Thermal Decomposition of Siderite–pyrite Assemblages: Implications for Sulfide Mineralogy in Martian Meteorite Alh84001 Carbonate Globules:

Introduction: Although magnetite crystals in ALH84001 have received considerable attention in the literature as a potential biomarker, the sulfide component is important because the coexistence of magnetite and sulfides has also been suggested as a marker for past biogenic activity [1]. Very little research, however, has focused on the formation of sulfides in ALH84001. Early research on the sulfides in ALH84001 indicated that pyrite grains (>10 μm in size) are associated with chromite, maskelynite, and/or carbonate globules [2,3]. Fine-grained sulfides, which are found in the black rims of the carbonate globules, were tentatively identified as pyrrhotite and greigite by [1]. The presence of greigite has not been confirmed but the presence of pyrrhotite has been confirmed by [4]. An alternative hypothesis to the biogenic origin for the carbonate-magnetite-sulfide assemblage in ALH84001 is the inorganic formation of this mineral assemblage by aqueous or hydrothermal precipitation of carbonate globules with Fe-sulfides followed by a heat pulse that was sufficient to decompose siderite to magnetite and pyrite to pyrrhotite [5,6,7]. We have previously precipitated Fe-sulfides (pyrite and pyrrhotite) and magnetite along with synthetic carbonate globules in laboratory experiments [5]. The pyrite precipitated along with the Fe-rich carbonate layers, and then was thermally converted to pyrrhotite upon heating to 470°C in a CO2 atmosphere. The thermal decomposition of pyrite-siderite assemblages to pyrrhotite-magnetite assemblages depends on a number of environmental variables, e.g., the partial pressure of S and CO2, presence/absence of volatile sinks, and open versus closed reaction environments. The objective of this paper is to report the effects of these environmental variables on the thermal decomposition of siderite-pyrite assemblages and to discuss their implications for the formation of the carbonate-sulfide-magnetite assemblage in ALH84001. Materials and Methods: Siderite (Nova Scotia, <150 μm size fraction with Mg and Mn impurities), pyrite (origin unknown, <150 μm with minor silicate inclusions), and mixtures thereof were heated to 550C in gas mixing furnaces under a stream of 95% CO2 + 5% CO. The endmember and mixture samples (200 mg siderite and/or 50 mg pyrite) were heated in open (quartz crucibles) and sealed (SiO2 glass tubes) containers to simulate open and closed systems, respectively. Temperatures were measured with thermocouples placed next to the sample containers. Another set of closed-system experiments was conducted with CaO placed in a separate crucible inside the sealed silica glass tube to act as a getter for CO2. The temperature in all experiments was ramped from room temperature to 550C at 1C/min, held at 550C for 1h, and then cooled to room temperature at 200C/min. Mineralogical and chemical properties of run products were characterized by X-ray diffraction analysis, scanning electron microscopy, electron microprobe analysis, and Moessbauer spectroscopy. Treatments and reaction products are summarized in Table 1. We also characterized sulfides in ALH84001 thin sections using electron beam techniques (see above). Results from Experimental Studies: Sideriteonly decomposition in the open-system experiments yielded magnetite as the only product (Table 1). Magnetite and residual siderite were present in closedsystem experiments. The extent of thermal decomposition is a function of the partial pressure of CO2, so that siderite decomposition stops once an equilibrium pressure is reached inside the sealed quartz tubes: