Application of combined fluid-inclusion and clumped isotope thermometry to biogenic and inorganic carbonates
Authors: van der Lubbe Jeroen , Nooitgedacht Cas , Staudigel Philip , Ziegler Martin 
Published: March 2021

&lt;p&gt;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 (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) of calcium carbonates (CaCO&amp;#173;&lt;sub&gt;3&lt;/sub&gt;) assuming the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of the parental fluid and isotopic equilibrium precipitation conditions. The development of the clumped isotope (&amp;#916;&lt;sub&gt;47&lt;/sub&gt;) thermometer allows for reconstructing equilibrium calcification temperatures without requiring a priori knowledge of the water &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values.&lt;/p&gt;&lt;p&gt;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 &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O as well as hydrogen isotopic (&amp;#948;&lt;sup&gt;2&lt;/sup&gt;H) composition. Subsequently, the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of fluid-inclusion and host carbonate may allow for the determination of paleotemperatures by providing constraint on the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O water value.&lt;/p&gt;&lt;p&gt;Reasonable equilibrium temperatures can be obtained for speleothem calcites from cave systems. On the contrary, anomalously high temperatures are derived from &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O fluid-inclusion and calcite pairs in soil carbonates possibly suggesting diffusion of trapped water from host CaCO&lt;sub&gt;3&lt;/sub&gt;. Deeply-buried and subsequently exhumed (inorganic) calcite veins have yielded discrepant paleotemperature estimates between fluid-inclusion and &amp;#916;&lt;sub&gt;47&lt;/sub&gt; 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&lt;sub&gt;3&lt;/sub&gt; at lower temperatures during uplift.&lt;/p&gt;&lt;p&gt;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&lt;sup&gt;o&lt;/sup&gt;C) for short timescales (90 minutes). In contrast, considerable positive shifts in the &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of fluid inclusions have been recorded in biogenic aragonites during experimental heating, which coincide with lower carbonate &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values (albeit to a lesser extent due to the overwhelming amount of oxygen in the CaCO&lt;sub&gt;3&lt;/sub&gt;), indicative of re-equilibration between host carbonate and pore fluids. This effect leads to apparently high equilibrium temperatures. In conjunction, the &amp;#916;&lt;sub&gt;47&lt;/sub&gt; derived temperatures do not change significantly after heating of inorganic aragonite, whereas a considerable higher &amp;#916;&lt;sub&gt;47&lt;/sub&gt; temperature is derived from aragonitic bivalve samples after heating. The positive shift in both thermometers has interpreted to reflect re-crystallization of CaCO&lt;sub&gt;3&lt;/sub&gt; 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.&lt;/p&gt;&lt;p&gt;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.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;