Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

Abstract

Abstract-Shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron-carbonate were initiated. Naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite. Samples were shocked in tungsten-alloy holders (W = 90%, Ni = 6%, Cu = 4%) to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder. Each sample was shocked to a specific pressure between 30 to 49 GPa. Transformation of siderite to magnetite as characterized by TEM was found in the 49 GPa shock experiment. Compositions of most magnetites are >50% Fe +2 in the octahedral site of the inverse spinel structure. Magnetites produced in shock experiments display the same range of sizes (~50-100 nm), compositions (100% magnetite to 80% magnetite-20% magnesioferrite), and morphologies (equant, elongated, euhedral to subhedral) as magnetites synthesized by Golden et al. (2001) and as the magnetites in Martian meteorite Allan Hills (ALH) 84001. Fritz et al. (2005) previously concluded that ALH 84001 experienced ~32 GPa pressure and a resultant thermal pulse of ~100-110 °C. However, ALH 84001 contains evidence of local temperature excursions high enough to melt feldspar, pyroxene, and a silica-rich phase. This 49 GPa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to > 470 °C. Therefore, magnetite in the rims of carbonates in Martian meteorite ALH 84001 could be a product of shock devolatilization of siderite as well.

Origins of magnetite nanocrystals in Martian meteorite ALH84001

“…Though it could be argued that re-equilibration, by solid state cation diffusion, of Mg between magnetite product and unreacted carbonate could have lead to the thermal stabilization of the residual carbonate by increasing its Mg mole fraction, such cation diffusion at 623 K would not have been sufficiently facile to allow such a process (as discussed earlier). Fortunately, the issue of the compositional homogeneity of Copper Lake siderite was reported previously by Bell (2007), who is affiliated with the same research group as Golden et al (2006) and so presumably used the same parent siderite sample. 20 Fig.…”

“…Consequently, this should not divert our attention from understanding the origin of the remaining majority of magnetites that would appear to have been derived from a detrital or allochthonous Bell (2007). The image on the left is a Hue-Saturation-Intensity (HSI) element ratio map for Fe:Mg (see Bensen and Lechene (2005)).…”

“…7F taken from Fig. 3A, 3B-3G of Bell (2007) illustrates the variation of Mg and Fe abundances in a single grain surface $175 Â 175 lm of Copper Lake siderite. In this single region, there are at least three distinct carbonate compositions as illustrated in the histogram shown in Fig.…”

“…The respective Fe and Mg signal intensities are derived from the grey-scale image pixel values of the electron microprobe Fe and Mg element maps shown in Fig. 3b and c of Bell (2007). Since neither electron microprobe map has an accompanying calibrated intensity scale the Fe:Mg values shown for the HSI image represents relative values and so are not absolute.…”

Conservation methods of iron artifacts recovered from the marine environment

“…Siderite is an iron-bearing carbonate common to many diverse sedimentary settings, particularly in marine and lacustrine environments. Substitution of Fe +2 by other metallic ions is common in siderite and the mineral is rarely found as pure FeCO 3 (Bel 2007 …”