Hi Javier,
Yes, the closure temperature for Zr diffusion in rutile appears to be significantly hgiher than the closure temperature for Pb diffusion. It’s maybe not surprising that they would be different because Zr and Pb have completely different charges and ionic radii.
That’s what was always so strange about the old titanite diffusion data. Sr diffusion looked very slow and Pb looked fast. But Pb and Sr have the same charge and ionic radius - why would they be so different? Daniele Cherniak, who ran the original experiments, was equally perplexed. However, the empirical estimates of Pb diffusion in titanite are more or less in line with the experimental data for Sr diffusion. This is shown in Kohn (2017; RiMG) and Kohn & Penniston-Dorland (2017; RiMG).
For titanite, it’s possible Zr and Pb also have very differenti diffusivities. But they appear to be so slow, we don’t have much leverage to distinguish them. What we really have are upper limits.
Best,
Matt
> On Jun 2, 2022, at 9:45 AM, Javier Rodriguez <[log in to unmask]> wrote:
>
> Dear Matt,
>
> I haven't read in detail Inal's information but let me make some thoughts on your comments... if this titanite grew over rutile at 700 ºC and if the nominal "closure temperature" of rutile is 700 ºC, then both minerals should have closed their lattice "doors" at the same time. Shouldn't they?
> Or am I missing something about rutile? Has it different closure temperatures for Zr diffusion and Pb diffusion? I must admit that I lost interest in this topic long time ago.
>
>
> -----Mensaje original-----
> De: Matt Kohn <[log in to unmask]>
> Enviado el: jueves, 2 de junio de 2022 17:25
> Para: Metamorphic Studies Group <[log in to unmask]>; Javier Rodriguez <[log in to unmask]>
> Asunto: Re: [geo-metamorphism] Metamorphic areas with very slow cooling
>
> Well, as Jesse mentioned earlier, the closure temperature of titanite is something like 800 °C (or maybe higher; reviewed in Kohn, 2017, RiMG, although maybe there are better constraints now?). So, if titanite grew at 700 °C (the Zr-in-titanite temperature), it simply records the time the rock was at 700 °C. The nominal closure temperature for Pb diffusion in rutile is much lower. So, if rutile can lose Pb (a matrix transport question), it should have a younger age than titanite.
>
> Of course, rutile grains don’t have to behave as open systems. For example, some rutile grains record Zr-in-rutile temperatures ≥900 °C (e.g., Jiao et al., 2011, CMP; Kooijman et al., 2012, JMG; Ewing et al., 2013, CMP, etc.), well above rutile's nominal closure temperature of ~700 °C. And, near-rim zoning in rutile can show clear evidence for matrix transport limitations (Kohn et al., 2016, CMP). So, not all rutile crystals behave as open systems.
>
> And, as so many have said, T-t histories can be complicated and still yield apparently simple age distributions.
>
> But why make a complicated interpretation unless there’s a reason?
>
> Best,
>
> Matt
>
>
>> On Jun 2, 2022, at 2:17 AM, Javier Rodriguez <[log in to unmask]> wrote:
>>
>> Dear Inal,
>>
>> Taking in consideration all the information you provided, you should suspect that some of your age numbers are not geologically meaningful. You've observed that rutile is partially resorbed by titanite, however your rutile age number is younger than that obtained for titanite.
>> Although it is very tempting to construct complex and innovative geological models with analytical data, first you should evaluate if the data are accurate (not only precise). In other words, numbers should match logical observations. If they do not, the simplest reasoning and probably the right one is that your numbers are wrong or have a different interpretation.
>>
>> Best wishes,
>>
>>
>> Javier Rodríguez, PhD.
>> Servicio de Geocronología y Geoquímica Isotópica SGIker – Facultad de
>> Ciencia y Tecnología Universidad del País Vasco (UPV/EHU) Barrio
>> Sarriena s/n
>> E48940 Leioa
>> Spain
>>
>> [log in to unmask]
>> http://www.ehu.eus/en/web/sgiker/geokronologia-eta-geokimika-isotopiko
>> a
>> http://www.linkedin.com/in/javi-rodriguez-032a5360
>>
>>
>>
>>
>>
>> -----Mensaje original-----
>> De: Metamorphic Studies Group <[log in to unmask]> En
>> nombre de Inal Demirkaya Enviado el: miércoles, 1 de junio de 2022
>> 22:21
>> Para: [log in to unmask]
>> Asunto: Re: Metamorphic areas with very slow cooling
>>
>> Dear Soucy, Sumit, Jesse, Yong-Fei, Daniela, Matt, Juergen and Denis,
>>
>> Thank you for your very detailed and helpful comments.
>>
>> Actually, this metamorphic area is located close to the Neo-Tethyan suture in northwest Turkey. Therefore it was surprising to determine that the exhumation of the metamorphic rocks close to the suture took so long (from Middle Permian (ca. 255 Ma) to Late Jurassic (ca. 155 Ma)).
>>
>> The chronometers used are U-Pb zircon (255-260 Ma), U-Pb titanite (ca. 230 Ma), Ar-Ar hornblende (ca. 205 Ma), U-Pb rutile (ca. 180 Ma), and Rb-Sr biotite (ca. 155 Ma). The metamorphic domain consists mainly of amphibolite (hornblende, plagioclase, epidote, titanite, rutile, and biotite). The rocks are well-recrystallized and are devoid of any obvious reaction texture. The only reaction texture is the resorption of rutile by titanite. Therefore we are not able to deduce a P-T-t path.
>>
>> Metamorphic temperatures are ca. 700 °C as calculated by Zr in rutile and Zr in titanite thermometers. We were in indecision whether U-Pb zircon ages represent the peak metamorphism or protolith ages. Th/U ratios are significantly higher than 0.1 (mostly in the range of 0.15-0.50). As the amphibolites display locally stromatic migmatitic structures in the field and this leucocratic (trondhjemitic) domains in composition, produced probably by water present melting) yielded identical U-Pb zircon ages we tend to interpret U-Pb zircon ages as the age of the peak metamorphism. Even if I regard the zircon ages as the protolith age, it is still a very long process ca. 230-155 Ma (75 Ma).
>>
>> Once again thanks a lot for your comments. They helped me a lot.
>>
>> İnal
>>
>>
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