A petrogenetic model for the comagmatic origin of chassignites and nakhlites: Inferences from chlorine‐rich minerals, petrology, and geochemistry

Abstract

Abstract-Twelve samples belonging to the chassignite and nakhlite subgroups of Martian meteorites were investigated using a variety of micro-beam analytical techniques to gain insight into the petrogenesis of these two meteorite classes. There are a striking number of geochemical similarities between the chassignites and nakhlites, including mineralogy and petrology, crystallization age, cosmic-ray exposure age, and radiogenic isotopic compositions. However, there are also geochemical differences, namely in trace element systematics of pyroxenes, that have led some authors to conclude that the nakhlites are comagmatic with each other, but not comagmatic with the chassignites. On the basis of data presented here, we propose a model in which these differences can be reconciled by the addition of an exogenous Cl-rich fluid to the chassignite-nakhlite magma body shortly after the formation of the cumulate horizon that was sampled by the Chassigny meteorite. This model is supported by the textural and chemical associations of the volatile-bearing minerals apatite, amphibole, and biotite, which record a history starting with the addition of a Cl-and LREE-enriched fluid to the magma body. As the magma continued to crystallize, it eventually reached chloride saturation and degassed a Cl-rich fluid phase. Depending on the provenance of the Cl-rich fluid, this model could explain how the chassignites and nakhlites originated from an LREE-depleted source, yet all exhibit LREE-enriched bulk-rock patterns. Additionally, the model explains the range in oxygen fugacity that is recorded by the chassignites and nakhlites because eventual exsolution and loss of Cl-rich fluid phases near the end of crystallization of the nakhlite sequence leads to auto-oxidation of the magma body due to the preferential partitioning of Fe 2+ into the fluid phase.

Lithium isotope constraints on crust–mantle interactions and surface processes on Mars

“…Progressive mineral equilibration shown by pyroxene quadrilateral and corresponding olivine equilibration, indicated by an arrow (Treiman, 2005;Day et al, 2006), is coupled with increasing d 7 Li. Correlations between systematics in the martian suite appear to reflect the highly incompatible character of Cl at magmatic temperatures (Fabbrizio et al, 2013), and is consistent with Cl-rich fluids operating during progressive crystallization of the nakhlite flow (Cartwright et al, 2013;McCubbin et al, 2013) and in other martian rocks (McSween and Harvey, 1998;Thomas-Keprta et al, 2009;Agee et al, 2013). A positive linear correlation observed between d 7 Li and Li/Cl for nakhlites (Fig.…”

“…Moreover, observational evidence suggests that nakhlites and chassignites may also have a co-magmatic origin (Lentz et al, 2011;McCubbin et al, 2013). A specific feature of nakhlites is their variable but large proportion of incompatible element-enriched mesostasis, representing quenched glass formed from inter-cumulus liquid that was trapped during accumulation of augite and olivine Day et al, 2006).…”

“…Instead, it is likely that less equilibrated nakhlites may have witnessed more extensive assimilation of altered materials within the mesostasis at the martian surface than more equilibrated nakhlites. The similarity in REE abundances and Nd isotopes in nakhlites (e.g., Day et al, 2006;McCubbin et al, 2013) constrains timing of processes of alteration to after the crystallization of augite and hints to the assimilation of materials similar in age and composition to the nakhlites themselves. McCubbin et al (2013) have proposed a model for the co-magmatic origin of chassignites and nakhlites through assimilation of a Cl-rich fluid into the chassignite-nakhlite parent magma shortly after formation of the olivine cumulates responsible for the Chassigny meteorite.…”

Validity of the Apatite/Merrillite Relationship in Evaluating the Water Content in the Martian Mantle: Implications from Shergottite Northwest Africa (NWA) 2975

“…The results of Ustunisik et al [56,57] and Howarth et al [39] suggest the loss of Cl to fluids. The late infiltration of Cl-rich brine can result in the appearance of Cl-rich minerals or Cl-enriched apatite [10,39,40,[64][65][66].…”

“…The NWA 2975 Cl-apatite is enriched in REEs in comparison to F-apatite ( Figure 5). Cl-rich fluids are commonly LREE-rich due to fluid/melt partitioning of LREEs in the presence of Cl under hydrothermal conditions [40,[67][68][69].…”