THE INFLUENCE OF GLASSY COATINGS ON VOLCANIC ROCKS FROM MAUNA IKI, HAWAII AND APPLICATIONS TO ROCKS ON MARS

Introduction: Yellow and orange glassy coatings have been observed on volcanic rocks in the Mauna Iki region of Hawaii. We have characterized these as part of a larger study to understand rock coatings on Mars. The coatings on these Mauna Iki samples are comprised of two layers an SiO 2 layer and an Fe-Ti-rich pigmenting layer. Reflectance and emittance spectra have been analyzed and are discussed in relation to spectra of rocks on Mars. Both visible/NIR and midIR spectra exhibit properties due to SiO2 coatings. Samples: A lava flow several meters long protrudes from a dusty barren landscape off the Mauna Iki trail. The lava is primarily black vitreous glass and is covered on the exposed surfaces by a patchy ceramiclike coating (Fig. 1). SEM imaging of this coated rock revealed that the surface actually has a coating layer ~12-18 μm thick (Fig. 2). This ceramic-like coating may have formed during high-temperature volcanic activity following the deposition of the lava or from post-emplacement alteration. Related rock coatings are discussed in [1]. The bulk glass chemistry is basaltic (Table 1). Several chemical measurements were made as well using a JEOL 845 SEM at ASU of spots in the coating layers and near surface glass of the Mauna Iki samples. The data were averaged for 6 spots on internal portions of the glass and found to be fairly uniform (Table 1). Most of the surface coating is nearly pure SiO2 and is translucent. A very thin (~1-2 μm) portion of the outer surface contains high concentrations of Ti and Fe. It appears that the amounts of these two elements vary substantially across the surface, which is consistent with the colors observed. Another study of white glassy coatings on volcanic rocks in the Mauna Iki region has found elevated Ti, but no Fe, on the surface [2]. We attribute black surfaces to regions where Ti and Fe levels in the coating are very low or absent. Table 1 Approximate chemical composition of the ground bulk glass (by LECO), near-surface volcanic glass (ave & σ from 6 spots by SEM) and coating layer (ave & σ from 3 spots by SEM). Wt.% Bulk Glass Surface Glass Coating SiO2 51.8 44.9 (1.7) 60 (21) Al2O3 10.2 6.9 (0.5) 0 FeO 12.0 32.8 (1.7) 16 (13) MgO 8.0 1.6 (0.3) 0 CaO 8.6 12.7 (0.7) 1 (1) Na2O 1.6 0.1 (0.2) tr K2O 0.4 0.0 tr P2O5 0.3 0.0 0 TiO2 2.7 0.0 22 (12) MnO 0.2 0.7 (0.7) 0 S 0.2 0.2 (0.0) tr Cl nd 0.1 (0.1) tr The Fe and Ti in the coating varied substantially across the surface and Ca was found at only one spot. Small amounts (<1) of other elements were observed in some cases and are listed as tr.; nd indicates not determined.