We have analyzed micrometeorites (MMs) retrieved from blue ice at Cap Prudhomme by EMP (15 particles) and INAA (25 particles) for bulk major and trace elements. Of the total of 40 particles in the size range 50-400 micrometers (resp. weight range 1-47 micrograms), 35 have refractory lithophile element contents very similar to CM and CI chondrites. Their abundances most commonly range between 0.7 and 3 x CI. Lithophile elements more volatile than Cr are highly variable and can either be depleted or enriched compared to CI abundances (Na, K, Rb, Zn, and Br). However, some of the major lithophile elements are also commonly depleted in MMs. All phyllosilicate-dominated MMs analyzed by us are depleted in Ca between 0.8 and 0.1 x CI, some are also depleted in Mg (to 0.2 x CI), Mn (to 0.08 x CI), and Na (to 0.09 x CI). Siderophile element abundances are usually subchondritic with a range mostly between 0.1 and 2 x CI for Os, Ir, Ni, Co, and Fe. However, the Ni/Ir ratios are usually subchondritic as are the Ni/Co and Ni/Fe ratios. This is due to a depletion in Ni, which is strongest developed in CI-like MMs and an overabundance, on average, of Fe. Siderophile elements more volatile than Fe have highly variable abundances (0.2-16 x CI for Au and As). Commonly depleted are Se (0.08-0.8 x CI) and S (0.06-0.5 x CI). These features have been described before from MMs (1-5) and IDPs (6-7) except for the depletions in Mg and Mn present in some of our MMs. There is no consensus regarding the origins of the depletions and enrichments (e.g., 2,3,5-8). Comparison of compositions of unmelted MMs with cosmic spheres (9 and our own data) shows that the depletions in Ca and Ni can in part be preterrestrial (2). No evidence was found for preterrestrial depletions in Mg and Mn. These could be due to terrestrial leaching of dolomite, a common phase of CM and CI chondrites that we could not find so far in MMs. Similarily, most Ca depletions could be due to terrestrial leaching of calcite. The depletion in S could also be due to terrestrial loss of sulphates, common in CI chondrites. All enrichments observed-- with the possible exception of C (e.g., 10)--appear to be terrestrial, analogous to what has been proposed for Br and Cl (11). These comprise enrichments in K, Rb, Zn, As, and Fe. The latter is visibly enriched in many MMs that are coated by magnetite rims, apparently condensates emplaced in the upper atmosphere whose E-layer is enriched in Fe (and other elements from evaporated meteoroids, e.g., 12). In some cases Fe diffusion from the surface into the particles is also observed. In conclusion, bulk compositions of most MMs resemble that of CM and CI chondrites but appear to have been altered in the terrestrial environment to different degrees. Some bulk chemical (10) and mineralogical (13) peculiarities, however, are apparently pristine features of MMs. Acknowledgement: This work was financially supported by FWF, Vienna References: (1) Maurette M. et al. (1991) Nature 351, 44-47; (2) Maurette M. et al. (1992) Lunar Planet. Sci. XXIII, 861-862; (3) Klock W. and Beckerling W. (1992) Lunar Planet. Sci. XXIII, 725-726; (4) Koeberl C. et al. (1992) Lunar Planet. Sci XXIII, 709-710; (5) Kurat G. et al. (1992) Lunar Planet. Sci XXIII, 747-748; (6) Brownlee D.E. (1985) Ann. Rev. Earth Planet. Sci. 13, 147- 173; (7) Sutton S.R. and Flynn G.J. (1988) Proc. Lunar Planet. Sci. Conf. 18, 607-614; (8) Flynn G.J. and Sutton S.R. (1990) Proc. 20th Lunar Planet. Sci. Conf., 335-342; (9) Christophe Michel-Levy M. (1992) Meteoritics 27, 73-80; (10) Perreau M. et al. (1992) Meteoritics, this issue; (11) Jessberger E. et al. (1992) MS submitted to EPSL; (12) Steinweg A. et al. (1992) J. Atmosph. Terrestr. Phys., in press; (13) Presper T. et al. (1992) Meteoritics, this issue.
- Pub Date:
- July 1992