THE HYDRATION AND MINERALOGY OF THE CLAY-SULFATE TRANSITION ZONE IN GALE CRATER

In 2021, the Mars Science Laboratory (MSL) Curiosity rover reached the “clay-sulfate transition” which is marked by a hydrated Mg sulfate signature with alternating layers of hydrated sulfates and clays as seen from orbit [1,2]. Variable hydration in this region, as measured by the Dynamic Albedo of Neutrons (DAN) instrument, is associated with a variety of sedimentary units and structures consistent with lacustrine depositional environments. Mineralogical trends are analyzed using the CheMin X-ray diffractometer instrument through drill samples which found decreasing clay abundance upsection [3], however the DAN instrument may be used to correlate hydration (H) data at drill sites and through the entire transition zone. The DAN instrument is an active neutron spectrometer that measures the abundance and distribution of hydration to 50 cm depth. In this work, H abundance is represented as water-equivalent-hydrogen (WEH) and the macroscopic thermal neutron absorption cross section (a proxy for the abundance of elements like Fe or Cl) is referred to as the bulk neutron absorption cross section (BNACs). Preliminary analyses of DAN measurements in the clay-sulfate transition region show periodic fluctuations in WEH and BNACs as the rover travels up section, which may be consistent with alternating layers of clays and sulfates. Additional work will determine the hydration of Mg sulfate within the layers and at depth by calculating the amount of absorbed water in clays and amorphous minerals. Finally, we will use WORM (Water, Organic, Rock, Microbe) Portal software to model the thermodynamic reactions between water and minerals that produce clay minerals, using hydration state constraints from DAN and mineralogical constraints from CheMin. We will explore adapting WORM for the types of depositional environments that would result in the clay-sulfate layering seen in the transition zone.

[1] R. Y. Sheppard et al. (2020) JGR

[2] Fraeman A. A. et al. (2016) JGR, 1713-1736.

[3] E. B. Rampe et al. (2022), LPSC