DON JUAN BASIN SEDIMENTS: SURFACE AND SUBSURFACE CHEMISTRY AND MINERALOGY

Don Juan Basin is located about 9 km West of Lake Vanda, Wright Valley, Antarctica. The basin floor is 117m above Mean Seawater Level with steep slopes to the north and south rising above 1000 m [1]. It contains an unprecedented amount of CaCl₂ salt [2]. Dickson et al. (2013) [3] consider the basin to be a model environment for calcium and chlorine weathering and distribution on Mars.

Eight soil cores and twenty-two surface samples were collected from Don Juan Basin [4]. Cores WV42, DJ21 and WV38 are from evaporite ponds successively east, distant, and at increasingly higher elevation from Don Juan Pond. Cores DJ39 and DJ33 were collected ~300 m and ~150 m southwest from the contemporary shoreline, while core DJ2074 was collected from the center of the pond. Reflectance spectra, XRD data, elemental abundances from INAA, and salt chemistry were applied to evaluate the degree of alteration and the aqueous processes most likely responsible for the alteration.

Don Juan Basin soluble salt concentrations show significant location and depth variability [5]. Reflectance spectra of samples from core DJ39 show peaks characteristic of OH and H₂O in the sulfate gypsum and aluminosilicate allophane [5]. XRD results confirm gypsum abundances between 0.3 and 1.9 % over a depth range of 16 cm, and the sulfate thernardite at ~2% for the top 8 cm. Halite is found at 2-5 cm depth at > 3%, in line with element abundance results. The mineral composition of core DJ42 is complex including high alteration product abundance within the upper 10 cm. High abundances of gypsum are found in almost all samples. Other cores show fewer alteration products and less or no gypsum. DJ42 also shows comparatively high thorium (12 ppm) and associated REE element abundances at a depth of 4-7 cm.

The variability of analytical results across the relatively small and confined area of Don Juan Basin suggests that both surface and groundwater alteration and movement of sediment constituents have operated long term, strongly influenced by local geochemical conditions.

[1] Harris H.J.H., 1981, Ph.D. Dissertation, Univ. of Illinois [2] Torii T. & Yamagata J., 1981, Dry Valley Drilling Project 33, 141-157. [3] Dickson J. L. et al., 2013, Sci. Rep. 3: 1166. [4] Gibson E. K. et al., 1983, Journal of Geophysical Research, 88, A912-A928 [5] P. Englert et al., GSA Annual Meeting 2013, abs. T65, 11-4.