Oxygen and carbon isotope variation between dolomite and co-existing micrite pairs, Gordon Group (Ordovician), Mole Creek, Tasmania, Australia
Titel:
Oxygen and carbon isotope variation between dolomite and co-existing micrite pairs, Gordon Group (Ordovician), Mole Creek, Tasmania, Australia
Auteur:
Rao, C. P.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 40 (1993) nr. 2 pagina's 131-139
Jaar:
1993-04
Inhoud:
Similar slopes of lines connecting δ18O and δ13C plots between co-existing micrite and dolomite pairs are due to small inheritance effects on δ13C from dissolution of CaCC3 and large variations in δ18O in dolomite due to dolomitizing fluids in a marine to mixing zone. The δ13C values of replaced precursor CaCO3 were variable due to early meteoric diagenesis. Dolomite is in equilibrium with marine, mixed-marine and meteoric CaCO3because dolomitization occurred when CaCO3 was metastable, simultaneously with or slightly after the transformation of metastable CaCC3 to calcite during early diagenesis. Recrystallization of dolomite involved the renewed growth of dolomite on early-formed dolomite and the replacement of co-existing CaCO3. This involved negligible depletion in δ13C and a small negative shift in δ18O. Three conceptual temperature models are proposed to show that similar original isotopic slopes of calcite-dolomite pairs will be retained with little modification in ? (dolomite-calcite) δ18O and δ13C values with increasing temperature. The trend of depletion of both δ13C and δ18O due to diagenesis of organic matter with increasing temperatures is partly applicable to the calcite-dolomite pairs studied. The Δ values of δ18O and δ13C are related to fluid composition, water-rock interaction, recrystallization of dolomite and some dedolomitization. Most of the δ ?13C and ? δ18O ratios are between 1:3 and 1:10+ indicating less 13C relative to 18O in the fluid, due to low to moderate water-rock interaction. Small influxes of meteoric and/or continental water into surface to shallowly buried marine sediment caused most of the dolomitization.
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