Application of combined fluid-inclusion and clumped isotope thermometry to biogenic and inorganic carbonates

Abstract

<p>Biogenic and inorganic carbonates are widely used to reconstruct past temperatures and fluid compositions. For decades, calcification temperatures have been inferred from oxygen isotope composition (δ<sup>18</sup>O) of calcium carbonates (CaCO­<sub>3</sub>) assuming the δ<sup>18</sup>O of the parental fluid and isotopic equilibrium precipitation conditions. The development of the clumped isotope (Δ<sub>47</sub>) thermometer allows for reconstructing equilibrium calcification temperatures without requiring a priori knowledge of the water δ<sup>18</sup>O values.</p><p>Carbonate minerals can also contain several weight percentages of water, which are typically trapped within microscopic pores. These fluid-inclusions may preserve remnants of the parental fluid, which can be analyzed for the δ<sup>18</sup>O as well as hydrogen isotopic (δ<sup>2</sup>H) composition. Subsequently, the δ<sup>18</sup>O of fluid-inclusion and host carbonate may allow for the determination of paleotemperatures by providing constraint on the δ<sup>18</sup>O water value.</p><p>Reasonable equilibrium temperatures can be obtained for speleothem calcites from cave systems. On the contrary, anomalously high temperatures are derived from δ<sup>18</sup>O fluid-inclusion and calcite pairs in soil carbonates possibly suggesting diffusion of trapped water from host CaCO<sub>3</sub>. Deeply-buried and subsequently exhumed (inorganic) calcite veins have yielded discrepant paleotemperature estimates between fluid-inclusion and Δ<sub>47</sub> thermometers. The distinctly lower fluid-inclusion derived temperatures might be attributed to kinetic fraction during initial vein cementation and/or isotopic re-equilibration between fluid-inclusions and CaCO<sub>3</sub> at lower temperatures during uplift.</p><p>Heating experiments demonstrate that the oxygen isotope exchange between fluid inclusions and host carbonate is limited for inorganic calcite and aragonite at high temperatures (175<sup>o</sup>C) for short timescales (90 minutes). In contrast, considerable positive shifts in the δ<sup>18</sup>O of fluid inclusions have been recorded in biogenic aragonites during experimental heating, which coincide with lower carbonate δ<sup>18</sup>O values (albeit to a lesser extent due to the overwhelming amount of oxygen in the CaCO<sub>3</sub>), indicative of re-equilibration between host carbonate and pore fluids. This effect leads to apparently high equilibrium temperatures. In conjunction, the Δ<sub>47</sub> derived temperatures do not change significantly after heating of inorganic aragonite, whereas a considerable higher Δ<sub>47</sub> temperature is derived from aragonitic bivalve samples after heating. The positive shift in both thermometers has interpreted to reflect re-crystallization of CaCO<sub>3</sub> and isotopic re-equilibration between the host carbonate and fluid-inclusions. This exchange might be facilitated by extremely small fluid-inclusions present in biogenic carbonates and/or water associated with organic substances.</p><p>Importantly, these isotopic exchange processes in biogenic aragonites took place in the absence of an external fluid and below the temperature thresholds for solid-state-reordering and the aragonite-to-calcite transition. The novel application of combined fluid-inclusion and clumped isotope thermometry has a proven utility in determining equilibrium precipitation temperatures, monitoring preservation of the primary fluid-inclusions and re-crystallization processes during diagenesis. However, additional experiments and analytical improvements are needed to further constrain the diagenetic behavior of this proxy.</p><p> </p>