Germanium isotopic variations in igneous rocks and marine sediments Olivier Rouxel a, * , Albert Galy b , Henry Elderfield b a Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry Department, Woods Hole, MA 02543, USA b Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Received 1 August 2005; accepted in revised form 27 April 2006 Abstract A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate rocks and biogenic opal. The analyses were performed using a continuous flow hydride generation system coupled to a MC- ICPMS. Samples have been purified through anion- and cation-exchange resins to separate Ge from matrix elements and eliminate potential isobaric interferences. Variations of 74 Ge/ 70 Ge ratios are expressed as d 74 Ge values relative to our internal standard and the long-term external reproducibility of the data is better than 0.2& for sample size as low as 15 ng of Ge. Data are presented for igneous and sedimentary rocks, and the overall variation is 2.4& in d 74 Ge, representing 12 times the uncertainty of the measurements and dem- onstrating that the terrestrial isotopic composition of Ge is not unique. Co-variations of 74 Ge/ 70 Ge, 73 Ge/ 70 Ge and 72 Ge/ 70 Ge ratios follow a mass-dependent behaviour and imply natural isotopic fractionation of Ge by physicochemical processes. The range of d 74 Ge in igneous rocks is only 0.25& without systematic differences among continental crust, oceanic crust or mantle material. On this basis, a Bulk Silicate Earth reservoir with a d 74 Ge of 1.3 ± 0.2& can be defined. In contrast, modern biogenic opal such as marine sponges and authigenic glauconite displayed higher d 74 Ge values between 2.0& and 3.0&. This suggests that biogenic opal may be significantly enriched in light isotopes with respect to seawater and places a lower bound on the d 74 Ge of the seawater to +3.0&.This suggests that seawater is isotopically heavy relative to Bulk Silicate Earth and that biogenic opal may be significantly fractionated with respect to sea- water. Deep-sea sediments are within the range of the Bulk Silicate Earth while Mesozoic deep-sea cherts (opal and quartz) have d 74 Ge values ranging from 0.7& to 2.0&. The variable values of the cherts cannot be explained by binary mixing between a biogenic compo- nent and a detrital component and are suggestive of enrichment in the light isotope of diagenetic quartz. Further work is now required to determine Ge isotope fractionation by siliceous organisms and to investigate the effect of diagenetic processes during chert lithification. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Germanium is a trace element in the Earth’s crust, aver- aging about 1 ppm in whole rocks and minerals. Because of nearly identical ionic radii and electron configurations for germanium (Ge) and silicon (Si), the crustal geochemistry of Ge is dominated by a tendency to replace Si in the lattice sites of minerals (Goldschmidt, 1958; De Argollo and Schilling, 1978). These two elements exist in seawater as similar hydroxyacids, i.e., Ge(OH) 4 and Si(OH) 4 (Pokrov- ski and Schott, 1998) and the uptake and regeneration pro- file of Ge is similar to that of Si (Froelich and Andreae, 1981), thus providing an interesting tracer for biogenic silica cycling in the ocean. The average Ge/Si ratio in the global ocean is about 0.7 · 10 6 (atom/atom) (Froelich and Andreae, 1981), which is significantly lower than the Ge/Si in basaltic rocks averaging about 2.6 ± 0.3 · 10 6 (De Argollo and Schil- ling, 1978). Because the two dominant Si and Ge sources to the oceans carry very different Ge/Si signatures, with (Ge/Si) rivers 0.54 · 10 6 (Froelich et al., 1992) and (Ge/Si) hydrothermal 8–14 · 10 6 (Mortlock et al., 1993), it has been proposed that the Ge/Si ratio buried in siliceous tests on the seafloor reflects the present and past source strength of river fluxes relative to hydrothermal fluxes (She- mesh et al., 1989; Froelich et al., 1992; Elderfield and Schultz, 1996). However, the use of Ge/Si as a paleoceano- graphic tool remains uncertain as Ge may be removed from 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.04.025 * Corresponding author. Fax: +1 508 457 2193. E-mail address: orouxel@whoi.edu (O. Rouxel). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 70 (2006) 3387–3400