Oxygen Isotopes in Spinels from Antarctic Micrometeorites

Spinel-rich inclusions were found in a large unmelted micrometeorite (MM) from Antarctica [1]. This particle (MM92/15-23) consists of a fine-grained matrix of dehydrated former phyllosilicates, which enclose a few small olivines, one large chromite, and several spinel-rich inclusions. The latter form elongated to rounded bodies up to 35 micrometers in length and consist of a spinel core enveloped by a Fe-rich silicate phase which probably is a (dehydrated?) phyllosilicate--too small to be analyzed with the electron microprobe. A few very small perovskite grains (<2 micrometers) are enclosed within the spinel. The chemical composition of the spinel is that of a Mg-Al spinel containing minor amounts of SiO2 (0.21 wt%), TiO2 (0.09%), Cr2O3 (0.11%), and FeO (0.83%). On top of the Fe-rich silicate envelops there is a discontinuous rim of aluminous Ca-rich pyroxene with a fairly high FeO content (8.0 wt%). The trace element content as determined by secondary ion mass spectrometry (SIMS) of these inclusions resembles that of group II Ca-Al-rich inclusions [2]. Meanwhile we have found a second Antartic micrometeorite containing a few spinel grains. This spinel is associated with some tiny ilmenite grains and embedded in the foamy melt matrix of scoriaceous micrometeorite particle MM94/1-28. This particle is about 80 micrometers long and consists mainly of a highly versicular melt of chondritic composition, which encloses the spinel grains (up to 10 micrometers long) and a few particles of thermally altered former phyllosilicates. The chemical composition of the spinel is that of a Mg-Al-spinel containing small amounts of FeO (0.6 wt%), but no Cr2O3. We have successfully analyzed the oxygen isotopic composition of two spinels from MM92/15-23 and one from MM94/1-28 following the procedures as outlined by [3]. The oxygen isotopic compositions were found to be (delta^17O/delta^18O relative to SMOW in permil) -25/-23, -24/-14, and -23/-24 in spinel 92/15-23/1 and 2 and 94/1-28,respectively. The data are plotted in the Figure with the approximate 2 sigma error bars of +/- 8 permil. Two of our data points plot directly onto the Allende mixing line (e.g., [4]) approximately halfway between the spinels richest in ^16O and the terestrial fractionation line. Such an isotopic composition of O is typical for spinel-rich type II inclusions (e.g., [5]). One spinel of MM92/15-23 plots to the right of the Allende mixing line. However, considering the analytical uncertainties, its oxygen isotopic composition is compatible with the Allende mixing line. All spinels are much less ^16O-rich than a CAI found among interplanetary dust particles [6]. In conclusion, the most common matter accreting onto the Earth today and represented by unmelted and partially melted micrometeorites consists of a matter similar, but not identical, to CM carbonaceous chondrites (e.g., [7]). The presence of spinel-rich CAIs with trace element contents and oxygen isotopic compositions of group II inclusions provides an additional support of that view. References: [1] Kurat et al. (1994) LPS XXV, 763-764. [2] Martin P. M. and Mason B. (1974) Nature, 249, 333-334. [3] Zinner E. (1989) USGS Bull., 1890, 145-162. [4] Clayton R. N. (1993) Annu. Rev. EPS, 21, 115-149. [5] Clayton R. N. et al. (1986) LPS XVII, 139-140. [6] Stadermann F. J. (1991) LPS XXII, 1311-1312. [7] Kurat et al (1994) GCA, in press. Fig. 1., which appears here in the hard copy, shows isotopic composition of oxygen in spinels from Antarctic micrometeorites. AML: Allende mixing line.