Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.236h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 236
Other
3
Scientific paper
Adelaide is an anomalous carbonaceous chondrite with a CV-like oxygen isotopic composition (Hutcheon and Steele, 1982), abundant small chondrules, and an anhydrous, low-Fe matrix. The bulk composition, which is highly depleted in volatiles and carbon (Kallemeyn and Wasson ,1982), and the H, N, and C isotopic compositions and abundances (Kerridge, 1985) do not match those of any known meteorite; nearest relatives appear to be Bench Crater and Kakangari (Fitzgerald and Jones, 1977). Adelaide contains a variety of refractory inclusions, including several rich in CaAl4O7 (Hutcheon and Steele, 1982). We report here initial results from a study of Mg and Si isotopes in Adelaide CAI. SMH-1 is a 150 x 200-micrometer compact ovoid inclusion of densely intergrown spinel (52% of the mode), melilite (33%), and hibonite (14%). Hibonite occurs as 10 x 25 micrometer blades projecting radially from the periphery into a melilite-rich core. A 5-micrometer-thick diopside rim surrounds SMH-1. Textural relationships suggest that SMH-1 crystallized from a melt. Mg is enriched in the heavier isotopes by ~2o/oo/amu and all phases have excess ^26Mg* from the in situ decay of ^26Al. Data from spinel, hibonite, and melilite define a linear array with slope ^26Mg*/^27Al = (4.5+-0.8) x 10^-5. HM-1 is a 200 x 500-micrometer compact inclusion of tightly intergrown hibonite (75%) and gehlenitic melilite (25%), with accessory perovskite. No spinel is present. HM-1 may be a fragment of a larger CAI; a ~30-micrometer-thick rim of secondary Ca-poor, hydrous alumino-silicate surrounds ~40% of HM1. Similar secondary material is also present in the interior. Melilite (Ak(sub)0.6 to Ak(sub)1.6) and hibonite show clearly resolved ^26Mg* with delta^26Mg up to 90o/oo in melilite, and define an array with slope ^26Mg*/^27Al = (4.3 +- 0.3) x 10^-5. The secondary alumino-silicate also exhibits ^26Mg*, but the inferred initial ratio falls considerably below 5 x 10^-5, suggesting partial exchange of Mg. Inclusion M12 is an irregularly shaped 100 x 200-micrometer CAI consisting almost entirely of melilite. M12 exhibits unusual isotopic heterogeneity. Mg is enriched in the heavier isotopes, but F(sub)Mg is not constant among melilite: 4.5 < F(sub)Mg < 19o/oo/amu. The heavier Si isotopes are also enriched but F(sub)si is nearly constant at 11 +- 2o/oo/amu. Excess ^26Mg* is present in some, but not all, melilite characterized by F(sub)Mg = 19o/oo/amu; melilite with F(sub)Mg = 4.5o/oo/amu has delta^26Mg = 0. The data suggest that, initially, F(sub)Mg was >= 20o/oo/amu and F(sub)Si was ~12o/oo/amu in M12. Subsequent reaction with an isotopically normal reservoir produced partial isotopic re-equilibration of Mg but not Si, creating a heterogeneous F(sub)Mg distribution and causing loss of ^26Mg*. Two 25- to 50-micrometer CAI consist almost entirely of CaAl4O7 with accessory perovskite and Fe-rich spinel. Large excesses of ^26Mg* (200-550o/oo) were observed for ^27Al/^24Mg of 600 to 1900. Data from both CAI fall along a single array on a Mg evolution diagram, with slope ^26Mg*/^27Al = (4.2 +- 0.5) x 10^-5. Excess ^26Mg* is widespread in Adelaide CAI. The presence of ^26Mg* in all eight CAI studied to date and the small number of CAI with disturbed Al-Mg isotope systems distinguish Adelaide from other carbonaceous chondrites. The high abundances of hibonite, CaAl4O7 and gehlenitic melilite suggest that Adelaide CAI were isolated from nebular gas at a higher temperature than most CAI in CV-chondrites. The near absence of isotopic disturbance suggests that isotopic and petrographic studies of Adelaide CAI may provide a clearer look at refractory inclusion formation. (Supported by NASA, NAG 9-43. Division contribution #5164 (781).) References Hutcheon I. D. and Steele I. M. (1982) LPSC XXIII, 352-353. Kallemeyn and Wasson (1982) Geochim. Cosmochim. Acta 46, 2217- 2228. Kerridge (1985) Geochim. Cosmochim. Acta 49, 1707-1714. Fitzgerald M. J. and Jones J. B. (1977) Meteoritics 12, 443-458.
Huss Gary R.
Hutcheon Ian D.
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