| 24302 |
abstract |
Impact-generated hydrothermal systems are driven by the heat in uplifted basement
material, fractured and brecciated target rock, and impact melt. The size and duration
of these systems scale with the size of the impact crater, resulting in lifetimes
>1 million years for basin-sized structures. Using CHILLER, we calculated mineral
assemblages that are produced between 13 and 300 °C upon alteration of a proxy for
Mars' crust: Martian meteorite LEW 88516. Three characteristic mineral assemblages
were produced: serpentine–chlorite–(amphibole–talc–magnetite ±garnet ±quartz) at low
water/rock ratios (W/R), hematite–clay–(pyrite ±quartz ±chlorite) at intermediate
W/R and almost pure hematite at high W/R. The amount of hydrous phases, e. g. clays,
varies with temperature and W/R. Clay minerals are kaolinite and nontronite. These
assemblages should occur more often in Noachian terrains, which were modified by a
higher impact rate than younger terrains, particularly if there was a cataclysmic
bombardment ~4 Ga like that seen on the Moon. Hydrothermal assemblages would be most
extensive in the central peaks, peak rings, and modification zones of complex craters.
Our results compare well to observations by instruments currently orbiting Mars (OMEGA,
CRISM) that have mapped hydrous minerals exclusively on Noachian surfaces: in crater
central peaks, crater rims, crater ejecta, and layered deposits as well as in outcrops
on otherwise mantled terrain. It is conceivable, if not likely, that primary impact-generated
hydrothermal assemblages were redistributed in impact ejecta produced by younger events.
Furthermore, geologic processes related to water activity, such as valley network
formation, are active in the Noachian, raising the possibility that hydrothermal deposits
were affected by erosion and sedimentary deposition in those terrains. Nevertheless,
the formation of impact-generated hydrothermal assemblages is independent of climatic
factors such as surface temperature, because the required heat is generated by the
impact. |