The combined short-lived ¹⁴⁶Sm–¹⁴²Nd and ¹⁸²Hf–¹⁸²W radiogenic isotope systems provide critical insights into early Earth differentiation and long-term preservation of mantle heterogeneities. Although Archean rocks often display resolvable ¹⁴²Nd and ¹⁸²W anomalies, these signatures diminish with time due to convective homogenization of the mantle. Proposed origins for these anomalies include early silicate-silicate or metal-silicate differentiation, and heterogeneous incorporation of late-accreted material [e.g., 1-4]. To further investigate these processes, we obtained high-precision ¹⁸²W isotope data for the 2.7 Ga old Theo’s Flow tholeiites. We show that, in addition to the previously reported ¹⁴²Nd excess [5], the Theo's Flow tholeiites also have a positive ¹⁸²W anomaly, indicating the survival of an early-formed mantle domain for nearly 1.8 Ga. The ¹⁴²Nd anomaly is best attributed to silicate differentiation at ~4.4 Ga, whereas the ¹⁸²W signature likely reflects incomplete mixing of late-accreted materials, consistent with a ‘grainy’ accretion scenario. Comparison with coeval komatiites from Boston Creek, which were derived from the same mantle plume [6], highlight spatial heterogeneity in the incorporation of late-accreted material at the single plume scale. These findings support inefficient mantle mixing during the Archean and are more consistent with a stagnant-lid tectonic regime than with modern-style plate tectonics.

References: [1] Boyet and Carlson (2006) EPSL 250, 254-268. [2] Puchtel et al. (2016) G3 17, 2168-2193 [3] Touboul et al. (2012) Science 335, 1065-1069 [4] Willbold et al. (2011) Nature 477, 195-199. [5] Debaille et al. (2013) EPSL 373, 83-92. [6] Puchtel et al. (2018) GCA 228, 1-26.

Molybdenum (Mo) isotopes are emerging as a valuable tracer of magmatic processes across diverse geological settings. Here we present Mo isotope data from the Kamchatka arc in the NW Pacific, spanning basaltic lavas along a SE – NW transect from the volcanic front to the back-arc. Most of the studied volcanic centres are situated above the subducting Hawaii–Emperor Seamount Chain.

Our results reveal systematic variations in δ⁹⁸/⁹⁵Mo correlated with trace element ratios (Ce/Pb, Ce/Mo, Nb/Zr, La/Sm) and ¹⁴³Nd/¹⁴⁴Nd across the arc. Lavas from the arc front exhibit higher δ⁹⁸/⁹⁵Mo and lower Ce/Pb, Ce/Mo, Nb/Zr, and La/Sm compared to back-arc lavas. As a subducted sediment contribution can be ruled out, we interpret these trends as reflecting a change in mantle source composition from arc front to back arc.

At the arc front, high δ⁹⁸/⁹⁵Mo and low Ce/Pb and Ce/Mo point to a dominant slab-derived fluid component. In contrast, back-arc lavas show signatures more typical of an enriched mantle, with mantle-like Ce/Pb and Ce/Mo, and elevated Nb/Zr and La/Sm. Notably, the combined δ⁹⁸/⁹⁵Mo, Nd, and Pb isotope signatures of back-arc lavas closely resemble those of modern Hawaiian ocean island basalts.

These observations suggest that the back-arc mantle may have been modified by an enriched asthenospheric component akin to that beneath Hawaii, potentially introduced by the subducted Hawaii–Emperor Seamount Chain.

Listvenites (carbonate+quartz) and soapstones (carbonate+talc) form by metasomatic transformation of variably serpentinized peridotites in the forearc mantle due to extensive reaction with CO₂-bearing aqueous fluids released from the subducting slab^[1]. This COH‑fluid/rock interaction plays a critical role in regulating deep carbon fluxes. Evidence for effective CO₂ storage includes: (1) naturally occurring, nearly fully carbonated peridotites; (2) high-pressure experiments that replicate fluid–rock interactions under subduction zone conditions; and (3) thermodynamic models simulating fluid infiltration. Fluid-driven carbonation sequesters large amounts of CO₂ since soapstones and listvenites commonly contain >20 wt% and >30 wt% CO₂, respectively. Although volumetrically rare rock types, they occur in many ophiolites throughout much of the geological record^[1]. In this presentation, the formation of listvenites and soapstones within the forearc as a relevant sink for CO₂ will be examplified.

Further, the stability of carbonates within the subducting slab and more generally within the upper mantle will be considered from recent experimental results on the anhydrous Ca-Mg-carbonate system. At 6 and 9 GPa, Ca-Mg-carbonates undergo incongruent melting producing dolomitic melt and periclase for temperatures above ~1250 ℃^[2,3]. At such high pressures, magnesite is expected to be the predominant carbonate phase. Magnesite does not melt, even in the presence of hydrous fluid or under hot subduction geotherms. ^[4].

[1] Menzel,M., Sieber,M.J., Godard,M. (2024) doi.org/10.1016/j.earscirev.2024.104828

[2] Sieber,M.J., Wilke,F., Koch-Müller,M. (2020) doi.org/10.2138/am-2020-7098

[3] Sieber,M.J., Wilke,M., Appelt,O., Oelze,M., Koch-Müller,M. (2022) doi.org/10.5194/ejm-34-411-2022

[4] Sieber,M.J., Reichmann,H-J, Farla,R., Koch-Müller,M. (2024) doi.org/10.2138/am-2023-8982

Mantle xenoliths give insights into igneous processes in the lithospheric mantle. Although spinel peridotite xenoliths from the Central European Volcanic Province have been studied extensively, the deeper central European sub-continental lithospheric mantle remains poorly understood due to the rarity of xenoliths from greater depths. Notably, the Delitzsch carbonatite complex in Germany hosts alnöite dikes (melilite-bearing ultramafic lamprophyres) that contain garnet-peridotite xenoliths [1]. To better characterise the deep lithosphere beneath central Europe, we investigated the radiogenic Sr isotope composition of a sheared garnet peridotite that was entrained from a depth of 190 km (5.9 GPa) by a Delitzsch complex alnöite [2]. We performed in-situ Sr isotope analyses on clinopyroxenes using a 193 nm Excimer laser ablation system coupled with a Neoma MC-ICPMS/MS. The use of SF₆ in the Neoma reaction cell enables interference-free measurement of SrF⁺ ions [3]. The analysed clinopyroxenes exhibit indistinguishable initial ⁸⁷Sr/⁸⁶Sr of 0.7051 ±16, which largely overlaps with the host alnöite (0.704 and 0.70336) [4,5]. The results are consistent with a scenario where refertilization of the lower lithosphere due to prolonged magmatic activity along with a shift to extensional tectonics during the Cretaceous triggered the destabilization and removal of the lower 50–70 km of northern Bohemian Massif lithosphere [2].

[1] Möckel et al. (2023), Geoprofil des LfULG 17, 82-136

[2] Röper et al., in review

[3] Craig et al. (2021), Anal. Chem. 93, 10519-10527

[4] Wand et al. (1987), Fifth Working Meeting Isotopes in Nature, 421-436

[5] Krüger et al. (2013), Chem. Geol. 353, 140-150

Three primitive alkaline SiO₂-undersaturated volcanic edifices from the southern Central European Volcanic Province (SW Germany) comprise coarse-grained, mm‑ to cm-thin veinlets and pockets of low-pressure differentiates, representing a missing link in an evolutionary trend towards phonolites. (1) In the Hegau region (Hohenstoffeln), a melilite-bearing olivine nephelinite contains ijolite pockets with skeletal perovskite, titanomagnetite, and euhedral fluorapatite. (2) Compositionally similar olivine melilitite from Urach (Sternberg) is crosscut by lileyite- and wadeite-bearing ijolite veinlets, in which titanomagnetite and perovskite lack skeletal shape. (3) Nepheline syenitic domains in a phlogopite-nepheline basanite from the Kraichgau region (Steinsberg) contain less clinopyroxene, titanomagnetite, and apatite, but accessory titanite. Despite variable host magma compositions, most differences between the occurrences are related to the exact timing of residual melt separation, a key factor in explaining the trace element evolution of late mineral and melt phases: at advanced clinopyroxene crystallization and perovskite saturation (Sternberg), massive trace element fractionation caused high Nb/Ta and Zr/Nb, but low LREE/HREE and Zr/Hf ratios in clinopyroxene and perovskite. Stabilization of LILE, Zr and Hf in the enriched residue, favoured by high F contents, finally lead to crystallization of an agpaitic assemblage. In contrast, melt separation before perovskite saturation (Hohenstoffeln) resulted in strong, uniform enrichment of LILE, HFSE, and REE in clinopyroxene and perovskite. The nepheline syenitic residues (Steinsberg) experienced a similar evolution with incompatible element enrichment in clinopyroxene and titanite, but their mineral assemblage differs from the ijolites due to higher aSiO₂ and aH₂O and lower F and P concentrations in the host melt.