THERMALLY ALTERED PALAGONITIC TEPHRA - A SPECTRAL AND PROCESS ANALOG TO THE SOIL AND DUST OF MARS

We studied six palagonitic soil samples (PH-1 through PH-6) which were collected at 30-cm intervals from a lava slab on Mauna Kea, Hawaii. T he samples represent an alteration sequence caused by heating during e mplacement of molten lava over a preexisting tephra cone. They are bot h spectral and weathering/alteration process analogs to the Martian su rface. Techniques employed included visible and near-IR spectroscopy, Mossbauer spectroscopy, and magnetic analysis. The two samples collect ed from beyond 90 cm from the slab (PH-5 and PH-6) did not see the tra nsient heating event and are composed of coarse-grained glassy basalti c (hawaiitic) particles, some of which have been moderately palagoniti zed since formation of the cone in the late Pleistocene. The four samp les closest to the slab (PH-1 through PH-4) have been strongly altered in response to heating during its emplacement; their iron oxide miner alogy is dominated by nanophase ferric oxide. The sample adjacent to t he slab (PH-1), which would have seen the highest temperatures, has a factor of 3 less H2O and contains crystalline hematite and magnetite i n addition to nanophase ferric oxide. Lesser amounts of magnetite, but not hematite, are present in samples 30 cm (PH-2) and 60 cm (PH-3) fr om the slab. The reflectivity spectra of samples PH-2 through PH-4 are similar to spectra of palagonites reported by other workers to be goo d Mars visible to near-IR spectral analogs. The reflectivity spectrum of PH-1 is an even better spectral analog to Mars in that it exhibits absorption features indicative of both nanophase and crystalline ferri c oxides (similar to several new Martian telescopic data sets). Our sa mpling site may also be a process analog for Mars, in that heating epi sodes by volcanism and/or impact cratering could produce crystalline f erric oxides from poorly crystalline palagonitic material. Thus, local ized thermal alteration events may provide a volumetrically important mechanism for the palagonitization of basaltic glass and the productio n of crystalline ferric oxides on Mars.