Rb/Sr, Ar/Ar and Sm/Nd Radiochronometer Behaviour during metamorphism: case study of the Bergen Arcs Eclogites (Norway)
COMPORTEMENT DES RADIOCHRONOMETRES Rb/Sr, Ar/Ar ET Sm/Nd AU COURS DU METAMORPHISME : CAS DES ECLOGITES DE L'ARC DE BERGEN (NORVEGE)
Résumé
Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of eclogite transformation. In particular, the role of deformation and fluid circulations appears essential. In order to better decipher this role, we investigated eclogitic samples from the Lindås Nappe in the Bergen arcs (Norwegian Caledonides) which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with extensive in-situ (EMPA and LA-ICP-MS) and mineral fraction (ICP-MS) major and trace element analyses, demonstrates a contrasting behaviour during the eclogite and the amphibolite transformations. A single mineral species from the eclogite facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the variations in composition are inherited from the chemistry of precursor minerals. Thus, the composition of eclogitic minerals is locally controlled by the composition of few hundred-micron microdomains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another. The contrasting behaviour for element mobility between peak eclogitic metamorphism and retrogression can be explained in term of rate process and fluid/deformation activity. During eclogitisation, deformation allows the formation of textural and mineralogical microdomains. Contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where quartz lenses crystallise including phengites. This, together with very fast recrystallisation of eclogitic minerals, results in a heterogeneous redistribution of elements at a sample scale and to a chemical disequilibrium between different microdomains of hundred-micron size. During retrogression in the amphibolite facies coeval with long term ductile deformation, the fluids act as an efficient vector to redistribute the elements from a given textural site to another and, therefore, permit to reach a new chemical equilibrium between the crystallising phases at a hand-sample scale. Rb/Sr, 40Ar/39Ar and Sm/Nd isotopic data are reported in Caledonian eclogites from the Lindås Nappe, Bergen Arcs, Norway, in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks. The Bergen Arcs exposes Sveconorwegian c. 950-930 Ma granulites, partially overprinted by Caledonian eclogite-facies metamorphism at c. 425 Ma and amphibolite-facies metamorphism at c. 410 Ma. Geochemical and Rb-Sr data from more than ten phengite fractions separated from one sample reflect the composition of the microdomain (a few hundred-microns in size) in which phengite crystallized. Phengite crystallized after garnet or plagioclase by dissolution-precipitation processes yield apparent age between 700 and 600 Ma. At the time of their crystallization, these phengites inherited the isotopic composition of their precursor minerals, at a microdomain scale. Phengite from quartz veins, which crystallized from elements mobilized by the circulating fluid, yield an age closer to the eclogite-facies metamorphic age. The closed system evolution of the eclogitizing fluid, the segregation of textural and reactional microdomains, the high Sr content of the studied phengite and the short duration of the recrystallization processes (<1Ma) are interpreted as the main factors responsible for the lack of a Rb/Sr isotopic equilibrium, at the scale of hand-samples. Such equilibrium is nevertheless reached in quartz veins where the crystallization of minerals implies that the fluid circulation acted as a factor of isotopic homogenization. The “in-situ” single mineral Ar/Ar datings revealed that both eclogite- and amphibolite-facies minerals are characterized by an excess of radiogenic argon (ages between 425 and 520 Ma). The excess of argon has been inherited from the previous granulite and has been only partially evacuated from the system by the circulating fluid. The apparent Sm/Nd ages from garnets inherited from the granulite-facies metamorphism (c. 930 Ma) are in agreement with previous estimates. This result confirms that the diffusion of Nd in garnet does not occur at temperatures lower than 700°C. This study highlights the complexity of radiochronometer behavior during HP metamorphism and demonstrates that coupling different radiochronometers, such as Ar/Ar and Rb/Sr, does not always guarantee the validity of the geochronological results.
Une bonne compréhension du comportement des radiochronomètres est essentielle pour mener des études de thermochronologie visant à contraindre des processus géodynamiques. Cependant, le comportement des radiochronomètres est complexe car dicté par un grand nombre de facteurs intrinsèques et extrinsèques au système étudié. Au cours de cette thèse, nous avons cherché à déterminer quels étaient les facteurs critiques lors des processus de rééquilibrage isotopique survenant durant un événement métamorphique de haut grade. Ainsi, nous avons étudié le comportement des systèmes isotopiques Rb/Sr, Ar/Ar et Sm/Nd lors de l'éclogitisation partielle (contrôlée par la déformation et la circulation de fluides) au Calédonien de roches granulitiques grenvilliennes dans l'Arc de Bergen en Norvège. Nous montrons, grâce à une étude géochronologique fine associée à différentes observations (pétrologiques, géochimiques in situ et sur fractions minérales, diffraction X et isotopes de l'oxygène) permettant de contraindre avec précision le flux des éléments d'un site textural vers un autre au cours des réactions d'éclogitisation, que le facteur principal garantissant un rééquilibrage isotopique complet pour les systèmes Rb/Sr et Sm/Nd lors d'un événement métamorphique de haut grade est la redistribution homogène des éléments à l'échelle de l'échantillon. Cependant, la circulation de fluides et la déformation qui peuvent être associées aux processus de recristallisation ne sont pas nécessairement suffisant pour garantir une telle réhomogénéisation. La mise en solution des éléments d'un système ou la fusion partielle apparaissent comme deux mécanismes de réhomogénéisation bien plus efficaces. Pour le système Ar/Ar, le facteur critique est l'évacuation totale de l'argon radiogénique précédemment accumulé dans le système. Cette évacuation est favorisée, mais non garantie, par la circulation de fluides en raison de la plus forte solubilité de l'argon dans les fluides. Cependant un grand nombre de facteurs vont contrôler ce processus : quantité d'argon radiogénique initialement présente (fonction de la composition chimique de la roche et de son âge), composition chimique du fluide, et circulation en système ouvert ou fermé.