Hi all,

A quick perusal of my Himalayan rocks shows that rocks whose peak metamorphic ages are ≤~10 Ma lack pleochroic haloes in biotite or chlorite (well, I don’t see them, anyway). Rocks whose peak ages are c. 15 Ma do have weak pleochroic haloes in biotite (around monazite).

But, as Bob points out, there are several factors to consider, including the U-Th content of the radioactive mineral, etc. I’ve seen haloes around titanite in Appalachian rocks, but not in Himalayan rocks. Also, it’s not really the peak metamorphic age that matters, but whatever closure temperature applies to the accumulation of radiation damage. Given the cooling history of the Himalayan rocks, and assuming some moderate Tc, I would assume it takes on the order of 5-10 Ma for haloes to develop in biotite around monazite.

If you search on Gentry (1974; Science) in Google Scholar, you’ll find c. 100 articles talking about radiohalos. I’d start there. But be careful what you read - there’s also a creationist argument that certain aspects of radiohalos indicate a young Earth.

Matt

> On Sep 9, 2016, at 8:24 AM, Robert Tracy <[log in to unmask]> wrote:
> 
> Mallickarjun,
> 
> Don’t know of any such references, but of course it would depend upon the concentration of alpha-particle producing elements (particularly 238U and 232Th) in the mineral producing the halo. But I can say that in my experience, pleochroic haloes around zircon are actually quite rare - I’ve only seen a handful out of thousands of pleochroic haloes I’ve looked at. Virtually always the culprit is monazite, allanite or xenotime. These have far higher U and Th in them than zircon and can produce the required alpha-particle flux. Others are also possible, like various U and Th oxides or silicates (e.g., uraninite, thorianite, thorite/huttonite, etc.) but these minerals are much less common than the three I mentioned. One other thing - other chemical factors are important. For example, the development of haloes in biotite and cordierite is dependent upon Fe/Mg ratio - typically, the more Fe-rich they are, the more developed the halo is. I have seen U- and Th-rich monazites in very magnesian cordierite (of Paleozoic age) that have virtually no observable halo at all.
> 
> Bob T
> 
>> On Sep 9, 2016, at 9:27 AM, Mallickarjun Joshi <[log in to unmask]> wrote:
>> 
>> Dear All,
>> 
>> Please let me know any references about how much minimum time does  a pleochroic haloe requires to develop around, say zircon.  Said the other way, if such a pleochroic haloe is observed what minimum time can safely be assumed for its development  within chlorites or biotites.
>> 
>> Thanks in anticipation for any ideas.
>> 
>> Mallickarjun Joshi
>> 
> 
> Robert Tracy
> Professor of Geosciences
> Associate Department Head
> Director, Museum of Geosciences
> Virginia Tech
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> 
>