|Title:||Complexly zoned Cr‐Al spinel found in situ in the Allende meteorite|
|Authors:||S.B. Simon, K.D. McKeegan, D.S. Ebel, and L. Grossman|
|Publication:||Meteor. Planet. Sci., v. 35, p. 215‐228.|
In addition to the Mg‐, Al‐, 16O‐rich spinels that are known to occur in refractory inclusions, the Murchison meteorite contains Cr‐rich, 16O‐poor spinels, most of whose sources are unknown because they are rarely found in situ. Here we report the in situ occurrence in Allende of Cr‐rich spinels, found in 13 chondrules and four \\\”olivine‐rich objects\\\” (OROs). The Allende spinels exhibit major and minor element contents, isotopic compositions, and zoning of Cr2O3 contents like those of the Cr‐spinels from Murchison. Some chondrules contain patchy‐zoned spinel (Simon et al., 1994), suggesting that such grains did not form by sintering but perhaps by formation of overgrowths on relict grains. Unlike the OROs, phases in all three chondrules that were analyzed by ion microprobe have uniform, near‐normal oxygen isotopic compositions. One ORO, ALSP1, has a huge (1 mm), fragment of chevron‐zoned spinel. This spinel has near‐normal oxygen isotopic compositions that are quite distinct from those of adjacent forsteritic olivine, which are relatively 16O−rich and plot on the CAI line, like some isolated forsterite grains found in Allende. The spinel and olivine in this object are therefore not genetically related to each other. Another ORO, ALSP11A, contains a rectangular, 150 × 100 µm, homogeneous spinel grain with 50 wt% Cr2O3 and 23 wt% FeO in a vuggy aggregate of finer‐grained (5‐90 µm), FeO−rich (Fo47‐55) olivine. The magnesian core of one olivine grain has a somewhat 16O‐rich isotopic composition like that of the large spinel while the FeO‐rich olivine is relatively 16O‐poor. The composition of the spinel in ALSP11A plots on the CAI line, the first Cr‐rich spinel found to do so. Chevron‐zoned spinel has not been observed in chondrules, and it is unlikely that either ALSP1 or ALSP11A were ever molten. Calculations show that a spinel with the composition of that in ALSP1 can condense at 1780K at a Ptot of 10−3 atm and a dust/gas ratio of 100 relative to solar. The Cr‐rich spinel in ALSP11A could condense at ⁓1420K but this would require a dust/gas enrichment of 1000 relative to solar. The data presented here confirm that, as in Murchison, the coarse, Cr−rich spinels in Allende are relatively 16O−depleted and are isotopically distinct from the 16O−enriched MgAl2O4 from CAIs. ALSP11A may represent a third population, one that is Cr‐rich and plots on the CAI line. That the oxygen isotopic composition of ALSP1 is like those of Cr‐rich spinels from chondrules indicates that oxygen isotopic compositions cannot be used to distinguish whether grains from such unequilibrated objects are condensates or are fragments from a previous generation of chondrules.