Hello All!
Yong-Fei makes some good points in his email on things to keep in mind when doing PT work on subduction zone metamorphic rocks, most of which I have seen echoed here and there in the literature on the topic. However, I am wondering about the differences when applying PT calculations to rocks in hydrothermal systems that are also related to ore deposits. I have a Master's Student who is currently working with some lovely garnet-bearing samples from such a system in northern Sweden. It is clear that not everything I learned in my own studies of regional metamorphic rocks in Tasmania will apply to his situation, and I would welcome discussion with anyone who has insights on special problems to be aware of in a hydrothermal setting, or situations where the hydrothermal setting will actually convey an advantage to persons trying to understand the conditions that applied during mineral formation. If anyone can point out any papers that relate to this topic that my student and I should be certain to read that will also be really appreciated.
Thanks,
--Riia
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Riia M. Chmielowski
Assistant Lecturer, LTU
Email: [log in to unmask]
Phone: +46 (0)920 492033
Mobile: +46 72-539 07 76
Fax: +46-920-491199
LTU Room: F833
http://ltu-se.academia.edu/RiiaChmielowski<http://ltu-se.academia.edu/ReiaChmielowski>
Postal address:
Luleå University of Technology
Department of Civil, Environmental and Natural Resources Engineering
Division of Geosciences
Riia M. Chmielowski
SE-971 87 Luleå Sweden
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From: Metamorphic Studies Group [[log in to unmask]] on behalf of Yong-Fei Zheng [[log in to unmask]]
Sent: Tuesday, April 29, 2014 4:07 AM
To: [log in to unmask]
Subject: Re: [geo-metamorphism] Thermobarometry applications
Dear metamorphic colleagues,
Various thermobarometers have been calibrated in terms of element/isotope exchange equilibria between different rock-forming and accessory minerals. While they have been widely applied to the estimate of peak metamorphic conditions, more and more problems have been encountered when applying to P-T-t studies of subduction-zone metamorphic rocks. With increasing applications of accessory mineral thermobarometers recently, it appears that reequlibration of the element exchange between rock-forming minerals and fluid/melt is evident during decompression exhumation. Therefore, a successful application of any thermobarometer to metamorphic petrogenesis must simultaneously justify of the following three conditions: (1) the element/isotope exchange between different minerals achieved thermodynamic equilibrium or reequilibrium under given metamorphic conditions; (2) the peak equilibrium was not disturbed by later geological events after formation of the metamorphic minerals; (3) the closure temperature and pressure of element diffusion in the target minerals are comparable to those at the peak metamorphic conditions of the rocks.
Good luck,
Yong-Fei
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Dr. Yong-Fei Zheng
Professor of Geochemistry
School of Earth and Space Sciences
University of Science and Tecnology of China
Hefei 230026, China
Tel & Fax: 0086 551 3603554
Email: [log in to unmask]
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From: Ganguly, Jibamitra - (ganguly)<mailto:[log in to unmask]>
Date: 2014-04-29 04:00
To: Matthew Kohn<mailto:[log in to unmask]>; [log in to unmask]<mailto:[log in to unmask]>
Subject: [SPAM] Re: [MSA-talk] Thermobarometry calculations
Dear Thermobarometry users:
Although my research has moved into different directions over the years, thermobarometry is one area where I have invested a considerable amount of time and effort. As Matt Kohn has correctly pointed out, "there's no simple answer" to the question of preferred calibrations/formulations. The user has to acquaint herself/himself at least to some extent with the different issues such as quality of the basic experimental data and thermodynamic solution models that go into expanding the scope of a thermo-/baro-meter in the compositional space, and make a reasonable judgment about the quality of thermobarometric formulations.
Some of our thoughts about judicious choice of compositions for zoned minerals like garnet, which is the single most important player in the arena of P-T-t history of rocks, and limitations of "average P-T method" of Holland and Powell have been summarized in a paper on the Himalayas in JMG (Dasgupta et al. 2004, 22, see section on GEOTHERMOBAROMETRY, P. 402-408) and by Tirone and Ganguly in Gondwana Research (2010, 18, 138-146); in the latter we have also addressed the issue of resetting of garnet composition during cooling. Dasgupta et al (2009, AJS, 309) and Sorcar et al. (2014, CMP) have also discussed the problems with the "average P-T method" on the basis of compatibility of the retrieved P-T data with the observed phase assemblages etc. in the Himalayas.
Although the field is quite old, thermobarometry is still an evolving field where people are still testing and refining the existing formulations, and also coming up with new ones. It is, therefore, very difficult to try to put together a database of the thermobarometers. Also, it may not be a good idea since it may bias the field in the direction of certain formulations that the compiler may think to be of superior quality. It is more desirable that different formulations are properly tested by the users and the results reported in the literature in an objective way.
In my website, I have compiled the thermodynamic formulations of some of the garnet based thermobarometers that were developed (from experimental studies to solution models) by me and my co-workers. These consist of Grt-Bt, GASP, Grt-Opx and Grt-Cpx thermobarometers and may be downloaded using the link:
http://www.geo.arizona.edu/~ganguly/pub/
There is a computer program for each thermo-/-barometer and one for finding the intersection of Grt-Bt/GASP that employ formulations for multicomponent solid solutions. Based on our own experience with applications to natural rocks, including Himalayas, and those of Dasgupta et al. ( 2009) and Sorcar et al. (2014), these formulations seem to work well and yield P-T conditions compatible with the phase assemblages. However, I would also like to know about failures so we can try to improve the formulations.
All the best
Jiba
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Jibamitra Ganguly
Professor, Department of Geosciences
University of Arizona, USA
http://www.geo.arizona.edu/Ganguly
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From: [log in to unmask] [[log in to unmask]] on behalf of Matthew Kohn [[log in to unmask]]
Sent: Monday, April 28, 2014 3:16 AM
To: [log in to unmask]
Subject: Re: [MSA-talk] Thermobarometry calculations
Hi,
Recall that Frank Spear has program GTB (for MAC) on his website http://ees2.geo.rpi.edu/spear/spear.html. This program compiles numerous thermometer and barometer calibrations. Also, Thermocalc has an average-PT option that some people like.
Regarding what's _preferred_ for calibrations, there's no simple answer. I have my own ...shall we say... "well-reasoned and insightful" (?) opinions about which to use, largely based on looking at the experimental data that go into the calibrations plus comparisons of natural data to phase equilibria. These calibrations routinely figure in my papers. But I know others do not agree with me and have their own preferred calibrations, in turn based on well-reasoned and insightful opinions.
[Warning: the following is a bit of a digression, but it's relevant to attempts to identify "best" calibrations]: One of the big issues in comparing calibrations in natural rocks is that different people use different criteria to identify thermobarometrically relevant compositions. Long ago many people reported "rim" compositions (meaning compositions collected as close to the physical edge of a crystal) for touching grains, reasoning that these were most likely to equilibrate. Which is probably true, but they're also the most likely to RE-equilibrate, and lose peak compositions. So you can't just go to the literature, pull out a bunch of data, and see "what works and what doesn't work" in terms of reproducing, say, the aluminosilicate phase boundaries or key reactions. You first have to explore the mechanisms of reequilibration and impact on mineral compositions in each rock, before thinking about thermobarometry.
Frank Spear talks about thermobarometric impacts of reequilibration in his book, and he references some great papers on this. You can also look at Kohn and Spear (2000; ReNTR insurance), which explores these issues in the context of metapelites and Himalayan tectonics. And if you're *really* fascinated by thermobarometry in the Himalaya (who isn't? - ha!), there's a review paper in Ann. Rev. Earth Planet. Sci. on Himalayan metamorphism that's due out in a couple weeks (I think) that provides some pointers for evaluating metamorphic/thermobarometric studies (for non-petrologists) and summarizes a huge pile of data.
Best,
Matt
Message: 4
Date: Fri, 25 Apr 2014 13:12:29 -0400
From: "Anne Argast" <[log in to unmask]<mailto:[log in to unmask]>>
To: "David Cornell" <[log in to unmask]<mailto:[log in to unmask]>>, "[log in to unmask]<mailto:[log in to unmask]>"
<[log in to unmask]<mailto:[log in to unmask]>>
Subject: Re: [MSA-talk] Monazite Xenotime thermometry spreadsheet?
Message-ID: <[log in to unmask]<mailto:[log in to unmask]>>
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More generally, is there anywhere a good compilation in a single place of the various geothermometers and barometers generally accepted and being used for different systems? If so a pointer in the right direction would be appreciated.
Anne Argast
Geosciences
Indiana Purdue University Fort Wayne
[log in to unmask]<mailto:[log in to unmask]>
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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]<mailto:[log in to unmask]>
http://earth.boisestate.edu/mattkohn/
phone: (208)-426-2757 fax: (208)-426-4061
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