Friedrich Lucassen

We investigated mafic igneous rocks found in the vicinity of the allochthonous Münchberg Massif in the Saxo-Thuringian Zone of the European Variscides, NE Bavaria. The basaltic magmas were injected during the Ordovician, Silurian and Devonian into contemporaneous marine sedimentary rocks of the Saxo-Thuringian Basin. Ordovician sills east and Devonian volcanic to subvolcanic rocks west of the Münchberg Massif show high Fe and Ti contents that place them in the rarely occurring group of high Fe–Ti basalts with FeO content of up to 17.6 wt% and up to 5.7 wt% TiO₂. Covariation of Nb, Y and Zr with major elements indicates compositional control by fractionation. Trace element patterns are similar to those of ocean island basalt and continental intraplate magmatic rocks. Nd isotope signatures of the mafic rocks from the Early Palaeozoic until the Devonian resemble those of the regional continental lithospheric mantle of the Saxo-Thuringian and Bohemian zones. Their Sr isotope signatures show the variable influence of post-magmatic mobilisation. The major source of these mafic rocks is likely the subcontinental lithospheric mantle. The Pb isotope compositions of the mafic rocks do not allow distinguishing between mantle and crustal sources but resemble the regional crustal signatures. Crustal contamination in the mafic rocks is minor. The geochemical signatures of the mafic igneous rocks and continuous deposition of their host sediments indicate a long-standing passive margin setting between the northern part of Gondwana and the Saxo-Thuringian. In contrast, the trace element patterns and isotope characteristics of post-Devonian mafic dykes in the allochthonous Münchberg Massif are significantly different: They are related to a late stage of subduction during the Variscan orogeny. Graphical abstract

Variations of strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) in river systems are increasingly utilised to geochemically trace origin and movement patterns of migratory fish species. Accretionary calcified structures, such as otoliths, preserve ⁸⁷Sr/⁸⁶Sr signatures of the surrounding water during a fish’s lifetime. In this study, we present ⁸⁷Sr/⁸⁶Sr measurements of water samples and catfish otoliths collected in the estuaries of the Sine-Saloum (Senegal), the Gambia River (The Gambia), and the Volta River (Ghana) to assess their systematics and relationships with salinity. The three rivers possess distinct hydrological properties resulting in variable degrees of correlations between ⁸⁷Sr/⁸⁶Sr and salinity. The Gambia River (⁸⁷Sr/⁸⁶Sr of ~ 0.71209) proved exceptionally preconditioned for the approach due to well-defined geochemical end-members, allowing for quantitative estimates of salinity based on otolith ⁸⁷Sr/⁸⁶Sr measurements. The Volta River (~ 0.71392) presents a more complex case due the possible influence of multiple water sources to the main channel, while the inverse salinity gradient and excessive evaporation in the Sine-Saloum estuary (~0.70915) impede any significant correlations between ⁸⁷Sr/⁸⁶Sr and salinity. Bulk otolith ⁸⁷Sr/⁸⁶Sr values in the Gambia River and Volta River clearly depicted a mixed influence of seawater and riverine compositions, strongly encouraging the application of this approach for geochemical fingerprinting of critical NW African species.

Geochemical and geochronological data reveal that late Oligocene‐early Miocene time is a break point in the evolution of Andean magmatism. The Patagonian Andes registered the onset of arc volcanism since the late Eocene forming part of the El Maitén Belt, whose development was driven by the subduction of the Farallon/Nazca plates beneath the Andean margin. During the Oligocene, the El Maitén Belt shows a change in the geochemical signature of its magmas from tholeiitic to calc‐alkaline compositions, reflecting a more mature stage in the magmatic arc evolution. Toward the early Miocene, a striking event is registered in Andean volcanic sequences as mafic tholeiitic lava flows of the El Maitén are interbedded with marine deposits, suggesting their development in the context of a fast subsiding regime. Geochemical analyses presented in this paper show that these rocks resemble enriched mid‐ocean ridge basalt‐like and ocean island basalt compositions, isotopically depleted, which strongly contrast with previous arc products. By this time, a global plate reorganization event had caused an increase in convergence rates, accelerated rollback, and a more orthogonal geometry of subduction, triggering widespread magmatism and the development of extensional basins in the overriding plate. Arc‐related volcanism during the early Miocene can be found only in the western slope of the Andes, suggesting the retreat of the volcanic front toward the trench. The proposed model highlights a strong linkage between the geochemical signature of magmatic products and changes in the subduction zone configuration and mantle dynamics during the evolution of the Patagonian Andes (41–44°S).

Timing, depth, and extent of high‐temperature hydrothermal alteration in the ocean crust is key to understanding how lower oceanic crust is cooled after accretion. Epidote veins were collected in spatially recurring zones of intense alteration of the lower crustal Wadi Gideah section and the root zone of sheeted dykes at Wadi Amadhi. ⁸⁷Sr/⁸⁶Sr ratios feature a narrow range from 0.70429 to 0.70512, while O isotope compositions vary between −0.7‰ and +4.9‰ in δ¹⁸O_SMOW. These compositions indicate water‐rock ratios between 1 and 5 and formation temperatures in the range of 300 to 450 °C. Fluid inclusion entrapment temperatures for a subset of samples linearly increase from 338 °C to 465 °C in lowermost 3 km of crust of the Wadi Gideah section and record 340 °C for the Wadi Amadhi basal sheeted dike sample. Salinities are uniform throughout and scatter closely around seawater values (3.2 ± 0.2 wt%). A model in which cooling down to 2 km (i.e., the depth of the melt lens) is followed by slow off‐axis hydrothermal cooling of the lower crust to 1 Myr predicts a thermal gradient for the lower crust that matches this observed trend for ages between 1 and 2.5 Myr. We suggest that the epidote veins formed in off‐axial hydrothermal systems that reach the base of the crust within 50‐ to 100‐km off axis. This deep circulation provides an efficient mechanism for mining heat that escapes the crust in the young flanks of mid‐ocean ridges.

Seabird excrements (guano) have been preserved in the arid climate of Northern Chile since at least the Pliocene. The deposits of marine organic material in coastal areas potentially open a window into the present and past composition of the coastal ocean and its food web. We use the stable isotope composition of nitrogen and carbon as well as element contents to compare the principal prey of the birds, the Peruvian anchovy, with the composition of modern guano. We also investigate the impact of diagenetic changes on the isotopic composition and elemental contents of the pure ornithogenic sediments, starting with modern stratified deposits and extending to fossil guano. Where possible, ¹⁴C systematics is used for age information. The nitrogen and carbon isotopic composition of the marine prey (Peruvian anchovy) of the birds is complex as it shows strong systematic variations with latitude. The detailed study of a modern profile that represents a few years of guano deposition up to present reveals systematic changes in nitrogen and carbon isotopic composition towards heavier values that increase with age, i.e. depth. Only the uppermost, youngest layers of modern guano show compositional affinity to the prey of the birds. In the profile, the simultaneous loss of nitrogen and carbon occurs by degassing, and non-volatile elements like phosphorous and calcium are passively enriched in the residual guano. Fossil guano deposits are very low in nitrogen and low in carbon contents, and show very heavy nitrogen isotopic compositions. One result of the study is that the use of guano for tracing nitrogen and carbon isotopic and elemental composition in the marine food web of the birds is restricted to fresh material. Despite systematic changes during diagenesis, there is little promise to retrieve reliable values of marine nitrogen and carbon signatures from older guano. However, the changes in isotopic composition from primary marine nitrogen isotopic signatures towards very heavy values generate a compositionally unique material. These compositions trace the presence of guano in natural ecosystems and its use as fertilizer in present and past agriculture.

The Andes in northern Patagonia are mainly formed by Mesozoic magmatic units: the mostly Jurassic–Cretaceous North Patagonian Batholith and volcanism of the Jurassic Lago La Plata (Ibáñez) Formation as well as the mid-Cretaceous Divisadero Group. These rocks represent the development of a magmatic belt through Jurassic–mid-Cretaceous time, during a switch of the tectonic regime from extension to compression. To study arc evolution during this transition, we carried out fieldwork and geochemical sampling at c. 43°S, clarifying structural relationships and characterizing the magmatic sources. Multi-element diagrams for both volcanic units suggest a slab-derived signature, whereas isotopic ratios (Sr–Nd–Pb) indicate parental melts sourced from the subduction-modified asthenospheric mantle interacting with crustal sources during their emplacement. An angular unconformity is identified between the synextensional Jurassic volcanic rocks and Lower Cretaceous sedimentary rocks beneath the mid-Cretaceous sequences. Although this deformational event was simultaneous with generalized overriding plate compression, geochemical ratios indicate an immature Aptian–Albian arc with no associated crustal thickening. Late Jurassic to mid-Cretaceous arc settlement after a trenchward retraction of magmatism from the foreland between c. 41 and 45°S, with an associated increase in slab dip angle, may have provoked crustal softening facilitating the subsequent initial fold–thrust belt growth. Supplementary material: Petrographic descriptions and geochemical–isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3677974