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Significance Due to active plate tectonics, there are no direct rock archives covering the first ca. 500 million y of Earth’s history. Therefore, insights into Hadean geodynamics rely on indirect observations from geochemistry. We present a high-precision 182 W dataset for rocks from the Kaapvaal Craton, southern Africa, revealing the presence of Hadean protocrustal remnants in Earth’s mantle. This has broad implications for geochemists, geophysicists, and modelers, as it bridges contrasting 182 W isotope patterns in Archean and modern mantle-derived rocks. The data reveal the origin of seismically and isotopically anomalous domains in the deep mantle and also provide firm evidence for the operation of silicate differentiation processes during the first 60 million y of Earth’s history.

The origins of Eoarchean peridotites found in the Itsaq Gneiss Complex (IGG) of southern West Greenland represent a crucial record of igneous and geodynamic processes on the early Earth. The igneous and geodynamic origins of these rocks have, however, been the subject of controversy, with some researchers arguing that they represent the first known slivers of mantle emplaced by tectonic processes in the crust and others contending that they represent cumulates associated with the local basalt units. The geodynamic context for the formation of these rocks has also been disputed, with some researchers arguing that they formed in a horizontal tectonic setting analogous to a modern subduction zone, while others propose a vertical tectonic origin for all Eoarchean rocks. Here, we provide new insights into the history of these peridotites using multiple sulfur isotope signatures combined with Hf isotope compositions. Anomalously high εΗf values in some IGC peridotites identified in previous studies [1], as well as in metabasalts with boninite-like compositions [2] found in the Isua Supracrustal Belt (ISB) within the IGC, point to contributions from a mantle source already depleted in the Hadean [2]. The multiple sulfur isotope data of the IGC peridotites found south of the ISB reveal small but significant Δ33S anomalies, consistent with incorporation of surface-derived material of Archean age or older. Furthermore, correlations between sulfur isotope data and major and trace element abundances as well as initial Hf isotope values of IGC peridotites support the hypothesis that high-degree melt depletion occurred under hydrous conditions, followed by variable degrees of melt metasomatism. The involved fluid and melt components precipitated sulfides that incorporated surface-derived sulfur with different depositional origins. We propose that these findings are best explained by a horizontal tectonic regime similar to modern arc settings. 1. van de Löcht, J., et al., Preservation of Eoarchean mantle processes in ∼3.8 Ga peridotite enclaves in the Itsaq Gneiss Complex, southern West Greenland. Geochimica et Cosmochimica Acta, 2020. 280: p. 1-25.2. Hoffmann, J.E., et al., Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland. Geochimica et Cosmochimica Acta, 2010. 74(24): p. 7236-7260.

Much of the continental lithosphere developed during the Archean, which was an Eon of change in terms of global geodynamics and geochemical cycles. Uncovering the causal links between crust forming processes and prevailing geodynamic mechanisms is crucial for understanding the origins and composition of the present-day continental lithosphere. Pristine Archean crust is scarce yet can be found in cratons worldwide. Many of these occurrences comprise rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which represent a prevalent component of the Archean continental crust. TTGs are generally considered to have formed by partial melting of amphibolite or eclogite source rocks that had basaltic precursors originally extracted from a depleted mantle (e.g., [1]). The age of the source rocks (i.e., the time between the basalt extraction from the mantle and TTG formation) can be determined from the initial radiogenic isotope compositions of TTGs, provided that the P/D ratio of the source can be reliably estimated and is significantly different from that of the depleted mantle.Based on this principle, we estimated the age of basaltic sources of TTGs from cratons of different age and paleogeography from initial 87Sr/86Sr compositions determined by in-situ Sr isotope analysis of primary igneous apatite (LA-MC-ICPMS). The 87Sr/86Sr of these apatites show that prior to 3.4 Ga TTGs were derived from relatively old mafic sources and that the average time between formation of basaltic material from the mantle and subsequent remelting under amphibolite to eclogite facies conditions decreased drastically during the Paleoarchean. This secular change indicates a rapid global increase in the efficiency of TTG production or the emergence of a new TTG-forming process at c. 3.4 Ga [2].In this contribution we explore this hypothesis by comparing the 87Sr/86Sr signature of the TTGs with their trace-element compositions, as well as with 176Hf/177Hf zircon data for these rocks and contemporary TTGs from other studies. This combined geochronological, isotope and geochemical analyses will provide new constraints on the age of TTG sources during the Archean and will allow investigation into the nature and probable causes of the apparent rejuvenation at 3.4 Ga, as indicated by Sr isotopes.[1] Hoffmann, J.E. et al. (2011) Geochim. Cosmochim. Acta 75, 4157-4178.[2] Caton, S., et al., (in review) Chem. Geol.

The relative and absolute abundances of highly siderophile elements (HSE) present in the modern terrestrial mantle has been set during late accretion of meteoritic materials that the Earth received after cessation of core formation. In particular, the study of HSE abundances in komatiite melts that erupted between 3.5 and 2.9 Ga revealed the slow mixing of HSE-rich late accreted material into the terrestrial mantle and the progressive homogenization of primordial domains by continuing mantle convection. Here, we report new Ru isotope and HSE abundance data for komatiitic rocks from the 3.46 Ga Dwalile greenstone remnant in the Kaapvaal Craton, SW Swaziland and from 3.5-3.2 Ga old komatiitic rocks from the Pilbara Craton, NW Australia. In contrast to 100Ru excesses identified in 3.5-3.2 Ga old komatiites from the Pilbara, contemporaneous komatiites melts from Swaziland were found to have 100Ru values indistinguishable from the modern mantle. Although HSE abundances in Dwalile komatiites show up to 2 times enrichment compared to primitive mantle estimates, the modern mantle-like Ru isotope composition of these rocks constrains that the Dwalile mantle source already fully equilibrated with the late accreted components by 3.5 Ga. These findings ultimately demonstrate that apparent HSE enrichments compared to the modern mantle may not necessarily reflect mass excesses of late accreted components. Therefore, our new data reveal that calculated HSE contents of mantle domains should be carefully interpreted with regard to the late accretion history of Earth and ideally combined with other sensitive tracers such as 100Ru isotope systematics.

The southern massifs of the Oman ophiolite were often considered as a model for the structure and composition of crust at fast-spreading ocean ridges. However, the influence of late-stage convergent plate margin magmatism on the evolution of lower crustal and upper mantle rocks in the ophiolite is still poorly understood. Here, we report new constraints from highly siderophile and other chalcophile element abundances and 187Os/188Os on samples from the CM-1 drill core of the Oman Drilling Project. Main results are: (1) Abundances of most elements in this study were not affected by low-temperature alteration. (2) In harzburgites and dunites from the top of the mantle section, chalcophile elements predominantly are controlled by traces of sulfides, precipitated from sulfide-saturated, incompatible element-depleted magmas. (3) The overlying dunite-rich crust-mantle transition zone (CMTZ) is extremely depleted in Pd, Se and Re, indicating that parent magmas were sulfide-undersaturated. (4) The mafic unit on top shows local sulfide saturation. (5) Ru/Ir and Os/Ir in CMTZ dunite are suprachondritic, whereas values in the overlying mafic unit are subchondritic. Hence, rocks of the mafic unit are products of magmas that initially crystallized cumulate olivine and a laurite-ehrlichmannite phase in CMTZ. (6) Suprachondritic initial 187Os/188Os of CMTZ, the mafic unit and other lower crustal gabbros suggest that most of the lower crust was affected by magmas that contained excess radiogenic 187Os, presumably a result of slab-derived inputs during the short-lived subduction zone stage.

The short-lived 146Sm-142Nd decay system is a powerful tool to investigate early geodynamic processes. Here, we introduce a new analytical protocol that allows measuring 142Nd/144Nd isotope compositions at the lowest ppm-level by MC ICP-MS. This new method is applied to various Archean to Proterozoic rock types (TTGs, boninites, tholeiites, dikes) from the Isua region, SW Greenland. While our results are consistent with previously reported elevated 142Nd/144Nd compositions[1,2], our study is the first to statistically resolve small 142Nd/144Nd isotope differences between rocks from the 3.7 Ga and the 3.8 Ga units of the Isua supracrustal belt. Further, we generally observe a strong decrease of 142Nd anomalies between 3.8 Ga and 3.4 Ga, likely linked to the progressive admixture of unfractionated bulk silicate Earth material that progressively replenished the mantle beneath the Isua region. Such a geodynamic model is also in accord with observational constraints from trace elements and long- and short-lived radiogenic isotopes. The recurrence of elevated 142Nd/144Nd compositions in younger Mesoarchean rocks (~2.97-3.08 Ga) and one Proterozoic dike (~2.0 Ga) can be explained by mantle sources that were influenced by older Eoarchean recycled components or crustal contamination, respectively. This is also consistent with unradiogenic initial ɛ176Hf and ɛ143Nd values and enriched incompatible trace element patterns that have been reported for some of these rocks[3]. [1] Caro et al. (2006), GCA, 70, 164-191. [2] Saji et al. (2018), GPL, 7, 43-48. [3] Szilas et al. (2015), MM, 79, 857-876.

The ultra-slow spreading South West Indian Ridge (SWIR) near Marion Rise comprises segments of thin crust and abundant abyssal peridotites exposed near the ridge axis. Zhou and Dick (2013) proposed that buoyant depleted mantle beneath the SWIR may represent Proterozoic melting residues recycled from Gondwana lithosphere, resulting in excess buoyancy at Marion Rise. In order to test this hypothesis, serpentinized harzburgites, dunites and scarce lherzolites were dredged and sampled by remotely operated vehicle along SWIR (36°54.0’ E to 39°13.5’ E) during expedition SO 273 in 2020. The major and trace element compositions of the peridotites (Mg# = 0.90-0.92) are similar to other depleted abyssal peridotites and mostly lack visible clinopyroxene. Here, we present new highly siderophile element (HSE) mass fraction and Os isotopes, that constrain the time of melt extraction, style of mantle melting and melt transport. Low HSE abundances (Ru = 0.07-0.3x primitive mantle) coupled with strongly fractionated HSE patterns (high Ru/Os, Ir/Os, Ru/Re) in studied peridotites exposed at normal faults suggest higher melting degrees than in most abyssal peridotites elsewhere. The HSE patterns are similar to those in harzburgites and dunites from ophiolites, interpreted to reflect sub-arc mantle lithosphere that was depleted by flux melting above subduction zones. Measured 187Os/188Os range between 0.119 and 0.142, possibly reflecting melt depletion at 1.4 Ga and interaction with sulfide-undersaturated melts or fluids with suprachondritic 187Os/188Os. The data provide evidence for locally strong depletion of mantle rocks at Marion Rise and the occurrence of ancient subarc mantle underneath a modern oceanic ridge.