The following is a continuation of a discussion of fluid inclusion dating (begun in Apr. 1997) that was held during early Sept-Oct 1998


 

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I'd like to continue the discussion on dating fluid inclusions. Does anyone have any ideas on how to date low salinity inclusions in quartz, calcite and fluorite that would be less than 10 or 15 Ma?

Jean

Jean S. Cline
Associate Professor, UNLV
Department of Geoscience
4505 Maryland Parkway, Box 454010
Las Vegas, NV 89154-4010


702/895-1091 (phone)
702/895-4064 (fax)
jcline@nevada.edu
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Hi Jean

My guess is that with the qtz f.i. you could try using the bulk sample crush leach (or dissolution) Rb-Sr isochron method, (see Scrivener, Darbyshire & Shepherd 1994, J. Geol Soc, London, vol 151, p587-590 and Pettke & Diamond 1995, G.C.A. v.59, p 4009-4027) it may be possible to obtain an age precision of +/- 1% . Off the top of my head I can't think of a means of dating f.i. hosted in a carbonate mineral like calcite, or indeed in fluorite. If the f.i. are primary though you could try and date the fluorite host itself using the Sm- Nd isochron approach, (Chesley, Halliday & Scrivener 1991, Science, v. 252, p949-951.) Not all fluorites are amenable to this method though and you may be opening a can of worms. Other people out there may have something to add, Thomas Pettke would be well worth contacting...are you out there Thomas?. By the way, could you explain a little more about what you are trying to do ??

Best of luck anyway
Wally Walshaw
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At 14:57 14-09-1998 +0000, Wally Walshaw wrote:

>My guess is that with the qtz f.i. you could try using the bulk
>sample crush leach (or dissolution) Rb-Sr isochron method, (see
>Scrivener, Darbyshire & Shepherd 1994, J. Geol Soc, London, vol 151,
>p587-590 and Pettke & Diamond 1995, G.C.A. v.59, p 4009-4027) it may
>be possible to obtain an age precision of +/- 1% . Off the top of my
>head I can't think of a means of dating f.i. hosted in a carbonate
>mineral like calcite, or indeed in fluorite. If the f.i. are primary
>though you could try and date the fluorite host itself using the Sm-
>Nd isochron approach, (Chesley, Halliday & Scrivener 1991, Science, v.
>252, p949-951.) Not all fluorites are amenable to this method though
>and you may be opening a can of worms. Other people out
>there may have something to add, Thomas Pettke would be well worth
>contacting...are you out there Thomas?. By the way, could you explain
>a little more about what you are trying to do ??

I think those rocks are too young anyway to date them with either Rb-Sr or Sm-Nd (less than 15 Ma!!!). It will be very hard to date them with any radiogenic method, except for the fi's in the calcite using U-disequilibrium methods, but only if that calcite is *not* metamorphic, because otherwise the amount of U will not be sufficient.

Try luminescence (anybody?).

Martijn
Martijn Moree

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Hi,
Dating fluorite itself instead of the FI can be a difficult task if your deposit has a long evolutionary history as you can verify in:

RONCHI, L.H., TOURAY, J.C., MICHARD, A. & DARDENNE, M.A. (1993) The Ribeira fluorite district - Southern Brazil: Geological and geochemical (REE, Sm-Nd isotopes) characteristics. Mineral. Deposita 28: 240-252.

regards,
Luiz Henrique Ronchi

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Jean,

As you know from our past discussions, my collegues and I have been trying to date fluid inclusions using Ar/Ar. We are still wrestling with the technique and have not been very successful yet. We are giving it one more try soon using different sample prep techniques. One of our problems is with the low signal that we get using a hundred or so milligrams of sample. The other big problem is with the lack of radiogenic argon. These samples have high salinities and are about 28 my old. Given what we have tried so far, and our lack of success, I doubt that this technique would be worth trying on your samples which you describe as low salinity and 10my old. We do have a few more things to try so I hope later I will be reporting greater success with our dating attempts.

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Dr. Andrew R. Campbell                       *
Professor of Geology                         *
Dept. of Earth and Environmental Science     *
New Mexico Tech                              *
Socorro NM 87801                             *
Phone: 505-835-5327                          *
Fax:   505-835-6436                          *
e-mail: campbell@mailhost.nmt.edu            *
www.ees.nmt.edu                              *
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Dear Jean,

Are your fluid inclusions primary or secondary? If they are secondary, you may try to measure the orientation of the microcracks (fluid inclusion planes) and compare them with the different stress fields as they are known in your area. This may give you an approximation (no more) on the age of your fluid inclusions.

Best regards
Anne-Marie

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Anne-Marie Boullier
C.N.R.S. - Laboratoire de Geophysique Interne et Tectonophysique
BP 53X 38041 GRENOBLE CEDEX 9
France
Tel +33 4 76 82 81 16
Fax +33 4 76 82 81 01
Anne-Marie.Boullier@obs.ujf-grenoble.fr
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Dear all,

Yes, Wally and others, I am (again) out here...

Direct dating of fluid inclusions -- Although highly desirable, I have many concerns, that this is not really feasible at present, despite the recent, great analytical advances in radiogenic isotope geochemistry ... The literature provides us with lots of unsuccessful examples (ours included). I hope things will improve in future...

As for any conventional isochron approach (Rb-Sr, Sm-Nd, ....) we need BOTH spread in the parent/daughter element ratio AND constant initial isotopic composition of the daughter element for producing any "fluid inclusion isochron" - Two prerequisites which appear to almost exclude each other in the hydrothermal environment which deals with highly complex, open systems (except maybe for magmatic-hydrothermal systems, e.g. Shepherd and Darbyshire, 1981, Nature 290, p. 578).

As a more promising approach I could think of trying to obtain a hydrothermal mineral isochron (not easy alike) and then analyze extracted bulk fluid inclusion fractions (given they are primary). If the bulk fluid inclusion samples which represent an additional phase of a given hydrothermal system, fall on the isochron described by the vein minerals, the obtained age is representative for the fluid inclusions as well. Of course, the two prerequisites mentioned above remain. This is not even speaking of the necessity to obtain pure mechanical separates of the hydrothermal minerals to be analyzed isotopically.

Attempts at dating fluid inclusion using 40Ar/39Ar (or any other method involving a noble gas element) probably suffers again from the problem of the INhomogeneous initially trapped 40Ar/39Ar component (i.e. the trapped component); variable 40Ar/39Ar ratios are commonly observed in a hydrothermal fluid. The trapped component, furthermore, is likely to be large due to the strong partitioning of noble gases into the liquid phase upon crystallization from a hydrothermal fluid. A large and variable initial component deals the deathblow to any dating attempts. We analyzed native gold samples that were crystallized from an aqueous low-salinity NaCl-KCl fluid for He and Ar (31 Ma old) and determined that MORE THAN 98% of our Ar and He was of a trapped origin .... (Pettke et al., 1997, Chem Geol 135, p. 173).

The most promising small-sample approach using 40Ar/39Ar in my view is using sub-milligram samples of hydrothermal white mica or clay separates (see e.g. Hall et al., 1997, EPSL 148, p. 287), an approach which may provide an age resolution of better than 1%. The obvious problems: (1) are there any such phases present, and (2) are they strictly cogenetic with the fluid inclusion population of interest?

For specific hydrothermal conditions (e.g. such that have an oxidizing fluid and the deposition of minerals was triggered by redox changes), a U-Pb approach might be feasible (but not U-series disequilibrium for samples in the several Ma range)...

Jean, I presume that you intend to obtain age constraints on your Carlin-type deposits - or is it anything else? If you have pyrite or other sulfides as a cogenetic phase, a Re-Os isochron approach might be worth a try (compare e.g. Freydier et al., 1997, Geology 25, p. 775).

I guess that an indirect approach to constrain the age of hydrothermal activity in Carlin-type deposits may be the most promising option.

Any other suggestions?

Cheers,
Thomas
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Thomas Pettke
Dept. Geol. Sci.
The University of Michigan
2354 C.C. Little Building
Ann Arbor, MI 48109-1063
USA
fax: (+1) 734 763 4690
phone: (+1) 734 763 9368
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Dear All F Inclusionists:
The recent very useful and important e-mail discussions on the possibilities of dating of fluid inclusions brings to mind an old story, from 33 years ago. At that time, Rama, Hart and Roedder (J. Geophys. Research, 1958, v. 70, no.2, p. 509-511) published one of the first (?) discussions of this matter. In this study, an inclusion-rich white quartz vein (from my back yard in Bethesda, MD) was run for K/Ar using isotope dilution methods. The K content was so low (14 ppm) and the Ar was so high (estimated partial pressure of Ar in the inclusions in the atmospheres range) that the raw "age" would have calculated to be thousands of times older than the enclosing rocks (Paleozoic phyllites). The Ar had obviously been "sweated out" of the enclosing phyllite during formation of the quartz veins. As expected, primary argon, in the fluid at the time of trapping, can vary over a wide range. Only if the host fluids had lost ALL their gases (e.g., by extensive boiling prior to trapping), would the K/Ar "clock" be reset to the time of trapping, and simple K/Ar age determinations, with reasonable error bars, be possible.


Ed Roedder

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Further to the foregoing discussions on XS Ar in fluid inclusions, it appears to be a common, even likely, phenomenon in hydrothermal systems, because, as Thomas Pettke (I think) pointed out, Ar will preferentially partition into any available fluid phase (including magma -- volatile-rich magmas are particularly likely). When trying to date mineralization at the Porgera gold deposit by K-Ar, we had terrible trouble with XS Ar, firstly in igneous minerals (hornblende), and then with hydrothermal minerals (roscoelite). Ar-Ar dating solved the hornblende problem, but the mistake I made with roscoelite, which was intimately and finely intergrown with quartz, was that the quartz would contain no radiogenic Ar (i.e., be blank). Wrong. It clearly contained loads of Ar with non-atmospheric composition, presumably in fluid inclusions. When I improved the separation of roscoelite from quartz and re-ran the samples, we got sensible dates (although we plan to firm these up using new Ar-Ar laser-dating techniques in the near future). As usual, ore deposits turn out to be harder things to work on than dry uneconomic systems! (But much more fun!)

Regards,

Jeremy Richards

Jeremy P. Richards, PhD
Associate Professor, Economic Geology
Earth Sciences Building, Rm. 3-02
Dept. Earth and Atmospheric Sciences
University of Alberta
Edmonton, Alberta
Canada, T6G 2E3
Tel: (403) 492-3430 (messages: 3265)
Fax: (403) 492-2030
E-mail: Jeremy.Richards@ualberta.ca

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