Spectral and Geochemical Characteristics of Lake Superior Type Banded Iron Formation: Analog to the Martian Hematite Outcrops

Introduction: The Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor discovered grey, crystalline hematite based on its fundamental oxide vibrational features at 300, 450, and 525cm at Sinus Meridiani (SM), Aram Chaos (AC), and various small outcrops within the Valles Marineris (VM) [1]. One of the proposed formation mechanisms for the grey, crystalline hematite is through precipitation of ferric oxides in iron rich waters as in terrestrial banded iron formation (BIF) [1]. Subsequent burial metamorphism would crystallize the deposit. Archean earth banded iron formation was first suggested as a possible terrestrial analog for early Mars based on the hypothesis that dark regions on Mars could be chemically altered, containing ferrous clay alteration minerals similar to those in BIF [2]. A recent study of the Mariner 6 and 7 spectra shows increased hydration signatures in the SM and AC hematite regions further supporting an aqueous formation mechanism [3]. Due to the lack of a volcanic source and subsequent volcanic features surrounding the Martian hematite regions, and the increased hydration signatures, a sedimentary, aqueous depositional environment is the preferred interpretation for this research, and thus terrestrial BIF is chosen as an appropriate analog. Terrestrial BIF: James (1954) [4] has defined iron formation as being a chemical sedimentary rock, typically thin banded and/or finely laminated, containing at least 15% iron of sedimentary origin and commonly but not necessarily containing layers of chert. Iron formation consists of four facies: sulfide, carbonate, silicate, and oxide. Their occurrence is controlled by the Eh/pH levels within the depositional environment. A reducing environment precipitates iron sulfide (pyrite), intermediate conditions precipitate iron carbonate (siderite) and iron silicates such as chamosite and stilpnomelane, and an oxidizing environment precipitates iron oxide (hematite). In the Lake Superior BIF, the iron oxide magnetite accompanies the carbonate, silicate, and oxide facies as it is a combination of ferrous (Fe) and ferric (Fe) iron and precipitates in intermediate to oxidizing conditions. Lake Superior Type BIF differs from other types of BIF due to its sedimentary (rather than volcanic) origin [5]. Samples from the oxide, carbonate, and silicate facies have been collected from Lake Superior Type BIF in northern Michigan from the Marquette and Gogebic iron districts. They have been measured for reflectance in the VNIR/SWIR (0.4 to 2.5 mm) and emittance in the TIR (5 to 50 mm). Geochemical analyses including thin section analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were also performed on the samples to attain more detailed compositional information and to corroborate mineralogy identified with spectroscopy. The minerals composing the separate BIF facies are possible auxiliary minerals to the hematite deposits on Mars. Data collected in this study can be directly compared to data gathered by the Mars Exploration Rover Opportunity (MER-B) at the Meridiani Planum landing site to determine whether or not a banded iron formation like process has occurred. This is significant because it requires the stability of a large body of water for an extended period of time at some point in Mars’ history. Oxide facies. The oxide samples occur in two distinct forms of grey, crystalline hematite along with chert and minor magnetite. The hematite exists as distinct, thin bands alternating with chert bands in typical BIF structure (figure 1), and in a more metamorphosed state as bulk, crystalline hematite with a schistose texture. The schistose samples are also associated with accessory metamorphic minerals.