Eclogites, although pretty rocks, can provide analytical challenges due to their unique mineralogy. This post discusses one of those problems and how I tried to overcome it… before I realised the problem wasn’t actually a problem.
Eclogites typically contain rutile. Some, such as the Engebøfjellet eclogite in Norway contain economic deposits of the stuff. Rutile can contain up to 95 % of the whole rock’s inventory of important trace elements (e.g. Nb, Ta, HFSEs) useful in fingerprinting the igneous protolith. The refractory nature of eclogite, however, means that it is somewhat hard dissolve and melt. Thus, when dissolving whole rock eclogite powders for ICP-MS analysis, using a series of powerful acids, how does one know you are actually dissolving the rutile? I needed to find an alternative method that I thought would potentially increase the dissolution of rutile during preparation.
The reason for the refractory nature of rutile is its dense tetragonal structure. Melting the rutile, along with all the other constituent minerals in the the rock, allows acids to dissolve the sample with ease. So that’s what I did. I crushed my samples, fused them into a glass (with lithium borate flux, 1100°C for 30 mins), crushed them again, and finally dissolved them with a series of acids on a hotplate. They were then analysed using ICP-MS at the NERC facility at East Kilbride, Scotland.
Only rare earth elements (REEs) were analysed. Abundances were compared to identical samples which underwent the “standard” preparation procedure. REE abundances of fused samples show 9-27 % depletions (from light-REEs to heavy-REEs respectively) with the exception of La which is significantly enriched by 1-2 orders of magnitude in each of the samples.
Conclusions & Implications
Well after all the effort that went into dreaming up this new method and preparing the samples, I was a little disappointed that it didn’t show increased abundances. However, the results show that rutile is being completely dissolved in the ‘standard’ sample preparation procedure at East Kilbride, and that the fusion technique achieves nothing but reducing abundances of the REEs by significant percentages whilst contaminating them with La. It is likely that La is being introduced from the flux, and that the other REEs are showing depletions after escaping the crucible during the high-T fusion process.
Now that I am sure that my samples are being completely dissolved, I am going to go ahead and analyse the other 20 or so samples in my collection from Norway. I’m looking at the REEs and traces such as Ta and Hf. There are already some interesting interpretations coming out of the REE data on the 17 samples I have at the moment!