I'm trying to measure Hf isotopes in CaCO3 rich (up to 98 %) sediment (and maybe uncleaned foram tests). To get reliable data on our machine I need ~20ng of Hf. However, it is really hard to leach Hf out of carbonate rich sediments. I don't know where in the leaching and purification process (Gutjahr et al., 2007) I loose or/and miss to dissolve the Hf. Has anybody an idea, why Hf could/would behave differently in presence of CaCO3?
We conducted some leaching experiments on carbonate rich sediments a while ago. It turns out that Hf is extremely low and extremely radiogenic in this fraction. You probably already know the Rickli et al 2013 paper (http://www.sciencedirect.com/science/article/pii/S0016703712005959) and judging from your affiliation you might be familiar with the leaching of Hf from sediments as well - so that might not be the big help for you after all. Do you have an idea how much Hf should be in your samples at all? Maybe you do not loose Hf - it's just not enough in your carbonate sample. You can email me if you have further questions on this and I'll see how much I can help, like I said it's been a while ago...
Cheers,
Torben
Like Torben said, it is unlikely that there is a lot of Hf in your sediments, just because calcite cannot fit that kind of ion in its lattice. The HREE fit slightly better, and that is the reason why you would get a radiogenic signature. A PhD student at Leuven University has been looking whether he can get enough Hf out of hydrothermal carbonates to use it as a dating tool (I think he hasn't been successful yet, because of the very low Hf concentrations). The only way to get enough Hf for analysis is to dissolve lots and lots of sample, unfortunately.
Hf is most likely to be present within particulate contaminants - e.g. zircon, requiring HF digestion. HFSE are very immobile. See something about its behaviour in extreme natural conditions in:
L. S. CAMPBELL, W. COMPSTON, K. N. SIRCOMBE and C. C. WILKINSON (2014). Zircon from the East Orebody of the Bayan Obo Fe-Nb-REE deposit, China, and SHRIMP ages for carbonatite-related magmatism and REE mineralization events. Contributions to Mineralogy and Petrology 168 23pp. http://link.springer.com/article/10.1007%2Fs00410-014-1041-3 (Open Access full text). DOI: 10.1007/s00410-014-1041-3
Thanks for the help so far. After some test with different decarbonation approaches, I dissolved quite large amounts of sediment (3-4 gramms) which gave me in the best cases about 20-40 ng of Hf. In some of the samples however I have lost almost all Hf (most likely due to some precipitates) which gave me around 2-4 ng in the end. Nevertheless, the isotopy of all samples could be measured using our Nu Plasma and Jörg Rickli's Neptune Plus, which returned data that are in an acceptable range for Southern Ocean Sediments with an tendency to being quite radiogenic (+3 to +10).
Why, do you think, there should be less Hf in Carbonate rich samples? I always thought there should always be a similar amount of Hf (depending on the conc in ambient seawater) in the Fe-Mn coatings, no matter what type of sediment. Could it be, that there are less coatings in carbonate-rich sediments or that they are more diluted?
Does the fact that HREE fit better into the calcite lattice lead to the conclusion that enriched radiogenic Hf signatures in carbonate sediments is imported via the carbonate and not from authigeniously from seawater?
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