Geochemical constraints on subduction-related mantle metasomatism of the Tiébaghi ophiolitic lherzolite in New Caledonia

Abstract

The geochemical evolution of mantle peridotite during subduction initiation (SI) remains an issue in geosciences. This study presents geochemical and Ca isotopic data for Tiébaghi lherzolites from the New Caledonia ophiolite to constrain their petrogenesis and the nature of melt-rock interaction during the nascent stage of subduction. Petrographic and geochemical analyses reveal that the Tiébaghi lherzolites are characterized by olivine with Fo contents of 89.5–90.7, high-Al Spinel (Cr# = 45.1–50.9; Al₂O₃ = 24.99–29.29 wt%), orthopyroxene with high CaO (0.87–2.47 wt%) and Al₂O₃ (2.19–4.91 wt%) coupled with relatively low Mg# (89.4–91.3), and clinopyroxene with high Al₂O₃ (2.72–6.44 wt%) and relatively low Mg# (90.4–92.5). Lherzolites from northwestern New Caledonia are thought to have escaped the suprasubduction re-melting, resulting in the formation of the highly depleted harzburgites that form the bulk of the ophiolite. They display a restricted range of δ^44/40Ca values (0.75–0.93 ‰), which are lower than the proposed δ^44/40Ca value of Earth's upper mantle. This isotopic signature is interpreted to reflect interaction with a low δ^44/40Ca metasomatic agent, possibly a carbonate-rich melt derived from the subducting slab. These findings suggest that the Tiébaghi lherzolites preserve a geochemical record of the early stages of melt-rock interaction during subduction initiation, emphasizing the role of carbonate melt metasomatism in altering the composition of the nascent mantle wedge.