Iron isotopic systematics of oceanic basalts

doi:10.1016/j.gca.2012.12.027

Geochimica et Cosmochimica Acta

Volume 107, 15 April 2013, Pages 12-26

https://doi.org/10.1016/j.gca.2012.12.027 Get rights and content

Abstract

The iron isotopic compositions of 93 well-characterized basalts from geochemically and geologically diverse mid-ocean ridge segments, oceanic islands and back arc basins were measured. Forty-three MORBs have homogeneous Fe isotopic composition, with δ⁵⁶Fe ranging from +0.07‰ to +0.14‰ and an average of +0.105 ± 0.006‰ (2SD/√n, n = 43, MSWD = 1.9). Three back arc basin basalts have similar δ⁵⁶Fe to MORBs. By contrast, OIBs are slightly heterogeneous with δ⁵⁶Fe ranging from +0.05‰ to +0.14‰ in samples from Koolau and Loihi, Hawaii, and from +0.09‰ to +0.18‰ in samples from the Society Islands and Cook-Austral chain, French Polynesia. Overall, oceanic basalts are isotopically heavier than mantle peridotite and pyroxenite xenoliths, reflecting Fe isotope fractionation during partial melting of the mantle. Iron isotopic variations in OIBs mainly reflect Fe isotope fractionation during fractional crystallization of olivine and pyroxene, enhanced by source heterogeneity in Koolau samples.

Introduction

The magnitude of equilibrium isotope fractionation decreases with increasing temperature and atomic mass, and is likely to be small for isotopes of heavy elements during high-temperature processes (Bigeleisen and Mayer, 1947, Urey, 1947). On the other hand, isotope fractionation associated with kinetic processes such as chemical diffusion, Soret effect, or evaporation/condensation can remain significant at high temperatures (Richter et al., 2009). Recent high-precision isotopic analyses of natural samples have shown that measurable Fe isotope fractionation could occur at both whole-rock (>0.2‰) and mineral scales (>1.6‰) during mantle melting (Williams et al., 2004, Williams et al., 2005, Williams et al., 2009, Weyer et al., 2005, Weyer and Ionov, 2007, Dauphas et al., 2009a, Zhao et al., 2010, Zhao et al., 2012, Huang et al., 2011c, Hibbert et al., 2012) and igneous differentiation (Poitrasson and Freydier, 2005, Heimann et al., 2008, Teng et al., 2008, Teng et al., 2011, Schoenberg et al., 2009, Schuessler et al., 2009, Weyer and Seitz, 2012, Telus et al., 2012). The fractionation of Fe isotopes at high temperatures could be produced by kinetic or equilibrium processes and may be associated with changes in the oxidation state of Fe.

Studying the mechanism associated with high-temperature Fe isotope fractionation is important to further our understanding of the theory on stable isotope fractionation and to use Fe isotopes as tracers of petrogenetic processes. For example, compared to chondrites (Poitrasson et al., 2005, Schoenberg and von Blanckenburg, 2006, Dauphas et al., 2009a, Craddock and Dauphas, 2010), terrestrial and lunar basalts have heavy Fe isotopic compositions (Beard et al., 2003, Poitrasson et al., 2004, Schoenberg and von Blanckenburg, 2006, Weyer and Ionov, 2007, Teng et al., 2008, Schuessler et al., 2009, Dauphas et al., 2009a, Craddock et al., 2010, Liu et al., 2010), which was initially ascribed to evaporation-driven kinetic Fe isotope fractionation during the giant impact that formed the Moon, resulting in a non-chondritic Fe isotopic composition of terrestrial and lunar mantles (Poitrasson et al., 2004). Polyakov (2009) and Williams et al. (2012) proposed instead that this reflected high-pressure equilibrium Fe isotope fractionation between metal and silicate during core formation or disproportionation of Fe²⁺ into Fe⁰ and Fe³⁺, which produced a non-chondritic mantle that was later sampled by terrestrial basalts. Alternatively, the difference in Fe isotopic composition between basalts and chondrites may result from Fe isotope fractionation during partial melting of the mantle (Weyer and Ionov, 2007, Dauphas et al., 2009a).

Additional complications arise from the fact that Fe isotopes can also be fractionated during fractional crystallization of magma (Teng et al., 2008, Teng et al., 2011, Schuessler et al., 2009, Weyer and Seitz, 2012). Consequently, basalts are not representative of their mantle sources. Indeed, global mantle xenoliths and high-degree partial melts have, on average, a Fe isotopic composition more similar to chondrites, suggesting a chondritic Fe isotopic composition of the Earth (Williams et al., 2004, Williams et al., 2005, Weyer et al., 2005, Weyer and Ionov, 2007, Dauphas et al., 2009a, Dauphas et al., 2010, Zhao et al., 2010, Zhao et al., 2012, Huang et al., 2011c, Hibbert et al., 2012, Craddock et al., 2013). Accordingly, complex models that invoke evaporation-induced kinetic isotope fractionation during moon-forming giant impact (Poitrasson et al., 2004) or equilibrium Fe isotope fractionation during core formation or Fe disproportionation (Polyakov, 2009, Williams et al., 2012) may not be needed.

In order to further constrain the extent to which partial melting and magmatic differentiation affect the Fe isotopic compositions of igneous rocks, mid-ocean ridge basalts (MORBs), ocean island basalts (OIBs) and back arc basin basalts (BABBs) were analyzed. The results show that MORBs and BABBs have homogeneous Fe isotopic composition whereas OIBs are isotopically heterogeneous. On average, MORBs, BABBs and OIBs are isotopically heavier than average mantle xenoliths, reflecting Fe isotope fractionation during partial melting of the mantle and magma differentiation.

Section snippets

Samples

Except for a few studies (Weyer and Ionov, 2007, Teng et al., 2008, Schuessler et al., 2009), most previously investigated oceanic basalts focused on geostandards (Poitrasson et al., 2004, Weyer et al., 2005, Williams et al., 2005, Schoenberg and von Blanckenburg, 2006, Dauphas et al., 2009a, Dauphas et al., 2009b, Craddock and Dauphas, 2010). Here, a geographically dispersed, chemically diverse set of well-characterized samples, including 43 MORBs covering major ridge segments, 47 OIBs from

Analytical methods

Iron isotopic analyses were carried out at the Origins Laboratory of the University of Chicago. The detailed protocols for sample dissolution, column chemistry and isotopic analyses have been reported in Dauphas et al. (2009b) and Craddock and Dauphas (2010). Only a brief description is given below.

Fresh basalt samples, usually 1–5 mg glass fragments, were crushed in an agate mortar, cleaned with Milli-Q water for 10 min, three times, in an ultrasonic bath before dissolution. All samples were

Results

Iron isotopic compositions are reported in Table 1 for MORBs and BABBs, Table 2 for OIBs and are summarized as histograms in Fig. 4 for all oceanic basalts as well as literature oceanic basalt data.

Discussion

Large linearly correlated Mg and Fe isotopic variations are found in Hawaiian olivines resulting from kinetic isotope fractionation during inter-diffusion of Mg and Fe, with olivines isotopically either heavier or lighter than basaltic melts depending on the directions of interdiffusion (Teng et al., 2011). Hence, the isotopic difference between olivines and basaltic melts cannot be used to estimate equilibrium fractionation factors. When compared to olivine fragments from Hawaiian basalts,

Conclusions

The main conclusions to be drawn from our high-precision Fe isotopic analyses of MORBs, OIBs and BABBs are:

(1)
MORBs (mostly quenched submarine glasses) display a limited Fe isotopic variation with δ⁵⁶Fe ranging from +0.07‰ to +0.14‰ and an average δ⁵⁶Fe = +0.105 ± 0.006‰ (2SD/√n, n = 43). The homogenous Fe isotopic composition reflects limited variation in the degree of partial melting and fractional crystallization of the MORBs investigated here, as evidenced by their limited range in MgO.
(2)
Three BABBs

Acknowledgements

We thank Haolan Tang for help in the clean lab, Karsten Hansse for unpublished major element data for two OIB samples, Roz Helz and Frank Richter for discussions. The constructive comments from Helen Williams, Jasper Konter, Sune Nielsen and Bill White, and careful and efficient editing from Janne Blichert-Toft are greatly appreciated. This work was supported by NSF (EAR-0838227 and EAR-1056713) and Arkansas Space Grant Consortium (SW19002) to Fang-Zhen Teng; NSF (EAR-0820807), NASA (NNX09AG59G

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