Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.220f&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 220
Other
6
Scientific paper
During the investigation of a Type A inclusion from Allende, preparatory to experimental studies designed to address the role of volatilization in CAI formation, we noted an unusual Ca-Ti-Al silicate, previously observed only as rare tiny grains in several CAIs. Here we report the petrography and chemistry of inclusion A44 and preliminary data for this mineral. Petrography: A44 is a large, highly convoluted inclusion that is remarkably unaltered compared to most Allende CAIs. Except for several broken fragments and faulted regions, it is surrounded by a well-defined rim sequence ranging from 50 to 100 microns in width. The core of the inclusion consists of irregular to almost perfectly circular patches of spinel poikilitically enclosed by fassaite and melilite, which are separated by large regions of spinel-free melilite. Several of these spinel framboids surround voids containing wollastonite needles similar to those described by Allen et al. (1978). Minor perovskite is primarily associated with spinel, but is also found within melilite. Several grains of the Ca-Ti-Al silicate are present in the inclusion; their occurrences and chemistry are discussed below. Chemistry: Two fragments (A: 249.6 mg and E: 241.1 mg) were analyzed by INAA. Fragment A is uniformly enriched in refractory lithophiles and siderophiles to 13-20 x C1, except for depletions of W and Mo, due to the increased volatility of both elements under oxidizing conditions (Fegley and Palme, 1985). Fragment E shows a wider range of enrichment factors (14-30 x C1) with a Yb anomaly (Yb/Lu = 0.52 x C1) characteristic of Group III patterns. The presence of a Yb anomaly in E and its absence in A may indicate that the inclusion consists of fragments formed under different nebular conditions. Mg isotopic fractionations are +5.5 +- 2.0 for melilite and +5.0 +- 1.6 for spinel, values typical for Allende CAIs. Ca-Ti-Al Silicate: An unusually Ti- and Ca-rich mineral occurs in three distinct modes. Numerous grains, up to 100 microns in size, are enclosed in altered melilite. The mineral also occurs as a long (120 microns) symplectite in unaltered melilite. Finally, it is found as a symplectitic rim around a large fassaite grain, again associated with alteration. In all occurrences it appears to replace perovskite. Major element chemistry is similar in all grains measured: CaO: 29.18-34.57 wt%; TiO2: 25.99-34.11 wt%; SiO2: 26.00-29.58 wt%; Al2O3: 3.76-9.16 wt%. Approximately 1 wt% MgO and 1.5 wt% V2O3 are also present. Both symplectites have slightly higher Al and lower Ti concentrations than the other grains. The symplectite surrounding fassaite also contains 1.5 wt% Sc2O3, not seen otherwise; the fassaite itself contains up to 1 wt% Sc2O3. El Goresy et al.(1984) and Paque et al. (1986) found small (<20 micron) grains of an unknown Ti- and Al-rich silicate as inclusions in melilite from a number of CAIs, although no association with perovskite was noted. Their mineral contains higher Al (16-19 wt% Al2O3) and lower Ti (20- 27 wt% TiO2) than the one described here. Based on a cation/oxygen ratio of 9/14 (Paque et al., 1986), it appears that up to 1/3 of the Ti may be present as Ti3+. Additional measurements will be done to determine the trace element (including REE) distributions in this mineral; X-ray investigations of the larger grains are also planned. Allen, J.M. et al. (1978) Proc. 9th Lunar Planet. Sci. Conf., 1209-1233. El Goresy, A. et al. (1984) Geochim. Cosmochim. Acta 48, 2283-2298. Fegley, B. and Palme, H. (1985) Earth Planet. Sci. Lett. 72, 311-326. Paque, J. et al. (1986) Lunar Planet. Sci. Conf. XVII, 646- 647.
El Goresy Ahmed
Floss Christine
Palme Herbert
Spettel Bernhard
Zinner Emst
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