Detailed characterization of diagenetic fluid behaviour and evolution is commonly hindered by shortage of appropriate and eco- nomic methods to carry out in situ analysis in the tiny diagenet ic mineral crystals. Using micromill samp領(lǐng) method, this study presents in situ stable isotopic data for the growth zones of saddle dolomite crystals in the hydrothermally-altered dolostones of Upper Cambrian (Furongian) at Keping, Tarim Basin, NW China. These data show minor and large variations in 13 C (− 0.7 to − 1.5 VPDB) and 18 O (−8.3 to − 13.8 VPDB), respectively, from the cores to outer rims of the dolomite crystals and sug- gest that saddle dolomites precipitated from dolomitizing fluids with similar carbon sources but oscillatory temperatures during different formation stages. This scenario is confirmed by microthe rmometry of fluid inclusions within the growth zones of dolo- mite crystals. This study indicates that in situ isotope analysis could provide more deta iled information related to the source and pathway of dolomitizing fluids, facilitating better characterization of dolomitizing fluids and processes.