Conference Agenda

Deciphering the formation history of Earth requires knowledge of the dynamical processes by which Earth accreted most of its mass. In the classical model, this occurred via collisions among planetesimals and Moon- to Mars-sized planetary embryos over a timescale of several tens of millions of years (Myr). In contrast, in the competing pebble accretion model, most of the Earth grew during the ~4 Myr lifetime of the gaseous disk by accretion of mm-cm sized, sunward-drifting pebbles, which themselves originated in the outer Solar System. Thus, a key difference between these two models is the provenance of the accreted materials. Nucleosynthetic isotope anomalies in meteorites and samples returned by space missions can be used to constrain the formation locations of their parent bodies. These mass-independent isotope variations arise from the heterogeneous distribution of presolar material in the solar protoplanetary disk and allow for the distinction between non-carbonaceous (NC) and carbonaceous chondrite (CC) type materials, which are presumed to represent the inner and outer Solar System, respectively. Here, I will discuss how comparing the isotopic compositions of different elements in the Earth’s mantle with extraterrestrial samples allows us to investigate the origin of the Earth’s building blocks and, thus, its formation history. Specifically, I will focus on recent advances made using Fe isotope anomalies, which support the notion that the Earth predominantly accreted material formed in the inner Solar System and that some of this material is not sampled by meteorites.

The term ‘late veneer’ refers to the final ~0.5% of Earth`s mass accreted to the mantle after core formation ceased. To investigate this process, mass-independent ruthenium (Ru) isotope compositions of Archean rocks can be used for identifying mantle domains that did not receive the full complement of late veneer material [1]. We report new high-precision Ru isotope data for Archean ultramafic rocks from Pilbara (Australia) and Singhbhum (India) cratons. The Ru isotope variations observed for komatiite formations from Pilbara define a temporal trend with a ¹⁰⁰Ru isotope excesses of 22 ppm at 3.5 Ga towards values overlapping with the modern mantle composition locally established at 3.0 Ga. For 3.2-3.1 Ga old Singhbhum samples we found a wider range of ¹⁰⁰Ru isotope excesses from +47 ppm to values overlapping with the modern mantle within a much shorter time span of ~100 Myr. The decreasing ¹⁰⁰Ru excesses at Pilbara and Singhbhum can be well explained by progressive homogenization of late accreted meteoritic material with ¹⁰⁰Ru deficits within the Archean mantle. In contrast to mantle sources at Singhbhum, Pilbara and SW Greenland [1], komatiites from Kaapvaal (South Africa) do not exhibit ¹⁰⁰Ru isotope excesses and have a modern mantle-like composition already at 3.5 Ga [2]. Combined, these observations from different Archean cratons reveal that convective homogenization of late accreted material in the Earth’s mantle was globally achieved at different times and occurred over different time scales.

[1] Fischer-Gödde et al. (2020), Nature 579, 240-244. [2] Tusch et al. (2022), PNAS, 119, 1-12.

Previous work has identified the presence of Hadean source components in Phanerozoic plume-derived rocks by the analyses of µ¹⁸²W and µ¹⁴²Nd of the extinct ¹⁸²Hf-¹⁸²W (T_1/2 ≈ 8.9 Ma) and ¹⁴⁶Sm-¹⁴²Nd (T_1/2 ≈ 92 Ma) decay systems [1,2]. This finding spurred a new debate on the homogenisation history of Earth's mantle and the origin of these Hadean signatures in the lowermost mantle, which have been explained either by core-mantle interaction [1] or the survival of Hadean silicate reservoirs [2,3].

To assess possible temporal trends in µ¹⁸²W and µ¹⁴²Nd, we conducted high-precision measurements of these isotope systems in Neoarchean to Mesoproterozoic volcanic provinces from West Africa, Western Australia, South Africa, and southern Sweden, which were linked to mantle plume involvement. A temporal variation may indicate the persistence of Hadean source components. Our data reveal complex relationships resulting from crustal assimilation and the influence of metamorphic fluids. However, we find small but resolvable negative µ¹⁴²Nd and µ¹⁸²W anomalies for samples least influenced by these processes. In addition, the average µ¹⁴²Nd of all samples reveals a small but still resolvable ¹⁴²Nd deficit which cannot be explained by core-mantle interaction. Consequently, our data suggest the presence of Hadean silicate reservoirs in the Proterozoic lower mantle that may have been influenced by recycled oceanic crust since the Archean and potentially (but not necessarily) core-mantle interaction, thereby altering the initial µ¹⁴²Nd and µ¹⁸²W compositions.

[1] Mundl et al., 2017

[2] Peters et al., 2018

[3] Tusch et al., 2022

Plume-derived large igneous provinces (LIPs) and their geochemistry can provide important insights into plume-crust-interaction and deep mantle dynamics and its components. With an area of 1.3 Mkm², the Paraná Traps constitute one of the most voluminous LIPs on Earth and were emplaced during the Early Cretaceous (132 Ma, [1]) during the Gondwana breakup and opening of the South Atlantic.

We investigated feeder dikes from Southern Paraná for major, trace element and radiogenic isotope compositions. We provide Hf isotope data for Paraná province lavas complemented by Pb-Nd-Sr isotope data. Depletions of Nb-Ta over Th and enrichment of Pb over Ce, coupled with negative Eu and Sr anomalies in most mafic samples indicate a crustal or mantle-lithospheric influence on the magmas, consistent with published data for feeder dikes from Eastern Paraná matching the Esmeralda magma type [1]. Negative age-corrected εNd (0 to –17) and εHf values (+7 to –21) coupled with initial Sr isotope compositions (0.70575 – 0.75082) in many mafic samples support crustal or lithosphereic recycling into the mantle sources. Co-variations between isotope compositions and MgO for some more differentiated samples also support a control of assimilation and fractional crystallization processes [2]. Still, some samples show mantle-like trace element ratios (e.g. Nb/Th) and isotope compositions similar to EM1-like compositions from Tristan and Gough, indicating recycled lithospheric material as a main mantle component in the plume source in its early stage [3].

[1] Marques et al., 2018

[2] Hoernle et al., 2015,

[3] Homrighausen et al., 2020