VESICLE SIZE ANALYSIS OF BASALTS USED TO DETERMINE THE PALEOPRESSURE OF CIMA VOLCANIC FIELD: A MARTIAN APPROACH

Knowing Mars’s paleo-atmospheric pressure and how it has changed over time can tell us valuable information about life on Mars, such as how long did mars have the ability to hold life based on the atmospheric gasses. Today Mars’s atmosphere is not even 1% as that of Earths at sea level. We have analyzed basalts using a textural technique that can constrain Mars’s paleopressure, using Sahagian’s (2002) method. Paleopressure is obtained with the modal vesicle volume because the pressure in gas-filled pockets is different at the base and top of a lava flow due to the sizes of gas bubbles at time of emplacement (now vesicles) being a product of atmospheric pressure and hydrostatic overburden.

We tested the method on a cinder cone basalt flow in Cima Volcanic Field and determined from preliminary data the technique will work in determining the paleopressure of Mars on similar cinder cones. Although Sahagian calls for a strict selection of a lava flow and discourages sampling from flows that overlap due to the possibility of deformed vesicles, we tested the method on a flow that had a few inches of ash above followed by a second flow. Samples were collected in-situ from the base and top and scanned using High-Resolution Computed Tomography (HRCT). The analysis software CTan was used to obtain the volume of hundreds of vesicles in the basalt samples. The calculated paleopressure obtained by the ratio of modal vesicle volumes, gravity, density and height of the lava flow was compared to the standard month to month barometric pressure from Automated Surface Observing Systems (ASOS) stations. The results showed to be analogous barometric pressure values, as it should be because flows sampled have been dated to be 100,000 years old and scientists have agreed atmospheric pressure has not changed much in Cenozoic time. It was also determined the ash in-between the two flows was a confining layer that prevented deformation of the vesicles.

Testing the method on overlapping lava flows was crucial in order to determine if the method could be used on a planet such as Mars, due to the lack of tectonic plates making overlapping lava flows an occurrence. Future rover missions will benefit in coming equipped with an HRCT instrument to scan in-situ cinder cone basalt flows. This can be valuable in constraining the change of paleopressure on Mars because rocks can be accurately dated.