Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.389m&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 389
Mathematics
Logic
Cais, Extinct Nuclides, Hibonite, Leoville
Scientific paper
Hibonite-bearing refractory inclusions preserve some of the most primitive chemical, isotopic, and petrologic properties from the earliest solar system [e.g., 1]. Among inclusions from CV3 chondrites, those in Leoville have been the objects of particular scrutiny ever since an early (now questionable) report [2] of hibonite with exceptionally high inferred initial 26Al/27Al of ~1 x 10-4, much higher than the generally accepted "canonical" value of 5 x 10-5 [3]. Leoville inclusions are interesting for other reasons as well, however. They contain little of the secondary mineralization (feldspathoids, gamet, etc.) that obscures primary textures and mineralogy in Allende CAIs, and many show the same pronounced flattening characteristic of other components in Leoville, leading to the possibility that the isotopic signatures of some of these inclusions may reflect the timing of the event causing the flattening [4]. Leoville 3535-3b is a compact and irregularly shaped Type A inclusion (maximum dimension >3 mm) with a very asymmetric structure, and is exceptionally rich in hibonite of two distinct textural types. The interior is mostly Al-rich melilite (Ak(sub)0.3-23). Enclosed in the melilite, but concentrated mostly toward one side of the inclusion, is a dense swarm of spinel grains (up to 20- 30 micrometers) and large (up to 230 micrometersm) angular fragments consisting of spinel and hibonite (0.6-1.8% V(sub)2O(sub)3, 0.3-3.7% MgO). At the center of this "swarm" is an aggregate of spinel-perovskite spherules (similar to ones in fine-grained spinel-rich inclusions) that are enclosed in a fine-grained mixture of secondary anorthite and pyroxene. This side of the inclusion is overlain by a relatively thin rim sequence of melilite, diopside, and olivine. On the opposite side of tbe inclusion the central melilite contains few inclusions of any kind but is overlain by an exceedingly thick and complex rim sequence. Within ~100-200 micrometers of the rim the melilite becomes crowded with myriad 10-20-micrometer-sized euhedral blue hibonite blades (<0.1-0.4% V(sub)2O(sub)3, 1.4-4.5% MgO), locally intergrown with spinel and/or perovskite. In places this zone expands into remarkable 200-300- micrometer thick intergrowths of mostly perovskite, together with hibonite and melilite. A complete Wark-Lovering type rim sequence is present only on the side of the inclusion containing the hibonite-spinel intergrowths, and terminates where the inclusion has apparently been broken. It consists (from innermost to outermost layers) of hibonite + perovskite, hibonite + spinel, melilite, Al-Ti pyroxene, and olivine. Broken surfaces on the inclusion are rimmed by a thin layer of pyroxene only. Ion microprobe analyses of spinel, melilite, and hibonite (both populations) yield an array of points on an Al-Mg isochron diagram that cluster tightly along a best-fit line with slope (initial 26Al/27Al) of (5.18 + 0.26) x 10-5 at 27Al/24Mg up to ~75, indistinguishable from the canonical value [3]. None of the phases in 3535-3b show significant Mg isotopic mass fractionation. Melilite in the interior of the inclusion has an unfractionated (Group I) REE pattern, but the hibonite-perovskite intergrowths in the thick rim sequence have a heavy-REE-enriched pattern (up to 400X Cl chondrites for Lu), with light-REE-enriched 100X. The spinel-hibonite clumps in the interior appear to be broken xenoliths enclosed by the later melilite, whereas the second-generation hibonite clearly formed as part of the rim-forming process. Yet the series of events leading to this complex structure occurred within a sufficiently short time that the two Al-Mg isotopic signatures are indistinguishable and "canonical." The thick hibonite-perovskite rim sequence probably did not form by volatilization of the melilite interior, because the trace-element enrichments in the rim relative to the interior would require a very large degree of evaporation that is not tenable in view of the lack of Mg isotopic fractionation. The very high abundance of Ti in the rim either requires addition of Ti (at least) during rim formation or else an unusual Ti-rich rim precursor. References: [1] Hinton R. W. et al. (1988) GCA, 52, 2573. [2] Lorin J. C. et al. (1978) Fourth Intl. Conf. on Geochron. Cosmochron. Isotope Geol., 257, U.S. Geol. Surv. Open-File Rept. 78-701. [3] Hutcheon I.D. (1982) Am. Chem. Soc. Symposium Ser. 176, 95. [4] Caillet C. et al. (1991) Meteoritics, 26, 326.
Davis Aileen M.
MacPherson Glenn J.
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