Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..572s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 572
Other
2
Scientific paper
Magnetite is a phase that may form by aqueous alteration and may therefore offer insights into processes of aqueous alteration in the early solar system. Rowe et al [1] have used oxygen isotopes of magnetite and other phases to constrain conditions of aqueous alteration of the CM and CI chondrites. Semarkona is one of the few ordinary chondrites known to have undergone hydrothermal alteration: Hutchison et al [2] found smectite and calcite in this meteorite as well as minor quantities of magnetite. In-situ oxygen isotope analyses of one magnetite grain from Semarkona were made with the ISOLAB 54 ion microprobe at the University of Manchester [3,4]. Four spots were analyzed, the spot size being 10-15 micrometers. A grain of terrestrial magnetite of known isotopic composition was used as a standard. Reproducibility of measurements of the standard was 1.6 per mil for ^(17)O/^(16)O, 2.0 per mil for ^(18)O/^(16)O and 1.5 per mil for Delta^(17)O. Results are shown in the figure, which also shows the composition of bulk Semarkona as given by Clayton et al [5]. The average Delta^(17)O for the magnetite is +2.9 per mil which is slightly higher (~2 sigma) than the bulk meteorite (Delta^(17)O=+1.07 per mil), but more measurements are desirable to confirm this result. The magnetite is clearly isotopically lighter (by ~9 per mil in delta^(18)O) than the bulk meteorite. It is also lighter (by ~8per mil in delta^(18)O) than the magnetite from CI and CM meteorites described by Rowe et al [1] and therefore cannot have been derived directly from any of the water reservoirs inferred by Rowe et al to have been involved in the evolution of the CI and CM meteorites. Since the maximum fractionation between magnetite and water is -13.6 per mil (at 120 degrees C, [1,6]) the Semarkona magnetite could have formed in equilibrium with liquid water at low temperature, provided the water had evolved to a composition close to that of the bulk meteorite (delta^(18)O within ~5 per mil). Such a water composition may have been achieved through exchange with the silicate minerals in the meteorite, albeit with a lower water/rock ratio than was the case for the carbonaceous chondrites. A low water/rock ratio during aqueous alteration is also consistent with the high D/H ratio of Semarkona [7], which suggests the indigenous (high D/H) water has not been diluted significantly by isotopically more normal water. Further measurements of the magnetite are in progress, in order to search for any inter-grain variability and to improve our measure of its Delta^(17)O value. References: [1] Rowe M. W. et al. (1994) GCA, 50, 1379-1395. [2] Hutchison R. et al., GCA, 51, 1875-1882. [3] Lyon I. C. et al. (1994) Rapid Comm. Mass Spec., 8, 837-843. [4] Saxton J. M. et al. (1995) Analyst, 120, 1321-1326. [5] Clayton R. N. et al. (1991) GCA, 55, 2317-2337. [6] Clayton R. N. and Kieffer S. W. (1991) in Stable Isotope Geochemistry: A Tribute to Samuel Epstein, Geochem. Soc. Spec. Publ., 3, 3-10. [7] Sears D. W. G. et al. (1995) Meteoritics, 30, 169-181.
Hutchison Robert
Lyon Ian C.
Saxton John M.
Turner Gary
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