As part of a study of geomorphic processes in the ice-free areas of Victoria Land, solution etch pits occurring on dolerite cobbles are being investigated in the Allan Hills and dry valleys. The ultimate objective is to model the formation mechanisms of these pits and use the calculated rates of development to determine the exposure ages of various ice-free surfaces in Victoria Land. This article presents some preliminary results. Samples were collected during the 1984-1985 field season and were analyzed using a variety of techniques.
The pits are gravity normal, exhibit abundant evidence of aqueous dissolution and alteration of primary minerals, and range in size from less than a millimeter to several centimeters.
The etch pits are essentially closed aqueous systems, and the chemistry of the aqueous phase and weathering products should indicate whether or not water/rock interactions in the Antarctic are in equalibrium or partial equilibrium.
Etch pits have a smooth, yellowish capillary coating deposited to varying degrees around the upper boundaries of the pits and along areas connecting pits to one another. Immediately underlying the pit is a thin zone, less than a millimeter, of weathered dolerite, consisting of highly altered primary grains with abundant hematite and amorphus products. X-ray diffraction gives only two extremely weak reflections at 3.34 Ängstroms (quartz) and 3.53 Ängstroms (possibly vermiculite or mixed-layer clays). IR spectroscopy identified quartz but is inconclusive for the phyllosilicate.
Whenever liquid water in within a pit, capillary action along the rough pit-interior surface drives water up to the lower boundary of the coating. This capillary action remains until evaporation of water in the pit is complete. Microscopic examination of these yellow coatings indicates that they are areas of deposition of material from solution.
The general composition of the coating is given in Table 1. The lack of sodium and low calcium, magnesium, and iron contents in a material derived from such a basic rock as the dolerite is remarkable. X-ray diffraction patterns of the coating are weak and indicate poor crystallinity, but enough lines are present to suggest illite and quartz as the major components. Infrared spectroscopy gives positive identification of nonmixed layer illite (90-100 percent illite) and quartz.
Oxide Weight percent Al203 18.5 Si02 63.1 K20 3. Na20 nd MgO 0.6 CaO 0.5 FeO 1.5 TiC2 0.8 S03 1.7 H20 9.3 Total 99.5% "nd" denotes "below detection limits"
Note: This gives an illite formula of: K^0.4Mg^0.1Al^2.1Si^3.30^10(OH)^2. The coating is an homogeneous mixture of 65 percent illite and 30 percent quartz and amorphous silica, with several percent hematite and sulfate salts unevenly dispersed within it.
Under scanning electron microscope examination, the illite-bearing coating consists of layers of microcrystalline material with an average crystallinity of less than 100 €ngstroms. However, the coating has an extremely constant chemistry and mineralogy throughout the capillary area. Table 2 gives four compositions in a profile through the coating. Note the constant aluminum/(potassium + magnesium) throughout the coating. This constant composition is surprising in a poorly crystalline material, formed by repeated evaporation of aqueous solutions under antarctic conditions, indicating that the illite and quartz (in the constant ratio of 2.2:1) are a result of partial or total equilibrium (Helgeson, Murphy, and Aagaard 1984) within the etch pits during and after evaporation of the pit-filling water under relatively nonvarying ambient conditions. Variations occurring in calcium, sulfate, iron, and titanium, which are present as trace phases of oxides and salts, suggest that these are not part of the same partial equilibrium reactions as the silicate phases.
1 2 3 4 Oxide (%) (%) (%) (%) AI2O3 19.9 18.2 18.4 20.3 SiO2 63.8 63.7 65.6 62.2 K2O 3.6 3.2 3.3 3.5 Na2O nd nd nd nd MgO 0.6 0.7 0.7 0.8 CaO 0.4 0.6 0.5 0.1 FeO 1.3 1.1 1.7 0.8 TiO2 0.2 0.7 0.2 <0.1 SO3 0.4 1.9 0.3 1.4 H2O6 9.3) (9.3) (9.3) (9.3) Totals 99.5 99.4 98.9 98.4 Al/(K + Mg)* 4.7 4.7 4.6 4.7 Al/Si** 0.31 0.29 0.28 0.33 "nd" denotes "below detection limits"
* Water analyses were performed on bulk sample giving an average water content for the coating as a whole. Spacial variation in water content are likely to be the greatest source of error and totals discrepencies in the above analyses.
** Aluminum (potassium + magnesium) and aluminum/silicon for an "ideal" illite (Helgeson et al. 1969) are 4.2 and 0.56, respectively. All the aluminum, magnesium, and potassium reside in the poorly crystalline illite, whereas some of the silicon is in poorly crystalline quartz and amorphous silica.
Kaolinite, montmorillonite and mixed-layer clays, iron oxyhydroxides, bauxite minerals, and various silica minerals comprise the usual insoluble phases that form during weathering of silicate material. Vermiculite forms as an intermediate product from a variety of silicates, but nonmixed layer illite has not been observed to form as a surface-weathering product directly from solution, only as an intermediate product derived from initial muscovite. The activities of potassium ion and silica are almost never high enough in weathering environments to form discrete illite, and the stability of montmorillonite/mixed-layer clays is favored by the presence of exchangeable cations (sodium, calcium, and magnesium) and tetrahedral aluminumsilicon polymerized chains remaining from feldspar breakdown. Therefore, Conditions within the pit solutions during evaporation must be fundamentally different from the more temperate weathering environment, not only with respect to temperature, but also solution chemistry.
The thinness of the material underlying the pits indicates that pit growth is keeping pace with infiltration and alteration. This, together with the indication of partial-equilibrium, allows a preliminary estimate for the formation rate. Mass transfer of material during hydrolysis of plagioclase can be roughly estimated based on work by Helgeson, Garrels, and Mackenzie (1969). To a first approximation, hydrolysis of plagioclase in the underlying material results in the destruction of 0.3 gram of plagioclase and precipitation of 0.002 gram of illite for every 1,000 grams of water. A 5-cubic-centimeter pit represents the removal of approximately 10 grams of plagioclase. If the average water flux experienced during growth of the pit is the presentday average of about 1 gram per year, then at least 30,000 years are required to form the pit.
Refinement of this first approximation requires recalculation of the rates from Helgeson, Brown, and Leeper (1969) for the dolerite under antarctic conditions, and direct measurements of the water flux at these surfaces as well as the solution chemistries of the water. It is expected that such refinement will lengthen the time required to form a 5-cubic-centimeter pit to more than 100,000 years.
This work was supported by National Science Foundation grant DPP 82-06391.
ReferencesHeIgeson, H. C., T. H. Brown, and R. H. Leeper. 1969. Handbook of theoretical activity diagrams depicting chemical equilibrium in geologic systems involving an aqueous phase at one ATM and 0 degrees to 300 degrees C, San Francisco: Freeman, Cooper and Co.
Helgeson, H. C., R. M. Garrels, and E. T. Mackenzie. 1969. Evaluation of irreversible reactions in geochemical processes involving minerals and aqueous solutions-II. Applications. Geochimica Cosmochimica
Acta, 33, 455-481.
Helgeson, H. C., W. M. Murphy, and P. Aagaard. 1984. Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. Geochimica Cosmochimica Acta, 48, 2405-2432.