A Two‐Step K‐Ar Experiment on Mars: Dating the Diagenetic Formation of Jarosite from Amazonian Groundwaters

• Published in: Journal of geophysical research. Planets 2017-12, Vol.122 (12), p.2803-2818
• Main Authors: Martin, P. E., Farley, K. A., Baker, M. B., Malespin, C. A., Schwenzer, S. P., Cohen, B. A., Mahaffy, P. R., McAdam, A. C., Ming, D. W., Vasconcelos, P. M., Navarro‐González, R.
• Format: Article
• Language: English
• Subjects: Amazonian; Argon; Curiosity (Mars rover); diagenetic; Fluid dynamics; Fluid flow; Geochemistry; Geochronology; Geological time; High temperature; Jarosite; K‐Ar; Low temperature; Mars; Mars craters; Mars rovers; Mars surface; Mineralogy; Modelling; Mudstone; Plagioclase; Planetary geology; Potassium; Potassium-argon dating; Radiometric dating; Sandstone; Sediments; Water
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1002/2017JE005445

Following K‐Ar dating of a mudstone and a sandstone, a third sample has been dated by the Curiosity rover exploring Gale Crater. The Mojave 2 mudstone, which contains relatively abundant jarosite, yielded a young K‐Ar bulk age of 2.57 ± 0.39 Ga (1σ precision). A two‐step heating experiment was implemented in an effort to resolve the K‐Ar ages of primary and secondary mineralogical components within the sample. This technique involves measurement of 40Ar released in low‐temperature (500°C) and high‐temperature (930°C) steps, and a model of the potassium distribution within the mineralogical components of the sample. Using this method, the high‐temperature step yields a K‐Ar model age of 4.07 ± 0.63 Ga associated with detrital plagioclase, compatible with the age obtained on the Cumberland mudstone by Curiosity. The low‐temperature step, associated with jarosite mixed with K‐bearing evaporites and/or phyllosilicates, gave a youthful K‐Ar model age of 2.12 ± 0.36 Ga. The interpretation of this result is complicated by the potential for argon loss after mineral formation. Comparison with the results on Cumberland and previously published constraints on argon retentivity of the individual phases likely to be present suggests that the formation age of the secondary materials, correcting for plausible extents of argon loss, is still less than 3 Ga, suggesting post‐3 Ga aqueous processes occurred in the sediments in Gale Crater. Such a result is inconsistent with K‐bearing mineral formation in Gale Lake and instead suggests postdepositional fluid flow at a time after surface fluvial activity on Mars is thought to have largely ceased. Key Points A third radiometric age dating experiment has been conducted on Mars The model formation age of plagioclase is greater than 4 Ga, while the model age for jarosite is less than 3 Ga The young jarosite age suggests the presence of liquid water in Gale Crater during the Amazonian period, most likely in the subsurface