9.3       Seawater hafnium

 

The first study of seawater hafnium was made by White et al. (1986), based on the analysis of four Fe–Mn nodules from the Pacific and one each from the Atlantic and Indian oceans. In contrast to the wide range of Hf isotope signatures in marine sediments, ferromanganese nodules were found to have homogeneous , Hf values around +2, with little variation outside analytical error, despite the observed wide range of Nd isotope compositions (Fig. 9.21). White et al. interpreted the isotope signatures of the nodules as indicative of seawater hafnium, which therefore appears to resemble seawater strontium in being homogeneous in the world’s oceans.

Fig. 9.21. Plot of hafnium isotope compositions against Nd and Sr to show variations in Fe–Mn nodules ( ! ) and marine sediments ( " ) relative to oceanic volcanics. After White et al. (1986).

 

            White et al. attributed the homogeneous seawater hafnium signature to mixing between crustal and mantle sources of dissolved hafnium. This implies that the crustal source should reflect the composition of non-zircon-bearing sediments, with , Hf around –9, probably carried in river water. On the other hand, the mantle-like end member was attributed to low temperature alteration of basaltic ocean floor crust, with a MORB , Hf signature around +16. This implies approximately equal mixing between the two end-members. However, because the deduced range of seawater Hf is much smaller than the range between the end-members, it also suggests that hafnium must have a long residence time in seawater, to allow such a high degree of homogenisation. This was surprising, since the concentration of hafnium in seawater is low. However, White et al. suggested that hafnium would be present in seawater as the hydrolysed species Hf(OH)5–, making it resistant to the process of particulate scavenging that gives rise to the short seawater residence time of the REE.

 

            The relatively large errors in Hf analysis by TIMS prevented the development of this work until the advent of MC-ICP-MS. In the first study with this instrument, Godfrey et al. (1997) used ferromanganese crusts as archives of recent seawater Hf compositions, and revealed a weak correlation with Pb isotope ratio. This was attributed to the mixing of continental and hydrothermal Hf in the global ocean system. Within this array the Atlantic had less radiogenic Hf, implying domination by sedimentary Hf, whereas the Pacific had more radiogenic Hf, suggesting domination by a sea floor hydrothermal Hf flux. However, the range of seawater Hf isotope signatures was more restricted than Pb (relative to the end-member compositions), which led Godfrey et al. to suggest that Hf has a longer seawater residence time than Pb.

 

            Albarede et al. (1998) analysed a larger suite of ferromanganese nodules from the Atlantic and Pacific oceans, and confirmed that these did show some degree of Hf isotope variation. In addition, they found that samples with over 7.5 ppm Hf displayed a significant co-variation with Nd isotope composition, in both the Atlantic and Pacific Ocean (Fig. 9.22). On the other hand, aberrant signatures in samples with lower Hf contents were attributed to the incorporation of less radiogenic Hf of a detrital or diagenetic origin. The trajectory of the right-hand end of the mixing line is not precisely defined, but it converges on the general field of ocean floor basalts, and is therefore compatible with a source in low temperature hydrothermal fluids, as proposed by White et al. (1986). On the other hand, the left hand end of the mixing line in Fig. 9.22 points to an end-member distinctly more radiogenic than most sediments. Similar results were obtained by David et al. (2001). Putting all the evidence together, it appears that the seawater residence time of Hf is somewhat longer than that of Nd, but much shorter than that of Sr.

Fig. 9.22. Plot of Hf versus Nd isotope composition for ferromanganese nodules from the Atlantic Ocean (!) and Pacific Ocean ( > ), compared with sedimentary rocks ( " ) and the oceanic mantle array. ( + ) = Mn nodules with low Hf contents. After Albarede et al. (1998).

 

            The first record of paleo seawater Hf isotope variations for the Cenozoic was presented by Lee et al. (1999). This record came from two Pacific ferromanganese crusts from ca. 2 km depth. These have been dated by 10Be over the past 20 Myr, with extrapolated growth rates back to 50 Myr. The two crusts were found to have flat , Hf evolution profiles for the past 20 Myr, ranging from +6 to +8, with more variations in the period 20 – 50 Myr ago that were partially correlated between the two crusts. However, there was essentially no correlation with Nd isotope ratio.

 

            More complex paleo-seawater Hf variations were observed in ferromanganese crusts from the North Atlantic Ocean by Piotrowski et al. (2000). Using a composite record from two different crusts, Piotrowski et al. observed a positive correlation between Hf and Nd isotope signatures over the past 5 Myr (Fig. 9.23). However, between 30 and 5 Myr ago, Hf and Nd were decoupled, as , Nd showed a gradual increase while , Hf decreased and then increased again. Piotrowski et al. invoked changes in the weathering of the non-zircon component of the continental crust to explain these variations. However, they rejected a suggestion by Albarede et al. (1998) that this end-member might reflect the input of zircon-deficient wind-borne material to the oceans. This was because suggested indices of the windborne flux, such as the Al contents of the crusts, were not correlated with changes in Hf isotope ratio. Therefore, Piotrowski suggested that the zircon-deficient continental Hf flux was mostly carried by rivers. This model is supported by Hf isotope data from Pacific ferromanganese crusts, which do not show a marked change in isotopic composition 3.5 Myr ago, when there was a marked increase in the flux of aeolian dust from the loess deposits of eastern Asia (Pettke et al., 2002).

Fig. 9.23. Plot of , Hf against , Nd showing a magnified (composite) record from two North Atlantic Fe–Mn crusts, attributed to temporal variations in the isotope composition of Hf and Nd of seawater due to varying riverine fluxes. After Piotrowski et al. (2000).

 

 

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