CONSTRAINTS ON THE HALOGEN CONTENT OF MARTIAN MAGMAS AND DEGASSING OF CHLORINE-RICH FLUIDS (Invited Presentation)

The Martian interior is thought to be enriched in halogens compared to the terrestrial mantle [1] and chlorine may replace water as the dominant volatile during magma genesis [2]. Here I will review what is known about the pre-eruptive halogen content of Martian basalts and evidence for degassing of halogen-rich, water-poor magmatic fluids.

The bulk chlorine contents of unaltered Martian basalts, as represented by the shergottite meteorites, are similar to depleted terrestrial basalts [3]. However, after taking into account igneous fractionation and partial melting, Martian basalts contain 2.5 (+/- 1) times more chlorine than terrestrial basalts and have a source region more consistent with the enriched terrestrial mantle [2, 3]. Similarly, fluorine in the Martian interior is enriched compared with the bulk terrestrial mantle [3]. Bromine in the Martian interior is less well constrained because of the paucity of data of Br contents of Martian meteorites [3].

Eruption of halogen-rich magmas has been used to explain the chlorine-rich nature of the bulk Martian crust [4] and variations in chlorine at the Home Plate outcrop in Gusev Crater [5]. Degassing of chlorine-rich magmatic fluids is also consistent with the mineralogy of Martian meteorites [6-8]. Martian meteorites contain end member Cl-scapolite marialite and chloro-potassic-hastingsite amphibole, which form from magmatic hydrothermal brines [6, 7].

Refs: [1] Dreibus G. and Wänke H. (1987) Icarus, 71. 225-240. [2] Filiberto J. and Treiman A.H. (2009) Geology, 37. 1087-1090. [3] Filiberto J et al. (2016) MaPS. DOI: 10.1111/maps.12624. [4] Keller J.M. et al. (2006) JGR. 111(E03S08). doi:10.1029/2006JE002679. [5] Schmidt M.E. et al. (2008) JGR. 113(E06S12). doi:10.1029/2007JE003027. [6] Filiberto J. et al. (2014) EPSL. 401. 110-115. [7] Giesting P. and Filiberto J. (2016) MaPS. in press. [8] McCubbin F.M. et al. (2013) MaPS. 48. 819-853.