| 24293 |
abstract |
In the Noachian, Mars’ crust was deeply affected by impact crater gardening, and subsequent
hydrothermal alteration. While water was present in the Martian crust, frequent impacts
provided prominent heat sources that drove hydrothermal systems. Those systems were
scattered randomly over the planet’s surface and variable in size, but generally spanned
the diameter of any ≥10 km complex crater. These systems were also deep reaching and
long-lived with central crater uplifts and crater modification zones affected by the
most intense water flow. Because circulating water driven by a temperature gradient
changes the thermochemical status of a system, mineralogical reactions inevitably
took place. Similar mineralogical consequences have been documented in terrestrial
impact-generated hydothermal systems. Approximately half of all known impact structures
on Earth show hydrous alteration phases produced by the interaction of water with
a melt sheet and/or hydrothermal systems that penetrated the crust beneath the crater
floor. Immense systems have been documented in drill cores from Earth’s largest impact
structures, such as the Chicxulub crater (Yucatán, Mexico) and the Sudbury structure
(Ontario, Canada). As we have shown earlier for the basaltic crust of Mars, thermochemical
changes caused by circulation of heated formation brines mainly result in the crystallization
of sheet silicates (serpentine, chlorite, nontronite, and kaolinite), amphiboles,
oxides and hydroxides (magnetite, hematite, diaspore), and additional minor phases.
Our results compare favorably with those from sattelite-based spectroscopic measurements
of the Martian surface by OMEGA and CRISM (aboard the European Mars Express and U.S.
Mars Reconnaissance Orbiter, respectively). They report many of the same minerals,
amongst them several smectites and micas – such as kaolinite, Fe/Mgsmectite, saponite,
illite/muscovite, and chlorite – but also hydrous silica, zeolite, and carbonate.
Potential testimony of impact-generated hydrothermal systems has been found in three
Martian craters so far: hydrous phases are located in central peaks and inner crater
walls in a ~100 km diameter crater in Mawrth Vallis, a crater of similar size in Terra
Meridiani, and an ~20 km diameter crater in the Western Isidis region. In addition
to producing mineral assemblages similar to those being observed on Mars, our calculations
constrain formation conditions. Our results suggest that alteration assemblages are
largely controlled by the water to rock ratio (W/R) and temperature, while the pH
is bufferd by the precipitating and dissolved species. Our previous studies showed
that three sheet silicates, which also have been found on Mars (chlorite, nontronite,
kaolinite), point to specific formation conditions: Chlorite is indicative of intermediate
to low W/R and occurs over the whole temperature range. The clay minerals nontronite
and kaolinite indicate intermediate and high W/R, respectively. Moreover, nontronite
forms below 250 °C only. While our previous studies focussed on the parameters pressure,
temperature, and W/R, in this study we explore the influence of host rock variability
on the resulting alteration assemblages. |
| 24293 |
label |
Schwenzer, Susanne P. and Kring, David A. (2009). Impact-generated hydrothermal
alteration on Mars: clay minerals, oxides, zeolites, and more. In: 40th Lunar and
Planetary Science Conference, 23-27 Mar 2009, Houston, TX, USA. |
| 24293 |
label |
Schwenzer, Susanne P. and Kring, David A. (2009). Impact-generated hydrothermal
alteration on Mars: clay minerals, oxides, zeolites, and more. In: 40th Lunar and
Planetary Science Conference, 23-27 Mar 2009, Houston, TX, USA. |