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The formation process of rodingite plays a crucial role in shaping the nature of slab fluids. Understanding this process is of great importance. Here we report high precision calcium (Ca) isotopic data of rodingites and associated rocks from the Xigaze ophiolite in the southern Tibet. These samples include eleven rodingites, one diabase-protolith, and one serpentinized harzburgite-country rock. They have previously been studied for whole-rock major and trace elements and mineral composition and magnesium (Mg) isotopic compositions. The rodingites yield a wide range of 44/40Ca values from 0.61‰ to 1.88‰. In contrast, the 44/40Ca value of country-rock fall within the range of abyssal peridotite, while the 44/40Ca value of protolith is lighter than those in (altered) oceanic crust. This suggests that the elevated Ca isotopic compositions of rodingites neither stem from their protoliths nor originate from country rocks. Consequently, we propose that the high 44/40Ca values of the Xigaze rodingites result from interactions with evolved external carbonate fluids. The Xigaze rodingites exhibit heavier Ca isotopes but lighter Mg isotopes compared to those of the normal mantle and oceanic crust, suggesting a decoupling between them resulting from differential fractionation. Although the Ca and Mg isotope of the Xigaze rodingites exhibit decoupling, both provide evidence supporting the necessity of carbonate incorporation. As a result, rodingite is a significant sink or reservoir for Ca during serpentinization of mantle rock.