Experimental quantification of the effect of Mg on calcite–aqueous fluid oxygen isotope fractionation Vasileios Mavromatis a, b, c, ⁎, Mark Schmidt a, d , Reiner Botz b , Laia Comas-Bru a, b, f , Eric H. Oelkers c, e a SFB574, University of Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany b Institute for Geosciences, University of Kiel, Olshausenstr. 40-60, D-24118 Kiel, Germany c Geosciences Environment Toulouse (GET), CNRS, UMR 5563, Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France d Helmoltz Centre for Ocean Research (GEOMAR), Wischhofstr. 1-3, 24148 Kiel, Germany e Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland f UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland abstract article info Article history: Received 18 January 2012 Received in revised form 23 March 2012 Accepted 28 March 2012 Available online 5 April 2012 Editor: U. Brand Keywords: Mg-calcite Oxygen isotope fractionation Paleo-temperature estimation Magnesium calcites were synthesized from aqueous solutions supersaturated with respect to calcite at 25, 40, 60, and 80 °C in gas tight batch reactors for up to 35 days. Any amorphous material still present in the precipitates was removed using a partial dissolution treatment. Resulting purified Mg-calcite had Mg contents ranging from 6 to 32 mol% MgCO 3. An isotopic steady-state was attained between the fluid and the precipitated solids within two weeks at 25 °C. δ 18 O values derived from the experiments at steady-state, depend on both temperature and the Mg content of the calcite in accord with: 1000 lnα MgÀcalcite–H 2 O ¼ 18; 030=T-32:42 þ 6 Â 10 8 =T 3 –5:47 Â 10 6 =T 2 þ 16; 780=T-17:21   Â C Mg where α MgÀcalcite–H 2 O represents the calcite–water oxygen isotope fractionation factor, T refers to the temperature in °K and C Mg denotes the mole percent of MgCO 3 in the calcite. These results indicate that the addition of 5 mol% MgCO 3 into the calcite increases 1000 lnα MgÀcalcite–H 2 O by 0.88 as compared to that of pure calcite at 25 °C. This difference could lead to a 4.2 °C decrease in estimated formation temperature estimates. These results demonstrate that the accurate interpretation of oxygen isotope fractionation in magnesium calcites requires explicit provision for the effect of magnesium on oxygen isotope fractionation factors. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Oxygen isotope fractionation between carbonate minerals and water is probably the most common method to estimate paleo- temperature during mineral formation. A large number of experi- mental (e.g., McCrea, 1950; O'Neil et al., 1969; Tarutani et al., 1969; Kim and O'Neil, 1997; Kim et al., 2006, 2007a; Dietzel et al., 2009) and theoretical (Watson, 2004; Schauble et al., 2006; Horita and Clayton, 2007; Chacko and Deines, 2008) studies have been performed to quantify this fractionation. These studies demonstrated that the oxygen isotope fractionation factor, α carbonate–water is affected both by fluid chemistry, including its saturation state with respect to the mineral phase, carbonate ion speciation, and carbonate mineral precipitation rates (see Dietzel et al., 2009 for details). Natural processes that result in rapid carbonate precipitation rates and thus kinetic isotope fractionation include degassing (Dandurand et al., 1982), evaporation, and mixing of supersaturated solutions (Aloisi et al., 2000). Rapid precipitation rates can also promote the incorporation of foreign ions into the solid phase. Several studies have demonstrated that Mg incorporation in calcite can significantly affect the degree of oxygen isotope fractionation between Mg-calcite and its coexisting fluid phase (Tarutani et al., 1969; Jimenez-Lopez et al., 2004). Tarutani et al. (1969) precipitated Mg-calcite containing from 4 to 12 mol% MgCO 3 and reported that 1000 lnα MgÀcalcite–H 2 O increases by 0.06‰ for each mol% of MgCO 3 incorporated in the crystal lattice at 25 °C. Many authors have used this result to calculate oxygen isotope compositions during mineral formation at lower temperatures (cf., Botz and von der Borch, 1984; Aloisi et al., 2000). In contrast, Jimenez-Lopez et al. (2004), found that 1000 lnα MgÀcalcite–H 2 O increased by 0.17‰ for each mol% of MgCO 3 incorporated in the crystal lattice at 25 °C. As the Mg content of natural calcites can range up to 30 mol% MgCO 3 , this variation of oxygen isotope fractionation factors with Mg content may have significant consequences for the interpretation of natural processes. This study was designed to improve our understanding of the effect of Mg on calcite isotopic Chemical Geology 310–311 (2012) 97–105 ⁎ Corresponding author at: Geosciences Environment Toulouse (GET), CNRS, UMR 5563, Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France. E-mail address: mavromat@get.obs-mip.fr (V. Mavromatis). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.03.027 Contents lists available at SciVerse ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo