Multi-Stage Evolution of the Oceanic Lithosphere beneath Heard Island, Southern Indian Ocean

Abstract The Kerguelen Plateau is the second biggest submarine large igneous province (LIP) on Earth, however, the nature of the lithospheric mantle source underlying it remains poorly constrained. In this contribution, we provide novel insights into the oceanic lithospheric mantle underlying Heard Island (southern Indian Ocean), which represents the most recent and active phase of volcanic activity (<1 Ma) in the Kerguelen Plateau. We present petrographic and geochemical data for a suite of spinel-bearing harzburgite xenoliths hosted in basanite lavas and provide detailed constraints for distinguishing in situ mantle metasomatism from post-entrapment modification of the xenoliths following interaction with the host magma. We demonstrate that the xenolith mineral compositions and textures preserve a complex multistage history of different modal and cryptic transformations that occurred in the mantle due to: i) high degrees of partial melting that produced highly refractory whole-rock Mg# (Mg# = (Mg + Fe)/Mg × 100; 88–92), major element (FeO/MgO = 0.17) and mineral compositions (e.g. highly forsteritic olivine; Fo = (Mg + Fe)/Mg × 100; 91–92 mol %); ii) solid-state re-equilibration reactions during decompression that caused exsolution of clinopyroxene and Cr-spinel from xenolith orthopyroxene to form symplectite intergrowths; iii) cryptic metasomatism affecting the composition of xenolith clinopyroxene (i.e. enrichment in Na, Th, U and light rare earth elements, and depletion in Rb, Nb, Zr, Hf and Ti) due to interaction with carbonatitic melts in the mantle. Mantle fragments, entrapped by ascending basanite magmas as xenoliths were further modified by reactions with the host magma. This resulted in the partial dissolution of mantle orthopyroxene and replacement by newly formed and compositionally distinct assemblages of clinopyroxene (Mg# 87–91), olivine (Fo: 81–88 mol %) and Cr-spinel (i.e. ‘wehrlitisation’ of the xenoliths). This study highlights the utility of combining petrography and mineral chemistry to decipher the complex and sometimes overprinting and masking effects that different processes (e.g. melting events, metasomatism) exert on the lithospheric mantle, as well as constrain the processes that modify the xenoliths during transport towards the surface.