Aqueous alteration of the Nakhla meteorite

Abstract

Abstract— Interior samples of three different Nakhla specimens contain an iron‐rich silicate “rust” (which includes a tentatively identified smectite), Ca‐carbonate (probably calcite), Ca‐sulfate (possibly gypsum or bassanite), Mg‐sulfate (possibly epsomite or kieserite), and NaCl (halite); the total abundance of these phases is estimated as <0.01 weight percent of the bulk meteorite. Rust veins are truncated and decrepitated by fusion crust and are preserved as faulted segments in partially healed olivine crystals, indicating that the rust is pre‐terrestrial in origin. Because Ca‐carbonate and Ca‐sulfate are intergrown with the rust, they are also indicated to be of pre‐terrestrial origin. Similar textural evidence regarding origins of the NaCl and Mg‐sulfate is lacking. Impure and poorly crystallized sulfates and halides on the fusion crust of the meteorite suggest leaching of interior (pre‐terrestrial) salts from the interior after Nakhla arrived on Earth but coincidental addition of these same salts by terrestrial contamination cannot be excluded. At least the clay‐like silicate “rust,” Ca‐carbonate, and Ca‐sulfate were formed by precipitation from water‐based solutions on the Nakhla parent planet although temperature and pressure conditions of aqueous precipitation are unconstrained by currently available data. It is possible that aqueous alteration on the parent body was responsible for the previously observed disturbance of the Rb‐Sr geochronometer in Nakhla at or near 1.3 Ga.

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“…In contrast most of the siderite in Governador Valadares and Nakhla is located in interstitial positions and has lower Ca contents. These two meteorites contain gypsum, Nakhla also containing anhydrite (Chatzitheodoridis and Turner, 1990;Bridges and Grady, 1999), halite and epsomite (Gooding et al, 1991). Figure 1b shows halite and anhydrite within a section of Nakhla.…”

“…On the basis of a TEM study, Gooding et al (1991) noted that the iddingsite veins within Nakhla olivine contained a probable smectite/illite, K-bearing mixed layer clay. Similarly, Treiman et al (1993) described smectite in Lafayette.…”

“…For instance, clay veins within the nakhlites are truncated by fusion crust and so are clearly preterrestrial (Gooding et al, 1991;Treiman et al, 1993). The association of some of the ALH84001 carbonate with preterrestrial fracturing is also incompatible with a terrestrial origin (e.g.…”

“…Subsequent studies (summarised in Table I) have enabled the mineralogical and stable isotope characteristics to be understood in more detail. The three nakhlites (Lafayette, Governador Valadares and Nakhla) contain smectite/illite, siderite, sulphates and halite (Chatzitheodoridis and Turner, 1990;Gooding et al, 1991;Treiman et al, 1993;Grady, 1999, 2000). The clay veins in these meteorites cross-cut the olivine and, in Lafayette (Figure 1a), they are intergrown with Ca-rich siderite (Vicenzi and Eiler, 1998;Bridges and Grady, 2000).…”

“…Terrestrial salts can be present and are abundant in the desert finds Dar al Gani 476 and 489 (Wadhwa et al, 2000). Minor efflorescence of salt from meteorite interiors onto the fusion crusts can also occur (Gooding et al, 1991) and so care is necessary in distinguishing martian from terrestrial mineral assemblages. Isotopic data suggests that some of the shergottites may contain traces of terrestrial salts along with martian secondary assemblages.…”