Chloramphibole in Nakhlite Northwest Africa 13368 and Implications for Halogens on Mars

Abstract:

The presence of halogens (especially chlorine) in rocks on Mars is well established from both landed missions and meteorites (e.g., [1-5]). Filiberto and Treiman [5] concluded that the martian interior contains small but significant amounts of chlorine, but relatively little water. Important evidence for the presence of halogens comes from analyses of accessory apatite, amphibole and mica [2-4] in igneous augite-olivine cumulate assemblages of several of the ~1.3 Ga old nakhlites and chassignites.

The 15^th member of the nakhlite-chassignite clan of meteorites was found in Mauritania in July 2020, and strongly resembles the paired nakhlite specimens recovered in 2003 and 2009 in the Miller Range, Antarctica. Northwest Africa 13368 consists predominantly of euhedral cumulus grains of augite (cores Fs_22.9-23.0Wo_39.2-39.7, FeO/MnO = 30-31, Al₂O₃ = 0.84 wt.%, TiO₂ = 0.27 wt.%; thin rims Fs_47.7-50.2Wo_33.2-37.9), plus sparse euhedral cumulus grains of olivine (~4 vol.%, cores Fa_61.5-62.7, FeO/MnO = 48-49; thin rims Fa_79.6-88.3) and titanomagnetite, accompanied by a fine grained intercumulus assemblage of cruciform titanomagnetite, fayalite, hedenbergite, silica polymorph and alkali feldspathic glass. Cl-bearing “iddingsite” occurs as thin veinlets within olivine grains, and melt inclusions within augite grains contain daughter crystals of chloramphibole:

Si_6.08Ti_0.05Al^IV_1.93Al^VI_0.22Fe³⁺_0.77Fe²⁺_3.70Mg_0.28Mn_0.05Ca_1.84Na_0.33K_0.69O₂₂[Cl_1.81(OH)_0.19]

This amphibole is a potassic-chloro-hastingsite very similar to that documented by Sautter et al. [2] in nakhlite MIL 03346, and provides further evidence for the presence of chlorine in the martian mantle.

References: [1] Rampe E. et al. (2018) in The Role of Halogens in Terrestrial and Extraterrestrial Geochemical Processes, pp. 959-995 [2] Sautter V. et al. (2006) EPSL 252, 45–55 [3] He Q. et al. (2013) MaPS 48, 474-492 [4] Hewins R. et al. (2020) GCA 282, 201-226 [5] Filiberto J. and Treiman A. (2009) Geology 37, 1087-1097.