First International Conference on Polar Science 3021.pdf

1 LUMINESCENCE DATING: A TOOL FOR AEOLIAN GEOCHRONOLOGY. K. Lepper and S. 2 1 W. S. McKeever ; Environmental Science Program / Department of Physics, 145 Physical Sciences 2 Bldg., Oklahoma State University, Stillwater, OK 74078. [email protected] Department of Physics, 145 Physical Sciences Bldg., Oklahoma State University, Stillwater, OK 74078.

Introduction: The Martian polar caps record a wealth of information about the environment and events on the surface of Mars. As on Earth, deciphering this rich record must include a sound chronology. The Martian ice caps clearly exhibit stratification, one of the most critical requirements for establishing a relative chronology (fig. 1), however, meaningful interpretation of data stored in the polar caps will require absolute dates. Stratification in the polar caps of Mars arises, at least in part, from the incorporation of aeolian material into the ice [1]. Active aeolian processes are also exhibited near the poles in the form of dune fields (fig. 2) [2]. Aeolian materials are ideally suited for luminescence dating and luminescence dating techniques have been used successfully to make absolute age determinations for numerous terrestrial Quaternary aeolian deposits (reviewed in [3]). Fig. 2. Map of the northern polar region on Mars. Dune Luminescence dating techniques could potentially be fields are shown in the hashed pattern (from [2]). developed to provide absolute age determinations for the aeolian sediments ubiquitous on the surface of Mars, including the sediments incorporated in the liberates charge carriers (electrons and holes) within Martian polar ice caps, thereby affording an opportunity silicate mineral grains. The charge carriers can subse- to correlate events recorded at the poles with the record quently become localized at crystal defects leading to of aeolian activity across the surface of Mars. accumulation of a “trapped” electron population. When General Principles of Luminescence Dating: exposed to solar radiation, the trapped charge Over geologic time, ionizing radiation from the decay population is depleted, thereby resetting the lumines- of naturally occurring isotopes and from cosmic rays cence clock. Recombination of the charge carriers results in photon emission. In the laboratory, thermal (TL) or optical (OSL) stimulation can be employed to liberate trapped charge and initiate the luminescence measurement process. The intensity of luminescence emission from the field sample is proportional to the elapsed time since burial. The response rate of the minerals to ionizing radiation and the local ionizing radiation dose rate of the deposit must also be deter- mined. Thorough reviews of the development of lu- minescence dating, detailed discussions of procedures and limitations can be found in the references [3,4,5]. The event dated by luminescence techniques is, ideally, the last exposure of the sediment grains to solar radiation. Aeolian sediments are generally well dispersed when transported, thus affording them the greatest opportunity for exposure to solar radiation and making them the best candidates for accurate lu- minescence age determinations. Fig. 1. Layered deposits of the northern polar ice cap on Mars (Image 56B86, from [2]). First International Conference on Mars Polar Science 3021.pdf

First International Conference on Mars Polar Science 3021.pdf Luminescence Dating for Martian Aeolian Geochronology: K. Lepper and S. W. S. McKeever

considered to be from ~1ka BP to ~150ka BP. Pore water in terrestrial sediments attenuates the external radiation dose received by sediment grains. In effect, this extends the upper age limit recordable by the sediment grains. The attenuation effect of the water ice and ice of the Martian ice caps and the local ionizing radiation dose rates are unknown. The potential for dating events beyond 150ka BP, however, is possible and should be investigated. Fig. 3. Small sand dunes photographed at the Mars Terrestrial investigations should be directed toward Pathfinder site (Sci. Am. 279:45). development of preliminary dating protocols. The Characteristics of Martian Aeolian Sediments: dating protocols could then be adapted for use with Data from the Mars Pathfinder predicts Martian samples procured from a “sample and return” mission lithologies analogous to terrestrial basalts and andesites or in the design guidelines for a luminescence dating [6]. Physical weathering alone of rock of this module for a rover-type vehicle. composition would yield sediment grains containing Conclusion: Aeolian materials, such as those in- pyroxene, calcic plagioclase, and biotie. Spectroscopic corporated in the Martian polar ice caps, are ideally data, however, indicates the presence of significant suited for luminescence dating. Luminescence dating amounts of poorly crystalline iron-oxides and clay techniques have been successfully applied to numerous minerals, suggesting chemically weathered surface terrestrial Quaternary aeolian deposits [3]. Further deposits [7]. In this case, quartz would also be predicted research on terrestrial analogs of Martian aeolian as a chemical weathering product [8]. materials is needed to develop luminescence dating The morphological similarity between terrestrial techniques for application to Martian samples. Lumi- and Martian dunes (fig. 3) supports the inference that nescence dating holds the potential to be a valuable tool the Martian dunes are composed of sand sized grains for absolute dating of the aeolian sediments ubiquitous [1]. Aeolian material incorporated in the polar ice caps on the surface of Mars, including the sediments is poorly determined at present, but is believed to be incorporated in the Martian polar ice caps. Lumines- sand and smaller particles [2]. cence geochronology could provide an opportunity to Development of Luminescence Dating for Mar- correlate the timing of events recorded at the poles with tian Applications: Luminescence dating procedures for the record of aeolian activity across the surface of Mars. terrestrial quartz and feldspar are well established [4,5]. References: [1] R. et al. (1992) in Mars ed. However, the applicability of luminescence dating to Kiefer H. H. et al. [2] Thomas P. et al. (1992) in Mars terrestrial pyroxenes and micas has not been ed. Kiefer H. H. et al. [3] Huntly D. J. and Lain investigated. Studies of the luminescence properties of O.B. (in press) Geological Survey of Canada Bulletin pyroxenes and biotites are needed as well as dating [4] Aitken M. J. (1985) Thermoluminescence Dating. studies of analogous terrestrial materials; sands and silts [5] Wintle A. G. (in press) Radiation Measurements derived from highly weathered basaltic terranes. [6] Rieder R. et al. (1997) Science 278:1771-1774. Sands and silts are optimum grain sizes for lumi- [7] Soderbolm L. A. (1992) in Mars ed. Kiefer H. H. et nescence dating [3] and recent developments make age al. [8] Gooding J. L. et al. (1992) in Mars ed. Kiefer H. determinations possible from extremely small samples, H. et al. [9] Duller G. A. T. (1995) Radiation including single grains [9,10]. Measurements 24:217-145. [10] Murray A. S. et al. The range of ages that is accessible to luminescence (1997) Radiation Measurements 27:171-184. dating of terrestrial materials varies with mineralogy and local ionizing radiation dose rate but is generally

Lepper, K.; McKeever, S. W. S. (1998) Luminescence Dating: A Tool for Martian Eolian Geochronology. The First International Conference on Mars Polar Science and Exploration, Proceedings of the Conference held at Camp Allen, TX. LPI Contribution No. 953. Houston, TX: Lunar and Planetary Institute, 1998, p. 24. Sears D. W. G., Lepper K. and McKeever S. W. S. (1999) Luminescence dating: A tool for martian eolian geochronology. Presented at the 30th Lunar and Planetary Science Conference (not on CD-ROM, abstract available on request).