Stability of Magnetite in Oxyhalogen-Rich Martian Fluids: Implications for Redox History on Mars

Session: Advancing Mineral Science and Exploring Planetary Surfaces: In Honor of MSA Dana Medalist, Elizabeth B. Rampe (Posters)

Presenting Author:

Authors:

Abstract:

Magnetite (Fe₃O₄) is a redox-sensitive mineral that contains both ferrous (FeII) and ferric (FeIII) iron, making it a valuable mineral for geochemical and astrobiological processes. On Mars, magnetite has been detected at multiple locations and is considered an important mineral for interpreting the planet’s magnetic, geochemical, and potentially biological history. Its mixed-valence nature allows it to respond sensitively to changes in redox conditions, which makes it especially useful for understanding the past presence of water and oxidative processes on the Martian surface. Previous studies have pointed out that magnetite underwent only surficial weathering in the presence of chlorate and bromate brines at room temperature conditions. While oxychlorine brines are most common on modern Mars, past geological settings on the planet routinely experienced hydrothermal conditions. Magnetite alteration in oxyhalogen-rich hydrothermal conditions remains unknown.

We examine how magnetite responds to long-term exposure to hydrothermal fluids that mimic Martian environments, particularly those rich in reactive oxyhalogen species such as chlorate (ClO₃⁻) and bromate (BrO₃⁻). To simulate these interactions, laboratory experiments were conducted over a 128-day period at elevated temperatures (95 °C) under strictly anoxic conditions. Synthetic Martian brines were prepared using sodium chlorate or sodium bromate in solutions containing a background fluid composition of magnesium sulfate and magnesium chloride. We used advanced analytical methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), to track any mineralogical and structural transformations in the magnetite.

While preliminary, the results demonstrate a remarkably resistant nature of magnetite to oxidation by chlorate and bromate brines, retaining its spinel crystal structure even after prolonged exposure to oxidizing fluids. However, subtle changes were observed, including a slight contraction in the unit cell and a reduced Fe(II)/Fe(III) ratio, thereby yielding non-stoichiometric magnetite. We detected no other secondary mineral products that may have formed as a result of magnetite alteration. While the bulk mineralogy of magnetite remains almost unaltered, further microscopic analysis of the mineral could provide additional information about subtle changes in the mineral structure. Owing to its robust crystallographic structure, any magnetite likely to be brought back during future Mars Sample Return Mission could provide information about the aqueous systems in which it was originally deposited.