Early Earth mantle heterogeneities
Early Earth mantle heterogeneities
by Nils Meßling
Date of Examination:2023-05-23
Date of issue:2023-07-04
Advisor:Prof. Dr. Matthias Willbold
Referee:Prof. Dr. Matthias Willbold
Referee:Prof. Dr. Andreas Pack
Referee:Prof. Dr. Volker Thiel
Referee:Prof. Dr. Thomas Müller
Referee:Prof. Dr. Gerhard Wörner
Referee:Prof. Dr. Andreas Stracke
Persistent Address:
http://dx.doi.org/10.53846/goediss-9974
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Abstract
English
Determining isotopic signatures established in the Hadean mantle offers far-reaching insights into the origin of Earth’s accreted building blocks, core formation, mantle differentiation as well as the formation of Earth’s first crust. Despite the masking effects of convective mantle homogenization and crustal recycling, rare isotopic heterogeneities established during Earth’s primordial evolution were not entirely erased. The extinct 182Hf-182W isotope system, only active during the first 60 Ma of Earth’s history, is an important tracer for primordial mantle components in the source of Archean and even modern mantle-derived rocks. Elevated 182W isotope compositions in Archean rocks are frequently interpreted to represent sluggish incorporation of Earth’s late-stage building blocks into the mantle, or, mantle domains subjected to early silicate differentiation. Through 182W variations, the incorporation of these primordial components into the mantle can be traced throughout Earth’s history. This provides valuable information on the timing of localized and global mantle homogenization, possibly tied to changes in geodynamic regimes. However, W is fluid-mobile and enriched in Earth’s crust. Due to these properties, W isotope systematics may be obscured through alteration or crustal contamination processes. To investigate W mobilization in Archean greenstone sequences, the first chapter of this thesis uses a combined 182W isotopic and major- and trace element approach to differentiate between fluid and magmatic sources of W in two Archean cratons. In the 3.53 Ga greenstone successions from the Onverwacht Group within the Kaapvaal Craton negative 182W anomalies were previously reported. These studies interpreted this unique signature to be the result of complex Hadean silicate differentiation processes. The 3.51 Ga volcanic units from the Badampahar Group from the Singhbhum Craton have not been constrained for their W isotopic budget so far. The results for mafic volcanic rocks from both cratons show W/Th ratios significantly higher than the canonical range, which suggests fluid-induced W enrichment. In contrast, the W budget of felsic volcanic rocks is primarily governed by magmatic processes. Samples, least affected by secondary W enrichment, show no resolvable W anomalies for both cratons and are indistinguishable from modern mantle values. Covariations of MgO, LOI, and W/Th ratios in mafic rocks indicate that serpentinization exerts a strong control on the scavenging of W from fluids. Serpentinized komatiites from the Onverwacht Group record 182W isotope deficits likely related to W-rich fluids derived from younger granitoid intrusions. The Badampahar Group exhibits low W isotope variability. A single komatiite sample has a negative 182W composition, indicating the presence of an unidentified fluid source characterized by negative W isotope anomalies, similar to granitoids found in the Kaapvaal Craton. The results of this chapter show that felsic intrusions are a major source of W-rich fluids in the Archean which need to be considered when evaluating the W isotope budget of volcanic rocks. Since negative 182W signatures are decoupled from their original magmatic source, processes other than Hadean silicate differentiation may explain their origin. Crustal recycling processes are an additional mechanism that can decouple W isotope anomalies from the geodynamic settings they are originally associated with. In modern mantle-derived rocks W isotope anomalies are exclusively associated with plume-related basalts and may therefore provide a unique tool to identify recycled plume material in subduction zone magmatism. In Central America, the Cocos and Coiba Ridges are subducting with the Cocos plate. These ridges may introduce material into the arc magma source that was derived from the Galápagos plume, which has been shown to carry anomalous 182W signatures. In the second chapter μ182W values and trace elements data for <5 Ma old adakites and back-arc basanites as well as accreted basalt terranes from Central America are reported. The accreted mafic terranes formed as a result of Galápagos plume activity in the last 70 Myr. They exhibit negative W isotope variations, attesting to the longevity of the primordial W isotope signature in the Galápagos plume. Conversely, negative 182W values in adakites and basanites indicate that a slab-derived component dominates the W isotope budget of these rocks. These arc rocks are extremely depleted in W, indicating that W was transferred via melts, rather than W-rich fluids. Fluid mobilization and crustal contamination significantly influence the reliability of W isotopes as a tracer for ancient mantle components in the Archean. The nucleosynthetic Ru isotope system provides a novel tool to trace the heterogeneous incorporation of Earth’s late-stage building blocks into the mantle. Compared to W, Ru is fluid immobile and depleted in the crust, providing a more robust tracer for primordial mantle components. In previous studies however, the sample selection was highly limited due to the large amounts of analyte required for high precisions isotope measurement, restricting these studies to meteorites, ore samples or Ru-rich ultramafic rocks. To extend the lithological range, to which this isotope system can be applied, chapter 3 provides new techniques for the quantitative isolation of Ru from a variety of rock matrices and precise measurement of the Ru isotopic composition. The separation involves pre-concentration of Ru using a NiS fire assay and subsequent isolation of Ru via cation exchange chromatography followed by steam distillation. Initial Ru isotope data for komatiites from the Singhbhum and Kaapvaal Cratons do not show any resolvable Ru isotope anomalies compared to the modern mantle. In line with the results from chapter 1, this indicates that mantle sources from both cratons fully incorporated meteoritic material added to Earth during its late accretionary phase. Combined, these studies aim to further our understanding of the possibilities and limitations of the W isotope system as a tracer for primordial mantle components. Through the addition of the novel Ru isotopic system, combined isotopic approaches can provide more precise constraints on the geodynamic evolution of Earth’s mantle and the origin and long-term survival of ancient mantle heterogeneities.
Keywords: Geochemistry; 182W; Ancient Mantle
