BASALT WEATHERING RATES IN A MARS ANALOG ENVIRONMENT: CLUES TO THE DURATION OF WATER ON MARS? E.M. Hausrath 1 , S.L. Brantley 2 and AMASE 3 1 302 Hosler Building, Penn State Univer- sity, University Park, PA 16802 emh191@psu.edu, 2 2217 Earth and Engineering Building, Penn State University, University Park, PA 16802 brantley@essc.psu.edu . 3 Arctic Mars Analog Svalbard Expedition Introduction: Extensive evidence, most recently from the Mars Exploration Rovers, documents that liquid water once existed on the surface of Mars [1], [2]. However, significant uncertainty still exists as to the duration of liquid water on the surface of Mars. Mineral dissolution rates may provide information on maximum durations of water on Mars [3],[4]. However, field and laboratory weathering rates on earth differ significantly, by up to 5 orders of magni- tude[5]. Few field weathering rates for basalt and oli- vine have been published. The climate history of Mars is not well-constrained; however, field weathering rates of minerals in Mars-analog environments may provide information on the duration of water on Mars. Spitsbergen (Norway) provides three examples of basalts weathering in a Mars-analog environment: out- crops of the Quaternary volcano, Sverrefjell, plateau basalt lavas emplaced approximately 10 million years ago [6], and a basalt dike. Sverrefjell has been well documented as a Mars analog containing carbonate globules that are very similar to those found in ALH84001 [7]. These three basalts with different chemistries will allow us to study basalt weathering in a cold, dry climate. Methods: Basalt samples were collected from each of the three locations as part of AMASE, the Arc- tic Mars Analog Svalbard Expedition in August of 2004. They were then observed by optical microscopy, backscattered electron microscopy, and energy disper- sive x-ray spectroscopy. Results: Chemical weathering. All three basalts are only slightly weathered, as expected from the cold (yearly average temperature approximately -5ºC), dry (less than 200 mm precipitation) climate, and the small amount of time since the surfaces were deglaciated (10,000 years) [8]. An altered surface is sometimes observed on sam- ples from each of the locations (Figure 1). However, most surfaces, although exposed to the environment, show little to no evidence for dissolution (Figure 2). In one case, the altered surface layer was 100 μm thick (Figure 1b). Evidence for dissolution includes porosity and preferential loss of minerals (Figure 1); however, some alteration may be related to prior hydrothermal events. Physical weathering. Cracks parallel to the surface indicate the potentially important role of physical weathering in a cold, dry environment. In cold dry regions, thermal fluctuations at the surface may be large [9], and may contribute to such cracking. The potential for artifacts due to preparation is also being investigated (see Figure 2). Olivine was observed to have physically weathered out of xenoliths at the basalt surfaces (see Figure 3), sometimes forming piles of mm-size olivine grains. We infer that physical disaggregation is as important or more important than chemical weathering in this local- ity. 1c 1b 1a Figure 1. BSEM photomicro- graphs of a) a potential dissolu- tion feature from a sample from the Sverrefjell vol- cano. Dark (ep- oxy) areas indi- cate porosity, possibly due to dissolution of glass matrix, leaving plagio- clase grains; b) a potential dissolu- tion feature from a sample from the plateau basalt lava flows; c) a potential dissolu- tion feature from a lichen-covered sample from the basalt di ke. Scale bars are in μm. Arrows indi- cate the thickness of the altered layer. 50 100 200 Lunar and Planetary Science XXXVI (2005) 2339.pdf