Low-Temperature Carbonate Concretions in the Martian Meteorite ALH84001: Evidence from Stable Isotopes and Mineralogy

Abstract

The martian meteorite ALH84001 contains small, disk-shaped concretions of carbonate with concentric chemical and mineralogical zonation. Oxygen isotope compositions of these concretions, measured by ion microprobe, range from delta18O = +9.5 to +20.5 per thousand. Most of the core of one concretion is homogeneous (16.7 +/- 1.2 per thousand) and over 5 per thousand higher in delta18O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous (delta18O = 4.6 +/- 1.2 per thousand). Secondary SiO2 has delta18O = 20.4 per thousand. Carbon isotope ratios measured from the core of one concretion average delta13C = 46 +/- 8 per thousand, consistent with formation on Mars. The isotopic variations and mineral compositions offer no evidence for high temperature (>650 degrees C) carbonate precipitation and suggest non-equilibrium processes at low temperatures (< approximately 300 degrees C).

Geochemistry of Carbonates on Mars: Implications for Climate History and Nature of Aqueous Environments

“…The results of these analyses for ALH 84001 show a wide variation in both δ 18 O and δ 13 C correlated with chemical composition (Fig. 4) (Eiler et al 2002;Holland et al 2005;Leshin et al 1998;Niles et al 2005;Saxton et al 1998;Valley et al 1997). The early-forming, Ca-rich phases were found to have the lowest δ 18 O and δ 13 C values and the Mg-rich, later-forming phases were found to have the highest δ 18 O and δ 13 C values (Leshin et al 1998).…”

“…2) and have a mixed Ca, Fe, Mg composition that is commonly found in low temperature diagenetic concretions on Earth (Mozley 1989). This is the result of kinetic effects associated with the dehydration of Mg (Lippmann 1973) and rapid crystallization (Valley et al 1997). The composition is consistent with other evidence Fig.…”

“…The scatter in the data likely reflects the variety of different carbonate phases within the martian meteorites. These multiple episodes vary strongly in carbon isotopes and may reflect evolution of the martian atmosphere through time that suggests that the carbonate-forming aqueous systems were short lived and subject to dynamic environmental changes (Changela and Bridges 2011;Niles et al 2005;Valley et al 1997). Experimental work performed to date has succeeded in forming carbonates with a similar chemical composition as the ALH 84001 carbonates (Golden et al 2000(Golden et al , 2001, but not with the correct variation in isotopic composition.…”

“…The carbonates contained in martian meteorites have isotopic compositions that can be highly variable on the micro-scale (Valley et al 1997) and also contain highly enriched carbon isotope compositions with δ 13 C values as high as +64 h-much more enriched than sedimentary carbonates on Earth, which rarely exceed 10 h (Knauth and Kennedy 2009;Niles et al 2005;Romanek et al 1994). The isotopic compositions have been measured using a variety of techniques including acid dissolution, pyrolysis-IRMS, and Secondary Ion Mass Spectrometry (SIMS).…”

“…One hypothesis contends that the ALH 84001 carbonates have a number of characteristics that indicate their low temperature (< ∼100 • C) deposition in a dynamic environment during perhaps several different episodes (Corrigan and Harvey 2004;Halevy et al 2011;Holland et al 2005;McSween and Harvey 1998;Niles et al 2005Valley et al 1997;Warren 1998). Another hypothesis suggests that the carbonates formed in a rapidly cooling environment which was initially at higher temperature (>150°C) and cooled to ∼30°C (Eiler et al 2002;Leshin et al 1998;Romanek et al 1994;Steele et al 2007), although this is now directly disputed by recently reported clumped isotope results (Halevy et al 2011).…”