Overview of Iron Oxide Concretions and Implications for Mars: Current Knowledge and Gaps

Introduction: Terrestrial studies of concretions have been strengthened from the NASA programs, with the study of “blueberries” in the Burns formation discovered by the Mars Exploration Rover (MER) Opportunity. Scientific interest on iron oxide concretions has increased from a point just a decade or two ago, when concretions were viewed simply as geologic “curiosities”. The purpose of this paper is to review what we currently know about concretions and the implications for Mars, and to explore the current gaps of what we still have left to learn. Mars is likely unique in its own particular setting of the sulfate and basaltic sandstone and extreme chemical solutions (by Earth standards). However, it is clear that terrestrial analog studies are pivotal to understanding basic geologic processes and relationships that have application to deduce fluid flow events and timing on Mars. These studies can provide input for evaluation of small diagenetic features in payload instrumentation on future Mars missions. Methods: There are a number of different approaches to studying concretions that should be used in concert to fully understand complex diagenetic processes. These approaches include: Field characterizations • Host rock (texture, fabric, composition/ mineralogy, porosity, permeability, role of clays) [e.g. 1-3] • Concretions (external morphology geometries, size, shapes, in situ self-organized spacing) [4] • Internal structure (solid, rinds, multiple layers) [4] • Ancient examples (Jurassic Navajo Sandstone with a lot of variety + others) [e.g. 5, 6] • Modern examples (Western Australia) [7] Mineralogy and Geochemistry • Visible near infrared (VNIR) reflectance spectroscopy [3, 8] • Thermal infrared spectroscopy (TIR) • Whole rock analysis [4] • Trace element geochemistry • XRD • Petrographic thin section • Iron, oxygen isotopes [9, 10] • QEMSCANquantitative evaluation via electron microscopy for minerals & element phases [8] • Tomography Modeling • Laboratory bench chemical experiments [11, 12] • Diffusion rates [13] • Numerical modeling and nucleation [11] Discussion: Characterization of both Mars “blueberries” [14] and terrestrial analog examples [e.g. 5-7, 15] provide a strong basis for interpreting broad Earth and Mars conditions. The common occurrence of terrestrial concretions in a wide range of mineralogies (from carbonates to iron oxides and iron sulfides) suggest that concretion formation is a common geologic process in near surface, porous sediments and sedimentary rocks, and thus is is not surprising that these were discovered in sedimentary deposits of Mars. While there were several proposed ideas for explaining hematite on Mars prior to the MER, only one group [5, 16] correctly predicted concretionary iron oxides to occur at Meridiani Planum. Ancient terrestrial examples provide a wide range of distributions, geometries and sizes to help us understand the variability of concretions and what might affect their growth and development. Field relationships indicate that there are two main types of mass transfer (Fig. 1): diffusion (which typically produces spherical concretions) and advection (which produces a wide range of forms that show anisotrophies either from inherent host rock properties or from preferential cementation due to fluid flow). The wide range of terrestrial geometries suggest that there can be multiple events that can be superimposed [4].