Dear Kenneth, thank you! Yes, this is a fundamental study for zircon analysis. BSE - CL images will be the next step in searching for zircon grains. There is still hope. An associated plagiogranite next to the gabbro body has plenty of zircons! 

Dear Dr. Vesela,

it is not very common that zircon occurs side-by-side with feldspar after a careful heavy liquid separation in combination with the FRANTZ isodynamic magnetic separator. There is a simple question as to the presence of zircon. How do you know that this is zircon? There are also other Zr-bearing minerals like baddeleyite and a wealth of rare Zr minerals mainly found in alkaline felsic intrusive rocks such as syenites. I suspect of dealing with a Zr mineral different from zircon. On the other hand, I do not know any Zr mineral pertaining to the group of light minerals. I suggest, first and foremost precisely clarifying the mineralogical nature of the mineral accommodating Zr in its lattice and then embarking on a process of mineral dressing. If you have clear information on the overall geological setting you might also limit the possibility of the Zr minerals in question.

Viel Erfolg

H.G.Dill

I completely agree with Mr. Dill.  When properly separated using heavy liquid  zircon and feldspar wont be present in the same media.

The specific gravity of zircon varies from 4.6 to 4.8, whereas that of feldspar from 2.6-2.8.

In fact a simple heavy liquid separates these two minerals in the crushed rock sample.

Zircon is a common accessory mineral in felsic igneous rocks, the presence of zircon in gabbro is a little rare. Baddeleyite is the zirconium bearing mineral one can expect in gabbro.

so, your 240 ppm Zr, could be from   baddeleyite, which is also a heavy mineral with specific gravity of  5.5 to 6. which can be separated easily if present in good concentration.

The formation of zircon during metamorphsm due to breakdown of baddeleyite in presence of silica is explained at places, in this case the original prismatic zircons wont be there.

As you have mentioned it as meta gabrro ....this kind of possibility also can be expected.

Please go through the following reference which has described in detail

Metamorphic zircon formation at the transition from gabbro to eclogite in Trollheimen–Surnadalen, Norwegian Caledonides

Victoria Beckman1,*, Charlotte Möller1, Ulf Söderlund1, Fernando Corfu2, Jan Pallon3 and Kevin R. Chamberlain4

Thank you all for your suggestions and links! Yes, there is still some feldspar - after Sodium Polytungstate separation - I am not using other heavier liquids and for most of the samples it works fine. First I will check the thin sections under the microprobe to search for Zircons and their size. I did not find baddeleyite by eye. Can this be also recognized easily under BSE - CL like zircons do?

Dear Petra,

There is another option, you can easily identify high-Zr domains scanning a thin section in the electron microprobe, then you can try in-situ dating (check the paper attached). In BSE imaging baddeleyite looks much brighter than zircon (look the picture attached). This is a sample where baddeleyite transformed into zircon in Ti-Fe ores associated to anorthosites.

Try using X-ray spectroscopy.

Zircon has the characteristic X-ray-excitation  luminescence spectra. The region 450 - 500 nm and 550-600 nm are observed peaks system from rare earth elements (dysprosium).

When using excitation electrons (cathodoluminescence) will be the same result.

Dear Petra

I hope that the dirty snow balls look like on my pictures? This is typical zircons from kimberlites.

As an additional method you can try to use Raman spectroscopy.

Zircon has a specific line in the Raman spectrum.

V. Mironov

P. S. for Kenneth

Besides English, I have an excellent Russian. What about you?

I fully agree with Leonid, see drawings. Use UV lasers for excitation is preferred.

Dear Dr Vesela,

  Many good responses have already been given. Additionally, if you have access to a cold-cathodoluminescence instrument, zircons in polished sections will be quick and very easy to find, like 'shining beacons'.

Example (Open Access): Campbell et al. (2014) http://link.springer.com/article/10.1007%2Fs00410-014-1041-3

Best wishes, Linda.

Even if it might be too late;

In my experience with mafic rocks I made the observation that the Zr content of your rock is not neccessarily linked to the occurrence of zircon.

Furthermore, I think "the typical" gabbro zircon does not exist, but very often zircons from mafic melts are more or less rounded due to somekid of "corrosion" resulting likely from Zr depleted melts and some other physico-chemical effects (e.g. Gärtner et al. 2016, History of the West African Neoproterozoic Ocean). Thus, you can often find rounded but elongated grains with mostly very smooth surfaces. On the other hand, there are often gabbros, which contain euhedral zircons.

See also attached images of gabbroic zircons.

The next characteristic is the colour: The grains are mostly transparent and then you can easily identify them under polarised light.

If you have metamictisation, the grains become translucent to opaque and can haver a broad variety of colors, e.g. white, brown, orange, purple... then it becomes tricky to identify them if they do not show any crytal faces. However, with EDX or latest during the CL imaging you will know definitely what kind of mineral you have.

Best wishes,

Andreas

I am not sure what the etiquette is on Research Gate regarding the discussion of answers ... but I would just like to add that, from a provenance perspective, the rounding of zircons by igneous processes as described by Andreas makes things complicated and very interesting! As you said, Kenneth, the grains are not being rounded by sedimentary processes, so if we were to discover these grains in a sediment, the first thing you might conclude is that the zircons have been through multiple orogenic cycles - and we'd be wrong! As to answering Petra's question ... perhaps crushing a few milligrams of your mineral separate and analysing by powder XRD might give some insight as to where (what mineral) your zirconium is concentrating - assuming, of course, that the mineral contains zirconium as a major element.

Sorry for late reply to you all and thank you for all constructive and helpful comments and suggestions!!! As my youngest son arrived in October, I am now busy with diapers and not zircons... BUT soon I will go through some steps to find a solution. Please continue sharing your expertise! Thank you!

There are several hypotheses concerning the "melt corrosion". Some authors think, if a melt gets depleted in Zr (for whatever reasons) or such fluids affect the rock, the existing zircon grains from this melt/rock suffer this kind "corrosion" until the melt is equilibrated. I've cited some of these studies in the paper given above.

Nevertheless, there is no clearance at all about the time scale of the "corrosion" process (days, weeks, decades, Ma???). As this process affects the surfaces of the zircon grains it obviously occurs after the zircon formation itself. Thus, there is an other question to rise: is the corrosion more or less coeval with the zircon formation or is it caused by later effects? Re-melting, fluids, metamorphism, etc.

On the other hand, the rounding is obviously no common feature in mafic rocks asthere are also a lot of euhedral zircon samples (see most of the pictures given in this blog).

I would appreciate it a lot, if some studies about the zircon shapes from mafic rocks would be launched!

However, the rounded grains are mostly NOT inheritance, which can be shown e.g. by Hf isotope studies indicating often juvenile TDM ages.

But Brentons question is also very important with respect to sediments derived from such rocks.

Up to now, the rounded zircons from mafic rocks show very smooth surfaces and often exhibit bright outer zones in CL images which are characterised by a depletion of U. maybe this could be a distintive feature for those grains in sediments, but a lot of work has to be done on this topic.

I hope that somebody can give some input to this discussion, as it seems to be a common, but largely ignored problem.

You can find anhedral Zircon grains using RAMAN microscope. It has specific curve. Here is an example: http://rruff.info/Zircon/R050286

Luminescence of zircons from highly diamondiferous kimberlites of Yakutia

• July 2021
• AIP Conference Proceedings 2392(1):020009
• DOI:
• 10.1063/5.0061968