Exploring for Ancient Hydrothermal Systems on Mars: A Case Study from
Southern Iceland
[N.H. Warner] (School of Earth and Space Exploration, Arizona State University,
P.O. Box 871404, Tempe, AZ 85276-1404; nicholas.warner@asu.edu); J.D. Farmer
(School of Earth and Space Exploration, Arizona State University, P.O. Box
871404, Tempe, AZ 85276-1404; jack.farmer@asu.edu)
The identification of the hydrothermal alteration products on Mars is of
particular importance because of implications for past surface or sub-surface
water, habitabile environments and life (Farmer, 2000). In this study, we
investigate the hydrothermal alteration materials deposited on sandur plains of
southern Iceland during subglacial catastrophic outfloods. Such deposits may be
process analogs for proposed Martian catastrophic outflood systems, whose
origins may be tied to geothermal melting of the Martian cryosphere (Carr,
1987). Several active basaltic volcanic centers are located beneath the ice
sheets in southern Iceland. Geothermal heating and melting of the ice has
produced several sub-glacial lakes that catastrophically drain along the ice
cap margin due to hydraulic over-pressure and ice dam breakage (Bjornsson and
Kristmannsdottir, 1984; Steinthorsson et al. 2000). In 1996, an eruption at the
Gjalp fissure produced a high discharge, sediment-laden catastrophic outflood
(jokulhlaup) that inundated Skeidararsandur, an ice marginal outwash plain
located south of the Vatnajokull ice sheet. Approximately 100 km west of
Skeidararsandur, the Myrdalsandur outwash plain was the site of a similar
magnitude catastrophic outflood following the 1918 eruption of Katla volcano
beneath Myrdalsjokull .
For the present study, we collected outflood sediments from surface and outcrop
localities at Myrdalsandur and Skeidararsandur. The 1996 Skeidararsandur
outwash surface is lithologically diverse. The > 2 mm size fraction is
dominated by moderately vesicular basaltic and intermediate volcanic clasts.
Vesicles are commonly infilled with secondary silica, zeolites, or clay
minerals. Approximately 2-5% of the > 2 mm fraction is comprised of
yellowish-tan palagonite clasts. Cobble-sized clasts of finely bedded
cryptocrystalline silica and coarsely crystalline (vein) calcite have also been
identified. The < 2 mm size matrix material contains basaltic glass, altered
palagonite glass, plagioclase (albite), fine grained silica, and calcite. The
1918 Myrdalsandur deposits exhibit better sorting and greater compositional
homogeneity. Approximately 97% of the surface is composed of gravel, to
granule-size basaltic scoria. Yellowish-tan palagonite and dense basaltic
clasts make up 2 – 5% of the gravel and granule-size surface materials. The
finer matrix (< 2 mm) contains abundant basaltic glass and minor amounts of
palagonitized glass and plagioclase.
Short wave infrared (1.6 – 2.4 microns) reflectance data from the Advanced
Spaceborne Thermal Emission and Reflection Radiometer (ASTER) show significant
absorption features at 2.2 and 2.3 microns for the surface of both sandur
plains. The spectral shape and absorptions are consistent with hydrated
aluminum silicates, including saponite, kaolinite, illite, montmorillonite,
natrolite, mesolite, and scolecite. Spectra suggestive of albite, pyroxene,
and goethite were also identified on the sandur surfaces. Calcite was
not identified remotely. Powder x-ray diffraction (XRD) analysis of homogenized
surface materials from both sandurs confirmed the presence of zeolite minerals
and clays (smectite, illite, and kaolinite) within the palagonite clasts.
Albite, pyroxene, and quartz were also identified in the basaltic scoria and
palagonite clasts. Oriented powder x-ray diffractograms (< 2 micron
fraction) for palagonite clasts revealed randomly ordered smectite (R < 1),
mixed layer smectite/illite with 30 – 40% illite concentration, zeolites,
quartz, and albite. The hydrated aluminum silicates identified in the remote
analysis likely correspond with the clay and zeolite minerals identified in the
palagonite clasts by thin section petrography and XRD. The mineral associations
present in the palagonite clasts suggest alteration at temperatures near 100 C.
Terrestrial analog studies that use remote, field, and laboratory techniques
provide a framework for future work at Mars. The sandur plains of southern
Iceland are ideal analogs for Martian fluvial systems whose origins may be tied
to geothermal activity (Carr, 1987). Remote identification of hydrothermal
mineral assemblages within the Martian outflood channels may provide important
information regarding the hydrologic, thermal, and (potentially) biologic
evolution of the planet.
References:
Bjornsson, H., H. Kristmannsdottir, 1984, The Grimsvotn geothermal area,
Vatnajokull, Iceland, Jokull, v. 34, p. 25 – 50.
Carr, M.H., 1987, Water on Mars, Nature (London), v. 326, p. 30 – 35
Farmer, J.D. 2000, Hydrothermal systems: Doorways to early biosphere evolution.
GSA Today, v. 10, p. 1-9.
Steinthorsson, S., B.S. Hararson, R.M. Ellam, G. Larsen, 2000, Petrochemistry
of the Gjalp-1996 eruption, Vatnajokull, SE Iceland, Journal of Volcanology and
Geothermal Research, v. 98, p. 79 – 90.
POSTER
CORRESPONDING AUTHOR: N. H. Warner