Oscillatory zoning in meteoritic forsterite

Abstract Some isolated forsterite grains in Allende (C3V) and ALHA 76004 (LL3) show oscillatory zoning when viewed with cathodoluminescence. Concentration profiles for these grains show that the oscillations correspond to cyclic variations in Al and Ti and, by inference with known correlations, V and Sc. Other elements, including Mg, Fe, Ca, and Cr, show only monotonic changes across the oscillatory-zoned area. Under reducing conditions assumed present during forsterite growth, the varying elements should be trivalent or higher in charge, whereas the Mg-Fe-Ca-Cr group would be divalent. Substitution into the olivine structure is considered to be two trivalent elements, dominated by Al, for Mg + Si, which represents a small substitution of the spinel component into forsterite. This same type of substitution is proposed for similar terrestrial oscillatory-zoned olivines in which no Fe or Mg variation is found. By analogy with terrestrial and lunar oscillatory-zoned minerals, crystallization of the forsterite from a liquid is probable. Currently, no such source material is known, although crystallization of porphyritic chondrules may produce grains of equivalent size with zoning. Although no oscillatory-zoned forsterite has been observed in chondrules, their recognition may be confirmation of the source of isolated forsterite grains; alternatively, the source of these forsterite grains has not yet been recognized.

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