Petrogenesis of Mafic to Felsic Lavas from the Oligocene Siebengebirge Volcanic Field (Germany): Implications for the Origin of Intracontinental Volcanism in Central Europe

Magmatism in the Cenozoic Central European Volcanic Province (CEVP) has been related to two geodynamic scenarios, either extensional tectonics in the north Alpine realm or upwelling of deep mantle material. The Oligocene (similar to 30-19 Ma) Siebengebirge Volcanic Field (SVF) is a major part of the German portion of the CEVP and consists of erosional remnants of mafic to felsic volcanic edifices. It covers an area of similar to 35 km (NW-SE) by similar to 25 km (SW-NE) with eruptive centres concentrated near the eastern shore of the Rhine river in the vicinity of the city of Bonn. Mafic rocks in the SVF comprise strongly SiO2-undersaturated basanites to alkaline basalts. Occurrences of alkaline basalts are confined to an inner NW-SE-striking zone, whereas the more SiO2-undersaturated basanites dominate the western and eastern periphery of the SVF. Radiogenic isotope compositions (Sr-87/Sr-86 0 center dot 70335-0 center dot 70371; epsilon Nd +3 center dot 1 to +4 center dot 5; epsilon Hf +6 center dot 5 to +8 center dot 0; Pb-206/Pb-204 19 center dot 46-19 center dot 69; Pb-207/Pb-204 15 center dot 63-15 center dot 66; Pb-208/Pb-204 39 center dot 34-39 center dot 62) indicate a common asthenospheric mantle end-member with HIMU-like characteristics for all mafic rocks, similar to the European Asthenospheric Reservoir (EAR). A lithospheric mantle source component with a residual K-bearing phase (phlogopite or amphibole) is inferred from negative K anomalies. Incompatible trace element modelling indicates that melting took place in the spinel-garnet transition zone with low degrees of melting at higher pressures generating the basanitic magmas (La-N/Yb-N = 20-25), whereas the alkaline basalts (La-N/Yb-N = 14-18) are the result of higher melting degrees at shallower average melting depths. Differentiation of basanitic primary melts generated tephritic to tephriphonolitic magmas that, for instance, erupted at the Lowenburg Volcanic Complex in the central SVF. Latites and trachytes, such as the prominent Drachenfels and Wolkenburg protrusions, are more common in the central portion of the SVF. These compositions originate from parental alkaline basaltic melts. All differentiated samples show evidence for crustal contamination, possibly with lower- to mid-crustal material comprising mafic granulites as found in Eifel basalt xenoliths and metapelites. Based on the spatial and temporal distribution of the various volcanic rock types, a model for the temporal evolution of the SVF can be proposed. During the initial phase of volcanism, low-degree basanitic melts were generated as a result of decompression following tectonic rifting and formation of the Cologne Embayment, a northward extension of the Rhine Graben. In a second stage, alkali basalts were generated at shallower depths and higher degrees of melting as a result of continued lithospheric thinning and passive upwelling of asthenospheric mantle. These conclusions strengthen previous models suggesting that intraplate volcanism in Central Europe is directly linked to regional lithospheric thinning and asthenospheric upwelling. Overall, the SVF constitutes an exceptionally well-preserved magmatic assemblage to illustrate these tectono-magmatic relationships

Precambrian sources of Early Paleozoic SE Gondwana sediments as deduced from combined Lu-Hf and U-Pb systematics of detrital zircons, Takaka and Buller terrane, South Island, New Zealand

Combined U-Pb-Lu-Hf isotope investigations on small detrital zircons (30-120 μm) using solution ID-MC-ICPMS and ID-TIMS were carried out on Cambrian-Ordovician sedimentary rocks from the Buller and Takaka terranes of the northeastern South Island of Ne

Separation of U, Pb, Lu, and Hf from single zircons for combined U-Pb dating and Hf isotope measurements by TIMS and MC-ICPMS

The U-Pb system has long been used to precisely date zircons because the high U-to-common-Pb ratio in zircon produces very radiogenic Pb isotope compositions over time. In contrast to U/Pb, zircon has very low Lu/Hf and therefore unradiogenic Hf, making this mineral ideally suited for determining the initial Hf composition of its original host rock. A new chemical separation technique presented here enables the determination of both U-Pb age and initial Hf isotope composition of individual zircon grains. The acquisition of such complementary information for single detrital zircons is especially useful for provenance analyses and crustal growth studies. Zircons are spiked with mixed 176Lu-180Hf and 233U-205Pb tracers and then digested in HF-HNO3. Lead, Lu, U, and Hf are sequentially separated from the zircon matrix on a single ion exchange column filled with Eichrom® Ln Spec resin. Using only ∼100 μl of resin for the separation keeps Pb blanks low (∼5 pg) while achieving better than 90% yields for each of the four elements. Hafnium isotope compositions and Lu concentrations are measured with multiple collector-inductively coupled plasma-mass spectrometry (MC-ICPMS), whereas U and Pb are analyzed by thermal ionization mass spectrometry (TIMS). The minimum grain size that can be processed is dictated by the amounts of Pb and Hf needed for an analysis. The smallest grains we currently analyze, as small as 50 μm (∼12pg of Pb and ∼3ng of Hf), can be analyzed with an external 176Hf/177Hf precision of ∼100 ppm (2 s.d.). The utility of this method is demonstrated with a population of detrital zircons from a Cambrian sediment of the Takaka Terrane, New Zealand. In addition, the technique has been used for 14 analyses of the standard zircon 91500, which yield a mean present-day 176Hf/ 177Hf of 0.282305 ± 12 (2 s.d., i.e., an external reproducibility of 43 ppm). The Hf isotopic compositions and U-Pb ages presented here are in good agreement with those of previous studies (e.g., Wiedenbeck, M., Alle, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., von Quadt, A., Roddick, J.C., Spiegel, W., 1995. Three natural zircon standards for U-Th-Pb, Lu-Hf, Trace element and REE analyses. Geostandards Newsletter 19, 1-23.), but our data suggest that the 91500 zircon is heterogeneous with respect to Lu and Hf contents and Lu/Hf

Hf-Nd-Pb isotope evidence from Permian arc rocks for the long-term presence of the Indian-Pacific mantle boundary in the SW Pacific

Trace element and initial Hf-Nd-Pb isotopic composition of a series of late Paleozoic arc rocks from South Island, New Zealand show evidence for the presence of Indian-type mantle in the Permian SW Pacific. The trace element budget points to a fluid-dominated arc setting such that Pb isotope compositions for both the volcanic and intrusive rocks were controlled by addition of fluids derived from the subducted slab to the mantle wedge. Relatively unradiogenic initial 207Pb/204Pb ratios indicate only a negligible contribution of pelagic sediments to the subduction component. Initial 208Pb/204Pb vs. 206Pb/204Pb co-variations therefore indicate that the subduction component originates from subducted oceanic crust having a Pacific-type composition. In contrast, Hf-Nd isotope correlations, corrected for slab fluid addition, reveal an Indian-type signature for the mantle wedge. Thus, the results indicate contribution of material from both Pacific- and Indian-type mantle sources to the island arc melts. From the source variability in Hf-Nd-Pb isotopes, it is therefore evident that a mantle domain boundary was present beneath the Permian Brook Street arc, similar to the prominent present-day isotope mantle boundary in the Earth's upper mantle, which can be traced along the western Pacific rim. These observations provide strong support that the isotopically defined mantle boundary between Indian and Pacific-type mantle was present in the SW Pacific since at least the late Permian. The existence of this boundary implies that convection cells of the Pacific and Indian mantle reservoirs co-existed in close proximity and yet remained distinct and isolated from each other since at least the late Permian. These results provide strong indirect evidence for the absence of significant chemical exchange between neighboring convecting regimes, at least for the approximate duration of one mantle overturn. Applying these distinct isotope features to the Permian plate tectonic configuration, the subduction polarity of the Brook Street arc was facing westwards, towards the active SE Gondwana margin

Reworking of Earth's first crust: Constraints from Hf isotopes in Archean zircons from Mt. Narryer, Australia

Discoveries of >4. Ga old zircon grains in the northwest Yilgarn of Western Australia led to the conclusion that evolved crust formed on the Earth within the first few 100. Ma after accretion. Little is known, however, about the fate of the first crust that shaped early Earth's surface. Here we report combined solution and laser-ablation Lu-Hf-U-Pb isotope analyses of early Archean and Hadean detrital zircon grains from different rocks of the Narryer Gneiss Complex (NGC), Yilgarn Craton, Western Australia. The zircons show two distinct groups with separate evolutionary trends in their Hf isotopes. The majority of the zircon grains point to separation from a depleted mantle reservoir at ~3.8-3.9. Ga. The second Hf isotope trend implies reworking of older Hadean zircon grains. The major trend starting at 3.8-3.9. Ga defined by the Hf isotopes corresponds to a Lu/Hf that is characteristic for felsic crust and consequently, the primary sources for these zircons presumably had a chemical composition characteristic of continental crust. Reworked Hadean crust appears to have evolved with a similar low Lu/Hf, such that the early crust was probably evolved with respect to Lu-Hf distributions. The co-variation of Hf isotopes vs. age in zircon grains from Mt. Narryer and Jack Hills zircon grains implies a similar crustal source for both sediments in a single, major crustal domain. Age spectra and associated Hf isotopes in the zircon grains strongly argue for ongoing magmatic reworking over hundreds of millions of years of the felsic crustal domain in which the zircon grains formed. Late-stage metamorphic zircon grains from the Meeberrie Gneiss unit yield a mean U-Pb age of 3294.5 ± 3.2. Ma with initial Hf isotopes that correspond to the evolutionary trend defined by older NGC zircon grains and overlap with other detrital zircon grains, proving their genetic relationship. This 'Meeberrie event' is interpret here as the last reworking event in the precursor domain before final deposition. The continuous magmatic activity in one crustal domain during the Archean is recorded by the U-Pb ages and Hf isotope systematics of zircon grains and implies reworking of existing crust. We suspect that the most likely driving force for such reworking of crustal material is ongoing crustal collision and subduction. A comparison of Hf isotope signatures of zircon grains from other Archean terranes shows that similar trends are recognised within all sampled Archean domains. This implies either a global trend in crustal growth and reworking, or a genetic connection of Archean terranes in close paleo-proximity to each other. Notably, the Archean Acasta gneiss (Canada) shows a similar reworking patterns to the Yilgarn Craton of Hadean samples implying either a common Hadean source or amalgamation at the Hadean-Archean transition

Stable W and Mo isotopic evidence for increasing redox-potentials from the Paleoarchean towards the Paleoproterozoic deep ocean

The scavenging of dissolved trace metals such as chromium (Cr), molybdenum (Mo) and tungsten (W) and their authigenic enrichment in sedimentary archives mainly occur by particle shuttling that cause resolvable isotope fractionation. Because their scavenging is also dependent on the local marine redox potential and the overall marine chemical environment, the stable isotope composition of these elements in Archean sediments is widely used as paleoredox proxy. Tungsten, a new element in this tool box, is dissolved as the oxyanion tungstate (WO42-) at extremely low marine redox potentials and can thus help to reconstruct the earliest changes in the marine redox state. We tested the applicability of stable W isotopes as a new paleoredox proxy of the atmosphere-ocean system. We analyzed the delta W-186/184 of Precambrian igneous rocks to investigate the detrital background delta W-186/184 signature. This Precambrian igneous inventory (PII) shows delta W-186/184 values between -0.007 and +0.097%o, identical to the range of modern igneous crustal rocks, which indicates that the average Earth's crust delta W-186/184 remained constant over billions of years. Furthermore, we present delta W-186/184 values of euxinic sediments from the Black Sea as a modern sedimentary analog, which we then compare with delta W-186/184 data of Archean and Proterozoic black shales (3.47-2.3 Ga). Modern Black Sea sapropels reveal crustal-like delta W-186/184 values between +0.050 and +0.071%o suggesting limited authigenic enrichment of W in euxinic environments. Similarly, post-Great Oxidation Event shales (2.3 Ga) show crustal- or PII-like delta W-186/184 values indicating the deposition in euxinic or oxic environment. However, the Archean shale sample suites show elevated W concentrations and fractionated delta W-186/184 values up to +0.246%o, which we attribute to authigenic enrichment of heavy seawater W in a ferruginous setting. Thus, black shales delta W-186/184 values can help to distinguish between anoxic-ferruginous and euxinic depositional environments. Furthermore, we combine previously published Cr and Mo isotope data with new Cr and W isotope data to quantify the evolution of the marine redox state during the Archean. By combining the Eh-pH stability fields of W and Mo with the delta W-186/184 and delta Mo-98/95 values of Precambrian sediments, we suggest a 3-step evolution of the Precambrian ocean. (1) From 3.47 to 3.0 Ga, the Eh of a dominantly ferruginous ocean was between -0.4 V and -0.25 V, enabling the persistence of soluble WO42- but not MoO42- and thus only the authigenic enrichment of isotopically fractionated W. (2) Starting from 3.0 Ga, the Eh of some shallow-marine environments increased above -0.25 V, as indicated by delta Mo-98/95 values above the detrital background in shallow-marine sediments, while the Eh of the deep ocean still remained below -0.25 V indicated by the lack of authigenic Mo enrichment and fractionated delta Mo-98/95 values in deep-sea shales. (3) After 2.7 Ga elevated delta Mo-98/95 and delta W-186/184 values in deep-marine sediments suggest that the Eh of the ferruginous open-ocean now also increased above -0.25 V. We therefore suggest that the combination of delta Mo-98/95 and delta W-186/184 values of shales is a very promising tool to investigate earliest changes in marine redox conditions during the Archean. (C) 2021 Elsevier Ltd. All rights reserved

The Quaternary monogenetic Bayuda Volcanic Field, Sudan – insights into mantle and crustal processes during magma petrogenesis

Please read abstract in the article.The National Research Foundation of South Africa (NRF).http://www.elsevier.com/ locate/lithos2023-12-28hj2023Geolog

The Quaternary monogenetic Bayuda Volcanic Field, Sudan - Insights into mantle and crustal processes during magma petrogenesis

The Holocene Bayuda Monogenetic Volcanic Field (BMVF) is located at the Great Bend of the river Nile in the north of Sudan. The analysed volcanic rocks are of basanitic to hawaiitic composition, characterising the BMVF as a sodic-alkaline volcanic field. Petrographic evidence documents olivine and clinopymxene as the main fractionating phases as well as FC processes that were accompanied by assimilation of mantle and crustal rocks in addition to mingling and mixing processes. Utilising systematic differences in trace element variations, two primary magmas of Series A and B can be distinguished. Uniform normalised incompatible element patterns, e.g. LREE enrichment accompanied by HREE depletion, negative anomalies of K and Pb, and modelling based on partition coefficients of La, Dy and Yb in mantle residues, suggest partial melting of an amphibole-garnet lherzolite. The presence of amphibole provides a constraint, implying that the melt was derived from a metasomatised lithospheric mantle. The presence of the Pb-depletion (documented by Ce/Pb ratios of >30) suggests that all magmas are of HIMU-OIB type. This is supported by their uniform( 87)Sr/Sr-86 (0.703010-0.703347), Nd-143/Nd-144 (0.512869-0.512995), Pb-206/Pb-204 (19.787-19.895), Pb-207/Pb-204 (15.656-15.653) and zospb/ampb (39.560-39.678) isotope characteristics. Additionally, we can describe the HIMU character of the NE African mantle with respect to Hf-176/Hf-177 (0.283006-0.283026). The mantle source of the BMVF magmas appears to have the composition of the common mantle C with an overprint by a HIMU component, resulting in higher radiogenic Pb-206/Pb-204 ratios above 19.5 (FOZO-HIMU-OIB type). We attribute this to the metasomatic character of the lithospheric mantle. The Hf/Nd isotope characteristics of the BMVF suggest that this metasomatism in the mantle source is most probably late Pan-African in age. When compared to circum-Mediterranean and regional volcanic fields, the high radiogenic Pb character of the BMVF indicates that their genesis cannot be related to magmatism associated with the activity of the Afar plume event and consequential magmatism along the Main Ethiopian Rift and the Red Sea Rift, as they are all of low radiogenic character