Detection of anorthosite rocks on Mars
Abstract
The surface of Mars is primarily made up of basaltic (volcanic) rocks comprised of pyroxene, olivine and intermediate felsic plagioclase minerals [e.g. 1,2] and additionally a smaller fraction of sedimentary rocks, at times composed of hydrated salt and clay minerals [3,4]. A few localized eruptive sequences may indicate some compositional evolution from basaltic to dacitic rocks [1], but these remain in essence volcanic rocks. Using the CRISM (Compact Imaging Reconnaissance Spectrometer for Mars) near-infrared imaging spectrometer orbiting Mars [5], we report the detection of a new rock type on Mars, anorthosite. Anorthosite is a highly felsic (>90% plagioclase, <10% mafic minerals) non-volcanic igneous rock which peculiar composition requires very specific formation processes. On Earth, anorthosite is a rare rock found mostly in plutonic rocks in continental areas sharing locations with granitoid rocks. Anorthosite is also a major component of the lunar crust and ubiquitous in the lunar highlands where it is interpreted to be the result of the crystallisation of the primordial magma ocean of the Moon > 4.3 Gyrs ago [6]. At least 8 anorthosite exposures have been found scattered over the southern highlands of Mars. These are found in the rims of large (D > 50 km) craters or as outcrops in massif units. The unit age for these anorthosite exposures places their formation early in the planet's history (> 4 Gyrs). The massifs exposures are interpreted as deep crustal material uplifted from the Hellas basin forming event [7], which together with the crater rim exposures, suggest a formation at depth in all cases. The preferential co-occurrence of Al-rich clays mixed with several anorthosite exposures also suggests that these rocks were later altered by water at or near the surface. On Mars, there are several reasons to explain why such rocks would not have been formed during its primordial differentiation. In particular, the production of significant quantities of highly differentiated, siliceous melt that would form anorthositic rocks requires specific mechanisms such as fractional crystallization, assimilation, or partial melting of an already evolved source incompatible with the ultramafic source composition inferred for Mars [8,9]. The unforeseen detection of anorthositic-rocks therefore places new and strong constraints on the formation and the evolution of the crust and the mantle of Mars. Two possible formation scenarios are discussed, either as a result of the formation of the primordial Martian crust or from later plutonic activity. [1] Christensen et al., Nature (2005). [2] Mustard et al., Science (2005). [3] Malin et al., Science (2000). [4] Murchie et al., JGR (2009). [5] Murchie et al., JGR (2007). [6] Wood et al., Proc. LPSC (1970). [7] Leonard & Tanaka, Geologic Investigations Series I-2694, USGS (2001). [8] Elkins-Tanton et al., JGR (2005). [9] Clarke, Granitoid Rocks, Chapman and Hall (1992).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.P44A..07C
- Keywords:
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- 3640 MINERALOGY AND PETROLOGY / Igneous petrology;
- 5410 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Composition;
- 5464 PLANETARY SCIENCES: SOLID SURFACE PLANETS / Remote sensing;
- 6225 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Mars