, but are predicted by theoretical calculations. Combining the calculated water/rock ratios with the δ7Li and δ11B evolution in the fluid, shows that modification of δ7Li during serpentinization requires higher water/rock ratios than modification of δ11B.
    Extremely heavy δ11B in serpentinized oceanic mantle can potentially be transported into subduction zones, as the B budget of the oceanic plate is dominated by serpentinites. Extremely light δ7Li is unlikely to survive as the Li budget is dominated by the oceanic crust, even at small fractions.
  • 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
    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.