Melt–Rock Interaction and Metasomatism of the Lithospheric Mantle beneath the Kerguelen Plateau: Evidence from a Veined Spinel Harzburgite Xenolith

Abstract Crosscutting veins in mantle-derived peridotite xenoliths, formed by melt infiltration and melt–rock interaction, provide key insights into lithospheric mantle processes, such as melt and fluid migration and metasomatism. To reconstruct the composition, reaction sequence, and evolution of the infiltrating medium and its interaction with the mantle peridotite wall rock, we conducted a petrographic, mineralogical, and geochemical investigation of a veined spinel harzburgite xenolith hosted in basanite lava from Heard Island (Kerguelen Plateau, southern Indian Ocean). The crosscutting veins exhibit a zoned structure, consisting of an orthopyroxene-rich layer at the vein–harzburgite boundary and a wehrlitic interior dominated by clinopyroxene and olivine, along with interstitial glass and minor Fe–Ni–Cu sulphides. We develop a model in which vein formation within the mantle via a multi-stage process, beginning with infiltration of a TiO2-rich, silica-rich melt that reacted with mantle olivine to produce the orthopyroxene-rich layer at the vein–host margin. In addition, we employ thermodynamic modelling (rhyolite-MELTS) to constrain the compositional evolution of this melt–rock reaction pathway. We suggest that this reaction progressively shifted the melt towards more silica-undersaturated and mafic compositions, resulting in crystallisation of the wehrlitic vein interior. Iron–Mg diffusion profiles in olivine at vein–host contacts indicate that vein formation occurred shortly before wall-rock disaggregation and xenolith entrainment, with timescales of weeks to a few tens of years. These short timescales, together with the bulk compositional affinity of the crosscutting vein with the host basanite, suggest that the crosscutting veins may represent melt fracture channels formed in the lithospheric mantle by the ascending basanite magma shortly before eruption. The globular morphology and highly variable compositions of Fe–Ni–Cu sulphides in the crosscutting veins, revealed by bulk laser ablation inductively coupled plasma mass spectrometric (LA-ICPMS) analyses, are inconsistent with in-situ equilibrium crystallisation, suggesting that sulphides were potentially introduced as suspended phases entrained within the infiltrating melt. These findings highlight silicate melt infiltration and associated metasomatism are key drivers of mantle refertilisation, enriching refractory peridotite in Ti, alkalis (Na, K), and incompatible elements (light rare earth elements – LREE; Ba, Sr, U, Th).