Jarosite Dissolution Rates and Lifetimes Under Mars-Analog Conditions

41st Lunar and Planetary Science Conference (2010) 1411.pdf

JAROSITE DISSOLUTION RATES AND LIFETIMES UNDER MARS-ANALOG CONDITIONS. S. K. Zahrai1, M.E. Elwood Madden1, A.S. Madden1, M. Miller1 and J.D. Rimstidt2, 1School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd, Norman, OK 73019, [email protected], 2Dept. of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksurg, VA 24061, [email protected].

Introduction: The lifetime of a dissolving metast- 55 kJ/mol. Dissolutioin rates were also considerably able mineral particle can be calculated using the initial lower in NaCl–saturated brine (Table 1). This suggests diameter, molar volume, and dissolution rates of the that the activity of water has a significant effect on the mineral particle and then by applying those values to a dissolution rate. shrinking sphere equation [1, 2]. Jarosite is an impor- Applying the shrinking sphere equation to the dis- tant ferric sulfate phase found in acid mine drainage solution rates determined experimentally or extrap- systems on Earth, and has also been obeserved by the loated using the measured activation energy, we pre- Mars Exploration Rover Opportunity in outcrops on dict that a 10 µm particle K-jarosite would survive less Meridiani Planum [3]. Jarosite has a very narrow sta- than 1 day at 373 K to approximately 100 years at 223 bility range and may precipitate as a metastable phase K in dilute aqueous environments. In NaCl-saturated in low temperature environments where more stable brines K-jarosite is predicted to survive approximately iron oxides are sluggish to form [4]. In this work, 22 days at high temperature to slightly over 9,000 endmember K-jarosite dissolution rates have been years at 223 K (Figure 1). measured under Mars-relevant conditions in the laboratory. The shrinking sphere model was applied to Table 1. Dissolution Rates determine the lifetime of jarosite paricles within aqueous systems on Mars. The results provide particle Experimental log k lifetimes over a range of Mars-relevent temperature Conditions and salinity conditions, which can be used to constrain 277 K, UP water -8.6 the duration of liquid water at Meridiani Planum and other regions where jarosite may be observed. 295 K, NaCl -9.8 Methods: K-endmember jarositewas synthesized 295 K, pH 4 -7.85 in the laboratory following the USGS method [5]. The 295 K, pH 2 -7.95 synthesized material was characterized using X-ray diffraction and BET surface area analysis. Products of 295 K, pH 6 -7.7 the jarosite dissolution experiments were analyzed 295 K, Oxalic acid -7.72 using atomic force microscopy (AFM) and transmis- 323 K, UP water -7.1 sion electron microscopy (TEM). 323 K, Oxalic acid -7 Dissolution experiments were conducted by add- ing 0.25 g of K-jarosite to 250 g of ultrapure (UP) wa- Reaction products: TEM and X-ray diffraction analy- ter, NaCl-saturated brine, oxalic acid, or a pH-buffered solution ranging from 2 to 6. Temperatures were set sis of the reaction products suggests that the jarosite ranging from 277 K to 323 K. 5mL samples were col- converts quickly to dominantly hematite on jarosite lected from the constantly stirring solution at set time grains and schwertmannite within the solution. Forma- intervals and passed through a 0.2 micron syringe fil- tion of reaction products was suppressed at pH 2, be- low the pKa of sulfate/bisulfate. Textures observed ter. Experiments spanned from 45 minutes to about 6 + 2- hours. Samples were kept refrigerated until analyzed suggest that at high temperatures K and SO4 are pre- by Atomic Absorption Spectrophotometry. ferentially leached from the jarosite particles, leaving a Results: Dissolution rates did not vary with re- honeycomb of hematite behind. Hematite is also ob- spect to pH or dissolved iron concentration in the pres- served forming on the surfaces of the jarosite particles ence of oxalic acid. However, rates varied signifcantly under low temperature dissolution conditions. with temperature, suggesting an activation energy of 41st Lunar and Planetary Science Conference (2010) 1411.pdf

Conclusions: Laboratory experiments demonstrate ‐5 that jarosite dissolution rates are independent of pH Dilute Aqueous 1 day ) 1

and oxalic acid concentration, but are significantly ‐

s Solution pH 2-6 ‐7 2

affected by salinity and temperature. Therefore, jaro- ‐ m

site may persist for thousands of years in low- 100 days temperature saline systems. However, jarosite is not ‐9 etime (mol f expected to be preserved in hydrothermal systems that

NaCl 27 yrs Li are active over time periods longer than 1 to 2 years. Saturated Brine TEM analyses also demonstrate that hematite is the Rate ‐11 dominant reaction product formed through jarosite 2700 yrs dissolution under the conditions investigated. This ‐13 suggests that the hematite spherules observed at Meri- 200 250 300 350 400 diani Planum may be a paragenetic product of jarosite Temperature (K) dissolution. However, the rate data reported here suggest that aqueous conditions required for jarosite dissolution and reprecipitation of hematite must have oc- Figure 1. Dissolution rates and particle lifetimes for curred over a fairly short time period in order to 10 µm K-jarosite particles in dilute aqueous solutions preserve the jarosite observed in the rocks at Meridiani and NaCl-saturated brines. Planum.

Acknowledgements: We are grateful to Sumudu Munasinghe, Greg Strout, John Guess, and Robert Turner for analytical and technical support. This project was funded by the University of Oklahoma Junior Faculty Award Program and NASA’s Mars Fundamental Research Program.

References: [1] Lasaga (1998) A. C. Kinetic Theory in the Earth Sciences. [2] Elwood Madden M.E., et al. (2009) Geology, 37, 635-638. [3] Squyres, S.W., et al. (2004) Science, 306, 1709–1714. [4] Elwood Madden, M.E.E., Bodnar, R.J., and Rimstidt, J.D. (2004) Na- ture, 431, 821–823. [5] Driscoll, R., and Leinz, R. (2005) U.S. Geological Survey Techniques and Me- thods 05–D1.