NONTRONITE OCCURRENCES IN COLUMBIA RIVER BASALTS: IMPLICATIONS FOR MARS

The ferric smectite nontronite occurs at a number of localities in the Columbia River Basalts. Nontronite of high-Fe composition frequently fills or coats vesicles, cracks, and other void spaces. This clay is generally considered to have formed by early weathering of the basalt host. In thin section it may be observed penetrating the walls of nontronite-filled vesicles, and replacing ferromagnesian phases such as olivine. Over time this dissolution expands the volume of the filled void spaces, and the nontronite infill is observed coating relict undissolved phases including feldspar, apatite, and titanomagnetite. At later stages of rock weathering, dissolution of these relict phases is accompanied by precipitation of aluminous clays including montmorillonite and kaolinite, Fe is present in oxides, and nontronite is no longer stable. Under the warm, wet conditions of the mid-Miocene Climatic Optimum, this weathering sequence led to formation of lateritic soils that are now preserved as paleosols between basalt flows.

Some unusually aluminum-rich nontronites, such as the CMS source clay SWa-1 ferruginous smectite of Grant County, are found at contacts where basalt flows capped these paleosols. Bulk chemistry and field relations indicate that these aluminous nontronites formed not directly by basalt weathering, but rather via hydrothermal alteration of the aluminous paleosol clays and the overlying basalt. This suggests that the chemical composition of a nontronite provides an indicator as to its formation environment and the overall degree of weathering experienced by the parent rock.

Occurrences of Fe-Mg smectites have been observed in ancient (>3.5 Ga) martian rocks. The presence of these clays shows conclusively that water-rock interaction occurred in the ancient martian past. Some Fe-Mg smectites are found as isolated strata, whereas others are observed in distinctive stratigraphic relationships with more aluminous clays or with other secondary weathering products. The weathering of Columbia River Basalts provides new possible interpretations for some of the observed stratigraphic sequences.