Oxygen and clumped isotope fractionation during the formation of Mg calcite via an amorphous precursor

Martin Dietzel,Bettina Purgstaller,Tobias Kluge,Albrecht Leis,Vasileios Mavromatis

doi:10.1016/j.gca.2020.02.032

Martin Dietzel, Bettina Purgstaller + Show 3 more

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https://doi.org/10.1016/j.gca.2020.02.032

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Abstract

The oxygen and clumped isotope signatures of Mg calcites are routinely used as environmental proxies in a broad range of surroundings, where Mg calcite forms either by classical nucleation or via an amorphous calcium magnesium carbonate (ACMC) precursor. Although the (trans)formation of ACMC to Mg calcite has been identified for an increasing number of settings, the behavior of both isotope proxies throughout this stage is still unexplored. In the present study ACMC (trans)formation experiments were carried out at constant pH (8.30 ± 0.03) and temperature (25.00 ± 0.03 °C) to yield high Mg calcite (up to 20 mol% Mg). The experimental data indicate that the oxygen isotope values of the amorphous and/or crystalline precipitate (δ18Oprec, analyzed as Mg calcite) are affected by the (trans)formation pathway, whereas clumped isotopes (Δ47prec = Δ47Mg-calcite) are not. The oxygen isotope evolution of the solid phase can be explained by the instantaneous trapping of the isotopic composition of the aqueous (bi)carbonate complexes. This entrapment results in remarkably high 103ln(αprec-H2O) values of ∼33‰ at the initial ACMC formation stage. During the ACMC transformation process the oxygen isotope equilibrium is approached rapidly between Mg calcite and water (Δ18OMg calcite-water = 30.3 ± 0.4‰) and no isotopic memory of the initial to the final Mg calcite at the end of the experiment occurs. The implications for oxygen and clumped isotope signatures of Mg calcite formed via ACMC are discussed in the aspects of various scenarios of (trans)formation conditions and their use as environmental proxies.

Highlights

The oxygen and clumped isotope signatures of Mg calcites are routinely used as environmental proxies in a broad range of surroundings, where Mg calcite forms either by classical nucleation or via an amorphous calcium magnesium carbonate (ACMC) precursor
(iii) Oxygen isotope composition of Mg calcite formed via ACMC can be highly affected by itsformation pathway, which is validated by three scenarios based on open to closed conditions in respect to the transformation solution (I and II) and on the transformation environment/conditions being different from that where ACMC formed (III)
(iv) Clumped isotopes of Mg calcite seem to be more promising than oxygen isotopes to be used as temperature proxy even at fast precipitation conditions, where ACMC precursors occur, but in particular scenario III has still to be investigated for this scope

Summary

Introduction

Oxygen- and clumped- isotope signatures of carbonate minerals are used as environmental proxies in a broad range of tasks comprising ancient ocean composition, paleoclimate reconstruction, origin of life- and dolostone, carbonate diagenesis, speleothem formation, hydrothermal carbonate deposits, technical settings, geochemical forensics etc. (e.g. Dietzel et al, 1992; Kosednar et al, 2008; Hoefs, 2015 and references therein; Defliese et al, 2016; Dietzel et al, 2016; Falk et al, 2016; Spooner et al, 2016; Fohlmeister et al, 2018; Kluge et al, 2018; Boch et al, 2019). Oxygen- and clumped- isotope signatures of carbonate minerals are used as environmental proxies in a broad range of tasks comprising ancient ocean composition, paleoclimate reconstruction, origin of life- and dolostone, carbonate diagenesis, speleothem formation, hydrothermal carbonate deposits, technical settings, geochemical forensics etc. To date oxygen isotope fractionation between calcite and water and clumped isotopes of calcite were studied via direct precipitation from an aqueous solution. In the past two decades, an alternative pathway of calcite formation has been shown to occur.

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