Mineralogy of three slightly palagonitized basaltic tephra samples from the summit of Mauna Kea, Hawaii

Certain palagonites from Hawaii are considered to be among the best analogs for Martian fines, based upon similar spectral properties. For this study, three distinctly colored layers were sampled from slightly palagonitized basaltic tephra just below the summit of Mauna Kea at 4145 m elevation. The mineralogy of size fractions of these samples was examined by diffuse reflectance (visible and near-IR) and far-IR spectroscopy, optical microscopy, X ray diffraction, Mossbauer spectroscopy, magnetic analysis, electron microprobe analysis (EMPA), and transmission and scanning electron microscopy. For the 20–1000 μm size fraction, sample HWMK11 (red) is essentially completely oxidized and has a hematite (Ti-hematite) pigment dispersed throughout the silicate matrix. The alteration is present throughout particle volumes, and only a trace amount of glass is present; no palagonitic rinds were detected. In addition to ferric Fe-Ti oxides, other phases detected were plagioclase feldspar and a trace of olivine. Sample HWMK12 (black) has the lowest proportion of ferric-bearing phases and is thus least weathered. It consists mostly of unaltered glass with embedded plagioclase and minor amounts of pyroxene, olivine, and Ti-magnetite. In some grains, a thin palagonitic rind is visible, indicating some surface alteration. The mineralogy for sample HWMK13 (yellow) is the same as that for HWMK12, except that it has distinct, well-developed palagonitic rinds consisting of erionite and smectite. For all samples, the amount of glass and plagioclase decreases and the amount of smectite increases with decreasing particle size for size fractions <20 μm. For HWMK11, the amount of hematite is essentially constant, and mica is present only in the coarse clay-sized fraction; smectites are low in structural Fe. For HWMK12 and HWMK13, the zeolite erionite is present along with smectites and nanophase ferric oxides (np-Ox). Erionite abundance decreases and np-Ox abundance increases with decreasing particle size. The smectite in both black and yellow samples contains some Fe3+ in octahedral layers. There were only two mineral phases containing iron in the fine clay fraction, namely, smectites and iron oxides. For HWMK11, relatively large iron oxide particles (0.1 to 0.4 μm) were dispersed on clay surfaces; for HWMK12 and HWMK13, much finer np-Ox particles were present in lesser concentrations. Formation of the zeolite erionite is consistent with the arid climate zone where these samples were collected. However, transient hydrothermal processes that occurred during the eruption of Mauna Kea volcano under its permanent ice cap during the Pleistocene may have resulted in minerals such as zeolites and smectites which may persist as relicts over a long period of time. Most of the iron released during weathering of basaltic tephra precipitated as poorly crystalline iron oxides and some of the Fe has substituted for the octahedral cations in the structure of authigenic smectites. The Ti-hematite in HWMK11, however, is the result of high-temperature oxidation of Ti-magnetite and exsolution from ironbearing silicate phases. Visible and near-IR reflectivity spectra for the <20 μm size fraction of HWMK11 is dominated by well-crystalline Ti-hematite. Corresponding spectra for HWMK12 and HWMK13, whose ferric mineralogy is dominated by np-Ox particles, are more similar to Martian bright-region spectra.