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Tectonics and geothermal gradients from subduction to collision in the NW Variscan Iberian Massif
(2022)
The earliest tectonometamorphic record of tectonic slices incorporated to the base of an orogen holds the key to understand how an orogen is built. The tectonic pile of the NW Iberian section of the Variscan Orogen includes tectonic slices separated by crustal-scale thrusts. The earliest tectonometamorphic record in the uppermost parautochthon is calculated at 11–14 kbar and 450–500°C (P-T gradient about 13°C/km), suggesting a subduction-related metamorphic recrystallization at lower pressure than the overlying Lower Allochthon. Early conditions calculated in the autochthon (9–10 kbar and 425–450°C; 16°C/km) point to a relatively ‘cold’ collisional setting. Higher thermal gradients obtained from some sections of the autochthon (11–12 kbar and 700–725°C; 21°C/km) and the Lower Parautochthon (7.5 kbar and 550–700°C; 24–31°C/km), correspond to more advanced and ‘hot’ stages of collision. New U–Pb monazite geochronology indicates a 318–311 Ma age for the final formation of HT domes in the region. We propose the rapid decrease in P-T gradient (from <10 to 16°C/km) documents a fail to sustain further burial along a regular subduction zone. We consider the subsequent increase in the geothermal gradient (from 16 to 31°C/km) as the culmination of previous crustal accretion and the onset of crustal underthrusting and later processes in a collisional stage. We propose these switches in the early tectonometamorphic record of individual tectonic slices as potential markers to track the transition from subduction to collision in collisional orogens.
While high-quality climate reconstructions of some past warm periods in the Cenozoic era now exist, the geological processes responsible for driving the observed longterm changes in atmospheric CO2 are not sufficiently well understood. The long-term change in atmospheric CO2 across the Cenozoic has been proposed to be driven by processes such as terrestrial weathering, organic carbon production and burial, reverse weathering, and volcanic degassing. One way of constraining the relative importance of the various driving forces proposed so far is to better understand the degree to which ocean chemistry has changed because the chemistry of seawater responds to geologic processes that drive atmospheric CO2. In addition, knowledge of the concentration of the major elements in seawater is crucial for accurately applying proxies such as those based on the boron isotopic composition and Mg/Ca of marine carbonates (a proxy for palaeo pH/CO2 and palaeotemperature, respectively). Previously reported records of seawater composition are primarily derived from fluid inclusions in marine evaporites; however, the results are sparse due to the limited availability of such deposits. In this thesis, changes in the Eocene seawater chemistry were reconstructed using trace element (elements/Ca) and isotopic (δ26Mg) proxies in a Larger Benthic Foraminifera (LBFs), i.e., Nummulites sp., to constrain the driving processes of long-term changes in seawater chemistry.
To achieve the objective of this thesis, first, a measurement protocol was established using LA-ICPMS to measure the K/Ca ratio simultaneously with other element/calcium ratios, which is challenging due to the interference of ArH+ on K+. Utilising this newly established measurement protocol, laboratory-cultured Operculina ammonoides grown at different seawater calcium concentrations ([Ca2+]), repeated at different temperatures, as well as modern O. ammonoides collected from different regions exhibiting a range of seawater parameters, were investigated. A significant correlation was observed between K/Casw and K/CaLBF, allowing K/CaLBF to potentially be used as a proxy for seawater major ion reconstructions. In addition, modern O. ammonoides demonstrated no significant influence of most seawater parameters (temperature, salinity, pH, or [CO32-]) on K/CaLBF. Modern O.
ammonoides were also assessed for their Mg isotopic composition (δ26Mg), revealing no significant effect of temperature or salinity on δ26MgLBF. Furthermore, the Mg isotopic fractionation in O. ammonoides was found to be close to that of inorganic calcite, indicating minimal vital effects in these large benthic foraminifera.
Operculina ammonoides is the nearest living relative of the abundant Eocene genus Nummulites, enabling the reconstruction of seawater chemistry using the calibration based on O. ammonoides. The trace elemental/calcium proxies, including Na/Ca, K/Ca, and Mg/Ca, as well as the δ26Mg proxy, were investigated in Eocene Nummulites. The result showed that during the Eocene, [Ca2+]sw was 1.6-2 times higher, while [K+]sw was ~2 times lower than the modern seawater composition. Furthermore, [Mg2+]sw decreased from the early Eocene (54.3− +9 7..69 mmol kg-1 at ~55 Ma) to Late Eocene (37.8− +4 4..3 4 mmol kg-1 at ~31 Ma), followed by
an increase toward modern seawater [Mg]. In contrast, the variability in δ26Mgsw values remained within a narrow range of ~0.3 ‰ throughout the Cenozoic. The reconstructed [Ca2+]sw agrees with the suggestion that Cenozoic seawater chemistry changes can be explained via a change in the seafloor spreading rate. When combined with existing records, the observed minimal change in δ26Mgsw with an increase in [Mg2+]sw suggests an additional possible role of a decrease in the formation of authigenic clay minerals coincident with the Cenozoic decline in deep ocean temperature, which is also supported by the increase in the [K+]sw reconstructed here for the first time. This finding highlights that the reduction in seafloor-spreading rate and decline in reverse weathering during the Cenozoic era has played a significant role in the evolution of seawater chemistry, emphasizing the importance of these processes in driving long-term changes in the carbon cycle.
Upper mantle shear zones are complex systems where deformation is commonly closely interacting with metamorphic (solid-solid) and/or melt/fluid-rock reactions. Here, feedback processes between deformation, reactions, grain size reduction and phase mixing result in strain weakening and the localization of deformation. The expression of these interlinked processes is portrayed by the microfabrics of strained peridotites and pyroxenites. The present thesis is focusing on these processes and their impact on the deformation in three upper mantle shear zones situated in the peridotite massifs of Lanzo (Italian Alps), Erro-Tobbio (Italian Alps) and Ronda (Betic Cordillera, Spain). In all three shear zones, the presence of melt led to phase mixing either by interstitial crystallization of pyroxenes from a Si-saturated and partially also highly evolved melt or by melt-rock reactions of pyroxene porphyroclasts with a Si-undersaturated melt. The effect of melt on the localization of strain is twofold and variable. Enhanced deformation by melt-wetted boundaries is assumed for all shear zones. Additionally, phase mixing by crystallization of interstitial pyroxenes or melt-rock reactions reduce or maintain the grain size by the formation of fine grained neoblasts and secondary phase boundary pinning. In this regard, pre- to early syn-kinematic, map-scale percolation of OH-bearing, evolved melts in the NW Ronda peridotite massif and the associated crystallization of interstitial pyroxenes result in the activation of grain size sensitive deformation mechanisms in the entire melt-effected area. In the rocks collected at Erro-Tobbio, syn-kinematic melt-rock reactions of pyroxene porphyroclasts and Si-undersaturated melt led to the formation of ultramylonitic neoblast tails (grain size ~10 μm). Compared to the adjacent coarser-grained olivine-dominated matrix, the activation of diffusion creep led to an increase in the strain rate by an order of magnitude within interconnected ultramylonitic layers. Strain localization and softening in ultramylonitic layers are also documented in the Lanzo samples. Neoblast tails of pyroxene porphyroclasts were likewise identified as their precursor. The phase assemblage of the tails, including ortho- and clinopyroxene, olivine, plagioclase, and spinel (± amphibole), and their geochemical trends suggest, unlike in Erro-Tobbio, a formation by continuous net-transfer reactions enhanced by the spinel lherzolite to plagioclase lherzolite transition.
The new results obtained from the three studied shear zones underscore the importance of reactions for the interlinked processes of grain size reduction, phase mixing, strain localization and strain softening in upper mantle shear zones. Concerning strain localization, the nature of the reaction (solid-solid, melt/fluid-rock) seems to play a subordinate role compared to its timing. Pre- to early syn-kinematic melt-triggered reactions result in strain localization along map-scale shear zones. Late stage syn-kinematic melt-rock or metamorphic reactions under high stress conditions are capable of localizing the deformation along discrete, sub-centimeter thick ultramylonites.
The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage.
The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/− PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80–120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.
Reconstructing Oligocene-Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth’s surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps, however, remain scarce. Here we present stable and clumped isotope geochemistry 15 measurements to provide a new paleoelevation estimate for the mid-Miocene (~14.5 Ma) European Central Alps. We apply stable isotope δ-δ paleoaltimetry on near sea level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different 20 alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30 ± 5°C) yield a near sea level precipitation δ18Ow value of -5.8 ± 0.2‰ and in conjunction with the high-Alpine phyllosilicate δD record suggest that the region surrounding the SFZ attained surface elevations of >4000 m no later than the mid-Miocene. Our near sea level δ18Ow estimate is supported by paleoclimate (iGCM Echam5-wiso) modeled δ18O values, which vary between -4.2 and -7.6‰ for the Northern Alpine Foreland Basin.
Stratospheric inorganic chlorine (Cly) is predominantly released from long-lived chlorinated source gases and, to a small extent, very short-lived chlorinated substances. Cly includes the reservoir species (HCl and ClONO2) and active chlorine species (i.e., ClOx). The active chlorine species drive catalytic cycles that deplete ozone in the polar winter stratosphere. This work presents calculations of inorganic chlorine (Cly) derived from chlorinated source gas measurements on board the High Altitude and Long Range Research Aircraft (HALO) during the Southern Hemisphere Transport, Dynamic and Chemistry (SouthTRAC) campaign in austral late winter and early spring 2019. Results are compared to Cly in the Northern Hemisphere derived from measurements of the POLSTRACC-GW-LCYCLE-SALSA (PGS) campaign in the Arctic winter of 2015/2016. A scaled correlation was used for PGS data, since not all source gases were measured. Using the SouthTRAC data, Cly from a scaled correlation was compared to directly determined Cly and agreed well. An air mass classification based on in situ N2O measurements allocates the measurements to the vortex, the vortex boundary region, and midlatitudes. Although the Antarctic vortex was weakened in 2019 compared to previous years, Cly reached 1687±19 ppt at 385 K; therefore, up to around 50 % of total chlorine was found in inorganic form inside the Antarctic vortex, whereas only 15 % of total chlorine was found in inorganic form in the southern midlatitudes. In contrast, only 40 % of total chlorine was found in inorganic form in the Arctic vortex during PGS, and roughly 20 % was found in inorganic form in the northern midlatitudes. Differences inside the two vortices reach as much as 540 ppt, with more Cly in the Antarctic vortex in 2019 than in the Arctic vortex in 2016 (at comparable distance to the local tropopause). To our knowledge, this is the first comparison of inorganic chlorine within the Antarctic and Arctic polar vortices. Based on the results of these two campaigns, the differences in Cly inside the two vortices are substantial and larger than the inter-annual variations previously reported for the Antarctic.
Biogenic organic precursors play an important role in atmospheric new particle formation (NPF). One of the major precursor species is α-pinene, which upon oxidation can form a suite of products covering a wide range of volatilities. Highly oxygenated organic molecules (HOMs) comprise a fraction of the oxidation products formed. While it is known that HOMs contribute to secondary organic aerosol (SOA) formation, including NPF, they have not been well studied in newly formed particles due to their very low mass concentrations. Here we present gas- and particle-phase chemical composition data from experimental studies of α-pinene oxidation, including in the presence of isoprene, at temperatures (−50 and −30 ∘C) and relative humidities (20 % and 60 %) relevant in the upper free troposphere. The measurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD) chamber. The particle chemical composition was analyzed by a thermal desorption differential mobility analyzer (TD-DMA) coupled to a nitrate chemical ionization–atmospheric pressure interface–time-of-flight (CI-APi-TOF) mass spectrometer. CI-APi-TOF was used for particle- and gas-phase measurements, applying the same ionization and detection scheme. Our measurements revealed the presence of C8−10 monomers and C18−20 dimers as the major compounds in the particles (diameter up to ∼ 100 nm). Particularly, for the system with isoprene added, C5 (C5H10O5−7) and C15 compounds (C15H24O5−10) were detected. This observation is consistent with the previously observed formation of such compounds in the gas phase. However, although the C5 and C15 compounds do not easily nucleate, our measurements indicate that they can still contribute to the particle growth at free tropospheric conditions. For the experiments reported here, most likely isoprene oxidation products enhance the growth of particles larger than 15 nm. Additionally, we report on the nucleation rates measured at 1.7 nm (J1.7 nm) and compared with previous studies, we found lower J1.7 nm values, very likely due to the higher α-pinene and ozone mixing ratios used in the present study.
Reconstructing Oligocene–Miocene paleoelevation contributes to our understanding of the evolutionary history of the European Alps and sheds light on geodynamic and Earth surface processes involved in the development of Alpine topography. Despite being one of the most intensively explored mountain ranges worldwide, constraints on the elevation history of the European Alps remain scarce. Here we present stable and clumped isotope measurements to provide a new paleoelevation estimate for the mid-Miocene (∼14.5 Ma) European Central Alps. We apply stable isotope δ–δ paleoaltimetry to near-sea-level pedogenic carbonate oxygen isotope (δ18O) records from the Northern Alpine Foreland Basin (Swiss Molasse Basin) and high-Alpine phyllosilicate hydrogen isotope (δD) records from the Simplon Fault Zone (Swiss Alps). We further explore Miocene paleoclimate and paleoenvironmental conditions in the Swiss Molasse Basin through carbonate stable (δ18O, δ13C) and clumped (Δ47) isotope data from three foreland basin sections in different alluvial megafan settings (proximal, mid-fan, and distal). Combined pedogenic carbonate δ18O values and Δ47 temperatures (30±5 ∘C) yield a near-sea-level precipitation δ18Ow value of ‰ and, in conjunction with the high-Alpine phyllosilicate δD value of ‰, suggest that the region surrounding the Simplon Fault Zone attained surface elevations of >4000 m no later than the mid-Miocene. Our near-sea-level δ18Ow estimate is supported by paleoclimate (iGCM ECHAM5-wiso) modeled δ18O values, which vary between −4.2 ‰ and −7.6 ‰ for the Northern Alpine Foreland Basin.
Stratospheric inorganic chlorine (Cly) is predominantly released from long-lived chlorinated source gases and, to a small extent, very short-lived chlorinated substances. Cly includes the reservoir species (HCl and ClONO2) and active chlorine species (i.e., ClOx). The active chlorine species drive catalytic cycles that deplete ozone in the polar winter stratosphere. This work presents calculations of inorganic chlorine (Cly) derived from chlorinated source gas measurements on board the High Altitude and Long Range Research Aircraft (HALO) during the Southern Hemisphere Transport, Dynamic and Chemistry (SouthTRAC) campaign in austral late winter and early spring 2019. Results are compared to Cly in the Northern Hemisphere derived from measurements of the POLSTRACC-GW-LCYCLE-SALSA (PGS) campaign in the Arctic winter of 2015/2016. A scaled correlation was used for PGS data, since not all source gases were measured. Using the SouthTRAC data, Cly from a scaled correlation was compared to directly determined Cly and agreed well. An air mass classification based on in situ N2O measurements allocates the measurements to the vortex, the vortex boundary region, and midlatitudes. Although the Antarctic vortex was weakened in 2019 compared to previous years, Cly reached 1687±19 ppt at 385 K; therefore, up to around 50 % of total chlorine was found in inorganic form inside the Antarctic vortex, whereas only 15 % of total chlorine was found in inorganic form in the southern midlatitudes. In contrast, only 40 % of total chlorine was found in inorganic form in the Arctic vortex during PGS, and roughly 20 % was found in inorganic form in the northern midlatitudes. Differences inside the two vortices reach as much as 540 ppt, with more Cly in the Antarctic vortex in 2019 than in the Arctic vortex in 2016 (at comparable distance to the local tropopause). To our knowledge, this is the first comparison of inorganic chlorine within the Antarctic and Arctic polar vortices. Based on the results of these two campaigns, the differences in Cly inside the two vortices are substantial and larger than the inter-annual variations previously reported for the Antarctic.
Biogenic organic precursors play an important role in atmospheric new particle formation (NPF). One of the major precursor species is α-pinene, which upon oxidation can form a suite of products covering a wide range of volatilities. Highly oxygenated organic molecules (HOMs) comprise a fraction of the oxidation products formed. While it is known that HOMs contribute to secondary organic aerosol (SOA) formation, including NPF, they have not been well studied in newly formed particles due to their very low mass concentrations. Here we present gas- and particle-phase chemical composition data from experimental studies of α-pinene oxidation, including in the presence of isoprene, at temperatures (−50 and −30 ∘C) and relative humidities (20 % and 60 %) relevant in the upper free troposphere. The measurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD) chamber. The particle chemical composition was analyzed by a thermal desorption differential mobility analyzer (TD-DMA) coupled to a nitrate chemical ionization–atmospheric pressure interface–time-of-flight (CI-APi-TOF) mass spectrometer. CI-APi-TOF was used for particle- and gas-phase measurements, applying the same ionization and detection scheme. Our measurements revealed the presence of C8−10 monomers and C18−20 dimers as the major compounds in the particles (diameter up to ∼ 100 nm). Particularly, for the system with isoprene added, C5 (C5H10O5−7) and C15 compounds (C15H24O5−10) were detected. This observation is consistent with the previously observed formation of such compounds in the gas phase. However, although the C5 and C15 compounds do not easily nucleate, our measurements indicate that they can still contribute to the particle growth at free tropospheric conditions. For the experiments reported here, most likely isoprene oxidation products enhance the growth of particles larger than 15 nm. Additionally, we report on the nucleation rates measured at 1.7 nm (J1.7 nm) and compared with previous studies, we found lower J1.7 nm values, very likely due to the higher α-pinene and ozone mixing ratios used in the present study.