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Published March 28, 2019 | Supplemental Material + Published
Journal Article Open

Mid-Proterozoic Ferruginous Conditions Reflect Postdepositional Processes


To evaluate the mechanics of mid‐Proterozoic environmental iron transport and deposition, we coupled microscale textural and bulk rock magnetic techniques to study the ~1.4‐Ga lower Belt group, Belt Supergroup, Montana and Idaho. We identified a pyrrhotite‐siderite isograd that marks metamorphic iron‐bearing mineral reactions beginning in subgreenschist facies samples. Even in the best‐preserved parts of the basin, secondary overprints were common including recrystallization of iron‐bearing sulfides, base metal sulfides, and nanophase pyrrhotite. Despite these overprints, a record of redox chemistry was preserved in the early diagenetic framboidal pyrite and detrital iron oxides including trace nanoscale magnetite that remained after sulfidization in anoxic and sulfidic sedimentary pore fluids. Based on these results, we interpret the Belt Basin as having oxic waters, at least in shallow‐water environments, with no indication of abundant ferrous iron in the water column; this is consistent with the cooccurrence of early eukaryotic fossils within the same strata.

Additional Information

© 2019 American Geophysical Union. Received 29 NOV 2018; Accepted 31 JAN 2019; Accepted article online 5 FEB 2019; Published online 18 MAR 2019. We thank D. Winston, G. Zieg, M. Jackson, C. Roach, T. Lyons, A. Chadwick, and 2013 Agouron Field Course members for assistance during sample collection and analysis. We thank J. Grotzinger, A. Sessions, and N. Swanson‐Hysell for insightful comments on earlier drafts of this manuscript as well as three anonymous reviewers. Data supporting the conclusions of this paper can be found in the supporting information. Samples from Glacier National Park were collected under NPS Collection Permits GLAC‐2012‐SCI‐0195 and GLAC‐2014‐SCI‐0008. Support for this work was provided by the Agouron Institute, the Tobacco Root Geological Society, the Belt Association, a GSA Student Research Grant, NASA Exobiology, the Packard Foundation, NSF Graduate Research Fellowship, NASA Earth and Space Science Fellowship, and a PEO Scholar Award. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, a DOE Office of Science User Facility, and as a U.S. Visiting Student Fellow at the Institute for Rock Magnetism at the University of Minnesota, a National Multi‐user Facility supported by NSF and the University of Minnesota.

Attached Files

Published - Slotznick_et_al-2019-Geophysical_Research_Letters.pdf

Supplemental Material - grl58615-sup-0001-2018gl081496-s01.pdf


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