Conference Agenda

Phlogopite-bearing mantle xenoliths from the cratonic, lithospheric mantle are direct evidence of alkali-, volatile-rich metasomatism related to lamproite and kimberlite magmatism. Only rarely studied is the enrichment in those xenoliths in sulfides. Here, we investigate 23 xenoliths (peridotites, peridotite-MARID interface, MARID) from the Kimberley kimberlite cluster (Kaapvaal craton), using in-situ major to trace elements of silicates, sulfides (pentlandite) and S-isotopes in sulfides in order to reconstruct their formation history.

The xenoliths derive from the shallow lithospheric mantle (<2.5 – 4 GPa, <700 – 1000°C). They have a depleted to enriched composition (Mg#_{olivine, phlogopite} = 86 to 96). Modal abundances of sulfides increase with increasing contents of phlogopite, clinopyroxene and amphibole. They range from <0.1 – 7.3 vol ‰ (peridotite), 48 – 470 vol‰ (peridotite-MARID interface), <1 vol‰ (MARID).

Pentlandite has variable PGE-pattern (Os <1 to 1350 µg/g, Pd/Ir_N = 0.002 to 3800), S/Se ratios (600 – 200000) and S-isotopes (δ³⁴S = +2 to -5.3 ‰). In silicates, chalcophile elements such as Ge, Cd, In, Ag or Au concurrently change with lithophile elements, Mg# and modal abundances of metasomatic phases. Additionally, Se, Te contents and S-isotopes in type-2 sulfides are connected to the changes of the silicates and bulk rock.

The concurrent changes indicate a coupled silicate and sulfide metasomatism and a magmatic/metasomatic evolution from MARID rocks to peridotites. We propose that the Sulfur-Alkali-Volatile-Oxidizing-Fe-Ti-Chalcophile-rich (SAVOFTiC) MARID agent, interacted with the reducing wall-rock, resulting in redox-freezing and formation of high amounts of sulfides at the peridotite-MARID interface in the shallow lithospheric mantle.

Heterogeneous assemblages within the lithospheric mantle represent some of the most enriched domains within the earth for alkaline, rare-earth and volatile elements. Volatile-rich mica-bearing pyroxenites are among these assemblages and were crucial in the recognition of metasomatism as a mantle process. However much of the experimental work until recently has focused on four-phase peridotites which are largely devoid of volatiles (i.e. C, H, N, S) and moderately volatile elements like fluorine unless they have been metasomatised. In domains where peridotite and pyroxenites coexist the chemical fingerprint of metasomatism is challenging to untangle, principally due to a lack of foundational phase relation studies in complex systems. Using a complex synthetic mica-pyroxenite system we will present new experimental results of melting and phase relations, and melt chemistry. Our new experiments span 900 – 1400°C at pressures of 1 and 2.5 GPa in a system containing carbon, water, sulfur, and fluorine, as well as 29 trace elements. Across these pressures we first observe the generation of a fluid phase at low temperature that acts as the precursor to a predominantly silicate melt. The silicate melts border the foidite field in TAS and straddle the leucitite and shoshonite divide in K2O vs. SiO2 space, while maintaining a K2O/Na2O value ranging from 0.9–3 in most experiments. The melts we've generated from our mica-pyroxenite assemblage can contribute to the explanation of a range of alkaline magmas while also having significant metasomatic potential at the point of melt generation.

Combined petrological and geochemical studies of ultramafic lamprophyres (UML) have great potential to unravel multi-stage histories of magmatic systems in continental rift settings. Our study focuses on the Klunst quarry in Ebersbach, Lusatia (Germany). Lower Cretaceous UML dykes (~130 Ma) intrude Cadomian granodiorites (~550 Ma) and ~400 Ma microgabbros. Importantly, these UML predate the Upper Cretaceous-Paleocene pre-rift magmatism along the Eger Rift System [1]. UML dykes from Ebersbach were examined for mineral chemistry using electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS); whole-rock compositions were determined by ICP-MS and X-ray fluorescence (XRF). Most samples classify as ‘alnöites’, according to the classification by Tappe et al. 2005 [2]. We describe the first olivine data from the locality. High-Mg (Mg# >90), Ni-rich, normal-zonedxenocrysts, likely sampled from spinel-peridotite mantle, are accompanied by reverse-zoned phenocrysts with low-Ni, high-Ca–Mn–Ti–Fe rims crystallized from ascending UML magmas. Zoned phlogopites, with Mg‑rich, Ti‑rich and Fe‑rich zones, document late‑stage interaction with carbonatitic melts. Cores of xenocrystic Cr‑spinel show compositions within the spinel‑peridotite field, whereas surrounding Fe-Ti‑rich rims also record interaction with increasingly oxidized UML/carbonatitic magmas [3, 4]. Our data demonstrate a multi‑stage history of xenocryst entrainment, melt-rock interaction and differentiation as found in other UML complexes nearby [3, 4].

References:

[1] Renno et al. (2003) Chem. Erde 63, 1–20

[2] Tappe et al. (2005) J. Petrol. 46, 1893–1900

[3] Ulrych et al. (2022) GSL Spec. Publ. 513, 237–269

[4] Krüger et al. (2013) Chem. Geol. 353, 140–150

Polymict mantle breccias are rare, complex xenoliths characterized by extreme disequilibrium among their constituent phases. They are currently interpreted as failed kimberlite intrusions, though it remains debated whether they are assembled and entrained by a single or multiple (proto-)kimberlite pulses. We investigated a polymict mantle breccia from Kimberley, South Africa, using EPMA and LA-ICP-MS to characterize major and trace element compositions and constrain melt interaction and associated metasomatism in the lithospheric mantle. Garnets span a wide compositional range, classified as group G1 (low-Cr megacrysts), G4 (pyroxenitic/eclogitic), G9 (lherzolitic), G10 (harzburgitic), and G11 (high-TiO2 peridotitic). Orthopyroxene, clinopyroxene, and olivine similarly define distinct populations corresponding to depleted, (re-)fertilized, and high-Cr megacryst groups. Two phlogopite populations are present, one magmatic and one closely resembling MARID-suite phlogopite. The MARID phlogopite apparently replaced or reacted with Cr-diopside megacrysts. Geothermobarometry of G1 garnets (assumed to crystallize within the proto-kimberlite) traces a cooling path (1300 °C to 1100 °C), leaving the adiabat at ~2.7 GPa (~90–100 km depth), indicating melt stagnation. While cores of G1, G9, and G10 garnets match reference data from the Kaapvaal Craton, all rims display a uniform, G1 composition with little apparent diffusion, indicating entrainment within a single, proto-kimberlitic pulse. Taken together, the polymict mantle breccia records the formation of megacrysts and possibly MARID-suite minerals and shows that melt stagnation occurred at the depth corresponding to the occurrence of mid-lithospheric discontinuities (metasomatic layers with hydrous minerals). These metasomatic processes may have preconditioned the lithospheric mantle for the ascent of subsequent kimberlite pulses.

Subduction-driven recycling of sulfur and chalcophile metals plays a crucial role in mantle enrichment, redox evolution, and the formation of metal deposits. However, the effects of subducted crustal materials on mantle sulfur and metal compositions remain debated. The eastern North China Craton, which has experienced both oceanic and continental subduction since the Paleozoic, offers an excellent natural laboratory to address this issue. Here, we investigate sulfur isotopes and chalcophile elements in mantle-derived magmas formed after these subduction events. Our results show that these magmas are variably enriched in fluid-mobile elements and display moderately to strongly enriched Sr–Nd–Os isotopic compositions, indicating contributions from both oceanic and continental sediments to their mantle sources. They also exhibit higher sulfur contents and heavier sulfur isotopic compositions than mid-ocean-ridge basalts (MORB), suggesting that slab-derived melts or fluids introduced significant sulfur into the affected mantle. However, despite clear evidence for subduction-related metasomatism, the contents and ratios of PGE and Cu (e.g., Os/Ir, Pd/Ir, Cu/Pd) are similar to those of MORB, implying that slab-derived components contribute little metal. Modeling further indicates that their metasomatized mantle sources contain chalcophile metal contents comparable to, or slightly higher than, the previously depleted sub-continental lithospheric mantle, but still lower than primitive mantle levels. Integrating our results with compiled global data on mantle peridotites/pyroxenites and sulfides, we conclude that subduction-related mantle metasomatism contributes substantial volatiles, and may also have restored strongly chalcophile metals in depleted lithospheric mantle to a level of fertile mantle for subsequent magmatic-hydrothermal processes and mineralization.