Abstract
Transmission electron microscopy and Fe-K X-ray absorption spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites, NWA 998, Lafayette, Nakhla, GV, Y 000593, Y 000749, MIL 03346, NWA 817, and NWA 5790. The secondary phases are a rapidly cooled, metastable assemblage that has preserved Mg# and Ca fractionation related to distance from the fluid source, for most of the nakhlites, though one, NWA 5790, appears not to have experienced a fluid pathway. All nine nakhlite samples have also been analysed with scanning electron microscopy, electron probe micro analysis, Bright Field high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction. By measuring the energy position of the Fe-K XANES 1s→3d pre-edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites. The crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable Fe3+/ΣFe values ranging from 0.4 to 0.9. In Lafayette, the central silicate gel parts of the veins are more ferric than the phyllosilicates around it, showing that the fluid became increasingly oxidised. The mesostasis of Lafayette and NWA 817 also have phyllosilicate, which have a higher ferric content than the olivine fracture deposits, with Fe3+/ΣFe values of up to 1.0. Further study, via TEM analyses, reveal the Lafayette and NWA 817 olivine phyllosilicates to have 2:1 T–O–T lattice structure with a the d001-spacing of 0.96nm, whereas the Lafayette mesostasis phyllosilicates have 1:1 T–O structure with d001-spacings of 0.7nm. Based on our analyses, the phyllosilicate found within the Lafayette olivine fractures is trioctahedral ferric saponite (Ca0.2K0.1)∑0.3(Mg2.6Fe2+1.3Fe3+1.7Mn0.1)∑5.7[(Si6.7AlIV0.9Fe3+0.4)∑8.0O20](OH)4·nH2O, and that found in the mesostasis fractures is an Fe-serpentine (Ca0.1Mg0.7Fe3+1.0AlVI0.4)∑2.2[Si2O5]OH4, with a ferric gel of similar composition in Lafayette and found as fracture fills throughout the other nakhlites.
Highlights
The eight nakhlites are a group of the SNC martian meteorites
The hydrothermal veins typically have a vein width of 5–10 lm, or can be up to $26 lm in the case of Lafayette and Northwest Africa (NWA) 817
A similar pattern is seen in Fig. 1b, where Lafayette and NWA 817 both have veining fractions of $10% of the olivine grains compared to the much lower fractions seen in the other nakhlites
Summary
The eight nakhlites (plus six pairings) are a group of the SNC martian meteorites. Their unique combination of known formation depths (Mikouchi et al, 2003, 2012) and hydrothermal veining (Gooding et al, 1991; Changela and Bridges, 2010) makes them an important source of information about water-crust interaction on Mars (Bridges and Schwenzer, 2012).The nakhlites are basaltic cumulate clinopyroxenite rocks, comprised mainly of sub-calcic augite with approximately 10% Fe-rich olivine, in all but one of the nakhlites (Miller-Range 03346). The eight nakhlites (plus six pairings) are a group of the SNC martian meteorites. Their unique combination of known formation depths (Mikouchi et al, 2003, 2012) and hydrothermal veining (Gooding et al, 1991; Changela and Bridges, 2010) makes them an important source of information about water-crust interaction on Mars (Bridges and Schwenzer, 2012). The nakhlites are thought to have formed in a thick basic–ultrabasic lava flow or shallow intrusion (Treiman et al, 1993).
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