Thomas Pettke

Universität Bern | UniBe · Institute of Geological Sciences

Characterizing fluid circulation in orogens is key to understanding orogenic processes because fluid–rock interaction modifies the physical properties of rocks, hence their response to deformation and, for example, their suitability for radioactive waste storage. Fluid circulation can be dated by applying geochronological methods to fluid-precipita...

Partial melting and melt evolution and migration play an important role in the differentiation of continental crust. These processes affect metal transport and concentration, volatile cycling, and the mechanical properties of the crust. The study of nanorocks, i.e. crystallised melt inclusions [1], has emerged in recent decades as a key approach in...

Many epithermal gold and gold-rich porphyry-type ore deposits are associated with potassic magmas. Hence, potassic magmas are commonly assumed to have been unusually Au-rich or to have contained high Au/Cu ratios. However, these hypotheses remain poorly tested. Here, we report Au concentrations and Au/Cu ratios in silicate melt inclusions analyzed...

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the compl...

crust in orogens from rifting to tectonic inversion. Since granitic rocks have low primary water contents and dominate the continental crust, their hydration exerts important control on metamorphism and deformation of the continental crust during orogeny. It is therefore of great interest to determine whether the granitic continental crust is hydra...

Oxide minerals contained in ultramafic rocks are useful tools to assess the redox conditions of the rock and fluids liberated upon progressive serpentinite dehydration during subduction, as these minerals contain a relevant redox-sensitive element, iron. Previous studies have revealed that magnetite predominates across the antigorite-out reaction....

The preserved Archean continental crust is dominantly comprised of tonalite-trondhjemite-granodiorite (TTG) suites associated with less abundant low-grade greenstone belts. The exact processes that form TTGs, as well as the source rock they are derived from, are difficult to constrain from the sparse Archean geological record. However, studies show...

Crystallization-differentiation of magmas and/or fluid migration in arc settings are important drivers of stable-isotope fractionation of elements like titanium and molybdenum. For both isotope systems, evolved magmas are heavier than average arc-basalts, which requires an isotopically light reservoir counterbalancing the heavy felsic lithologies....

Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals under given P–T conditions. Some rock-forming minerals like quartz and feldspars have been intensively studied in terms of deforma...

Hydrothermal veins and altered feldspar are evidence for fluid circulation in granitic rocks in the continental crust. The hydrothermal alteration of feldspar affects the deformation behavior of granitoids, especially if it occurs before orogeny. Geochronology can establish the timing of fluid circulation to determine if this fluid-driven alteratio...

Eclogite-hosted metamorphic veins mark former fluid migration pathways during a subduction- exhumation cycle, and allow to trace fluid-mediated element transfer across lithologies, to ultimately metasomatize the mantle wedge. Fluids can be generated by dehydration reactions at different P-T conditions in various lithologies, all influencing how dif...

Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals at given P–T conditions. Common rock-forming minerals like quartz, feldspars and sheet silicates have been intensively studied in...

The Fe isotope systematics of subducted lithologies are crucial for the understanding of redox-dependent mass transfer in subducting slabs, with consequences for the compositions of arc magmas and of the deep mantle. We investigated eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif to unravel whether their Fe isotope compo...

Serpentinites are central to water (re)cycling in subduction zones and thus effect fluid-mediated element transfer between the hydrosphere and the Earth's mantle and back into continental crust via calcalkaline magmatism. Diverse and often controversial models exist on the relevance of various source contributions to the budget of fluid mobile elem...

Low-temperature chronometers offer potential to gain insights into the temporal evolution of hydrothermal systems. The long-lived fault-bound Grimsel pass hydrothermal system (including a fossil and an active part) in the Central European Alps serves here as a key site to test such an application. Zircon and apatite grains were separated from sampl...

Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in su...

The Cima Lunga unit in the Central Alps is dominated by quartzofeldspathic gneisses with subordinate mafic, ultramafic, and metacarbonate rocks. Only mafic and ultramafic lithologies were thought to preserve clear evidence of Alpine high-pressure metamorphism. This led to the questions of whether the different rock types were subducted and exhumed...

Eclogites, metagabbros, and paragneisses from the Variscan Münchberg Massif record a complex succession of igneous, hydrothermal and metamorphic processes. The geodynamic setting related to the protolith formation and the impact of different types of fluid-rock interactions have been uncertain up to now. We use major and trace element chemistry as...

Terrestrial and extraterrestrial rocks exhibit significant mass-dependent Ti isotopic variations, with basalts being isotopically lighter than evolved lithologies. The observed trend from light to heavy Ti isotopic compositions from more primitive to more differentiated rocks agrees with theoretical predictions that light Ti isotopes are sequestere...

Molybdenum isotopes are an established proxy for paleoredox conditions in low-temperature surface systems. However, the mechanisms behind demonstrated Mo isotope fractionation during igneous and hydrothermal processes at elevated temperatures are still controversial. This study focusses on a comprehensive dataset documenting the late stage magmatic...

Molybdenum isotopes have emerged as novel tracers for high-temperature igneous and metamorphic processes. The debate remains to what extent different subducted slab lithologies, such as oceanic crust and marine sediments, contribute to the Mo isotope signature of arc magmas and, hence, exert different controls on the terrestrial Mo cycle. Here we i...

Epidote – here defined as minerals belonging to the epidote–clinozoisite solid solution – is a low-μ (μ=238U/204Pb) mineral occurring in a variety of geological environments and participating in many metamorphic reactions that is stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U-...

The processes of carbon mobilisation at convergent plate boundaries are hotly debated. Recent findings suggest that carbon release along subduction geothermal gradients may well be more relevant than previously thought; however, it has remained difficult to achieve steady state atmospheric CO2 conditions over geological time scales based on current...

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon cr...

An amendment to this paper has been published and can be accessed via the original article.

Ion probe 208Pb/232Th fissure monazite ages from the Argentera External Massif and from the high-pressure units of the Western Alps provide new insights on its Cenozoic tectonic evolution. Hydrothermal monazite crystallizes during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolutio...

Monoclinic epidote is a low-µ (µ = 283U / 204Pb) mineral occurring in a variety of geological environments, participating in many metamorphic reactions and stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U–Pb geochronometer has been hindered by the commonly high contents of initi...

The ~4500-m-deep Iceland Deep Drilling Project (IDDP) borehole IDDP-2 in Iceland penetrated the root of an active seawater-recharged hydrothermal system below the Mid-Atlantic Ridge. As direct sampling of pristine free fluid was impossible, we used fluid inclusions to constrain the in situ conditions and fluid composition at the bottom of the hydro...

The chemical and physical processes operating during subduction-zone metamorphism can profoundly influence the cycling of elements on Earth. Deep-Earth carbon (C) cycling and mobility in subduction zones has been of particular recent interest to the scientific community. Here, we present textural and geochemical data (CO, Sr isotopes and bulk and i...

The mass balance of molybdenum (Mo) was studied in the Strengbach catchment. Monitoring of rainfall, vegetation, and soil characteristics in this 0.8 km2 catchment was started decades ago. We present Mo concentrations and isotope compositions of about 60 samples including bedrock types, perennial springs, soil profiles, roots and leaves, and the ou...

The Erro Tobbio olivine-antigorite serpentinites and associated dehydration veins represent hydrated oceanic mantle rocks that escaped complete dehydration and recycling into the mantle after subduction to ~ 550-600 °C and 2.0-2.5 GPa. These rocks thus offer valuable insights into the petrological evolution of a slice of hydrated oceanic mantle and...

The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpent...

The Lago Superiore Unit (LSU, Monviso Massif, Italian Western Alps) is a section of fossil oceanic lithosphere equilibrated to eclogite facies conditions (550 °C – 2.8 GPa) during Alpine subduction (45–40 Ma). It is cut by two major shear zones, namely the Intermediate (ISZ) and Lower Shear Zone (LSZ), mostly consisting of serpentinite. The lowermo...

Key Points Mylonitic metagabbros cemented by unfoliated Omp + Grt ± Lws matrices show that pristine brecciation may occur at eclogitic conditions Geochemical variations and sharp increase in fluid content of matrices points to local embrittlement prior to external fluids ingression Structural and geochemical evidence suggest that eclogite‐facies br...

In the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing serpentinites (the...

Serpentinites are central to carry water and fluid-mobile elements down subduction zones. The breakdown of antigorite represents the most prominent aqueous fluid release, boosting fluid-mediated element cycling from the slab to the mantle wedge. At Cerro del Almirez, Spain, an antigorite dehydration reaction front is preserved in subducted serpenti...

All instruments are suitable for LA. All instruments are capable to produce good results in combination with LA. No clear recommendation can be given. Selection depends on focus of the instrument (e.g. fastest quadrupole, low argide formation, best abundance sensitivity, best low mass LOD) and personal preference (e.g. hardware handling and softwar...

Of the isotopically distinctive mantle domains, the so-called HIMU (“high-μ”; μ = ²³⁸U/²⁰⁴Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high ²⁰⁶Pb/²⁰⁴Pb (reaching 56) but unchanged ²⁰⁸Pb/²⁰⁴Pb. Similar,...

The subduction-related volcanic front in Nicaragua consists of the Tertiary “Coyol” member in the eastern highlands and the Quaternary to recent volcanic arc within the Nicaraguan depression. Although the Holocene to recent explosive volcanism has been studied extensively no detailed work has been done on the products of explosive volcanism from Qu...

Relationships between cation substitution and hydrogen-bond system in hydrous pyroxenoids with a twisted chain consisting of three SiO4 tetrahedra are investigated by means of chemical and X-ray single-crystal structural analysis. Minerals with the common crystal-chemical formulaM3(Me¹⁺)M1,M2(Me²⁺)2[Si3O8(OH)], Me¹⁺ = Na, Li and Me²⁺ = Ca, Mn, Mg,...

Recent studies report a large Mo isotope variability of up to 1‰ (expressed in δ98/95MoNIST3134) in convergent margin lavas. These isotopic variations have been associated with subduction zone processes and ultimately may account for heavy and variable isotope signatures in evolved continental crust. Arc lavas show both, lighter and heavier Mo isot...

We experimentally investigated the dissolution of forsterite, enstatite and magnesite in graphite-saturated COH fluids, synthesized using a rocking piston cylinder apparatus at pressures from 1.0 to 2.1 GPa and temperatures from 700 to 1200 °C. Synthetic forsterite, enstatite, and nearly pure natural magnesite were used as starting materials. Redox...

Estimates of dissolved CO2 in subduction-zone fluids are based on thermodynamic models, relying on a very sparse experimental data base. Here, we present experimental data at 1–3 GPa, 800 °C, and ∆FMQ ≈ −0.5 for the volatiles and solute contents of graphite-saturated fluids in the systems COH, SiO2–COH ( + quartz/coesite) and MgO–SiO2–COH ( + forst...

Fluid-mobile element (FME) systematics in serpentinites are key to unravel the environments of mantle rock hydration, dehydration, and element recycling in subduction zones. Here we compile serpentinite geochemical data and, for the first time, report discriminative FME enrichment trends for mid ocean ridge vs. forearc serpentinisation by applying...

The study of coexisting inclusions of multiple fluid phases (aqueous liquid, vapour, salt, and silicate melt) allows for direct quantification of the partitioning behaviour of elements between residual water-saturated melt and aqueous fluid at the magmatic-hydrothermal transition. This stage is decisive for the mass transfer of elements between geo...

To make use of the full geochronological potential of accessory minerals such as zircon, allanite, monazite, and titanite, high spatial resolution isotopic analysis of the Th-U-Pb system is required. Laser ablation ICP-MS techniques are increasingly applied for this purpose, yet the matrix-dependence of analysis and the paucity of high-quality stan...