CHLORIDE DEPOSITS ON MARS: CHLORINE FROM THE SKY, OR CHLORINE FROM THE ROCKS?

Orbital spectroscopy has identified chloride (most likely halite)-bearing deposits in paleolakes and other geological settings on Mars [e.g. 1,2]. The origin of the chlorine and fluids that formed the deposits is largely unconstrained; chloride-rich inverted channels suggest surface runoff-fed ponds [3], alternatively, the abundance of chloride (~ 10–25 wt. %) and absence of associated evaporites (carbonates, sulfates) are consistent with late-stage groundwater upwelling [2]. We use HiRISE DTMs and THEMIS decorrelation stretched images to map a set of chloride-bearing deposits and calculate their volume, and a geochemical reaction-transport modeling code (CHIM-XPT [4]) to test possible sources for the chlorine (dissolution of igneous chlorapatite in basalt or weathering of volcanically-derived Cl-phases deposited in the top few meters of Mars soil/dust).

For a short duration (< 1 Mars yr) warming event (above freezing point), chlorapatite dissolution cannot provide sufficient Cl^- to satisfy mass balance. High water-to-rock ratios (W/R) are also ruled out because sulfates would be transported into the lakes and precipitate close to or with the chlorides. If the source of chlorine was the weathering of volcanic Cl-phases (e.g. perchlorate), long warm events are not required, since the volcanic Cl-phases only reside in the top-most meters of soil/dust. For a ~ 1.4 × 10¹¹ kg NaCl deposit near Miyamoto Crater in Meridiani Planum [5], we find that masses of degassed volcanic HCl reported by [6] are insufficient to form the deposit. However, chlorapatite weathering to a depth of ~ 15 m across the 1.2 × 10⁹ m²basin (i.e. ~ 1 Mars yr) could form the deposit but would be inconsistent with seasonal melting.

[1] Osterloo+ (2010) JGR 115, E10012. [2] Glotch+ (2016) JGR 121, 454–471. [3] Osterloo and Hynek (2015) LPSC 46, #1054. [4] Reed (1998) Rev. Econ. Geol. 10, 109–124. [5] Hynek+ (2015) Geology 43, 787–790. [6] Craddock and Greeley (2009) Icarus 204, 512–526.