Constraints on the emplacement of Martian nakhlite igneous rocks and
their source volcano from advanced micro-petrofabric analysis
Abstract
The Martian nakhlite meteorites, ormed from a single magma source
region, and emplaced during multiple events spanning at least 93 ± 12
Ma, represent a key opportunity to study the evolution of Martian
volcanic petrogenesis. Here 16 of the 26 identified nakhlite specimens
are studied using coupled large area electron backscatter diffraction
(EBSD) and energy dispersive X-ray spectroscopy (EDS) mapping to
determine shape preferred orientation (SPO) textures of contained augite
(high Ca-clinopyroxene) phenocrysts by considering crystallographic
preferred orientation (CPO). Textural parameters derived from EBSD and
EDS analyses were used to calculate maximum and minimum magma body
crystallization thicknesses via three endmember emplacement scenarios:
thermal diffusion, crystal settling, and crystal convection. Results
from CPO textural analyses indicate weak to moderate fabric textures
that are comparable to those in terrestrial clinopyroxenites. In all
samples, a consistent foliation within the {001} axis of augite is
observed. In all but two of the studied nakhlites this {001} foliation
is typically coupled with a weaker lineation fabric in one or more of
the {100}, {010}, {001} axes. Results from the calculated magma
body thicknesses are consistent with an emplacement mechanism for the
nakhlites driven by crystal settling. These crystal settling results
infer magmatic body thicknesses ranging from <1 m to several
10’s m, forming two distinguishable groups that appear random when
assessed against observed texture, geochemical, and age parameters.
Coupled textural and modelling results therefore suggest that the
nakhlite source volcano varied in thickness over time yet consistently
solidified via the mechanism of crystal settling.