Reika Yokochi, University of Illinois (Chicago)
The time scale of fluid migration in the crust is a crucial factor in addressing the development of oil and hydrocarbon reservoirs as well as assessing the risks of artificially injected CO2 leakage from subsurface reservoirs. This project examines the feasibility of using the noble gas radionuclides as chronometers to determine fluid residence time in the crust. The isotopic abundances of noble gas radionuclides in CO2(and hydrocarbon, if possible) reservoirs will be determined for the first time, and will be interpreted in the context of fluid migration in the crust together with stable noble gas chemical and isotopic compositions. The progresses over the second grant period are two folds:
(i) Search for the most weight-effective CO2absorbents
One of technical difficulties in measuring noble gas radionuclides is the large quantity of noble gases required. An efficient removal of the carbonic gases in the field is essential for successful sampling. During the first year, we evaluated our field gas-purification method by laboratory experiments and based on our field experiences. We found that the capacity of the Ca(OH)2-based absorbent was only about a half of what is expected from the stoichiometry. We presumed that this was due to the formation of CaCO3 near the surface of the Ca(OH)2 which deters the CO2 diffusion thus the reaction. During the second grant period, we compared the efficiency of CO2absorbent for different hydroxides.
LiOH pellets was by far the most weight effective (i.e. absorb the largest amount of CO2 for a given weight) absorbent, whereas Sodasorb and Sofnolime, pelletized Ca(OH)2-products, has about 1/3 of CO2 absorption capacity compared to LiOH. NaOH and KOH 4N solutions have commonly been used for removing CO2 from gas phase of volcanic and geothermal gases during sampling, but their capacity of CO2 absorption were about 10 times less than that of LiOH pellets. Unfortunately, the LiOH is about 10 times more expensive than Ca(OH)2-based pellets absorbents, thus the latter is more economical for collecting large-volume samples as needed for this project. As we supposed that the absorption capacity of this absorbent could be limited by a geometric factor such as diffusion, we also examined Ca(OH)-based pellets of two different grain sizes. The CO2absorption capacity was however comparable between the two, indicating that (i) the low capacity compared to the stoichiometric expectation is not related a geometric factor, and (ii) using smaller pellets would not improve the efficiency of gas sampling.
An undergraduate student, Ms. Rubina Hafeez, participated in this part of the project as her first laboratory work experience, and made important contributions. Ms. Maria Valdes, a summer student supported by this grant, performed most of the experiments, and prepared a manuscript reporting the results of our experiments on (i) the adsorption rate of carbon dioxide on calcium hydroxide pellets (works done before the previous grant period), and (ii) CO2absorption capacity on different hydroxides (results obtained during this quarter). We submitted the manuscript to G-cubed as Technical Brief. The student took responsibility in putting data together, adding discussions that includes thermodynamic calculations, and writing a substantial fraction of the main text of the manuscript.
A system that applies pressure-swing adsorption method is also being developed, and a progress has been made by an undergraduate student, Mr. Kyle Cronin. During the previous grant period, the unit was controlled using a programmable logic controller. Because of the limited operational flexibility, we replaced the control unit by a multi-function data acquisition unit that is controlled by a LabView program. We plan to start field sampling as soon as we complete the optimization of the sampling apparatus.
(2) Geochemical modeling of noble gas radionuclide in crustal fluids
During the first grant period, we developed and published a geochemical model that estimates distribution of a nucleogenic noble gas radionuclide, 39Ar (atmospheric 39Ar/Ar ~ 8.1´10-16, Half life = 269 years) and suggested the possibility of this nuclide to become a new chronometer of fluid residence/transport time in the crust. During the second grant period, we applied the model and analyzed a unique data set of noble gas radionuclide from Yellowstone National Park. The data had been obtained prior to this project, but geochemical interpretation to the data was provided and published for the first time using the model developed during the first grant period. The key findings are three folds:
(i) The 85Kr/39Ar ratios of subsurface production and that of atmosphere are ~0.0018 and 2.6, respectively, indicating that 85Kr/39Ar ratio is a proxy of an atmospheric component recently mixed with the crustal fluid.
(ii) The 85Kr/39Ar ratios do not only depend on the fraction of young atmospheric component but also on the on the ventilation age of the young component as 85Kr abundance in the atmosphere has steadily been increasing since 1940’s. In order to determine the residence time of the crustal fluid prior to the mixing, it is necessary to have another tracer of similar nature (i.e. young chronometer of atmospheric origin).
(iii) The uncertainty on 39Ar/40Ar* (where “*” denotes radiogenic) ratios in lithospheric reservoir is large due to the uncertainty in both the 39Ar production rate, suggesting the need for a systematic study of 39Ar production rate. The extent of the souce rock supplying these isotopes to the fluid can also be an important factor.
In summary, we have made further progresses in sampling technique and strategy, as well as in understanding the behavior of noble gas radionuclides in the crust and the strategy of data interpretation. The field sampling will take place on completion of optimizing the sampling apparatus.