What we should worry about for diagenesis

16 Sep 2011

Ok, now that we have a burgeonning community, let's see if we can start a serious thread and engage in useful discussions. I think that one useful thing to do is to discuss what parameters we should worry about in order to apply clumped isotopes to diagenetic environments, and how to calibrate/remediate them.

Here are my top 5 "worries" or things that I believe we should address as a community:

  1. High temperature: there is currently no calibration published that go beyond 80 C. This needs to be remediated by thorough calibrations and inter-lab verifications of said calibrations.
  2. Chemistry: many of the carbonate species we study have a wide range of chemistry. Think about dolomite and ankerite, i.e how iron content can vary, or other chemical. How much will chemistry of the carbonate impact on clumping, and to what extent can we apply one unique calibration curve to a range of chemistries? I guess, again, that calibrations will be needed.
  3. pH: pH will vary dramatically between the ocean, hydrothermal systems, burial brines and/or hydrocarbon-rich regions. How much will pH impact on clumping?
  4. Diffusion: Ben gave a fantastic talk about diffusion in solid crystals at the clumped isotope workshop. This offers the opportunity for clumped isotopes to be used as a tool for reconstructing the thermal history of basins, but it also implies a complication: how much will solid-state diffusion "change" the original clumped isotope signature of your targetted mineral?
  5. Kinetic fractions: where, and when can we expect kinetic fraction effects? This is a tricky one, because it is not limited to a specific mineralogy. As demonstrated by Matthieu and Hagit in speleothemes, depending on the conditions in which your mineral precipitate, you can have reactions out of equilibrium. What happens, for instance, in capillary flows and pore spaces? This can be significant for the applications of clumped isotopes to diagenesis.

So, let's hear it from you now. What are your top 5 concerns, and how do you think we should address them?

Paul Dennis's picture
Paul Dennis
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Calibration of the clumped

Calibration of the clumped isotope thermometer at temperatures between say 100 and 250 degrees C is essential. This temperature range also represents a gap in the empirical carbonate-water isotope fractionation calibration. I think I'm right in saying that the O'Neill, Clayton and Mayeda (1969) experiments were all conducted at either less than 100, or more than 200 degrees C.

It's a difficult temperature range to work in. Below 100 degrees C slow precipitation can be carried out by diffusive degassing of CO2 from solution. At the higher temperatures recrystallisation/Ostwald ripening experiements involving dissolution and reprecipirtation can be carried out in reasonable times (up to several weeks/months) in conventional cold-seal Tuttle vessels. 

We're trying to access this temperature range using a small pressure vessel with a gas permeable barrier able to withstand a total pressure of up to 20 bars. We think we can control PCO2 across the barrier (provided diffusion kinetics are fast enough) and thus promote carbonate precipitation from solution.

Paul Dennis, Stable Isotope Laboratory, Environmental Sciences, UEA

Cedric's picture
Cedric
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Paul, this is really

Paul, this is really interesting.

Anne-Lise is currently looking at existing methods in the literature. One possibility that we consider is to use a pressurised vessel maintained at constant temperature. We aim for 200 bars, and 200-300C. The aparatus has a pump and two separate inlet also maintained at temperature, so we can inject two supersaturated solutions (say, one supersaturated solution of dissolved Ca++ and a supersaturated solution containing dissolved CO3--). This way, when the solution mix in the main chamber they should precipitate. The whole system can be maintained at temperature with a tolerance of 0.5 C.

Does anyone have experience with this, and can comment on the approach or any possible pitfalls?

Stuart
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Joined: Sep 22 2011
Cedric, Will the

Cedric,

Will the supersaturated solution be injected already at temperature or cold into the vessel which is already at pressure and temperature? If you inject cold into hot will precipitate form instantly when the liquids mix at there temperature, as they don't have time to heat up. I might be concerned about not getting a homgenous temperature of precipitation maybe?

Stuart

 

 

 

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Cedric
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Stuart,  Yes, we did think

Stuart, 

Yes, we did think about that: both solutions would be heated to the target temperature before being injected into the reaction vessel. So thermal equilibrium should not be a concern.

Cedric