Thermally altered palagonitic tephra: A spectral and process analog to the soil and dust of Mars

Abstract

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. The samples represent an alteration sequence caused by heating during emplacement of molten lava over a preexisting tephra cone. They are both spectral and weathering/alteration process analogs to the Martian surface. Techniques employed included visible and near‐IR spectroscopy, Mössbauer spectroscopy, and magnetic analysis. The two samples collected from beyond 90 cm from the slab (PH‐5 and PH‐6) did not see the transient heating event and are composed of coarse‐grained glassy basaltic (hawaiitic) particles, some of which have been moderately palagonitized since formation of the cone in the late Pleistocene. The four samples closest to the slab (PH‐1 through PH‐4) have been strongly altered in response to heating during its emplacement; their iron oxide mineralogy is dominated by nanophase ferric oxide. The sample adjacent to the slab (PH‐1), which would have seen the highest temperatures, has a factor of 3 less H2O and contains crystalline hematite and magnetite in 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) from the slab. The reflectivity spectra of samples PH‐2 through PH‐4 are similar to spectra of palagonites reported by other workers to be good 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 ferric oxides (similar to several new Martian telescopic data sets). Our sampling site may also be a process analog for Mars, in that heating episodes by volcanism and/or impact cratering could produce crystalline ferric oxides from poorly crystalline palagonitic material. Thus, localized thermal alteration events may provide a volumetrically important mechanism for the palagonitization of basaltic glass and the production of crystalline ferric oxides on Mars.