Chemical, multispectral, and textural constraints on the composition and origin of rocks at the Mars Pathfinder landing site

Rocks at the Mars Pathfinder site are probably locally derived. Textures on rock surfaces may indicate volcanic, sedimentary, or impact-generated rock s, but aeolian abration and dust coatings prevent unambiguous interpretatio n. Multispectral imaging has resolved four spectral classes of rocks: gray and red, which occur on different surfaces of the same rocks; pink, which i s probably soil crusts; and maroon, which occurs as large boulders, mostly in the far field. Rocks are assigned to two spectral trends based on the po sition of peak reflectance: the primary spectral trend contains gray, red, and pink rocks; maroon rocks constitute the secondary spectral trend. The s patial pattern of spectral variations observed is oriented along the prevai ling wind direction The primary spectral trend arises from thin ferric coat ings of aeolian dust on darker rocks. The secondary spectral trend is appar ently due to coating by a different mineral, probably maghemite or ferrihyd rite. A chronology based on rock spectra suggests that rounded maroon bould ers constitute the oldest petrologic unit (a flood deposit), succeeded by s maller cobbles possibly deposited by impact, and followed by aeolian erosio n and deposition. Nearly linear chemical trends in alpha proton X-ray spect rometer rock compositions are interpreted as mixing lines between rock and adhering dust, a conclusion supported by a correlation between sulfur abund ance and red/blue spectral ratio. Extrapolations of regression lines to zer o sulfur give the composition of a presumed igneous rock. The chemistry and normative mineralogy of the sulfur-free rock resemble common terrestrial v olcanic rocks, and its classification corresponds to andesite. Igneous rock s of this composition may occur with elastic sedimentary rocks or impact me lts and breccias. However, the spectral mottling expected on conglomerates or breccias is not observed in any APXS-analyzed rocks. Interpretation of t he rocks as andesites is complicated by absence of a "1 mu m" pyroxene abso rption band. Plausible explanations include impact glass, band masking by m agnetite, or presence of calcium- and iron-rich pyroxenes and olivine which push the absorption band minimum past the imager's spectral range. The inf erred andesitic composition is most similar to terrestrial anorogenic icela ndites, formed by fractionation of tholeiitic basaltic magmas. Early meltin g of a relatively primitive Martian mantle could produce an appropriate par ent magma, supporting the ancient age of Pathfinder rocks inferred from the ir incorporation in Hesperian flood deposits. Although rocks of andesitic c omposition at the Pathfinder site may represent samples of ancient Martian crust, inferences drawn about a necessary role for water or plate tectonics in their petrogenesis are probably unwarranted.