The 'Porky' Inclusion from the Axtell Carbonaceous Chondrite: A Hercynite-bearing Condensate with Large 48Ca and 50Ti Excesses
Only a few hercynite-rich refractory inclusions have been identified to date [1,2]. Porky (450 micrometers long) is an unusual CAI with texturally distinct regions and dominant intergrown oxide mineral phases . The core is a symplectitic vermicular intergrowth of ubiquitous ferroan spinel and ilmenite. Inwards branching of ilmenite crystals and projecting fan-shaped hibonite from the borders suggest crystallization from a fast cooling melt. Three marginal regions are mineralogically identical to the core: two fine-grained areas with acicular corroded crystals contrast with an altered spinel area. The inclusion is entirely rimmed by yellow luminescing Al-diopside. Hibonite exhibits pale orange luminescence. Broad beam analyses reveal high FeO (22.5 wt %) and low CaO/Al2O3 (0.024) for the whole CAI . Si, Al and Ti concentrations are very similar to those of CM inclusions. There are no significant chemical differences between the coarse-grained core and the fine-grained regions. Hibonite shows widely varying concentrations (in wt %) of TiO2 (1.5-6.8), MgO (1.2-3.6) and FeO (0.7-4.7). Spinel contains much FeO (20.5-27). Hercynite close to the border of the inclusion next to hibonite has high MgO (11 wt%) and small amounts of Cr2O3 (0.15-1.55) and V2O3 (0.1-0.3), implying a melt origin. The Al-rich, Si-poor composition indicates that Porky experienced higher temperatures than CV3 inclusions. The presence of Fe-rich mineral phases, the lack of metal grains, the relatively high NiO concentration (average = 0.25), and the orange luminescence of hibonite indicate formation or re-equilibration in an oxidizing environment. Trace element patterns are basically volatility-fractionated and mostly ultrarefractory-depleted but vary from a Group II pattern in hercynite (to a lesser extent in hibonite and diopside) to extreme excesses of the less refractory elements Nb, Eu, Tm and Yb in a fine-grained region (Fig. 1). These patterns indicate condensation from a UR-depleted gas at varying temperatures (highest for the core, lowest for the fine-grained regions) and under varying redox conditions and trace element compositions. The refractory trace elements measured in hercynite are likely to reside in ilmenite (the abundances in this phase would then be ~15 times higher than shown in Fig. 1 for hercynite). In contrast to large 48Ca and 50Ti depletions in hibonite-hercynite inclusion Lanc HH-1 , Porky shows extreme excesses in 48Ca (37.3+/-5.9 (2s) per mil in hibonite and 27.4+/-4.8 per mil in diopside) and 50Ti (~62 per mil in hibonite and ilmenite). No clear 26Mg excess and no Mg and Fe isotopic mass fractionation were detected. Fe isotopic ratios in hercynite are normal; the Allende inclusion EK1-4-1 thus remains the only object in which a large 58Fe excess is associated with 48Ca and 50Ti excesses . References:  Brigham C. A. et al. (1986) LPSC XVII, 85-86.  Fahey A. J. et al. (1994) GCA, 58, 4779-4793.  Caillet C. (1994) Meteoritics, 29, 453-454.  V"lkening J. and Papanastassiou D. A. (1989) Astrophys. J., 347, L43-L46.
- Pub Date:
- September 1995
- CALCIUM-ALUMINUM-RICH INCLUSIONS;
- RARE EARTH ELEMENTS;
- REFRACTORY INCLUSIONS