A SYSTEMATIC CLASSIFICATION OF METEORITES
PART III

According to Papike et al. (2009), both depleted and enriched material were formed early in the planet's history as a result of magma ocean crystallization. Although complex two- and three-stage petrogenetic models have been proposed, martian lithologies can be resolved into four basic reservoirs based on geochemical, isotopic, and mineralogical characteristics. These reservoirs represent the shergottites through contrasting models: one involving mantle magma assimilation of crustal material, and another, perhaps more plausible, involving the mixing of multiple mantle magma reservoirs linked by fractional crystallization. These four basic reservoirs also represent the nakhlites and chassignites, conceivably originating from a deep reservoir, as well as the single known orthopyroxenite ALH 84001, conceivably originating from a shallow reservoir.

Although ALH 84001 is an orthopyroxenite, and as such was characterized by the Planetary Chemistry Laboratory at Washington University as a subgroup of the nakhlites, its parental source magma has a composition that is consistent with the same mixtures of depleted and enriched REE end-member components that are used in a geochemical classification of the shergottites (Lapen et al., 2012). It was determined that the source magma of ALH 84001 contained a higher proportion of the enriched REE component than all other shergottites studied thus far. Therefore, ALH 84001 might be most appropriately classified as a subgroup of the shergottites.

The four basic reservoirs distinguished by Papike et al. (2009) are shown in the following table:

A new hybridized model developed by Borg and Draper (2003) suggests that the martian magma ocean crystallized and produced a depleted mantle (45% opx, 38% ol, 14% cpx, and 3% majoritic garnet) plus an enriched, trapped, late-stage liquid (after 99.5% crystallization of magma ocean). This stage was followed by cumulate melting which generated partial melts compositionally similar to the most primitive martian meteorites. The other meteorites were generated by fractionation of olivine and orthopyroxene to form parental melts in initial conditions of high pressure (≥12 GPa), superchondritic CaO/AlO ratio, high Mg# (~80), and an FeO component of ~13.5 wt%. The late-stage liquid was trapped in the cumulate pile after 98–99.5% crystallization, representing a component analogous to lunar KREEP (potassium–rare earth element–phosphorus). Basaltic martian meteorites were derived from the melting of mixtures of cumulates and late-stage liquids that crystallized ~4.5 b.y. ago.

The above hybridized model was refined by Lapen et al. (2010) to describe the mantle source of shergottites as well as the orthopyroxenite ALH 84001. Their model generally agrees with that of Borg and Draper (2003), which propounds that the various martian lithologies were produced from variable mixtures of depleted cumulate material and trapped, enriched residual liquids; however, improved age and isotopic data indicate that residual liquid remaining after ~93–98% crystallization of the magma ocean was not part of the mixture that produced enriched shergottites. The average mixture of the shergottites was determined to consist of 94% cumulates and 6% trapped residual liquids, and the depth of the mantle reservoir—consistent with partial melting characteristics and observed incompatible element abundances—was calculated to be 250–400 km. The research team recognized two distinct mantle reservoirs: one in which the shergottites and ALH 84001 were formed, and another in which the nakhlites were formed.

Nonetheless, Andreasen et al. (2015, #2976) demonstrated that a three-component mixing model for martian mantle source regions remains as the most consistent given the growing amount of isotopic data from an increasing number of shergottite analyses. These are described as a depleted reservoir with high Sm/Nd and high Lu/Hf ratios, a depleted reservoir with high Sm/Nd and low Lu/Hf ratios, and an enriched reservoir with low Sm/Nd and Lu/Hf ratios. The known shergottites presently encompass virtually the entire range of mantle compositions established in the three-component mixing model, delineated by the depleted end-member NWA 7635 and the enriched end-member ALH 84001. Such a three-component mixing model involving depleted, enriched, and KREEP-like end-members is supported by the results of a statistical analysis based on Sr-Nd-Hf-Pb isotopes in shergottites conducted by Jean and Taylor (2017, #1666).

The geochemical diversity among the martian basalts was examined by Treiman and Filiberto (2015), and a new model of petrogenesis was proposed. Utilizing elemental abundance ratios (primarily involving incompatible elements, but also Al and Ni), they divided the pyroxene-phyric and olivine-phyric shergottites into three subgroups in order of increasing evolution: olivine-phyric ⇒ low-Al basalt ⇒ high-Al basalt. They reasoned that an evolutionary relationship exists between these subgroups, and asserted that the diversity among them occurred as a result of differential mixing of two primary magma (or source) components—one depleted and the other enriched in incompatible elements. The enriched component was described as a highly evolved product of fractionation of a magma ocean analogous to lunar KREEP. Further processing followed the mixing stage, which involved variable degrees of silicate fractionation and/or accumulation of olivine megacrysts. Although the full range of geochemical variability of the martian basalt meteorites may be represented in our present collections, a third complementary mixing component hypothesized to exist could still be missing.

Classification schemes and data for this page were adapted from the following sources:

Borg and Draper, 2003
Warren and Bridges, 2005
Irving et al., 2007
Symes et al., 2008
Bunch et al., 2008
Irving and Kuehner, 2008
Shih et al., 2009
Rumble III and Irving, 2009
Papike et al., 2009
Irving et al., 2010
Lapen et al., 2010
Treiman and Filiberto, 2014
Andreasen et al., 2015
Jean and Taylor, 2017

A more in-depth treatment of the classification of martian meteorites can be found on Dr. Anthony Irving's Martian Meteorites website.