Surface alteration of basalt due to cation-migration

Basaltic lava from Kilauea, Hawaii may have a red-brown surface, indicative of Fe-(hydr)oxides. This surface is not found where exposed to weathering, but at the interface between lava lobes, or in the interior of lava channels. We use several analytical techniques to determine how these Fe-(hydr)oxide surfaces may have developed. WDS-elemental distribution line profiles from the lava surface towards the lava′s interior detect an Fe-rich film of less than 5 μm thickness. Heat treatment of quenched, fresh lava samples of the same chemical composition between 600–1,090°C helps to replicate temperatures under which such an Fe-rich film might have formed. These experiments suggest that Fe-enrichment occurs above 1,020°C, whereas at lower temperatures Ca is enriched relative to Fe. One sample was treated below the glass transition temperature, at 600°C for 164 h. A depth profile with secondary neutral mass spectrometry shows an enrichment of Mg at the outer 50 nm of the glass surface. The formation of films requires cation migration, which is driven by an oxygen chemical potential between air and the reduced basalt (Fe²⁺/Fe³⁺ ratio of 13.3). The change of surface alteration from Mg to Ca film at lower temperatures, to predominantly Fe at high temperatures, is determined by a change of cation availability, largely controlled by crystallization that already occurs below 850°C, and volume crystallization that occurs above 925°C.