High-precision triple oxygen isotope analysis of Archean and Proterozoic carbonates
Authors: Jäger Oliver , Surma Jakub , Albrecht Nina , Marien Chris S. , Xiang Wanli , Schier Katharina , Bau Michael , Reitner Joachim , Pack Andreas 
Published: March 2020

&lt;p&gt;Oxygen isotopes are a widely used tracer in the field of paleoceanography and provide unique information on mineral formation and environmental conditions. Carbonate sediments record a shift in &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O of 10 to 15&amp;#8240; from the Archean towards higher values in the Phanerozoic. Three different scenarios are suggested to explain this observation: (I) hot Archean oceans, (II) depletion of &lt;sup&gt;18&lt;/sup&gt;O in Archean oceans compared to present day and (III) diagenetic alteration of the primary isotopic signature [1]. Recent advances in high-resolution gas source isotope ratio mass spectrometry provide a new tool that may allow to decipher the origin of this isotopic shift observed in the early rock record. We performed high-precision &lt;sup&gt;18&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O and &lt;sup&gt;17&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O measurements on oxygen ion fragments (&lt;sup&gt;16&lt;/sup&gt;O&lt;sup&gt;+&lt;/sup&gt;, &lt;sup&gt;17&lt;/sup&gt;O&lt;sup&gt;+&lt;/sup&gt;, &lt;sup&gt;18&lt;/sup&gt;O&lt;sup&gt;+&lt;/sup&gt;) generated in the ion source from CO&lt;sub&gt;2&lt;/sub&gt; gas [2]. Isobaric interferences on m/z=17 (&lt;sup&gt;16&lt;/sup&gt;OH&lt;sup&gt;+&lt;/sup&gt;) and m/z=18 (H&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;16&lt;/sup&gt;O&lt;sup&gt;+&lt;/sup&gt;) are separated by means of high mass resolution. The CO&lt;sub&gt;2&lt;/sub&gt; gas is first liberated from carbonate samples by orthophosphoric acid digestion and then analyzed on a &lt;em&gt;Thermo Scientific Ultra&lt;/em&gt; dual-inlet gas source isotope ratio mass spectrometer [3]. By adding the dimension of &lt;sup&gt;17&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O to the classical&lt;sup&gt; 18&lt;/sup&gt;O/&lt;sup&gt;16&lt;/sup&gt;O system, equilibrium trajectories of carbonates that are defined by the equilibrium fractionation factor (&lt;sup&gt;18&lt;/sup&gt;a&lt;sub&gt;eq&lt;/sub&gt;) and the triple isotope fractionation exponent (&amp;#952;) can be predicted as a function of temperature. Minerals that were altered by or formed in meteoric water can be distinguished from those that precipitated in equilibrium with ambient sea water. Therefore, triple oxygen isotope analysis of carbonates does not only hold the potential for a new single-phase paleothermometer, but may also be used to trace the origin of carbonates. Here, we present high-precision triple oxygen isotope data for carbonates from the Pilbara and the Kaapvaal cratons that cover nearly one billion years from the Paleoarchean to the Paleoproterozoic. Marine carbonates from the Phanerozoic complement the dataset. The carbonates were formed in different marine settings, from shallow marine stromatolites to carbonates grown in the interstitial space of basaltic pillows. Phanerozoic carbonates record equilibrium conditions with modern sea water at moderate temperatures. The majority of Precambrian carbonates plot below the predicted equilibrium curve in the &amp;#948;&amp;#8217;&lt;sup&gt;18&lt;/sup&gt;O-&amp;#916;&amp;#8216;&lt;sup&gt;17&lt;/sup&gt;O space and do not reflect equilibrium conditions with modern sea water at elevated temperatures that were proposed for the Archean oceans. Modeling the triple oxygen isotope composition of carbonates in equilibrium with sea water, that is depleted in &lt;sup&gt;18&lt;/sup&gt;O also cannot explain the observed isotopic shift. Further modeling of post-depositional alteration suggests that most carbonates interacted and re-equilibrated with meteoric waters at variable water-rock ratios and temperatures.&lt;/p&gt;&lt;p&gt;[1] Shields and Veizer, 2002, Geochem., Geophy., Geosyst., 10.1029/2001GC000266&lt;br&gt;[2] Getachew et al., 2019, Rapid Commun. Mass. Spectrom., 10.1002/rcm.847&lt;br&gt;[3] Eiler et al., 2013, Int. J. Mass. Spectrom., 335, 45-56.&lt;/p&gt;