Stable Zirconium Isotopic Compositions of the Metasomatized Mantle

Mantle metasomatism, which affects the geochemical and lithological properties of the Earth, is crucial to the evolution and modification of the Earth's interior. Variations in chemical proxy of metasomatism, including high-field strength elements (HFSE, e.g., Zr-Ti isotopes and isovalent Zr-Hf), have been commonly utilized as a tool to unravel the processes associated with the evolution of the Earth. However, the impact of mantle metasomatism by diverse agents on the Zr isotopic compositions of metasomatized mantle remains poorly understood. Here we carried out high-precision double-spike delta 94/90Zr measurements for a suite of basalts, which were derived from the mantle sources metasomatized by typical metasomatic agents of fluid, silicate melt and carbonate melt originating from the deep mantle or recycled crustal materials. Mantle metasomatism results in lithological diversities within the mantle, ranging from hybridized peridotites to pyroxenite/eclogite and carbonated eclogite, which can significantly fractionate the isovalent HFSEs. However, our data show that these basalts have primitive mantle-like delta 94ZrNIST values (-0.033-0.043 parts per thousand), regardless of their superchondritic Zr/Hf. The primitive mantle-like delta 94ZrNIST, irrelevant to any proxy of metasomatism (e.g., Sr-Nd isotopes, Zr/Hf and La/Sm), indicate that mantle metasomatism and subsequence melting produce no measurable Zr isotopic variations in metasomatized mantle-derived basalts. The delta 94ZrNIST of basalts thus can be representative of the mean metasomatized mantle compositions. Combined with previously reported komatiites and oceanic basalts, we show that the Earth's mantle displays consistent delta 94ZrNIST (-0.013 +/- 0.053 parts per thousand), even if it has a secular dynamic evolution with the interaction between Earth's exterior and interior. Mantle metasomatism has a significant impact on the geochemical and lithological properties of the Earth's interior. The chemical variations, including the high-field strength elements (HFSE), caused by this process, could provide key insights into such modification. However, the variation of Zr isotopes during mantle metasomatism remains enigmatic. Here, we analyzed the Zr isotopes of fresh basalts derived from mantle sources metasomatized by diverse metasomatic agents. We find that although mantle metasomatism leads to the substantial fractionation of Zr (Nb) from Hf (Ta), no measurable Zr isotopic variation occurs for metasomatized mantle-derived melts, as reflected by the homogeneous and primitive mantle-like Zr isotopic compositions of different samples. Combining our findings with previous data, we conclude that the Earth's mantle, from large-scale and secular perspectives, maintains a consistent Zr isotopic composition, although the Earth underwent complex and dynamic evolution. A first comprehensive investigation of Zr isotopes of mantle metasomatized by diverse agents of fluid, silicate melt and carbonate melt Basalts derived from metasomatized mantle show variable radiogenic isotopes and trace element patterns, but limited Zr isotope variation The Earth's mantle exhibits consistent delta 94Zr, despite its secular dynamic evolution with interaction between Earth's exterior and interior