Mafic Rocks

Similar to Earth and Moon, Mars demonstrates a dichotomy. This dichotomy is based on compositional and geologic discrepancies as mafic rocks differ in the northern lowlands and southern highlands [Bandfield et al., 2001]. These mafic rocks are analogous to Terrestrial andesites and basalts, which are common Terrestrial volcanic rocks. This analogy implies the same bulk volcanic compositions and conditions existed on both planets.

Andesite is an intermediate volcanic rock (Fig. 1, 2) that is common at subduction zones such as the Andes. It is mainly composed of plagioclase, pyroxene, and amphibole [Matthes, 2001]. Contrary to basalt, it bears higher silica- but a lower mafic-content. Its melting temperature is lower and the viscosity is higher due to increased silica content. Basalt is a basic volcanic rock that consists of 40-70 Vol. % mafics, which is mainly pyroxene (in parts olivine) and felsic minerals like anorthitic plagioclase. It is present at subsurface and submarine lava flows (Island arc basalts IAB, mid-ocean-ridge basalts MORB, continent margins), rift zones, as plateau basalts, and at the Lunar surface.

Bandfield et al. [2000] classified the Martian surface into two general spectral classes ofvolcanic rocks (Fig. 1):

  • The basaltic type → Surface Type 1, and
  • the andesitic type → Surface Type 2.
Figure 1: Comparison between two Martian end member Surface Types (black) and Terrestrial analogues (blue). Surface Type 1 corresponds to basalt (Martian Terra Cimmeria basalt vs. Terrestrial Deccan Traps flood basalt), Surface Type 2 to basaltic andesite. Broad absorptions are observed between ~800-1200 cm-1 and ~200-500 cm-1 wavenumber regions with a depth of~0.04 and 0.09 in emissivity. There, the first component features a broad, slightly square-shaped absorption between 800-1200 cm-1 and absorption with a negative slope superimposed by minor, narrow absorptions. The second component has a rounded V-shaped absorption between 800-1200 cm-1. A rise in emissivity at wavelengths > 500 cm-1 is observed. Shape, depth, and wave length of absorption features are in good correlation with terrestrial samples [Bandfield et al., 2000].

Surface Type 1 dominates in the ancient Martian southern highlands whereas Surface Type 2 is concentrated in the younger northern lowlands. This potentially indicates that basaltic volcanism is restricted to the past and andesitic volcanism to times that are more recent. Considering the basaltic Surface Type 1 detected by TES, there is a general correlation with pyroxene-rich regions found by OMEGA [Poulet et al., 2007].

Figure 2: Chemical classification diagram of model-derived chemistries for terrestrial 10 cm-1 data and Martian spectra (stars) indicating Martian rocks match well into Terrestrial basaltic and/or andesitic compositions; [Hamilton et al., 2001].

Both surface types represent two distinct compositions that are common in hot-spot, flood volcanism, and rifting environments on Earth. These compositions are different to those of Lunar basalts or Martian meteorites [Bandfield et al., 2000]. Furthermore, as there is no evidence for Martian plate tectonics, andesite must have formed differently than is common on Earth along subduction zones.

References:

  • Bandfield, J.L., V.E. Hamilton, and P.R. Christensen (2000): A Global View of Martian Surface Compositions from MGS-TES, Science 287, pp. 1626-1630.
  • Hamilton, V.E., M.B. Wyatt, H.Y. McSween, and P.R. Christensen (2001): Analysis of terrestrial and Martian volcanic compositions using thermal emission spectroscopy: 2. Application to Martian surface spectra from the Mars Global Surveyor Thermal Emission Spectrometer, Journal of Geophysical Research 106, pp. 14733-14746.
  • Matthes, S. (2001): Die magmatischen Gesteine (Magmatite). In: Matthes, S. (eds.), Mineralogie. Springer-Verlag, Berlin, pp. 187-192.
  • Poulet, F., C. Gomez, J.P. Bibring, Y. Langevin, B. Gondet, P. Pinet, G. Belluci, and J. Mustard (2007): Martian surface mineralogy from Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité on board the Mars Express spacecraft (OMEGA/MEx): Global mineral maps, JGR (Planets) 112.

Last update: 31/05/2010 15:56