26Mg-13C-18O systems as geochemical tracers for dolomite recrystallization: A case study of lower Ordovician dolomite from Tarim Basin

Widespread replacement dolomite occurs commonly in the geologic record. These dolomites are generally characterized by different crystal sizes and shapes that have at times been interpreted to represent multistage dolomitization; although the variation of dolomite texture could also have resulted from the recrystallization of precursor dolomite. It is important, but challenging, to differentiate these two scenarios to better understand the nature of dolomite and the associated processes of dolomitization. Our study explores this problem by using delta 26Mg, delta 13C, and delta 18O isotopes to characterize the different crystal morphologies exhibited by Ordovician do-lomites from the Tarim Basin, China.Although the dolomites show distinct textures and crystal morphologies, there are no discernible trends in their delta 26Mg values (from-1.66 to-2.39%o), suggesting that these dolomites were formed by the same dolo-mitizing fluid. This interpretation is supported by an overlapping range of delta 13C values (from 0.46 to-1.89%o). By contrast, the delta 18O data demonstrate a wide range of values, from-3.8%o to-8.8%o, reflecting the different degree of recrystallization with increasing burial temperatures. We suggest that the Mg and C isotopes remained unchanged during recrystallization because Mg and C were rock-buffered, so the recrystallized dolomite inherited Mg and C from the precursor dolomite. Based on these results, it appears that Mg isotopes, together with conventional O-C isotopes, can provide a diagenetically robust geochemical tracer for identifying dolomite recrystallization in the geological record.