All,
Bob's point about the multi-phase process is correct, but only if
the Plag is nearly pure anorthite. However, I think that such a
heterogeneous reaction is poorly described as exsolution. We have
seen that rutile needles in mantle pyrope sometimes have REE-bearing
loveringite-crichtonite intergrown with rutile (Wang et al., 1999,
CMP). That is hard to accept as an internal reaction.
cheers,
eric
On Sep 26, 2008, at 12:38 PM, Robert Tracy wrote:
> In this light, if one has access to a microprobe with elemental
> mapping capabilities, a Ti map of a polished surface at appropriate
> scale would produce an image that could be used with image-
> processing software to get a very accurate estimate of areal
> proportion of rutile needles (assuming random and not preferred
> orientation of such needles). If I were doing this, I'd probably
> map several areas to test for consistency. But as Eric suggests,
> high-resolution BSE micrographs could also be used with image-
> processing to do the same thing, although the contrast between
> rutile and garnet would be more clearcut in an elemental map.
>
> With regard to Eric's excellent point about "exsolving" an RO2 phase
> from an R2O3 one, there may be cases where "paired" exsolution may
> occur. We worked some years ago on Opx megacrysts from the
> Adirondacks which had apparent exsolution lamellae (they were
> definitely oriented lamellar intergrowths) of olivine, calcic
> plagioclase and ilmenite. The ilmenite was not a problem from the
> point of view Eric noted (R2O3 in R2O3), but olivine and plag were.
> However, an equi-molar mixture of anorthite and forsterite (modeling
> plag and olivine as pure end-members) would produce a hypothetical
> model composition of (CaAlMg2)(AlSi3)O12 which could be viewed as
> having a pyroxene-like stoichiometry. Not a real happy-looking
> stoichiometry, but if these megacrysts represented very rapid high-T
> crystallization (a Ron Emslie-like mechanism), unusual molecular
> constituents might have been included in initial magmatic
> crystallization.
>
> Bob T.
>
> Eric Essene wrote:
>>
>> Alexander,
>> The term exsolution should not apply unless the structures of
>> the host and newly formed phases are similar structures, e.g.,
>> feldspars, pyroxenes, amphiboles, carbonates. Some people use the
>> term exsolution rather carelessly however. The term precipitate
>> comes from metallurgy.
>> It is rather difficult to get rutile alone to form from garnet
>> by a closed system process because rutile is an RO2 phase and
>> garnet is R2O3. A small amount of reacted TiO2 could be
>> accommodated by simultaneous oxidation of Fe in the garnet. Pate
>> precipitation should also involve a halo around the needle, as has
>> been shown by Dave Wark for rutile needles in some quartz. The
>> amount one sees of needle rutile in transmitted light gives a
>> mistakenly large overestimate of how much is really there because
>> it includes needles from depth inside the host into what seems to
>> be just two dimensions. This mistake has been made repeatedly by
>> authors describing quartz in eclogitic Cpx, ilmenite in olivine,
>> etc. The areal percent in reflected light or BSE images is a much
>> better estimate. Spotting the needles in reflected light is OK
>> once you learn how to identify the often 1-5 micron wide cross-
>> sections of needles. They can be oriented on tetrahedral edges
>> inside the garnet. So could they be epitaxial or topotaxial in
>> origin, or do they represent a reaction upon cooling? Both
>> mechanisms involve preferred orientation of the precipitate on the
>> host.
>> cheers,
>> eric
>>
>>
>>
>> On Sep 26, 2008, at 4:45 AM, proyer wrote
>>
>>
>>
>> :
>>
>>> Hello everybody,
>>>
>>> hope some of you can help me out on two questions related to each
>>> other:
>>> 1) Is there a source which clearly defines when to call a mineral
>>> phase unmixing from a solid solution "exsolution" or "precipitate"?
>>> 2) Do you know of any article expaining explicitly (reaction
>>> equations etc.) how rutile unmixes from high-tempertaure garnet?
>>> I know of three such papers for ultrahigh-pressure garnets (Van
>>> Roermund, 2000; Zhang et all. 2003 and Yang & Liu, 2004),
>>> but none for granulite facies rocks etc.
>>> Please let me know if you have come across or even authored any
>>> such references.
>>>
>>> Sincerely,
>>> Alexander Proyer
>>> University of Graz, Austria
>>>
>>>
>>
>
> --
> Dr. Robert J. Tracy
> Professor of Geosciences
> Department of Geosciences
> 5064 Derring Hall, Virginia Tech
> Blacksburg VA 24061-0420
>
> Office Phone: 540-231-5980
> Fax: 540-231-3386
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