Calcium isotopic fractionation between clinopyroxene and orthopyroxene from mantle peridotites Shichun Huang ⁎, Juraj Farkaš, Stein B. Jacobsen Department of Earth and Planetary Sciences Harvard University 20 Oxford St., Cambridge MA 02138, United States abstract article info Article history: Received 17 November 2009 Received in revised form 25 January 2010 Accepted 26 January 2010 Available online 23 February 2010 Editor: R.W. Carlson Keywords: Ca isotopes stable isotopic fractionation mantle geochemistry We report the first observation of Ca isotopic fractionation between co-existing clinopyroxene and orthopyroxene from Kilbourne Hole and San Carlos mantle peridotites. The 44 Ca/ 40 Ca in orthopyroxenes is 0.36 to 0.75‰ heavier than that in the co-existing clinopyroxenes. Using these isotopic constraints and the relative proportions of major Ca-bearing minerals in the upper mantle, the estimated 44 Ca/ 40 Ca of the upper mantle is 1.05 ± 0.04‰ heavier relative to NIST SRM 915a. This is slightly higher than our average for basalts (0.97 ± 0.04‰ heavier relative to NIST SRM 915a). Combined with published 44 Ca/ 40 Ca data on low temperature Ca-bearing minerals (calcite, aragonite and barite), we infer that the inter-mineral fractionation of Ca isotopes at both low- and high temperatures is primarily controlled by the strength of Ca–O bond. Accordingly, the mineral with a shorter Ca–O bond and a smaller Ca coordination number (i.e., stronger Ca–O bond) yields a heavier Ca isotopic ratio (i.e., higher 44 Ca/ 40 Ca). Since stable isotopes of major elements, such as Ca and Mg, exhibit small fractionations during igneous processes, the estimate of stable isotopic compositions of the bulk differentiated planetary bodies, including the Earth and the Moon, needs to take into account the relative proportions of major rock-forming minerals and their respective isotopic signatures. © 2010 Elsevier B.V. All rights reserved. 1. Introduction With the advancement of modern analytical techniques, non- traditional stable isotopes of Mg, Si, Ca and Fe, which were previously not believed to fractionate during magmatic processes, have become powerful tools in the fields of cosmochemistry (e.g., Georg et al., 2007; Fitoussi et al., 2008; Chakrabarti and Jacobsen, 2009) and high temperature geochemistry (e.g., Williams et al., 2004; Teng et al., 2008; Dauphas et al., 2009). As the fifth most abundant element in the Earth, Ca has six isotopes ( 40 Ca, 42 Ca, 43 Ca, 44 Ca, 46 Ca and 48 Ca), making it a geochemical and cosmochemical tracer with considerable potential (e.g., DePaolo, 2004). With the exception of H and He, Ca has the largest relative mass difference (Δm/m = 20%) between the heaviest and the lightest isotopes. Thus, similar to stable isotopic studies of Si, Mg and Fe (e.g., Georg et al., 2007; Dauphas et al., 2009), the comparison of Ca isotopic ratios between the Earth and other planetary bodies, including the Moon, could yield important infor- mation regarding the early evolution of the Solar System and the origin of the Earth–Moon system (e.g., Simon and DePaolo, 2010). Knowledge of the Ca isotopic ratio in the Earth's mantle is also critical in investigating the chemical and isotopic evolution of seawater through geological time, as several lines of evidence suggest that the chemistry of the Archean and Paleoproterozoic oceans was strongly “mantle-buffered” due to massive circulation of seawater via oceanic crust and submarine hydrothermal systems (e.g., Veizer, 1982; Jacobsen and Kaufman, 1999). Previous Ca isotopic studies have focused mostly on modern and ancient marine carbonates and sulphates, documenting large and systematic isotopic variations (e.g., DePaolo, 2004; Heuser et al., 2005; Kasemann et al., 2005; Farkaš et al., 2007; Griffith et al., 2008a), yet detailed work on igneous rocks is fairly limited (Russell et al., 1978; Skulan and DePaolo, 1999; DePaolo, 2004; Amini, 2007; Amini et al., 2009a, b). Russell et al. (1978) presented the first and the most extensive Ca isotopic study that covered a wide range of igneous rocks from the inner Solar System. More recently, DePaolo (2004) showed ~0.7‰ variation in 44 Ca/ 40 Ca in oceanic basalts (see his Fig. 5). Amini et al. (2009b) reported ~0.5‰ variation in 44 Ca/ 40 Ca in a series of silicate rocks, including both felsic and ultramafic rocks. Huang et al. (2009a) reported that Makapuu-stage Koolau lavas have slightly lower 44 Ca/ 40 Ca (by 0.2‰) than other Hawaiian tholeiitic lavas. The observed 44 Ca/ 40 Ca variation in basalts may be interpreted either as a result of recycling ancient carbonate into the mantle (Fig. 15 of DePaolo, 2004; Huang et al., 2009a), or due to the fractionation of stable Ca isotopes during igneous processes. In order to constrain the Ca isotopic composition of the Earth's mantle and to investigate the possible Ca isotopic fractionation during igneous process, we report 44 Ca/ 40 Ca measurements on a series of terrestrial igneous rocks, including two nephelinites from Oslo Rift (Norway), six Hawaiian shield stage tholeiites (USA), one dunite (DTS-1) from Twin Sisters (Washington, USA), and clinopyroxene and orthopyroxene separates Earth and Planetary Science Letters 292 (2010) 337–344 ⁎ Corresponding author. Tel.: +1 617 496 7393. E-mail address: huang17@fas.harvard.edu (S. Huang). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2010.01.042 Contents lists available at ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl