Jansen, Mike Wolfgang
(2023).
Ancient mantle heterogeneities in modern intraplate
lavas – insights from new analytical tools.
PhD thesis, Universität zu Köln.
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Abstract
Since more than 4.57 billion years, several geodynamic processes are shaping the Earths ex- and interior. While several geodynamic processes occurring on the Earth’s surface can be addressed using first order geological methods, geochemical studies on volcanic rocks provide important constraints on its inaccessible interior. Already since the 1950’s, several studies have investigated long-lived radiogenic isotope systems (Sr, Nd, Hf, Pb) and trace-element compositions to study the composition and nature of the Earth’s mantle. While it was long thought that the Earth’s mantle is compositionally rather homogenous, studies on Ocean Island Basalts have indicated a rather heterogenous composition and several compositional endmembers that can be identified using geochemical evidence. Most recently, the application of short-lived isotopes, including 182Hf-182W, 146Sm-142Nd and 129I-129Xe in modern basalts have further indicated the survival of primordial mantle reservoirs that must have formed during the early Hadean Eon (during the first ~100 Ma in solar system history) and survived mantle convection until today. Among the previously mentioned short-lived isotope systems, the 182Hf-182W (half-life = 8.9 Ma, (Vockenhuber et al., 2004)) has been increasingly used to detect the participation of primordial mantle reservoirs in terrestrial rocks from the Archean as well as modern Ocean Island Basalts. The occurrence of deficient 182W isotope compositions and a positive correlation with 3He/4He ratios found in modern plume related volcanic rocks has thereby been interpreted as evidence for the long-term survival of mantle reservoirs that record fractionation of Hf and W during Earth’s primary differentiation (silicate-silicate or metal-silicate) or core-mantle interaction occurring either early – or at any time later in Earth’s history. The combination of seismic- and geochemical data as well as modelling constraints have further provided evidence for Large Low Shear Velocity Provinces and Ultra Low Velocity Zones, seismically anomalous zones that are present at the roots of deep mantle plumes, as potential source reservoirs that are capable of long-term isolation. Many young ocean island basalts show small 182W deficits, in particular if they are sourced from deep-rooted mantle plumes. The origin of this anomaly is still ambiguous and proposed models focus on core-mantle interaction or the presence of reservoirs in the lower mantle that have been isolated since the Hadean. In order to evaluate the role of upper mantle reservoirs, we report the first 182W data for intraplate basalts where a deep plume origin is still debated (Ascension Island, Massif Central, Siebengebirge and Eifel) and intraplate volcanic rocks associated with either plume or subduction zone environments (Italian Magmatic Provinces). Further, we compare these data with volcanic settings where seismic investigations have indicated a deep mantle plume origin (La Réunion, Baffin Island) (Chapter I). Additionally, we present detailed 182W data on oceanic intraplate volcanic rocks from the South Atlantic Ocean, including the three primary mantle plumes Tristan-Gough, Discovery and Shona, that are sourced by deep-mantle plume(s) and have shown to tap temporally changing magma sources (Chapter II). In summary, the 182W composition in all intraplate volcanic provinces of questionable plume origin overlap the composition of modern upper mantle to within 3 ppm. The absence of resolvable 182W anomalies in these intraplate basalts, which partially tap the lithospheric mantle, suggests that primordial components are neither present in the central and southern European lithosphere nor in the upper mantle in general. In contrast, 182W compositions in samples from La Réunion and the Tristan-Gough hotspot track span a range from modern upper mantle values to deficits as low as μ182W = −8.8 ppm and indicate a heterogenous contribution of primordial reservoirs to these mantle plumes. Given that seismic data has shown these plumes to be rooted at the core-mantle boundary, our data provide further evidence that Large Low Shear Velocity Provinces (LLSVPs) and Ultra Low Velocity Provinces (ULVPs) play a key role as potential hosts for primordial reservoirs in the lower-mantle. Given the heterogenous distribution of primordial signatures in rising plumes, our data provide evidence for lower-mantle dynamics as the driving force for material incorporation. Further, observed co-variation of 182W with Sr-Nd-Pb isotope values further confirms that the deep mantle sources of the South Atlantic volcanism comprise variable amounts of primordial material that is diluted by younger, post Hadean recycled material. Interestingly, the proto-Iceland plume basalts from Baffin Island have uniform and modern mantle-like 182W of around 0 despite extremely high (3He/4He) and thus provide evidence for decoupled He-W systematics in rising plumes. Quaternary rocks of the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province and into continental intraplate volcanism in general. Coupled trace-element and radiogenic Sr-Nd-Hf-Pb-Os isotope compositions provide detailed evidence for heterogenous mantle sources and compositionally distinct magmatic pulses (Chapter III). While a mantle plume origin for volcanism has been questioned, our new data and modelling constraints are in good agreement with admixtures of litospheric and asthenospheric mantle sources. Using geochemical evidence, our data further indicate the presence residual eclogitic components present within the upper mantle underneath central Europe and also indicate a potential influence of carbonatite metasomatism. Compiling available geochemical data for Quaternary as well as Tertiary volcanic provinces in central Germany (e.g., Siebengebirge, Vogelsberg, Rhön) shows that similar spatial, temporal and compositional relationships can be traced throughout all eruptive stages of central European volcanism. Conclusively, our data argue in favor of discrete melt pulses and pooling of polybaric melts from asthenospheric (plume-like) and lithospheric mantle sources underneath central Europe.
Item Type: | Thesis (PhD thesis) | ||||||||||||||||
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URN: | urn:nbn:de:hbz:38-651575 | ||||||||||||||||
Date: | March 2023 | ||||||||||||||||
Place of Publication: | Kölner UniversitätsPublikationsServer | ||||||||||||||||
Language: | English | ||||||||||||||||
Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||||||||||
Divisions: | Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute of Geology and Mineralog | ||||||||||||||||
Subjects: | Natural sciences and mathematics Chemistry and allied sciences Geography and travel |
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Date of oral exam: | 24 November 2022 | ||||||||||||||||
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Funders: | European Research Commission, Grant 669666 | ||||||||||||||||
Refereed: | Yes | ||||||||||||||||
URI: | http://kups.ub.uni-koeln.de/id/eprint/65157 |
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