Mineral Spectroscopy: A Tribute to Roger G. Burns © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D. Dyar, C. McCammon and M. W. Schaefer Are there abiotically-precipitated iron-formations on Mars? M. W. SCHAEFER Yale University, Department of Geology and Geophysics, P. O. Box 208109, New Haven, CT 06520-8109, U.S.A. Abstract- In early times, surface conditions on Mars and Earth were probably quite similar, however, the two planets took different paths in their evolution. Banded iron-formations (BIFs) were prevalent on Earth in the Archean, and the likelihood of their formation on Mars at the same time is investigated. There are several abiotic mechanisms for producing banded iron-formations on Earth. These are: (1) oxidation of ferrous iron by free oxygen, (2) photooxidation of ferrous iron, (3) oxidation of ferrous iron by hydrogen peroxide, and (4) anoxic precipitation of ferrous iron by changes in pH. Any of these mechanisms could have been active on Mars in its distant past, to produce BIFs there as well, which should be observable today. The minerals one would expect to observe on the surface of a BIF on Mars are indeed thought to exist on Mars, but unfortunately BIFs are not unique in containing these minerals, and confirmation of the existence of BIFs on Mars will have to wait for more data. Proof of the existence of BIFs on Mars would give us valuable information about the early history of the planet Mars as well as Earth. It would act to confirm models of Mars that include a warm, wet period, and the existence of bodies of standing water. INTRODUCTION It is likely that Earth and Mars were not dissimi- lar when they were first formed. By studying pro- IN THE SURFACEof Mars we see a relict of the early evolution of the planet. During the early history cesses that occurred on the early Earth, we may of the solar system, conditions were probably not gain insight into what processes might have been dissimilar on the surface of Earth and Mars, but as important on early Mars. And conversely, as we time passed, the two planets diverged. Earth, larger obtain more detailed information about the present and with a greater source of internal heating, stayed surface of Mars, we will learn more about the early active and its surface kept evolving. Mars, on the history of both planets. BURNS (1993) originally other hand, smaller and with less internal heating, suggested that banded iron-formations (BIFs) froze and its surface almost completely stopped could have occurred on Mars. The present paper evolving. Therefore, when we look at the surface discusses the likelihood of the occurrence of BIFs of Mars, we have the opportunity not only to ob- on Mars, and what their existence might imply serve the early history of Mars itself, but also to about the early environment of Mars. observe a landscape perhaps more similar to that of TERRESTRIAL BANDED IRON-FORMATIONS early Earth than is the present terrestrial landscape. The surface of Mars seems to have changed rela- Some of the oldest surviving sedimentary fea- tively little over the past several billion years. Pro- tures on Earth are the banded iron-formations, cesses that have acted on its surface in the past chemically-precipitated sedimentary deposits con- include meteorite bombardment, which was at its taining layers of silica-rich and iron-rich bands. peak over 3.5 Gy ago; volcanism, which is thought GROSS (1980) classifies banded iron-formations to have largely ceased by about 1 Gy ago; and into two groups, based on associated types of rock, fluvial and glacial processes, about which there is lithological features, and interpretation of their active speculation (BAKER et al., 1991; BAKER et depositional environment. These two groups are al., 1992; KARGELet al., 1995), but which neither Algoma-type and Lake-Superior-type. Both of appear to have been active within the last 1 Gy these types exhibit cherty layers interbedded with (although periglacial processes may be active today iron in four facies: oxides, carbonates, silicates, (ROSSBACHERand JUDSON, 1981). The role of hy- and sulfides (JAMES, 1954). In addition to the drothermal processes is as yet unknown, however banded iron-formations, there is another type of they could be very important, both to the chemical iron-formation called an oolitic ironstone. Oolitic evolution of the regolith and to the isotopic evolu- ironstones are higher in aluminum than BIFs, and tion of the atmosphere (JAKOSKYand JONES, 1996). have a more clastic association. They are found in Physical and chemical weathering, which is still rocks of a variety of ages from Proterozoic to Plio- active today, acts only to alter the characteristics cene (MAYNARD, 1983). of the uppermost surface layers (GOODING, 1978; Although BIFs are agreed to be chemical precipi- GOODING et al., 1992). tates of iron, the mechanism by which the precipita- 381 382 M. W. Schaefer tion occurs is a controversial topic. Both biotic iron in solution. (4) Anoxic precipitation offerrous (CLOUD, 1973; GARRELS et al., 1973; LABERGE, iron. Under anoxic conditions, it is possible to pre- 1973; LABERGE et al., 1987; ROBBINSet al., 1987) cipitate ferrous iron carbonates and silicates under and abiotic (BRATERMANand CAIRNS-SMITH,1987; the right conditions of pH and Pco" These mecha- CAIRNS-SMITH, 1978; EUGSTER and CHOU, 1973; nisms are shown schematically in Fig. 1 and will FRAN<;:OIS,1986; HOLLAND, 1973) mechanisms be discussed in detail below. have been proposed. The question of whether banded iron-formations The major idea behind both of these classes of on Earth formed primarily by a biotic or an abiotic theories is that the atmosphere of the early Earth mechanism is still an open question. Another open question is the likelihood of life ever having existed was depleted in O2, compared to the present atmo- spheric level (PAL). Under conditions of low oxy- on Mars. The main thrust of the present paper is gen fugacity, especially in an acidic environment, the discussion of abiotic mechanisms of making iron exists in the ferrous form, and is,highly soluble BIFs. Such mechanisms require fewer assump- in water. Thus any iron input into a lacustrine or tions, when translated to Martian conditions, than oceanic system by weathering or other process will do the biotic models. However, if life did exist on stay in solution in the water until it reaches satura- Mars and was instrumental in causing iron precipi- tion with respect some iron silicate or carbonate, tation in bodies of water there, then the iron forma- or is oxidized to form ferric iron, which is relatively tions developed would be a likely place to look for insoluble in water. Then, for any of a variety of fossil evidence of such life. reasons, depending on the model, the iron is oxi- dized, and precipitates out, forming the extensive Dissolved oxygen deposits that still exist today. There is geological evidence that free oxygen In the classic biotic models the factor that causes was present in the early atmosphere (even as early this precipitation of iron is the increase in atmo- as 3.8 Gy ago), although not in quantities ap- spheric O caused by the development of photosyn- 2 proaching PAL (ToWE, 1983). HOLLAND (1973) thetic organisms. However, it is not necessary to demonstrated that neither subaerial weathering or invoke the development of life to achieve oxidation volcanic emanations were likely to have been the of iron in Precambrian waters. Also, there is evi- immediate sources of soluble iron to form the Ham- dence that very little oxygen was present in the ersley Basin BIF of Australia. He then proposed atmosphere during the formation of many of the that upwelling of bottom waters in the ocean could deposits. The presence of detrital siderite in Lake- Superior-type deposits in Labrador and the occur- rence of detrital grains of uraninite and pebbles of pyrite in the 2.5-Gy-old Witwatersrand conglomer- UVradiation ate indicate a vanishingly low P02 content in the atmosphere at that time (GARRELS et al., 1973). III However, biotic models explain this by postulating procaryotic microorganisms that reduced CO2 to surface runoff and CH20 in the hydrosphere by taking electrons from ferrous iron (CLOUD, 1972, 1973). ABIOTIC MECHANISMS OF MAKING IRON-FORMATIONS There are four abiotic processes that are capable of forming BIFs. (1) Oxidation of ferrous iron by dissolved oxygen. Although this is also descriptive of what happens when photosynthesis produces ox- FIG. 1. Schematic diagram of the four abiotic mecha- ygen, there are abiotic reactions that may produce nisms of BIF formation. Ferrous iron may be oxidized to free oxygen. (2) Photooxidation of ferrous iron by ferric iron in the mixing layer by means of dissolved ultraviolet irradiation. Ultraviolet (UV) radiation oxygen or hydrogen peroxide, or nearer the surface by UV radiation. Anoxic water (not necessarily bottom water, as is capable of directly oxidizing ferrous iron in solu- in the figure, if the oxygen content of the atmosphere is tion. (3) Oxidation of ferrous iron by hydrogen low) may precipitate ferrous iron if the pH is very high peroxide. Hydrogen peroxide (H202) is a strong (caused by evaporation) or very low (caused by an input oxidant, and is also able to directly oxidize ferrous of acidic ground or surface water). Iron-formations on Mars? 383 have provided the necessary quantities of dissolved thought (CANUTOet al., 1983). Although the Sun's iron to produce this deposit, and generalized this total luminosity is thought to have been lower in mechanism to explain other Lake Superior-type de- the past, the UV flux was likely much higher (Ta- posits where there is little clastic debris (HOLLAND, ble 1).