THE ALTERATION OF IMPACT MELT - A FIRST STEP

Large amounts of clay minerals have been detected in relation with impacts and impact glass, the possible result of impact glass alteration. In this project experimental analyses and modeling of impact glass transformation are done on impact melt from several impact craters. Most of the samples are glass- or melt-rich rocks from several different impact structures of various ages, in different target lithologies at the various impact sites.

The experiments are conducted at hydrothermal temperature (200 ^oC) by 3 weeks percolation in a titanium batch reactor without flowing or stirring. We are using a saline solution containing 30 mg/l NaCl, close to the composition of normal, marine water. The experiments are aiming at representing a reproduction of the conditions found in the melt bearing rocks the first few thousand years after deposition. The first results show the formation of smectite, chlinochlore, Fe-pyrophyllite, saponite, analcime, talc and philippsite. The geochemical composition of the samples are studied before and after dissolution and then the experimental results will be compared to geochemical modeling (PHREEQC).

During the studies we aim at explaining and demonstrate the formation of smectite, saponite and clay minerals related to impact glass alteration, minerals found as natural alteration products of impact melt. These minerals and associated phyllosilicates have also been indicated to commonly appear on the surface of Mars. In this study we show the alteration products clearly to reflect the composition of both the source glass and percolation water. The natural appearance, both on the Earth and Mars, are also clearly controlled by other processes as surface weathering and postdepositional diagenesis.