Martian meteorites shergottites

Martian meteorites ( SNC for shergottite, nakhlite, chassignite) comprise a diverse suite of igneous rocks (McSween, 2004). With only one exception, they are geologically young. Presently, 34 of these meteorites are recognized, most recovered from Antarctica and North African deserts. 

Martian meteorites, viewed in plane polarized light with FOV = 5.4 mm. (a) Zagami basaltic shergottite, composed of pyroxene and plagiodase (white), which has been converted to maskelynite by shock, (b) Lafayette nakhlite, composed mostly of high-calcium pyroxene, (a) from Lauretta and Killgore (2005), with permission. 

Borg and Drake (2005) have determined the timing of aqueous alteration events in Martian meteorites from the ages of secondary minerals. Carbonates in ALH 84001 formed at 3.9 Ga, iddingsite in nakhlites formed -630 Myr ago, and salts in shergottites formed sometime after the crystallization of these rocks, 170 Myr ago. 

Ejection ages (sum of cosmic-ray exposure age + terrestrial age) for Martian meteorites. The ages cluster by meteorite type, suggesting that each cluster represents a distinct impact (ejection) event. The only outliers are the EETA 79001 and Dhofar 019 shergottites and ALHA84001. Modified from McSween (2008). 

Although the rocks in Meridiani Planum analyzed by the Opportunity rover are fascinating (see below), they are unusual. Therefore, we will focus on the rocks and soils in Gusev crater analyzed by the Spirit rover, which are spectrally similar to the bulk of the Martian crust. We will compare these compositions with those of Martian meteorites and Bounce Rock in Meridiani, which is similar to shergottites. We will also consider orbital geochemical data obtained by GRS. 

The shergottites are the most abundant (18 out of 26) and the most diverse of the martian meteorite subgroups. They are commonly divided into two types basaltic and Uierzohtic. 

Much ink has flowed over the number of impacts required to produce the shergottites. The controversy arises partly out of technical issues concerning production rates and partly because of the apparent scarcity on Mars of target sites with rocks that appear likely to have the crystallization ages measured for the martian meteorites, which tend to be young, —200 Myr (Nyquist et al., 1998, 2001). Clearly, if martian terrain with the right crystallization age is rare, then each new discovery of... 

Figure 5 Mg/Si versus Al/Si weight ratios show that martian meteorites and Mars rocks and soils are depleted in Al, relative to terrestrial rocks. Filled circles are basaltic shergottites, open circles are Pherzolitic shergottites, filled squares are nakhlites and chassignites, open square is orthopyroxenite, and larger symbols are for Mars rocks and soils analyzed in situ (after Rieder et aL, 1991 McSween, 2002).

Figure 11 Chemical classification of martian volcanic rocks. Squares show basaltic materials in the southern highlands (surface 1) and andesitic materials in the northern lowlands (surface 2), derived from deconvolved TES spectra from Mars Global Surveyor (Hamilton et al., 2001). Analyzed compositions of the Mars Pathfinder dust-free rock (Wanke et al., 2001) and martian meteorites (basaltic shergottites are filled circles and nakhlites are...

Recent, more detailed investigations of oxygen isotopic compositions of martian meteorite components have focussed on secondary minerals and water contained in the samples. Studies that report analyses of all three oxygen isotopes consistently show that secondary minerals and water in the martian meteorites do not lie on the MSFL. Karlsson et al. (1992) observed elevated A O values, up to +0.9 %o (or -0.6 %o above the MSFL) in water extracted by stepped heating from Nakhla, Lafayette and Chassigny, with the most pronounced effects observed in Nakhla. The water extracted from the shergottites generally records A O values between than the MSF and the TF line. Farquhar and... 

Folco L, Franchi lA, D Orazio M, Rocchi S, Schultz L (2000) A new martian meteorite from the Sahara The shergottite Dar al Gani 489. Meteoritics Planet Sci 35 827-839 Franchi lA, Wright IP, Sexton AS, Pillinger CT (1999) The oxygen-isotopic composition of Earth and Mars. Meteoritics Planet Sci 34 657-661... 

Kiefer WS (2003) Melting in the martian mantle Shergottite formation and implications for present-day mantle convection on Mars. Meteorit Planet Sci 39(12) 1815-1832 King EA, Wagstaff J (1981) Micrometeorites from Antarctic ice cores. Ant J US 16(5) 92-93... 

Taylor LA, Nazarov MA, Shearer CK, McSween HY Jr, Cahill J, Neal CR, Ivanova MA, Barsukova LD, Lentz RC, Clayton RN, Mayeda TK (2002) Martian meteorite Dhofar 019 A new shergottite. Meteorit Planet Sci 37 1107-1128 Taylor S, Brownlee DE (1991) Cosmic spherules in the geologic record. Meteoritics 26 203-211... 

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