The following discussion was initiated by Nicole Guilhaumou in April 1996. I have taken the liberty to slightly edit some of the posts. Click of the one of the following names to go directly to that persons remarks.



Date: Wed, 10 Apr 1996
Subject: preservation of FI in quartz

From Nicole Guilhaumou

A question about reequilibration in quartz: Does anybody have example of low temperature formed quartz where monophase fluid inclusions have not been reequibrated by burial at temperatures up to 150 C? Does anybody think that in some cases of sedimentary basins, observation of two cogenetic minerals one with Th up to 150 C and one with monophase FI may be interpreted in this way ?

Nicole

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Date: Wed, 10 Apr 1996

Dear Nicole: The answer on your question depends on which "cogenetic minerals" you are talking about. Note, however, that an explanation for two- and one-phase inclusions in cogenetic quartz grains might be simple failure to nucleate a vapor bubble during cooling. In our experiments with synthetic water inclusions, we often observe metastability (failure to nucleate a vapor bubble at room temperature) for inclusions that lie along isochores corresponding to homogenization temperatures as high as 130-150 C.

Maxim

Maxim Vityk
Department of Geological Sciences
Virginia Tech
Blacksburg, VA
24060 USA
maximv@vt.edu
tel.: (540) 231-6521

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Date: Wed, 10 Apr 1996

Nicole (and others):

There are dozens/hundreds(?) of papers in which the authors express the belief that their fluid inclusions have not been re-equilibrated (where trapping or post-trapping temperatures are thought not to have exceeded 150 C). Whether or not these interpretations are accurate is a different matter.

If your sample was at the surface, it has probably been exposed to temperatures below 0 C. If the inclusions were indeed trapped below 150 C and contain aqueous liquid, then it is possible that the trapped liquid did not separate into a liquid and vapor or if a vapor phase was evolved its volume fraction would be small. If the two cases described in the previous sentences are true, then the host mineral can be fractured when ice forms and its volume exceeds that of the inclusion (i.e. Ice isn't very compressible). Voila, what you end up with is an inclusion with a higher volume/lower bulk density and consequently may now develop a vapor phase or larger volume fraction of vapor. Whether or not a particular inclusion will or will not fracture depends on host mineralogy, inclusion geometry, and some other variables.

Charlie Oakes (CSOakes@LBL.gov)

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Date: Thu, 11 Apr 1996

Nicole,

Another possibility for explaining your observations is the mechanism by which quartz overgrowths form and trap fluid inclusions (I take it that this is one of the types of quartz you are referring to).

SEM studies reveal that before the clast is completey enveloped by authigeneic quartz the "overgrowth" comprises many small (<10 micron) quartz crystals. These gradually coalesce to form a single continous overgrowth. However, this process can leave an "interconnected" network of FI's at the clast-overgrowth contact, rather than a series of "hermetically" sealed FI's. The result of this is that there is a strong potential for these inclusions to re-equilibrate during further burial.

In my experience diagentic quartz overgrowths exhibit three main types of fluid inclusion (these are of course not the only ones):

1. FI's that decorate the clast-overgrowth contact. Here, the FI's commonly exhibit highly variable L to V ratios (including monophase and bi-phase types). These are generally the most common.

2. FI's located in the main body of the overgrowth away from the clast-overgrowth contact. Here, in an individual sample, FI's generally exhibit constant L to V ratios and consistent Th's. Unfortunately?, these seem to be the least common type.

3. FI's located along healed fractures. These healed fractures can cut the overgrowth and penetrate the clast. In this case, they tend to have constant L to V ratios and consistent Th's. However, an alternative case is where the fractures stop at the clast-overgrowth contact. Here, the FI's are generally vapour-rich and seem to be related to "blow-outs" from FI's hosted at the clast overgrowth-contact.

These are just some general observations that I have made during my FI studies of quartz overgrowths -- I hope that they help explain some of the phenomena that you are observing.

Cheers
Jon

Jon Naden
Fluid Inclusion Researcher, Geochemist and Ore Petrologist
British Geological Survey, Keyworth
Notts NG12 5GG
Tel:#+44 (0)115 936 3163
Fax:#+44 (0)115 936 3302
E-mail j.naden@bgs.ac.uk

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Date: Thu, 11 Apr 1996

A comment on the freezing of one-phase liquid inclusions, as mentioned by Oakes on Nicolle's question. Although theoretically correct, I have grave doubts that freezing at the surface, in nature, will work the way it is suggested. Surface temperatures, particularly rock temperatures, probably don't get very far below zero. As you all know, when cooling an inclusion, freezing almost never occurs within a few degrees of Th ice, i.e., for a low salinity inclusion, near zero. It frequently takes 20 or 30 degrees of supercooling to nucleate ice. Small inclusions (as would be common in the low-temperature sediments involved) take even more. Although natural freezing could result in sub-zero temperatures for hours or days, time alone is not always adequate to overcome the metastability. I have kept inclusions at -10 to -15 for a week or more (in my home freezer) and examined them while still cold -- still liquid!

Ed Roedder

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Date: Thu, 11 Apr 1996

Briefly echoing Jon's comments, specifically with respect to preservation of quartz "dust-rim" hosted inclusions in the burial environment, Dick Larese and I have seen the following through observation and hydrothermal experimentation.

1) Based on the strength of quartz-hosted inclusions from work done in Bob Bodnar's shop, it is difficult to generate the necessary differential pressures to deform BULK quartz-hosted inclusions in the burial environment (e.g., in a sedimentary basin). The relationship of typical thermobarometric gradients in sedimentary basins to isochoric slopes are such that fairly substantial burial depths and temperatures are required--even assuming hydrostatic pore pressures. Pore pressures rarely remain hydrostatic below 10K ft of burial, based on worldwide correlations by Bradley and Powley. Hence the potential for stretching is correspondingly decreased, because differential pressures may be lower in overpressured environments.

2) Having said that, Pittman and Larese, among others, have demonstrated the inherent weakness of overgrowth boundaries (the dust rim), and the propensity for preferential replacement of quartz by carbonate, for instance, along this surface. The fluids responsible for replacement, would also be available to flush the early-formed inclusions and possibly refill them with later, deeper, hotter fluids. It is our opinion that the trends shown for quartz-hosted inclusions which have been attributed to closed-system deformation (e.g., in the North Sea) are probably more easily explained by this flush and fill mechanism, as Bob Goldstein has demonstrated for carbonate hosted inclusions. These studies rarely report salinities or gas contents, hence the supposition cannot be checked. It has been our experience that hot inclusions at dust rims have compositions that are more consistent with late formation rather than stretching of early-formed inclusions. Furthermore, it is thermodynamically impossible for early-formed inclusions to stretch to give near present-day maximum burial temperatures, as are seen in many North Sea samples (e.g., Th= bottom-hole temperatures. Late dust-rim inclusions tend to be fairly large, and irregular, whereas the dust-rim inclusions we have interpreted to be early formed are small, regular, and, may be single phase liquids even when recovered from fairly hot environments (>150 C present day).

3) Finally, the timing of quartz cementation in many environments is often circumstantial and relies on calculations of intergranular volumes in the best constrained cases, and basin modeling results in the worst constrained cases. In some areas of the North Sea, where early initiation of quartz cementation is reported (e.g., at 60 C or so) we have seen primary oil and aqueous inclusions within bulk overgrowths and along dust rims which have relatively high, unaltered homogenization temperatures (approaching inferred maximum temperatures). This suggests to us dominantly late quartz precipitation, at least in these samples.

Some of these data will be presented at PACROFI. See you there.

Don Hall
Amoco E&P Tech Group
dhall@amoco.com

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Date: Thu, 11 Apr 1996

Ed Roedder expressed 'grave doubts' about my suggestion to Nicole that some of her single phase, liquid inclusions may have been fractured by freezing the included water.

Unfortunately I deleted Ed Roedder's comment before I decided to make use of my talent for obnoxiousness; so please excuse or feel free to correct any misrepresentations I might make in my reply to his comment.

With all due respect to Ed Roedder's refrigerator, no one has yet been successful in maintaining pure water in its liquid form below something like -40 C. I'm too lazy to look up the references (and they're all in boxes now anyway) but I believe that someone named Angell has published a number of papers on the topic. I'll agree that fluid inclusions commonly remain liquid below their stable equilibrium melting pts but pure water generally freezes well before reaching -40 C and my experience with natural, aqueous fluid inclusions trapped in silicates at <150 C is that they tend to be extremely dilute.

As for the temporal variable: while winters tend to be of roughly the same temporal order of magnitude as the time involved in Ed's refrigerator experiment, my meager memory of geologic time and events is that the various ice ages were of substantially longer duration. And since the continents seem to have moved around a bit it is quite possible that even samples recently retrieved from currently balmy places have experienced a bit of chillier weather.

As for temperature variable: I'd venture that -40 C is not or has not been all that uncommon in various parts of the world (especially above and below roughly the 50 degree lines of latitude). I'm sure some of you folks in less temperate places than Berkeley, California can vouch for that better than I.

Thus another case where more information about the sample is necessary before a credible answer to a post can be formulated.

Charlie Oakes (CSOakes@LBL.gov)

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Date: Fri, 12 Apr 1996

I have to disagree with Charlie about liquid water being able to exist at temperatures below -40 C.

We have synthetic "pure" water inclusions that can be held at liquid nitrogen temperatures for hours and they still don't freeze. So, there is evidence that liquid water can exist at temperatures below -40 C. We also know that it is liquid water (i.e., not simply a failure to recognize ice) because we have analyzed the flincs on the Raman probe at low T and get a nice water spectrum, which differs from the spectrum of water ices. People who work on water in minerals have also observed the same phenomenon. They have submicroscopic "fluid inclusions" that still show a liquid water spectrum at -196 C - not ice. However, many have argued that such small amounts of water (which occurs as films) do not have the same physical and thermodynamic properties as bulk water.

Whether or not these same inclusions would freeze if held at low T over geologic time is another question that is difficult (impossible) to address.

Dr. Robert J. Bodnar
Virginia Tech
Blacksburg, VA 24061
Tel: (540) 231-7455 (O)
Fax: (540) 231-3386
e-mail: bubbles@vt.edu

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Date: Fri, 12 Apr 1996

Dear Charlie,

The concept of decrepitation of monophase aqueous inclusions in quartz by chilling during ice (glacial) times is even more unrealistic, if we take into account that the temperatures, such as -40 C, can never be reached at depths more than a few feet below the surface.

Perhaps it would be more interesting to extend the discussion about preservation of monophase aqueous inclusions from diagenetic to other environments.

We have described monophase aqueous liquid and coexisting monophase CO2 vapour inclusions in decrepitation clusters around larger H2O-CO2 inclusions which originated during greenschist-facies metamorphism (Contrib. Mineral. Petrol. 112, 414-427). The inclusions were rather small (1 micron) and even much thinner and it seemed to be plausible to invoke an older Russian hypothesis about anomalous properties of fluids in thin films to account for the existence of these inclusions.

Surprisingly, we have faced the similar problem during our recent study on cumulate xenoliths entrained in Pliocene alkali basalts. The plagioclase+quartz xenoliths contain up to 50 vol. % of intergranular melt with numerous vesicles (bubbles). The majority of these vesicles seem to be empty but in fact they are composed of low-density CO2 vapour. Some vesicles contain also aqueous liquid. The water-filled vesicles have various liquid-to-vapour ratios at room T, but a large number of other vesicles are filled only with aqueous solution and they are monophase at room T !. Several lines of evidence suggest that these aqueous fluids must have been expelled from the interstitial melts at magmatic temperatures (around 900 C) during ascent from the depth around 15 km to the surface. The formation conditions as well as inclusion diameters (up to 20 microns for monophase liquids, up to 100 microns for two-phase vesicles) are high enough to exclude any possibility of a kind of failure to nucleate a vapour on cooling. I emphasize again that the melts must have experienced nearly isothermal decompression during volcanic eruption - despite this, they still contain extremely dense aqueous inclusions, which are moreover incompatible with any point of their PT evolution.

Cheers

Vratislav Hurai

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Date: Fri, 12 Apr 1996

Several points in Charlie's "obnoxious" reply need further comment. Nobody has disputed the -40 C maximum supercooling of plain water. So that point in reference to my home freezer experiment is, to pardon the expression, a real red herring.

Now the application to Nature is a different kettle of fish (i.e., red herring?) Is there any credible evidence that even IF the rocks Nicole referred to had been under an ice sheet (rather than just the subareal "surface" conditions most people think of during exposure of rocks at the surface), do such subice rocks ever get to -40 C, before, during, or after the iceman cometh? I can well imagine a small point of rock projecting into moving air at -40 might get cooled to that point, but an expanse of sediments -- hardly. However, I remember a comic saying, many years ago, "Was you there, Charlie?" We can never really know, and can only say what was likely -- I say that -40C for a MASS of sediments is unlikely.

Who is next in the discussion?

Ed Roedder

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Date: Tue, 16 Apr 1996

To some request, I will try to (be more) precise (in the description of) the context of my samples. I am sorry I did not respond before but I have a lot of troubles with my mail connection which is always interrupted.

Quartz as siliceous overgrowths and neoformed crystals coexist as cements with dolomite in sedimentary depositions at about 1500 meters depth. The cementations are considered by petrography as cogenetic. The quartz has monophase fluid inclusions and the dolomite has two phase fluid inclusions displaying Th between 80 and 180 degrees celsius with a maximum around 180. Additionally, we have (noted) the coexistence of anhydrite bearing monophase fluid inclusions and biphase (fluid inclusions) showing Th between 125 and 180 degrees celsius.

The question is : What do the measured Th's in dolomite and anhydrite mean?

Regards to all
Nicole

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Date: Tue, 16 Apr 1996

Nicole, a few questions:

1) Is the dolomite and anhydrite bedded or cement; primary or secondary?

2) Are the compositions (salinities, gas contents) of Th 80-180 biphase inclusions in dolospar similar?

3) Same question for biphase inclusions in anhydrite with Th 125-180 C.

4) Can you artificially deform the monophase inclusions in anhydrite and quartz to get their salinities, etc.? If you can, you can compare these to the biphase compositions to add confidence to your hypothesis of cogenesis.

5) Are the quartz-hosted inclusions along overgrowth boundaries or within the overgrowth proper; and are they primary or secondary?

6) Do you have any independent information about burial/thermal history of the area.

Your inclusions in dolospar and anhydrite are giving a pretty large range in Th. If the area has been deeply buried or subjected to local thermal anomalies, then you may be recording stretching effects in dol and anhy with resistance of quartz-hosted inclusions to stretching....provided the inclusions are cogenetic.

Don

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