Exhalative origins of iron formations

Abstract

Iron formations are stratigraphic units which are largely composed of iron-rich chemical sedimentary rock, here called ironstone. Most aspects of iron formations continue to be controversial and so one must read voluminous literature to appreciate either the range of iron-formation characteristics or the conflicting interpretations of those characteristics. Most protagonists in the ongoing debate may be classified into two groups, i.e. those who support a shallow weathering source for the iron (weathering of land or surficial seafloor sediment) and those who invoke deep weathering (hydration of new crust or late diagenesis of sediments) followed by exhalation of ferriferous fluids through the seafloor. The present review concludes that deep weathering has been the source of all iron formations. Cherty iron formations are attributed to hydration of new crust. Noncherty iron formations are attributed to exhalation of late-diagenetic fluids which have been driven through a continental margin by seismic pumping. Iron-formation controversies are reviewed herein through the development of flow charts which illustrate relationships among the many controversies. Preferred routes through these flow charts are suggested for both cherty and noncherty iron formations but the reader readily may select other routes. The mode of iron supply (deep or shallow weathering) is the most fundamental among many other issues, e.g., the mechanism for long-term maintenance of abundant dissolved iron within a large water body. The iron in any extensive iron formation which is consistently thicker than 10 m is attributed to a long-lasting suboxic mass of seawater which lacked H 2S. The paucity of H 2S either has been due to a paucity of all sulfur species or to an inhibition of sulfate reduction under reducing conditions, as in the modern Orca Basin under the Gulf of Mexico. Water in the Orca Basin contains up to 20 ppm Mn 2+ just below the oxic-suboxic interface and an average of 1.6 ppm Fe 2+ throughout the suboxic region. Cherty iron formations are attributed to low-temperature (<300°C) hydration of newly formed igneous crust by seawater. Peak production of cherty iron formations, e.g. during the beginning and end of the Proterozoic, is attributed to particularly rapid crustal accumulation in opening rifts, followed by abrupt failure of the rift and low-temperature hydration of the new crust. Rifts presumably opened, failed, and became sheared by transform faults more rapidly on a radioactively hotter young planet. Broad submarine transform fault zones are characterized by seismic pumping of seawater. Exhalative sedimentation of small cherty iron formations within rifts has continued into the Phanerozoic and a partial modern analog exists in the Red Sea. Noncherty iron formations are attributed to seismic pumping of seawater through an ophiolite-bearing sedimentary pile along a continental margin. Iron-dissolving fluids are hypothesized to have been hypersaline because of pumping through evaporites or cooling plutons within the sedimentary pile. The production rate of noncherty iron formations has not changed much through Earth history. A modern analog exists in the continental margin of Venezuela where the soft-sediment equivalent of ferrous-silicate ironstone is accumulating near Cabo Mala Pascua. Ferrous-silicate (berthierine) ironstone is accumulating where exhalations rise quickly and reach the shallow ocean before precipitating iron. More slowly rising coastal and many deep-water exhalations in Venezuela precipitate glauconite just below the sediment-water interface. If all iron formations have formed by exhalation, then manganese and phosphate deposits probably also are exhalative.