Phyllosilicate-poor palagonitic dust from Mauna Kea Volcano (Hawaii): A mineralogical analogue for magnetic Martian dust?

The mineralogical and elemental composition of dust size fractions (<2 and <5 μm) of eight samples of phyllosilicate-poor palagonitic tephra from the upper slopes of Mauna Kea Volcano (Hawaii) were studied by X-ray diffraction (XRD), X-ray fluorescence (XRP), visible and near-IR reflectance spectroscopy, Mössbauer spectroscopy, magnetic properties methods, and transmission electron microscopy (TEM). The palagonitic dust samples are spectral analogues of Martian bright regions at visible and near-IR wavelengths. The crystalline phases in the palagonitic dust are, in variable proportions, plagioclase feldspar, Ti-containing magnetite, minor pyroxene, and trace hematite. No basal reflections resulting from crystalline phyllosilicates were detected in XRD data. Weak, broad XRD peaks corresponding to X-ray amorphous phases (allophane, nanophase ferric oxide (possibly ferrihydrite), and, for two samples, hisingerite) were detected as oxidative alteration products of the glass; residual unaltered glass was also present. Mössbauer spectroscopy showed that the iron-bearing phases are nanophase ferric oxide, magnetite/titanomagnetite, hematite, and minor glass and ferrous silicates. Direct observation by TEM showed that the crystalline and X-ray amorphous phases observed by XRD and Mössbauer are normally present together in composite particles and not normally present as discrete single-phase particles. Ti-bearing magnetite occurs predominantly as 5-150 nm particles embedded in noncrystalline matrix material and most likely formed by crystallization from silicate liquids under conditions of rapid cooling during eruption and deposition of glassy tephra and prior to palagonitization of glass. Rare spheroidal halloysite was observed in the two samples that also had XRD evidence for hisingerite. The saturation magnetization J_s and low-field magnetic susceptibility for bulk dust range from 0.19 to 0.68 Am²/kg and 3.4x10^-6 to 15.5x10^-6 m³/kg at 293 K, respectively. Simulation of the Mars Pathfinder Magnet Array (MA) experiment was performed on Mauna Kea Volcano in areas with phyllosilicate-poor palagonitic dust and with copies of the Pathfinder MA. On the basis of the magnetic properties of dust collected by all five MA magnets and the observation that the Pathfinder MAs collected dust on the four strongest magnets, the value for the saturation magnetization of Martian dust collected in the MA experiments is revised downward from 4±2 Am²/kg to 2.5±1.5 Am²/kg. The revised value corresponds to 2.7±1.6 wt % magnetite if the magnetic mineral is magnetite (using J_s=92 Am²/kg for pure magnetite, Fe₃O₄) or to 5.0±3.0 to 3.4±2.0 wt % maghemite if the magnetic mineral is pure maghemite (using J_s= 50 to 74 Am²/kg for pure maghemite, γ-Fe₂O₃). Comparison of the magnetic properties of bulk Mauna Kea palagonitic dust to those for dust collected by MA magnets shows that the MA magnets extracted (culled) a subset (25-34 wt %) of composite magnetic particles from bulk dust. The extent of culling of Martian dust is not well constrained. Because the Mauna Kea palagonitic dust satisfies the essential constraints of the Pathfinder magnetic properties experiment (composite and magnetic particles capable of being collected by five MA magnets), a working hypothesis for the strongly magnetic mineral present in Martian dust and soil is magnetite (possibly Ti-bearing) formed by rapid crystallization from silicate liquids having volcanic and/or impact origins. Subsequent palagonitization of the glass produces the nanophase ferric oxide phases that dominate the spectral properties of Martian bright regions at visible and near-IR wavelengths. Magnetic and phyllosilicate-poor palagonitic dust from Mauna Kea Volcano is thus a spectral and magnetic analogue for magnetic Martian dust.