Lithium isotopes reveal mantle plume heterogeneity: Insights from the Emeishan Large Igneous Province

Abstract

Mantle plumes, rising from deep within the Earth, provide key insights into the interior heterogeneity. Previous studies of ocean island basalts have demonstrated the role of recycled crustal materials in modifying elemental and isotopic signatures in mantle plume sources in oceanic settings. However, the volume and nature of recycled crustal contributions to plume sources in continental settings remain poorly constrained. The Emeishan Large Igneous Province in southwestern China, widely recognized as the product of a mantle plume in a continental setting, offers an ideal natural laboratory to address this question. Here we report lithium (Li) isotopic data from high-Ti picrites at Ertan and Maoniuping (δ⁷Li = 5.1–9.6 ‰) and low-Ti picrites at Wulongba (δ⁷Li = 4.7–7.2 ‰), all of which exhibit heavier Li isotopic compositions than the typical mantle value (3.5 ± 1.0 ‰). After ruling out the effects of weathering and crustal and lithospheric mantle contamination, we attribute the elevated δ⁷Li values to the source characteristics of the Emeishan plume, which we estimate to contain 10–15 % recycled altered oceanic crust and 0.05–5 % marine silicate sediments. High-Ti picrites exhibit higher δ⁷Li values than the low-Ti picrites, likely reflecting differences in the proportions of pyroxenite and peridotite involved in mantle partial melting. A global comparison further reveals that plume-derived magmas from both continental and oceanic settings systematically display high δ⁷Li values, underscoring the widespread incorporation of recycled crust and the formation of pyroxenite with higher δ⁷Li values in the plume source. The even higher δ⁷Li values in continental settings are attributed to an “overlying lid effect”, whereby suppressed mantle melting enhances the relative contribution of pyroxenite-derived melts. Together, these findings provide isotopic evidence for the presence of recycled crust in mantle plume sources and establish Li isotopes as a robust tracer of deep crustal recycling.