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

• Published in: Science (American Association for the Advancement of Science) Vol. 275; no. 5306; pp. 1633 - 1638
• Main Authors: Valley, John W., Eiler, John M., Graham, Colin M., Gibson, Everett K., Romanek, Christopher S., Stolper, Edward M.
• Format: Journal Article
• Language: English
• Published: Legacy CDMS American Society for the Advancement of Science 14.03.1997; American Association for the Advancement of Science; The American Association for the Advancement of Science
• Subjects: Atmosphere; Carbon Isotopes; Carbonate rocks; Carbonates; Carbonates - chemistry; Chemical composition; Chemical Precipitation; Composition; Cosmochemistry. Extraterrestrial geology; Earth sciences; Earth, ocean, space; Evidence; Exact sciences and technology; Extraterrestrial geology; Extraterrestrial life; Geology; High temperature; Ion probes; Isotopes; Life on other planets; Lunar And Planetary Science And Exploration; Magnesite; Mantle; Mars; Mars (Planet); Martian atmosphere; Meteorites; Meteoroids; Meteors & meteorites; Mineralogy; Minerals; Minerals - chemistry; Music analysis; Oxygen Isotopes; Precipitation; Rocks, Carbonate; Silicon Dioxide - chemistry; Space life sciences; Temperature; Nasa Discipline Exobiology; Nasa Discipline Number 52-10; Nasa Center Jsc; Nasa Program Exobiology; meteorites; stable isotopes; O-18; orthopyroxenite; pyroxenite; polished sections; SEM data; igneous rocks; low temperature; ion probe data; carbonates; mineral assemblages; Mars; concretions; C-13; Allan Hills Meteorites; zoning; isotope fractionation; plutonic rocks; electron microscopy; shock metamorphism; cumulates; ultramafics; electron probe data; NASA Discipline Exobiology; NASA Center JSC; NASA Discipline Number 52-10; NASA Program Exobiology
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.275.5306.1633

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 $\delta^{18}$O = +9.5 to +20.5 ‰. Most of the core of one concretion is homogeneous (16.7 ± 1.2 ‰) and over 5 ‰ higher in $\delta^{18}$O than a second concretion. Orthopyroxene that hosts the secondary carbonates is isotopically homogeneous ($\delta^{18}$O = 4.6 ± 1.2 ‰). Secondary SiO$_2$ has $\delta^{18}$O = 20.4 ‰. Carbon isotope ratios measured from the core of one concretion average $\delta^{13}$C = 46 ± 8 ‰, consistent with formation on Mars. The isotopic variations and mineral compositions offer no evidence for high temperature (>650°C) carbonate precipitation and suggest non-equilibrium processes at low temperatures (<∼300°C).