Temperature regimes during formation of Miocene island dolostones as determined by clumped isotope thermometry: Xisha Islands, South China Sea

Abstract

Carbonate clumped isotope thermometry is a relatively new type of analysis that has been used to obtain a better understanding of the temperature and salinity of the water that mediated the development of dolostones in the Cenozoic successions of isolated oceanic islands. There is, however, still some uncertainty about the factors that could modify the clumped isotopes even in shallow burial conditions and thereby affect the temperatures and salinities derived from them. Here, focus is placed on a 210.5 m thick succession of Miocene dolostones that is found on the Xisha islands in the South China Sea that have experienced rapid subsidence since the Early Miocene. These dolostones are typically formed of crystals that have a “dirty” core that is encased by a clear outer rim. The formation of the “dirty cores”, the primary replacement dolomite, was probably mediated by normal seawater in near-surface conditions, whereas the later cements formed at higher temperatures from slightly evaporated seawater. Given that it is impossible to separate the “dirty cores” from the clear rimes for analytical purposes, it must be recognized that the clumped isotopes and other geochemical data (e.g., δ18O, 87Sr/86Sr) are averages of these two types of dolomite. This also means that the development of dolomite in these dolostones involved two successive stages of dolomite development that was probably related to sea-level cycles. Despite these issues, clumped isotope thermometry can still provide valuable temperature and salinity information for understanding the dolomitization processes that led to the development of island dolostones.