Aqueous alteration and bioalteration of a synthetic enstatite chondrite

Abstract

Understanding the transformations of highly reduced enstatite chondrites (EC) in terrestrial environments, even on very short timescales, is important to make the best use of the cosmochemical and mineralogical information carried by these extraterrestrial rocks. Analogs of EC meteorites were synthesized at high pressure and high temperature. Then, their aqueous alterations, either abiotic or in the presence of the bacteria Acidithiobacillus ferrooxidans or Acidithiobacillus thiooxidans, were studied under air, at pH~2, 20°C, and atmospheric pressure. They stayed in shaken batch reactors for 15 days. Reference experiments were carried out separately by altering only one mineral phase among those composing the synthetic EC (i.e., sulfides: troilite or Mg-Ca-rich sulfides, enstatite, and Fe 70 Si 30 ). Composition of the alteration aqueous media and microstructures of the weathered solids were monitored by inductively coupled plasma atomic emission spectroscopy and by scanning electron microscopy, respectively. Alteration sequence of the different mineral components of the synthetic EC was found to occur in the following order: magnesium-calcium sulfides > troilite > iron-silicon metallic phase > enstatite regardless of the presence or absence of the microorganisms. Such small biological effects might be due to the fact that the alteration conditions are far from biologically optimal, which is likely the case in most natural environments. The exposed surfaces of an EC meteorite falling on Earth in a wet and acidic environment could lose within a few hours their Ca-and Mg-rich sulfides (oldhamite and niningerite). Then, in <1 week, troilite and kamacite could be altered. In a wet and acidic environment, only the enstatite would remain intact and would weather on a much slower geological timescale.