Dear all,

I have observed the same type of oriented inclusions in garnet from high-grade metapelites collected in the Rhodope mountains in N‘ Greece, and they turned out in almost any case to be rutile (microprobe data). As mentioned by others in this discussion already, they seem to be rather typical for high-grade rocks of the entire pressure range and their often regular and three-dimensional distribution throughout the garnet hosts supports an exsolution origin rather than overgrowth of a pre-existing sagenitic biotite/chlorite precursor where the orientation of rutile needles is planar.

Hence my preferred interpretation as “COPs” (crystallographically oriented precipitates – term coined by Stephen Haggerty). Aside from their aesthetic value these rutiles got me interested because their exsolution (or rather precipitate) origin can by no means be explained as an isochemical process within garnet. Based on this and on other precipitates in high-grade clinopyroxenes and titanites I coined the term “open system precipitation” for this type of behavior where precipitation of a daughter mineral in a host is possible only if assisted by diffusional exchange of ions with the rock matrix – the slowest diffusion component(s) staying behind in the form of precipitates.

These observations and considerations on clinopyroxenes are already published (Proyer et al. 2009, JMG 27, 639-654), those on garnet are just being written up as we have done extensive EBSD and TEM work on the subject meanwhile – (following and adding to the pioneering paper by Hwang et al. (2007) presented to this forum by Tzen-Fu Yui).

The actual reaction mechanisms and the aspect of diffusion, as mentioned by July, are highly interesting and quite tricky to figure out.

At any rate, this is a fascinating subject, and I would be glad about any report from your side with specifics about observed rutile COPs in garnet and type of host rock and/or estimate of peak P-T conditions. As most of these observations may be unpublished, this could be a way to get an overview in what P-T range such Ti-rich garnet precursors are actually formed. If you like, I could pack these observations into a Table in the forthcoming paper and so make them available for everybody.

Best regards,

Alexander Proyer,

Department of Earth Sciences

University of Graz, Austria

PS: A remark on “testing”: Raman is valid, extinction angle is not because many of these rutile needles have oblique extinction, as observed already many years ago, e.g. by Bill Griffin (1971). AP

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I don't think the simple presence of rutile exsolution in garnet is
specifically indicative of a particular P-T path.
Rutile exsolution is widely reported in garnet from eclogite facies,
granulite facies, UHT, and I think also occasionally in amphibolite-facies
occurrences.

That said, if anyone knows anything about the diffusion rate controls on
Ti transport leading to rutile needle separation out of garnet, I would love
to know.  I have seen some spectacular oscillatory zoning that shows up in
thin section as differing concentrations of exsolved rutile needles in a
garnet from a very rapidly-exhumed eclogite.  The same garnet contains
healed fractures in which the rutile needles are gone, and the new clean
garnet contains occasional larger rutile.

Julie Vry


On 9/12/11 7:56 AM, "Shauket Baltybaev" <[log in to unmask]> wrote:

  

Hassina and others, thanks a lot!
Yes, it's most probably rutile, next week I'll check its composition.
Let me concretize the question - I've never seen such rhomboidal location.
Is it decompressional structure, lamella or what?

Shauket


    

Hi Shauket, I could not open the pictures properly, but could that not be
Rutile?
      

regards
      

hassina