Experimental Shock Decomposition of Siderite to Magnetite

摘要

The debate about fossil life on Mars includes the origin of magnetites of specific sizes and habits in the siderite-rich portions of the carbonate spheres in ALH 84001. Specifically were able to demonstrate that inorganic synthesis of these compositionally zoned spheres from aqueous solutions of variable ion-concentrations is possible. They further demonstrated the formation of magnetite from siderite upon heating at 550 deg C under a Mars-like CO2-rich atmosphere according to 3FeC03 = Fe304 + 2CO2 + CO and they postulated that the carbonates in ALH 84001 were heated to these temperatures by some shock event. The average shock pressure for ALH 84001, substantially based on the refractive index of diaplectic feldspar glasses is some 35- 40 GPa and associated temperatures are some 300-400oC. However, some of the feldspar is melted, requiring local deviations from this average as high as 45-50 GPa. Indeed, observes the carbonates in ALH 84001 to be melted locally, requiring pressures in excess of 60 GPa and temperatures > 600 deg C. Combining these shock studies with the above inorganic synthesis of zoned carbonates it seems possible to produce the ALH 84001 magnetites by the shock-induced decomposition of siderite. We are testing this hypothesis experimentally by exposing siderite to experimental shocks. As reported earlier we produced intimately mixed zones of spinelstructure Fe- and Mg-oxides, based on preliminary transmission electron microscope (TEM) analyses. We have now concentrated magnetic separates from this mix for more detailed identification and characterization of the shock products.