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The Genesis of Siderite in Archean and Paleoproterozoic Oceans

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The Genesis of Siderite in Archean and Paleoproterozoic Oceans Ninth Annual V. M. Goldschmidt Conference 7632.pdf THE GENESIS OF SIDERITE IN ARCHEAN AND PALEOPROTEROZOIC OCEANS. A. J. Kaufman, Department of Geology, University of Maryland, College Park, MD, 20742 ([email protected]) The genesis and the exodus of Archean and References: [1] Veizer, J. and Compston, W. (1976) Paleoproterozoic banded iron-formations (BIFs) is Geochim. Cosmochim. Acta., 40, 905-914. [2] generally considered to be the logical result of the Jacobsen, S.B. and Pimentel-Klose, M.R. (1988) Earth biological production of oxygen. In this view, the Planet. Sci. Lett., 87, 29-44. [3] Jacobsen, S.B. and origin of these enigmatic marine deposits is related to Kaufman, A.J. (1999) Chem. Geol. (in press). [4] the immobilization of soluble iron by fixation with O2 Bahrig, B. (1988) Palaeogeogr. Palaeocli. produced during oxygenic photosynthesis, and Palaeoecol., 70, 139-151. [5] Kaufman, A.J., Hayes, deposition of BIF ceased when the deep oceans J.M. and Klein, C. (1990) Geochim. Cosmochim. became pervasively oxidized around 1.8 Ga ago. The Acta, 54, 3461-3473. [6] Kaufman, A.J. (1996) Sr and Nd isotopic records indicate that Archean and Precam. Res., 79, 171-193. [7] Winter, B.L. and early Paleoproterozoic oceans were dominated by Knauth, L.P. (1992) Precam. Res., 59, 283-313. [8] hydrothermal fluids [1-3], and were therefore anoxic, Klein, C. and Beukes, N.J. (1989) Econ. Geol., 84, acidic, and rich in soluble ferrous iron; isolated 1733-1774. [9] Beukes, N.J., Klein, C., Kaufman, plumes of iron-poor, oxygenated water spread out A.J. and Hayes, J.M. (1990) Econ. Geol., 85, 663-690. from stromatolitic platforms. Eventually these plumes [10] Eugster, H.P. and Chou, I-M. (1973) Econ. spread, coalesced, and deepened. In their wake iron Geol., 68, 1144-1168. was swept from the oceans. However, rather than being driven by biological sources from above, the primary control of the oxidation state of the deep ocean appears to be exerted by tectonic forces controlling the flux of reduced ions from below. Based on modern studies in volcanic lakes [4], as well as petrographic and carbon isotopic analyses [5-7] of BIF siderite microbands, the primary control on the deposition iron-bearing BIF minerals appears to be related to changes in pH and iron availability across the transition from anoxic to oxygenated waters. The current model suggests an abiological process for the primary precipitation of siderite in the Archean and Paleoproterozoic oceans [cf. 8, 9]. Soluble ferrous iron as well as CO2-rich and sulfide-poor solutions were upwelled from deep hydrothermal sources to shallow shelves. At a chemocline these acidic solutions were neutralized by the dissolution of calcium carbonate raining down from the surface or swept off from shallow water subtidal and peritidal environments. Dissolution would result in a noticable increase in alkalinity; because siderite is 100-fold less soluble than calcite [10] and ferrous iron was readily available in the ascending water column, siderite would precipitate. The rain of siderite down from the chemocline would accumulate at the sediment-water interface. This iron-bearing mineral would have persisted to a depth where the acidity of ascending waters neutralized the less soluble carbonate. The Archean and Paleoproterozoic oceans may thus have been characterized by having both calcite and siderite compensation depths..
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