Enstatite metachondrites as Mercury surface analogue materials: Potential and limitations

A number of past experimental studies have shown that plausible Hermean surface compositions may be derived via melting of enstatite chondrite percursors, in particular EH chondrites (e.g., 1,2). A current serious difficulty in studying Mercury is the complete absence of known material samples (meteorites or sample returns). While known enstatite meteorites themselves are not plausible as current constituents of Mercury, their broad geochemical and mineralogical kinship with Mercury may make them useful analogues for some aspects of Hermean geological processes, as has been argued for other enstatite metachondrites (3). Here we present new mineral chemical and petrographic information for several metamorphosed enstatite meteorites and suggest Hermean environments and processes for which they may provide useful analogues.

Lapaz Icefield (LAP) 02225: Impact melting and rapid solidification near the surface. The meteorite LAP 02225 is an unbrecciated, lightly-shocked rock of enstatite chondrite parentage consisting of hollow enstatite laths (up to 100s of microns in the long dimension, parallel to the b-axis) in a matrix of metallic iron, sulphides, and fine-grained plagioclase. The LAP 02225 meteorite is the product of rapid solidification of a total melt of EH composition. While plausible near-surface rocks on Mercury are generally unlikely to contain such high amounts of Fe metal and sulfide, this sample provides a useful analogue for the silicate mineralogy of rapidly-cooled impact melts on a broadly EH-chondrite like Mercury, such as the upper surfaces of large impact melt sheets.

 

F igure 1: Reflected light (A), Plane-polarized light (B), and crossed-polarized light (C) images of a representative area of LAP 02225. Large, hollow and commonly twinned laths of enstatite are hosted in a fine-grained matrix dominated by enstatite, plagioclase, metal and sulphides. The textures in this meteorite are consistent with rapid solidification of a total (impact) melt, corresponding to the upper part of a large impact melt sheet.

 

Northwest Africa (NWA) 8173: Extreme metal sulphidation and metasomatism. The exsolution of fine graphite flakes in Fe₀ may result from the competition between Si and C as alloying elements. The preponderance of MgS-dominant sulphides (with Fe-bearing niningerite the most common sulphide) may reflect extreme sulphidation. Exsolution of troilite occurs both along the octahedral (111) planes of niningerite, as well as in anastomosing veins cross-cutting and overprinting the existing sulphide textures. Exsolved troilite itself commonly encloses blebs of oldhamite (CaS). NWA 9173 may provide analogues for regions of focused S-rich alteration and metasomatism associated with endogenic or exogenic heating (magma bodies and impact melts).

 

F igure 2: Textural relations between silicates and sulphides in NWA 8173 (A). The host sulphide phase is cubic (Mg,Fe)S, the exsolved phase is troilite (hexagonal stoichiometric FeS). Exsolved troilite occurs either as plates along the (111) planes of host niningerite, or as anastomosing veins that cross-cut and overprint the octahedral exsolution (B). Graphite occurs as minute flakes hosted in Si-bearing Fe₀, consistent with the exsolution of graphite (C).

 

NWA 4301 and Zakłodzie: Impact-melting followed by cooling, assimilation, re-equilibration, and extensive separation of graphite. Annealing and incorporation of cold country rock into melt sheets. Both NWA 4301 and Zakłodzie contain significant amounts of graphite that are associated with Si-bearing Fe₀ (or weathering products of Fe₀), consistent with separation of a graphitic melt from the metal-sulphide, possibly the result of Si enrichment in the Fe₀. These meteorites are consistent with slow cooling of large impact melt sheets, a process also likely to have been important throughout Mercury’s geological history. Alternatively, the segregation of graphite here could provide an analogue to the first stages of the formation of a potential graphite flotation crust on early Mercury (4).

 

Figure 3: Backscattered electron (BSE) images of representative silicate-metal-sulphide-graphite textures in NWA 4301 (A) and Zakłodzie (B). In both meteorites, both metal and sulphides have been strongly affected by terrestrial weathering, however, the association between graphite and (former) metal is clear. Silicates are dominated by equant-textured enstatite.

 

Queen Alexandra Range (QUE) 94204: Incipient differentiation of plagioclase-silica-pyroxene partial melts, and Fe-FeS from an enstatite-dominant restite. With progressive heating and equilibration, Fe₀ becomes progressively enriched in Si potentially leading towards a very Si-rich core. The occurrence of cristobalite at the interface between metal and enstatite may be connected to the disproportionation of Si with some entering the Fe₀ structure and some forming SiO₂ (thus further reducing the fO₂ of the bulk system).

 

F igure 4: Reflected light (A), Plane-polarized light (B), and crossed-polarized light (C) images of a representative area of QUE 94204. Enstatite is the dominant mineral, with metal and sulphides either enclosed within as rounded bodies, or concentrated in the interstices of enstiatite grains along with sodic plagioclase and cristobalite.

 

While enstatite meteorites should not be expected to provide perfect geochemical, mineralogical, or spectral analogues for Mercury, they can provide insights into the kinds of geological processes that have shaped the current Hermean surface. We eagerly anticipate both the future recovery or identification of meteorites from Mercury, or sample returns from the Hermean surface.

 

1. Boujibar et al. (2025) Icarus 437, 116602

2. McCoy et al. (1999) MAPS 34(5) 735-746

3. Udry et al. (2019) MAPS 54, (4) 785-810

4. Vander Kaaden, & McCubbin (2015), J. Geophys. Res. Planets, 120, 195–209.