Hi Ralf,
Yes, I agree substantially - there’s evidence for diffusional reequilibration of REE at T ~ 700-750 °C extending hundreds of microns into garnet interiors (e.g., Kohn, 2009; CMP). Such profiles are how Carlson (2012) derived his estimates of REE diffusion rates.
So, depending on time-scales and grain sizes, I wouldn’t automatically rule out diffusional homogenization of Penglei’s garnets.
Still, Nature has a way of producing unexpected phenomena, and I seem to recall fish-net garnets from low- to mid-grade garnet schists and amphibolites from New England and the Himalaya that I’ve worked on that have almost no major element zoning across grains (although they also have slight oscillations on scales of microns. I’m traveling, so I can’t look this up at the moment). So, I think it’s worth considering what other mechanisms might create homogeneity (which was Penglei’s original question), even if, ultimately, it’s decided they’re unlikely.
Warm regards,
Matt
> On Jun 25, 2022, at 11:49 AM, Ralf Milke <[log in to unmask]> wrote:
>
> Hi Penglei, Matt, and others,
>
> we had a parallel discussion these days about microprobe reference
> standard materials. Everyone experienced in garnet analysis knows about
> the zoning in natural garnets in terms of main elements. Personally, I
> have synthesized more that one million garnet crystals (Ca-garnet), and
> although the starting mixture was nearly pure, the garnets were zoned and
> anisotropic. So I ask myself, why should REE have flat concentration
> profiles in garnet. If not due to later homgenization by temperature?
>
> Ralf
>
>
>> Hi Penglei,
>>
>> It kind of depends on how “flat” they really are, but Skora et al. (2006;
>> CMP) comment on the potential for matrix-limited diffusion of trace
>> elements to cause deviations from Rayleigh-like distillation behavior.
>> Basically, the growing garnet produces a HREE-depleted halo around itself.
>> For the right combination of garnet growth rate and REE diffusion rate
>> through the matrix, it’s theoretically possible to create a flat REE
>> profile. However, it seems unlikely that a profile would ever be truly
>> homogeneous, because garnet growth rates are rarely simple. That’s why
>> “flatness” matters.
>>
>> If this is the explanation, I wouldn’t expect flat profiles for all
>> elements, because they have different inter-granular diffusivities. For
>> example, maybe Mn looks more Rayleigh-like because its intergranular
>> diffusion rate is much faster than REE.
>>
>> Of course, a steady influx of fluid with constant REE concentration might
>> create a flat profile.
>>
>> And, perhaps a neighboring rock with high REE concentration could serve as
>> a homogeneous source to the rock you’re studying.
>>
>> Best,
>>
>> Matt
>>
>> Note: While I may send email outside of regular business hours, I do not
>> expect the same from others.
>> ******************************************************************
>> Dr. Matthew J. Kohn, University Distinguished Professor
>> Department of Geosciences, Boise State University
>> 1910 University Dr.; MS1535
>> Boise, ID 83725-1535
>> [log in to unmask]
>> https://www.boisestate.edu/earth/people/matthew-j-kohn/
>> phone: (208)-426-2757 fax: (208)-426-4061
>> Pronouns: He/him/his
>> ******************************************************************
>>
>>> On Jun 25, 2022, at 6:52 AM, liupengleiincug
>>> <[log in to unmask]> wrote:
>>>
>>> Dear all,
>>> I'm working on some medium-T (peak T = 700 oC) UHP eclogites and found
>>> that garnet in these rocks commonly displays a flat HREE zoning pattern.
>>> I personally ruled out that diffusion homogenized the HREE composition.
>>> Are there any other better ways to interpret this feature? Thanks in
>>> advance.
>>> Cheers
>>> Penglei
>>>
>>>
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