Chromium, Nickel, and Iron in Deep-Sea Spheres

Details Chromium, Nickel, and Iron in Deep-Sea Spheres Chromium, Nickel, and Iron in Deep-Sea Spheres

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.558n&link_type=abstract

Meteoritics, vol. 30, no. 5, page 558

Astronomy and Astrophysics

Astrophysics

1

Ablation, Atmospheric, Deep Sea Spheres, Fractionation, Isotopic, Isotopes, Chromium, Iron, Nickel, Vaporization

Scientific paper

Spheres of iron and nickel oxides from deep sea sediments are thought to be extra-terrestrial material which was melted and oxidized during atmospheric passage [1]. The heavy isotopes of oxygen, iron, and nickel are often highly enriched in these "Type I" spheres [2-6]. From the fractionation of the Ni and Fe isotopic compositions, Xue et al. [6] inferred pre-atmospheric Fe/Ni ~13 for such spheres. This result does not uniquely determine the source material as several types of meteoritic metal have similar Fe/Ni ratios. We have analyzed some deep-sea spheres for chromium as well as for nickel and iron. The vapor pressure of metallic Cr, a possible measure of its tendency to evaporate, exceeds that of Fe and Ni. Type I spheres KK1-9 (256 micrograms) and KK1-10 (222 micrograms) were dissolved in 500 microliters 6N HCl and a 5% aliquot of each solution was taken for elemental analysis by ICP/MS [6]. The Fe, Ni, and Cr in the remaining solutions were separated by ion exchange. The isotopic abundances of Fe and Cr were determined by thermal ionization MS [7,8] and those of Ni by ICP/MS [6]. Elemental concentrations were: KK1-9 -- Fe 64.6%, Ni 5.5%, Cr 160 ppm; KK1-10 -- Fe 63.3%, Ni 0.038%, Cr 450 ppm. The composition of KK1-9 is typical of Type I spheres. The low nickel content of KK1-10 is unusual among spheres we have analyzed [6], but not unprecedented (cf [9]). The isotopic mass fractionation, Phi, in sphere KK1-9 is 2.2+/-0.4%/AMU for Fe (Fig. 1) and 1.9+/-0.4%/AMU for Ni. Assuming Rayleigh-type distillation, these values correspond to evaporative losses of ~90% for each element, and a source Fe/Ni ratio similar to that measured; i.e., ~12. The Ni data conform to the anticorrelation of Phi with Ni concentration [4,5]. Cr in KK1-9 was too low for isotopic analysis, but Cr/Fe = 0.00025, slightly higher than in most iron meteorites. This result is consistent with iron meteorite source materials, if Fe was lost somewhat preferentially to Cr, counter to expectations from the vapor pressures of the metals. The Ni available from KK1-10 was insufficient for isotopic analysis, but Fe was unfractionated (Fig. 1). Phi(Cr) = 0.27+/-0.21%/AMU, implying a modest Cr loss of ~30%. A terrestrial origin for KK1-10, suggested by low Ni/Fe <10^-3, is counter to mass fractionation of Cr and the presence of cosmogenic 53Mn in similar spherules [10]. The possibility of an extra-terrestrial source with low Ni/Fe merits further investigation. The Fe isotopic abundances in KK1-10 were within error limits of the terrestrial composition when normalized to 54Fe/56Fe = 0.062669 [11], but the data for KK1-9 hint at an 57Fe deficit of -2+/-1 epsilon-units, similar to values reported for non-FUN inclusions of Allende [11]. References:[ 1] Blanchard M. B. et al. (1980) EPSL, 46, 178-190. [2] Davis A. M. and Brownlee D. E. (1993) LPS XXIV, 373-374. [3] Davis A. M. et al. (1991) LPS XXII, 281-282. [4] Clayton R. N. et al. (1986) EPSL, 79, 235-240. [5] Herzog et al. (1994) GCA, 58, 5319-5323. [6] Xue et al. (1995) LPS XXVI, 1527-1528. [7] Shih C.-Y. et al. (1995) LPS XXVI, 1289-1290. [8] Nyquist L. E. et al. (1994) Meteoritics, 29, 872-885. [9] Czajkowski J. (1987) Ph.D. dissertation, Univ. of California, San Diego. [10] Nishiizumi K. (1983) EPSL, 63, 223-228. [11] Volkening J. and Papanastassiou D. A., Astrophys. J., 347, L43-L46. Figure 1 showing the Fe-isotopic fractionation in two Type I spheres appears here in the hard copy.

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