All,
John Valley noted a systematic analytical artifact for UHT rocks
based on Sato & Santosh's (2007) study of Ru-Qz-Zc in granulites from
South India. Sato & Santosh (2007) did not collect their samples but
purchased them in a local market (Fig. 2). This is a dangerous way to
do thermobarometry. The samples were not even plotted on a map. With
large sprays of rutile or hematite, the samples do not look promising
to this writer for metamorphic thermobarometry. One wants samples
with fine intergrowths of the "exsolved" inclusions to be sure that
reintegrations are meaningful in terms of recovering evidence of
former high T. It is very difficult to explain T values in excess of
say 1100-1200°C, and yet the authors ignored much high values. The
research should be undertaken on in situ samples, not those that
conceivably could have derived from other locales.
Are the samples of Sato and Santosh (2007) were derived from the
belt where several diagnostic UHT assemblages have been reported
(Clark et al., 2009, Precambrian Research 174, 129-144; Nishimiya et
al., 2010, Lithos 114, 490-502)? What diagnostic assemblages, how
much resetting, why were the hematite quartzites ignored, what are the
phases with the different textures? Why are the Sato and Santosh
(2007) quartzites so heterogeneously distributed in terms of rutile
(Fig. 3a) and a quartz matrix? What are the black minerals in Figs.
2f, 2g, 2h? In Fig. 3, what is the cause of the patchy texture with a
quartz matrix and rutile (+other minerals) included? Sato and Santosh
(2007) applied two Ti thermometers giving results from 1000-1600°C
and 800-1500°C, ranges of 600-700°C, even though they emphasized the
preciseness in these systems. There is also a suggestion of a
checkerboard pattern in Fig. 4 of Sato and Santosh (2007). The
arguments about the subsequent stability of fluorine-rich paragenesis
seem contrived to this reader.
Clark et al. (2009) noted sapphirine, some retrogressive
symplectites, and a few garnet profiles, and they found interesting
spikes in a few HREE minerals in garnet. However, their Zc-Ru-Qz T are
only 660-740°C. Are these UHT conditions? That is not evident to
this reader, who would have liked to have seen a much more careful job
done on many more in situ samples.
Nishimiya et al. (2010) noted complex textures in aluminous rocks
and inferred the reaction staurolite + garnet → sapphirine + quartz +
H2O, a corundum-spinel assemblage, spinel-quartz, intergrowths of
gedrite-spinel and gedrite-sapphirine-quartz, hogbomite, and
complicated P-T nets involving other minerals not shown in the study
(osumilite, cordierite, yoderite). Unreversed syntheses of a rutile-
garnet thermometer by Kawasaki and Motoyoshi (2007) were employed and
gave 1050-1070°C (Sato and Santosh, 2007), but it is not clear how
compositional variations were accommodated in that system.
Available phase equilibrium data for other bulk compositions are
better established in S. India (Hensen and Green, 1973; Bertrand et
al., 1991; Kelsey et al. 2004; Kelsey, 2008; Koshimoto et al., 2004;
Shimpo et al., 2006), Tsunogae et al., 2008; Tsunogae and Santosh,
2003; Santosh and Sajeev, 2006; Collins et al., 2007b). They are
consistent with HT to UHT in this zone with metamorphic T (900-1000°C)
using most of the available geothermometers. The effect of variable
Fe3+/Fe2+ on reactions with garnet, gedrite, cordierite, spinel,
osumilite is unaddressed however. This reader has no cause to
question most of the UHT inferences, but he repeats that using samples
sampled by others is no way to initiate another study in the area.
cheers,
eric
Professor of Geology
On Jan 27, 2010, at 11:57 PM, M. Santosh wrote:
>
> Prof. John Valley
>
>
>
> Dear John,
>
> Thank you for your knowledgeable comments and caution on the problem
> of Ti secondary fluorescence.
>
> Sato and Santosh (2007, Mineral. Mag.) also observed that some of
> the temperature estimates derived from Ti concentrations in their
> study are exceedingly high and beyond the realm of UHT metamorphism
> (pages 150-151). These values therefore need to be viewed with
> caution, and the problem of Ti secondary fluorescence which you and
> some of the other list members pointed out is important in this
> context. However, the samples analysed by Sato and Santosh (2007)
> come from a well known UHT belt from where several "diagnostic" UHT
> assemblages have been reported (e.g. Clark et al., 2009,
> Precambrian Research 174, 129-144; Nishimiya et al., 2010, Lithos
> 114, 490-502, among others).
>
> Nevertheless, your comments and the subsequent discussion by the
> list members provide valuable insight and future guidelines on this
> topic.
>
> Thanks and best regards,
>
> Santosh
>
>
>
> ________________________________
>
> Prof. Dr. M. Santosh
>
> Faculty of Science
> Kochi University
> Kochi 780-8520
> JAPAN
>
> Editor-in-Chief
> Gondwana Research
> (www.elsevier.com/locate/gr)
>
>
>
>
> On Fri, Jan 22, 2010 at 11:24 PM, John W. Valley <[log in to unmask]
> > wrote:
>
> Dear Mike
>
> I was very interested by the paper with Sato that you sent to the
> list on Dec. 31. Metamorphic temperatures of >1190 C are very
> surprising in such rocks and if correct, would be quite important.
> However, I think that the basis of this estimate is an analytical
> error. The Ti compositions in quartz adjacent to rutile needles
> shown in your Fig. 5 exactly match what John Fournelle has
> calculated for the effect of secondary fluorescence of Ti X-rays by
> bremsstrahlung, see below. It appears that the Ti content of this
> quartz is actually very low in contrast to the UHT minerals of the
> host rock; perhaps it was reset by unmixing of rutile needles.
> John's calculations are described at:
> http://www.geology.wisc.edu/~johnf/Ti-quartz.html
>
> We would be interested in your thoughts on this.
>
> I am copying this message to the list because many people are
> interested and have already received the pdf of Sato and Santosh
> (2007).
>
> Secondary fluorescence effects can extend further from a mineral
> grain boundary than many people realize. This is especially
> important to evaluate for trace element analysis.
>
> with best wishes, John
>
> <P35322276.png>
>
>
>> Dear All,
>>
>> We had identified a similar occurrence as hematite from laser Raman
>> study. Please see Fig. 2 (f,g) in the attached pdf file.
>>
>> Best wishes,
>>
>> Santosh
>>
>>
>> ________________________________
>>
>> Prof. Dr. M. Santosh
>>
>> On sabbatical from:
>> Faculty of Science
>> Kochi University
>> Kochi 780-8520
>> JAPAN
>>
>> Editor-in-Chief
>> Gondwana Research
>> (www.elsevier.com/locate/gr)
>>
>> Content-Type: application/pdf; name="Sato and Santosh 2007 Min
>> Mag.pdf"
>> Content-Disposition: attachment;
>> filename="Sato and Santosh 2007 Min Mag.pdf"
>> X-Attachment-Id: f_g3v3wszk0
>>
>> Attachment converted: Macintosh HD:Sato and Santosh 200#4583D1.pdf
>> (PDF /CARO) (004583D1)
>
>
> --
>> John W. Valley, phone: 608-263-5659
>> Charles R. Van Hise Professor fax: 608-262-0693
>> Dept. of Geoscience email: [log in to unmask]
>> University of Wisconsin
>> 1215 W. Dayton St.
>> Madison, WI 53706, USA http://www.geology.wisc.edu/people/valley.html
>>
>> Zircons Are Forever- http://www.geology.wisc.edu/zircon/zircon_home.html
>> Wisc-SIMS http://www.geology.wisc.edu/facilities/wiscsims/wisc_sims.html
>
>
>
|
