Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid–rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet–phengite quartzite. The B isotopic composition is variable (δ11B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid–rock interaction during peak metamorphic conditions. This interpretation is supported by fluid–rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ11B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ11B values (δ11B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ11B values that cluster around 0% (δ11B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid–rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones

Effects of fluid-rock interaction on 40Ar/39Ar geochronology in high-pressure rocks (Sesia-Lanzo Zone, Western Alps)

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 126 (2014):475-494, doi:10.1016/j.gca.2013.10.023.in situ UV laser spot 40Ar/39Ar analyses of distinct phengite types in eclogite-facies rocks from the Sesia-Lanzo Zone (Western Alps, Italy) were combined with SIMS boron isotope analyses as well as boron (B) and lithium (Li) concentration data to link geochronological information with constraints on fluid-rock interaction. In weakly deformed samples, apparent 40Ar/39Ar ages of phengite cores span a range of ∼20 Ma, but inverse isochrons define two distinct main high-pressure (HP) phengite core crystallization periods of 88-82 Ma and 77-74 Ma, respectively. The younger cores have on average lower B contents (∼36 mg/g) than the older ones (∼43-48 mg/g), suggesting that loss of B and resetting of the Ar isotopic system were related. Phengite cores have variable d11B values (-18 to -10 ‰), indicating the lack of km scale B homogenization during HP crystallization. Overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40Ar/39Ar ages than the respective cores. They also show variable effects of heterogeneous excess 40Ar incorporation and Ar loss. One acceptable inverse isochron age of 77.1 ±1.1 Ma for rims surrounding older cores (82.6 ±0.6 Ma) overlaps with the second period of core crystallization. Compared to the phengite cores, all rims have lower B and Li abundances but similar d11B values (-15 to -9 ‰), reflecting internal redistribution of B and Li and internal fluid buffering of the B isotopic composition during rim growth. The combined observation of younger 40Ar/39Ar ages and boron loss, yielding comparable values of both parameters only in cores and rims of different samples, is best explained by a selective metasomatic overprint. In low permeability samples, this overprint caused recrystallization of phengite rims, whereas higher permeability in other samples led to complete recrystallization of phengite grains. Strongly deformed samples from a several km long, blueschist-facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40Ar/39Ar ages (64.7 to 68.8 Ma), yielding an inverse isochron age of 65.0 ±3.0 Ma. Almost complete B and Li removal in mylonitic phengite is due to leaching into a fluid. The B isotopic composition is significantly heavier than in phengites from the weakly deformed samples, indicating an external control by a high-d11B fluid (d11B = +7 ±4 ‰). We interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone. This event also caused partial resetting of the Ar isotope system and further B loss in more permeable rocks of the adjacent unit. We conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40Ar/39Ar data in partially metasomatized rocks.Funding of this work by the Deutsche Forschungsgemeinschaft (grant KO-3750/2-1) is gratefully acknowledged

Ein Beispiel aus der Sesia Zone (Westalpen)

0\. Title page and table of contents 1\. Chapter 1 1 PRÄAMBEL 1.1. What is the dissertation about? 1.2. Structure of the work 1.3. Scientific manuscripts 1.4. Organisation of the appendix 2\. Chapter 2 6 2.1. Abstract 2.2. Zusammenfassung 2.3. Introduction 3\. Chapter 3 20 INSIGHT INTO THE PHYSICO-CHEMICAL PROPERTIES OF A SUBDUCTED SLAB FROM GARNET ZONATION PATTERNS (SESIA ZONE, WESTERN ALPS) 4\. Chapter 4 59 KINEMATICS AND MECHANISMS OF EXHUMING SUBDUCTED CONTINENTAL CRUST IN THE SESIA ZONE, WESTERN ALPS 5\. Chapter 5 99 VOLATILES, MINERAL STABILITIES AND THEIR EFFECT ON THE FORCES DRIVING EXHUMATION OF SUBDUCTED CONTINENTAL CRUST IN THE SESIA ZONE (WESTERN ALPS) 6\. Chapter 6 129 THERMODYNAMIC MODELLING OF DIFFUSION-CONTROLLED GARNET GROWTH 7\. Chapter 7 154 SUMMARY AND OUTLOOK 8\. Chapter 8 160 REFERENCES Appendix 182 Colour Plates 203 Danksagung 211The Sesia Zone (SEZ) in the Western Alps is a large sliver of continentally derived crustal rocks that was subducted to depth exceeding 50 km. Because of its excellent preservation of high-pressure textures and structures, the SEZ is well-suited for the investigation of subduction- and exhumation-processes. To investigate the processes and mechanisms responsible for the subduction of the SEZ and its subsequent exhumation, we carried out structural mapping across large portions of the SEZ, petrologic investigations of structurally well defined samples as well as thermodynamic modelling and rheological calculations to better constrain the interplay of deformation, metamorphism and the physico-chemical properties of the high-pressure rocks. Constraints on the prograde Alpine metamorphism in the SEZ can be derived from the interpretation of garnet zonation patterns. Chemical growth zonation patterns in garnets from the central SEZ have therefore been investigated. These zonation patterns show a strong correlation between the shape of the garnet zonation and the sample locality. The comparison of observed garnet zonation patterns and those from thermodynamically models shows that the different zonation patterns are strongly influenced by differences in the water content of the subducted rocks as well as by the shape of the prograde P-T trajectory. The water content during burial significantly influences the physical properties of the subducted rocks, such that water-undersaturated rocks become denser than their water-saturated equivalents, facilitating the subduction of continental material, such as the SEZ rocks. The structural investigations revealed five regionally significant deformation phases. Initial exhumation of the Sesia Zone occurred along steeply dipping transpressive shear zones. At greenschist facies conditions sub-horizontally to moderately dipping normal faults develop in the structurally highest parts of the exhumed block, indicating a significant change in the orientation of the principal stress axes and possibly a change in the exhumation scenario from buoyancy-enhanced transpressional exhumation to extensional exhumation along moderately dipping normal faults. Growth zonation patterns in calcic amphibole indicate a change from near-isothermal decompression during transpressional exhumation to cooling during exhumation along the moderately dipping normal fault. Extensional exhumation from mid-crustal depths associated with further cooling indicates an exhumation mechanism that is possibly enhanced by hinge roll- back. Shear strength calculations together with petrological and structural constraints indicate a combination of buoyancy- and tectonically-driven exhumation that enables exhumation to mid-crustal levels along transpressive shear zones followed by further exhumation along moderately dipping normal faults. Ongoing cooling during normal faulting suggests further exhumation to be caused by syn-convergent crustal scale extension. Further, the numerical thermodynamic modelling showed that fractional garnet crystallisation as well as disequilibrium element incorporation in garnet leads to pronounced episodic garnet growth and may even cause growth interruptions. Discontinuous growth, together with pressure- and temperature-dependent changes in garnet chemistry cause zonation patterns that are indicative of different degrees of disequilibrium element incorporation. These typical garnet zonation patterns can be used to detect hindered element transport in metamorphic rocks.Die West-alpine Sesia Zone (SEZ) ist ein großer Span kontinentaler Kruste, der in Tiefen bis über 50 km subduziert wurde. Da in der SEZ alte Mineraltexturen und deformative Strukturen, in verschiedensten Maßstäben hervorragend erhalten sind, eignet sich die SEZ bestens zur Untersuchung von Subduktions- und Exhumierungsprozessen. Zur Untersuchung der Prozesse, die zur Subduktion und zur anschließenden Heraushebung der SEZ geführt haben, haben wir große Teile strukturell kartiert, haben detaillierte petrologische Untersuchungen an klar strukturell definierten Proben durchgeführt und außerdem thermodynamische und rheologische Modellierungen dieser Gesteine berechnet. Erkenntnisse über die prograde, subduktionsgebundene Metamorphose der Sesia-Gesteine können aus der Interpretation von Granat-Zonierungsmustern gewonnen werden. Wir untersuchten in der zentralen SEZ die chemischen Wachstumszonierungen in Granaten, die während der prograden Metamorphose gewachsen sind. Diese Wachstumszonierungen zeigen eine deutliche Korrelation zwischen Probenort und Form der Zonierung. Vergleiche zwischen natürlichen und modellierten Granat-Zonierungsmustern zeigen, dass die Unterschiede in der Zonierung auf unterschiedliche Wassergehalte des Wirtsgesteins und auf unterschiedliche Formen des P-T Pfades zurückzuführen sind. Der Wassergehalt während der Metamorphose hat signifikante Auswirkungen auf die physikalischen Eigenschaften der Gesteine. Wasseruntersättigte Gesteine werden dichter als ihre wassergesättigten Equivalente, was die Subduktion von kontinentalem Material erleichtert. Die Ergebnisse der strukturellen Untersuchungen zeigen, dass die SEZ durch fünf regional signifikante Deformationsphasen beeinflusst wurde. Die erste Exhumierung der SEZ fand entlang von steil stehenden transpressiven Strukturen statt. Unter grünschiefer-faziellen Bedingungen entwickeln sich dann in den strukturell höchsten Bereichen der SEZ sub-horizontale Abschiebungen, die eine Änderung der Spannungsachsen andeuten und einen Wechsel von auftriebsgesteuerter transpressiver Exhumierung unter Mantelbedingungen hin zu extensionsbetonter Exhumierung entlang von flachen Abschiebungen andeuten. Wachstumszonierungen in Ca-Amphibolen deuten einen Wechsel von isothermaler Dekompression während der transpressiven Exhumierung hin zu einer Abkühlung während der Heraushebung entlang der flachen Abschiebungen an. Exhumierung entlang von flachen Abschiebungen aus der mittleren Kruste, assoziiert mit Abkühlung, lässt darauf schließen, dass die Exhumierung durch hinge-rollback unterstützt wurde. Unsere Festigkeitsberechnungen zusammen mit den petrologischen und struturgeologischen Daten deuten auf eine Kombination aus auftriebs- und tektonikgetriebener Exhumierung hin, die es erlaubt, die Gesteine entlang von transpressiven Scherzonen bis in die mittlere Kruste zu bringen und dann entlang von flachen Abschiebungen weiter herauszuheben. Des Weiteren zeigen unsere thermodynamischen Modellierungen, dass fraktionierte Granat Kristallisation und auch Ungleichgewichtseinbau von Elementen in Granat zu betont episodischem Wachstum führt und Wachstumsunterbrechungen hervorrufen kann. Diese typischen Zonierungsmuster können verwendet werden um behinderten Stofftransport im Gestein zu detektieren

Fluid migration above a subducted slab-constraints on amount, pathways and major element mobility from partially overprinted eclogite-facies Rocks (sesia zone, western alps)

The Western Alpine Sesia-Lanzo Zone (SLZ) is a sliver of eclogite-facies continental crust exhumed from mantle depths in the hanging wall of a subducted oceanic slab. Eclogite-facies felsic and basic rocks sampled across the internal SLZ show different degrees of retrograde metamorphic overprint associated with fluid influx. The weakly deformed samples preserve relict eclogite-facies mineral assemblages that show partial fluid-induced compositional re-equilibration along grain boundaries, brittle fractures and other fluid pathways. Multiple fluid influx stages are indicated by replacement of primary omphacite by phengite, albitic plagioclase and epidote as well as partial re-equilibration and/or overgrowths in phengite and sodic amphibole, producing characteristic step-like compositional zoning patterns. The observed textures, together with the map-scale distribution of the samples, suggest open-system, pervasive and reactive fluid flux across large rock volumes above the subducted slab. Thermodynamic modelling indicates a minimum amount of fluid of 0 center dot 1-0 center dot 5 wt % interacting with the wall-rocks. Phase relations and reaction textures indicate mobility of K, Ca, Fe and Mg, whereas Al is relatively immobile in these medium-temperature-high-pressure fluids. Furthermore, the thermodynamic models show that recycling of previously fractionated material, such as in the cores of garnet porphyroblasts, largely controls the compositional re-equilibration of the exhumed rock body

Combined thermodynamic and rare earth element modelling of garnet growth during subduction : examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway

Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Western Gneiss Region (Norway). All investigated garnets show multiple growth zones and preserve complex growth zonation patterns with respect to both major and rare earth elements (REE). Due to chemical differences of the host rocks two types of major element compositional zonation patterns occur: (1) abrupt, step-like compositional changes corresponding with the growth zones and (2) compositionally homogeneous interiors, independent of growth zones, followed by abrupt chemical changes towards the rims. Despite differences in major element zonation, the REE patterns are almost identical in all garnets and can be divided into four distinct zones with characteristic patterns. In order to interpret the major and trace element distribution and zoning patterns in terms of the subduction history of the rocks, we combined thermodynamic forward models for appropriate bulk rock compositions to yield molar proportions and major element compositions of stable phases along the inferred pressure-temperature path with a mass balance distribution of REEs among the calculated stable phases during high pressure metamorphism. Our thermodynamic forward models reproduce the complex major element zonation patterns and growth zones in the natural garnets, with garnet growth predicted during four different reaction stages: (1) chlorite breakdown, (2) epidote breakdown, (3) amphibole breakdown and (4) reduction in molar clinopyroxene at ultrahigh-pressure conditions. Mass-balance of the rare earth element distribution among the modelled stable phases yielded characteristic zonation patterns in garnet that closely resemble those in the natural samples. Garnet growth and trace element incorporation occurred in near thermodynamic equilibrium with matrix phases during subduction. The rare earth element patterns in garnet exhibit distinct enrichment zones that fingerprint the minerals involved in the garnet-forming reactions as well as local peaks that can be explained by fractionation effects and changes in the mineral assemblage.11 page(s

Microplastics in Glaciers: First Results from the Vatnajökull Ice Cap

Microplastic particles, as a second-phase material in ice, may contribute to the effect such particles have on the melting and rheological behaviour of glaciers, and thus influence the future meltwater contribution to the oceans and rising sea levels. Hence, it is of the utmost importance to map and understand the presence and dispersal of microplastics on a global scale. In this work, we identified microplastic particles in snow cores collected in a remote and pristine location on the Vatnajökull ice cap in Iceland. Utilising optical microscopy and µ-Raman spectroscopy, we visualised and identified microplastic particles of various sizes and materials. Our findings support that atmospheric transport of microplastic particles is one of the important pathways for microplastic pollution