Phosphates in Banded Iron Formations (bif). a Tool for Exploring Ancient

Introduction: Microbial metabolic activity is significant in the cycling of phosphorus in aquatic and sedimentary environments. The requirement of microorganisms to use elemental phosphorous for energy transfer and to bind, concentrate and cause crystalliza-tion of phosphates is well established [e.g. 1,2]. The crystallization of phosphates in a sedimentary environment commonly occurs when the proper physical conditions and biological activity coincides [2]. Phosphates and biogenic apatites are used in paleoclima-tological and environmental studies to obtain critical information on temperature of formation and environmental conditions under which the phosphate minerals were formed or deposited. Many of these studies have explored the possibility that trace elements and stable isotopes in sedimentary apatite may be used as a proxy for water chemistry. Martian sample return missions will focus upon both the search for life and reconstructing the history of water on the martian surface. Initial missions could center upon iron-rich lithologies on the martian surface. Phosphates could potentially provide a means of exploring the composition of the surface water and the phosphorus cycle on both the ancient Earth and Mars. Here, we report the results of our initial study of phosphates in the very low-grade metamorphic lithologies of the Gunflint BIF. Our focus is to understand the presence and distribution of phosphates in BIFs and to evaluate the possibility that their major and trace element characteristics reflects conditions of BIF deposition or early diagenesis. Gunflint BIF: The 1.9 billion year old Gunflint BIF is approximately 120 m thick and extends over more than 180 km from the north shore of Lake Superior westward along the Ontario-Minnesota boundary. Stratigraphically, this formation is part of the Animikie Group and consists mainly of clastic and chemical sedimentary rocks. Both the lowermost and the uppermost facies of the Gunflint consist of algal chert that is overlain by tuffaceous shale, which in turn is succeeded by a thick, granular taconite facies that grades into banded chert-carbonate layers. The Gun-flint is a virtually unmetamorphosed Precambrian iron-formation in which primary textures have been well preserved. It is estimated that the Gunflint experienced burial temperatures of 100 to 130ºC, [3]. However , areas of higher metamorphic grade are better represented by the pyroxene-hornfel facies [4].