Abstract
The conditions controlling the formation of sedimentary dolomite are
still poorly understood despite decades of research. Reconstructing
formation temperatures and δ18O of fluids from which dolomite has
precipitated is fundamental to constrain dolomitization models.
Carbonate clumped isotopes are a very reliable technique to acquire such
information if the original composition at the time of precipitation is
preserved. Sedimentary dolomite first mostly forms as a poorly-ordered
metastable phase (protodolomite) and subsequently transform to the more
stable ordered phase. Due to this conversion its important to determine
if the original clumped isotope composition of the disordered phase is
preserved during diagenetic conversion to ordered dolomite, and how
resistant clumped isotope signatures are against bond reordering at
elevated temperatures during burial diagenesis. Here, we present a
series of heating experiments at temperatures between 360 and 480 °C
with durations between 0.125 and 426 hours. We uses fine-grained
sedimentary dolomites to test the influence of grains size, and cation
ordering on bond reordering kinetics. We analyzed a lacustrine dolomite
with poor cation ordering and well ordered a replacement dolomite, both
being almost stoichiometric. The poorly ordered dolomite shows a very
rapid alteration of its bulk isotope composition and higher
susceptibility to solid state bond reordering, whereas the well-ordered
dolomite behaves like a previously studied coarse-grained hydrothermal
dolomite. We derive dolomite-specific reordering kinetic parameters for
ordered dolomitea and show that ∆47 reordering in dolomite is material
specific. Our results call for further temperature-time series
experiments to constrain dolomite ∆47 reordering over geologic
timescales.