Abstract: The classic giant impact model is currently the most widely accepted hypothesis for explaining the lunar formation process. It posits a collision between a proto-Earth in its late accretion stage and a Mars-sized impactor named Theia. According to this model, the majority of the Moon’s material is derived from Theia. However, there is still a lack of precise constraints on the contribution percentage of the impactor to the lunar mass. In this study, high-precision measurements of mantle peridotites and komatiites are employed to reevaluate the V isotope composition of the Bulk Silicate Earth (BSE). Unlike previous studies, the new data indicate δ51VBSE=-0.91±0.02‰（2SE, n=18）. We incorporated this into a two-component mixing model for the Earth-Moon system, considering a system with pre-impact (proto-Earth, Theia) and post-impact (Earth, Moon, escaping mass) components. The best estimate for the mass fraction of Theia in the present Moon ranges from 73% for M_Theia=0.8M_Mars to 83% for M_Theia=0.45M_Earth This represents a reduction of approximately 5% in Theia’s contribution compared to earlier studies. These findings provide more reliable parameters for the classic collision model, contributing to a deeper understanding of the lunar formation process.

Key words: classical collision model, bulk silicate Earth, Moon, vanadium isotopes