Highly Enriched and Heterogeneous Mantle Signals Recorded in Mantle Pyroxenite Veins

Abstract The Earth's mantle is inherently heterogeneous, reflecting ongoing mantle depletion and crustal recycling driven by plate tectonics. Radiogenic isotopes in oceanic basalts are commonly used to characterize mantle reservoirs, but the full chemical diversity of mantle‐derived melts may be obscured by magma mixing during melt transport and storage. Pyroxenite veins formed by melt segregation or melt–peridotite interaction in the lithospheric mantle provide an alternative archive that may better preserve the compositional diversity of mantle‐derived melts. We present Nd–Hf isotopic data for bulk rock, clinopyroxene, and orthopyroxene from pyroxenite veins in three mantle massifs exposed along the Yarlung‐Tsangpo Suture Zone in southern Tibet. Clinopyroxene and orthopyroxene from the same samples display distinct initial isotopic compositions, indicating crystallization from compositionally different melt pulses. Orthopyroxene commonly records more enriched and less radiogenic signatures than clinopyroxene, highlighting its potential to preserve isotopic information on enriched melt components. Additionally, the pyroxenite veins define a wide range of initial Nd–Hf isotopic compositions (initial ε Nd = −10.3 to +8.6 and initial ε Hf = −8.4 to 18.3), extending beyond the variability reported for basaltic lavas and cumulates from the same ophiolitic system. These variations are consistent with mixing between melts derived from depleted mantle and melts originating from enriched mantle domains. Our results suggest that mantle pyroxenite veins can preferentially preserve the isotopic heterogeneity of the enriched mantle source. Overall, this study highlights the value of mineral‐scale isotopic analyses in pyroxenites for documenting mantle heterogeneity and provides new constraints on melt migration and aggregation processes in the upper mantle.