Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.605z&link_type=abstract
Meteoritics, vol. 30, no. 5, page 605
Computer Science
7
Chondrites, Primitive, Condensation, Metamorphic Temperatures, Meteorite, Allende, Bishunpur, Krymka, Renazzo, Semarkona, Opaque Association, Sulfides
Scientific paper
Sulfide veneers are frequent on chondrules [1-4] and often contain partly or totally oxidised and/or carburised metal grains. Haggerty and McMahon [1] observed veneers, composed of troilite +/- awaruite +/- pentlandite +/- heazlewoodite, around ~3/4 of the chondrules in Allende. They attributed them to a discrete post-chondrule event, possibly vapor deposition, and Blum et al [5] showed that the opaque mineral assemblages (OAs) in Allende are due to low-temperature reequilibration (thermal metamorphism). Hutchison et al. [2] observed very similar features in Semarkona, where in situ alteration of the silicates had taken place around 260 degrees C, and they attributed the OAs to the same cause. Similar associations are commonly found in the serpentinisation of peridotites [e.g. 6]. We find the same low temperature minerals to [2,5] in sulfide veneers in Semarkona, but also an Fe-Co alloy (close to Fe3Co, and possibly wairauite) known to exist in terrestrial serpentinisation associations [6]. From the Ni in pentlandite in the assemblage troilite-pentlandite-awaruite we derive metamorphic temperatures [7] of 230 degrees C (s.d 20) for Semarkona and, using the data of [8], 335 degrees C (s.d. 50) for Allende. Chondrule veneers in more metamorphosed (and less altered) objects such as Krymka and Bishunpur show no carbides and pentlandite, but Ni-free troilite with minor amounts of tetrataenite. [2] implied that OAs were derived from a metal/troilite association reacting with C and O, the Ni liberated by the oxidation (and possibly carburization) of Fe reacting with the troilite to make the low temperature phase pentlandite. We disagree. We have interpreted sulfide veneers as the product of recondensation of S vaporized during chondrule formation on metal grains on the chondrule surface [3,4]. In Renazzo, veneers consist of an intimate troilite-pentlandite intergrowth derived from Ni-rich monosulfide solid solution (mss). Recent experiments by Lauretta and Fegley [9] show that condensate sulfide made from the reaction of gaseous H2S with kamacite contains significant dissolved Ni, with concentrations increasing away from the metal-sulfide interface and depending on the temperature. We therefore believe that most of the initial OA material in all these meteorites was Ni (and possibly Co) -bearing troilite (mss). The evolution of such a phase in a parent body depended on the prevailing conditions (low T hydrous alteration or thermal metamorphism). In [4], we showed that S migrated in the early metamorphism, FeS appearing inside low-FeO chondrules in Bishunpur and becoming frequent in more metamorphosed objects, though absent from low FeO chondrules in both Renazzo and Semarkona. The initial presence of Ni inside a large fraction of the chondrites' sulfide is the reason for such a migration, some S being released during the metamorphic breakdown of this phase into Ni-free troilite and tetrataenite and ultimately reacting with metal inside chondrules. References: [1] Haggerty S. E. and McMahon B. M. (1979) Proc. LPSC 10th, 851-870. [2] Hutchison R. et al. (1987) GCA, 51, 1875-1882. [3] Zanda B. et al. (1994) in Chondrules and the Protoplanetary Disk, 47-48. [4] Zanda B. et al. (1995) GCA, submitted. [5] Blum J. D. et al. (1989) GCA, 53, 543-556. [6] Lorand J. P. (1985) Tschermaks Mineral. Petrol. Mitt., 34, 183-209. [7] Misra K. C. and Fleet M. E. (1973) Econ. Geol., 68, 518-539. [8] Eisenhour D. D. (1994) Ph.D. dissertation, Arizona State Univ. [9] Lauretta D. S. and Fegley B. Jr. (1995) LPS XXVI, 831-832.
Bourot-Denise Michèle
Hewins Roger H.
Zanda Brigitte
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