The following is additional discussion of apparent low temerature observations that was held in February, 1998.

See also the 1st discussion.

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From: <kwitko@cvrd.com.br>
To: Multiple recipients of list <fluid-inclusions@relay.doit.wisc.edu>
Subject: MgCl-H2O system


Dear fellows,
I'm working with vein dol-qz FI, mainly hosted by Fe-dolomites. Despite their low average salinity (near zero), in some of them I found eutectic temperatures as low as -52 C, compatible with the system CaCl2-H2O.

However, I have a negligible contribution of Ca for the hydrothermal chemistry. I want to test thus a MgCl-H2O thermodynamic model to fit the data, and didn't found a reference about it. Do you have any suggestion?

Thank you, and have a hopeful year.
Rogerio Kwitko

Cia Vale do Rio Doce - Technology
BR 262, km 296 PO box 09
Santa Luzia/MG Brazil

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MgCl2-H2O only goes to eutectic at -35 degrees -- see Chapt. 8 in my 1984 book Fluid Inclusions.

Edwin Roedder

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Fellow inclusionists:

Over the last several years we have been doing much work on various water-salt systems using synthetic fluid inclusions. We have looked at many systems including H2O-CaCl2-NaCl, H2O-NaCl-MgCl2, H2O-FeCl2, H2O-FeCl2-NaCl, etc. In all of these systems and others, we see "something happen" at temperatures well below the expected eutectic temperatures. These changes in appearance of the inclusion are reproducible, and occur in the same temperature range for each composition. Yet, I am fairly certain that we are NOT seeing eutectic melting. Rather, I believe that it is either a metastable (but reproducible) eutectic or, more likely, some type of solid state transition or glass transition. Any of you who have worked with synthetic H2O-CO2 fluid inclusions have probably seen the very obvious and reproducible change in appearance of the inclusions that occurs at about -90 to -80 during heating from very low temperatures. This change in appearance is apparently a solid state transformation and is not a melting event.

All of these observations that we have made using synthetic fluid inclusions make me very skeptical of low eutectic temperatures that are being reported more and more commonly in the fluid inclusion literature. In the past, with poorer quality optics and cooling stages that either could not be cooled to very low temperatures or in which the temperature could not be controlled very easily, these "phase changes" would have been easily overlooked. With today's better equipment, we are seeing more subtle changes in the fluid inclusions, but I am not completely convinced that most of these changes are in fact eutectic melting events.

I would be intersted to hear what other people think about this, especially those of you who have extensive experience using synthetic fluid inclusions in which the real eutectic is known.

Bob Bodnar

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Fellow inclusionists:

I completely agree with Bob Bodnar's comments about observations of low temperature phase behavior in aqueous inclusions. In a previous discussion on this list I said that I'm convinced that many "eutectic" melting observations are really devitrification of saline, aqueous glass. The glass transition temperature in saline aqueous systems is well documented in the chemistry literature. Austin Angell has published a number of papers on this subject (there is a citation for Angell and Kanno, in the Goldstein and Reynolds book. I don't remember it off hand.)

If anyone wants to understand the low temperature phase transitions that can be observed in aqueous inclusions, and "practice" making the observations, I recommend getting some halite, finding some large inclusions, and carefully observing the transitions. After you've seen the stuff in the big inclusions, then try making the observations on the small inclusions in the same sample. This can be a humbling experience. The interpretation is not easy. Keep in mind that there is lots of metastability as Ed Roedder pointed out many years ago.
In any case, have fun with microthermometry.

Bob Burruss

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A possibly informative alternative to Bob Burruss's suggestion (though not as simple in sample prep.). aqueous solutions of Ca(NO3)2 are good 'glass' formers (and thus some of the Angell work Bob cited deals with Ca(NO3)2). It has a much higher eutectic than CaCl2 so IF it is possible to realize any of its metastable eutectics vs glass transitions/devitrification, the eutectics should be much more easily observable/attainable. Its solubility also increases very rapidly with temperature (increasing from about 8 molal at 25 C to >24 molal at 100 C). There are several hydrate phases which melt congruently (the tetrahydrate at about 55 C) and incongruently and they have the nasty tendency to metastably persist. So while the system isn't particularly relevant geologically, it could be a decent teaching tool in that it's phase behavior is messy enough to make things interesting (or frustrating) while retaining chemical simplicity.

For you folks that do Raman and IR work there has been a fair amount of work on Ca(NO3)2(aq) though I can't think of any at <0 C. Seems to me that while fine grained crystalline aggregates and glasses in flincs can be visually indistinguishable they would have vastly different Raman characteristics (at least for this case). One of you folks can tell me whether a glass and a melt (e.g. the tetrahydrate) would be distinguishable or for that matter are physically distinct.

Charlie Oakes

P.S. I believe that I sent an earlier reply, inadvertantly, ONLY to Rogerio (I know some of you hate it when I do that). Anyway, I gave him some references to thermo models for MgCl2 and a recent paper by Dubois and Marignac which deals with MgCl2 in flincs. If anyone wants the refs - ask. Or if he wants to forward the list back to the list 'tis fine with me.

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Dear all,

I just want to add some comments to the discussion about the meaning of eutectic temperature. I completely agree with Bob Burruss concerning the behaviour of water-salt(s) systems, when Bob is writing that many (supposed) eutectic behaviour are due to devitrification. I have to mention the work of Vuillard and Kessis (1960) about the (de-)vitrification in the H2O-LiCl system :

"Vitrification is observed in binary mixtures in the eutectic composition range in systems where the eutectic temperature is low. The solid phase is supposed to appear in a very viscous medium [...]. The vitrification temperature is more or less constant for a given system and, when always operating in the same way, it is possible to obtain for this temperature a reproducibilitty comparable to the melting temperature."

During heating, vitrification is followed by a devitification ("recuit") which consists of the crystallization of the expected stable phases. However the authors are talking about one to two days to reach the complete crystallization!

I think the best way to measure correct (or not too false) eutectic temperature is to use the sequential freezing (Haynes, 1985), i.e. when one big ice crystal is present. It is surprising to note that the (supposed) eutectic behaviour occurs differently (i.e. at different temperatures) when 'eutectic temperature' is measured during the initial freezing/heating run or during the sequential freezing procedure, and sometimes during successive sequential freezing runs ! This supposes that in the case of systems with succesive hydrates, metastable eutectics are possible. This fact was never observed in simple binary systems such as H2O-KCl, H2O-CsCl or H2O-RbCl (Dubois et al., 1993, 1997). An application of Haynes' technique to the determination of precise eutectic temperatures is given in the former citation.

Concerning the reference given by Charlie Oakes about the H2O-NaCl-MgCl2 system (which was the initial question !), the complete reference of my communication is:

Dubois, M. and Marignac, C. (1997): The H2O-NaCl-MgCl2 ternary phase diagram with special application to fluid inclusion studies. Econ. Geol., 92, 114-119.

I will be happy to send a reprint to any interested person.

Have fun with FI's !! :-)

Michel Dubois

Ref listed :

Dubois, M., Royer, J. J., Weisbrod, A. and Shtuka, A. (1993): Reconstruction of low-temperature phase diagrams using a constrained least squares method: Application to the H2O-CsCl system. European. J. Mineral., 5, 1145-1152.

Dubois, M., Weisbrod, A., Shtuka, A. and Martinez-Serrano, R. (1997): The low temperature (T<1200C) H2O-RbCl phase diagram. Comparison with other water-alkali chloride systems. European. J. Mineral., 9, 987-992.

Haynes, F. M. (1985): Determination of fluid inclusion composition by sequential freezing. Econ. Geol., 80, 181-186.

Vuillard, G. and Kessis, J. J. (1960): Equilibres solide-liquide et transformation vitreuse dans le systhme eau-chlorure de lithium. Bull. Soc. Chimique de France, , 2063-2067.

Michel DUBOIS
Universite des Sciences et Technologies de Lille
U.F.R. des Sciences de la Terre - SN5
URA 719, Sedimentologie et Geodynamique
59655 VILLENEUVE D'ASCQ CEDEX
FRANCE

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Dear all,

in reponse to the two Bob's, I think a practical solution to the problem of apparent low eutectics, and an alternative to the sequential freezing of Haynes mentioned by Michel Dubois, is to attempt to identify the burst of rapid melting which commonly occurs at the stable eutectic (e.g. see description by Davis et al., Geochimica, 54 p. 600.). In many natural inclusions where good bulk analyses are available, a marked increase in melting behaviour (crystal coarsening, changes in appearance of vapour bubble, appearance of patches of vapour etc.) is observed at the stable eutectic expected for the dominant salts present in solution.

With regard to Bob Bodnar's enquiry about synthetic inclusions we have observed clear melting below the stable eutectic in synthetic inclusions in the KCl-H2O system (-10.7 C reported by Hall et al., 1988, Econ. Geol., 83, 197-202). In fact, euhedral solid (presumed to be sylvite) was observed to coexist with ice and liquid for several hours without noticeable change at temperatures between -14.3 and -11.3 C (I have some good photographs of this). It is possible this indicates the presence of a KCl hydrate, only reported in the Russian literature.

Dr Jamie Wilkinson

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Fellow inclusionists:

I enjoyed recent discussions on the question raised by Rogerio Kiwitko. I would like to point out that the best reference (and discussion) for the MgCl2-H2O system is the abstract by Ron Spencer and Tim Lowenstein on "Phase Equlibria in the System MgCl2-H2O" published in PACROFI IV Program and Abstracts (Lake Arrowhead, California, May 21-25, 1992) p. 138-141. In that abstract, many eutectic (stable and metastable) temperatures were listed, including ice-MG12 (-33.6 C), MG12-MG8 (-16.8C), MG12-BISCH (-19.4C), ice-BISCH (near -80 C), ice-MG8 (near -50C).

Reaction between bischofite and ice beginning at metastable eutectic (no liquid was observed) was also suggested as possible explanations for the recrystalization of solids within frozen fluid inclusions on warming between -80 and -70 C. For the ternary system NaCl-MgCl2-H2O, see Davis et al. (1990, GCA 54, 591-601).

I-Ming Chou

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Jamie and others:

You are right about the burst of melting if the inclusion has a fairly high salinity. But, remember that the amount of liquid generated at the eutectic is directly related to the salinity (i.e., the lever rule). If you have a composition close to the eutectic salinity you get an obvious burst of melting, but if the salinity is very low the amount of melting at the eutectic may be so small that a melting event may not be obvious, especially in small inclusions.

Bob Bodnar

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Bob,

your comment is appreciated, nonetheless the same also applies to metastable hydrate melting phenomena - I think the relative increase in melting rate which occurs at the stable eutectic can be observed even if metastable melting or glass transition effects have taken or are taking place (at least for inclusions of reasonable size and/or salinity, typically in excess of say 10wt%). In small and/or very dilute inclusions I don't think that it is possible to confidently observe either stable or metastable eutectic melting for the reason you give. There's no solution for these inclusions...

Jamie Wilkinson

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I agree completely.

Bob Bodnar

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Dear fellows,

Thank you for the precious suggestions about the eutectic misleads

I have faced another doubt in my studies, regarding ThCO2 measuring. In continuous freezing/heating, some inclusions showed CO2 homogenization in the presence of clathrate. Stepped cooling/heating measures of the same inclusions provide ThCO2 without clathrate 9 C lower, as an average.

What is the meaning of a ThCO2 measured with clathrate, and its consistently higher T values?

Thanks,

Rogerio Kwitko

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