Origin of plagioclase-olivine inclusions

• Main Author: Sheng, Yuntai Jack
• Format: Others
• Language: en
• Published: 1992
• Online Access: https://thesis.library.caltech.edu/6705/1/Sheng_yj_1992.pdf; Sheng, Yuntai Jack (1992) Origin of plagioclase-olivine inclusions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/cjkd-xe14. https://resolver.caltech.edu/CaltechTHESIS:10042011-103334484 <https://resolver.caltech.edu/CaltechTHESIS:10042011-103334484>

Plagioclase-Olivine Inclusions (POIs) are an abundant group of chondrule-like objects found in carbonaceous chondrites. Despite textural evidence of a molten or partially molten origin, approximately half of the POls studied exhibited variations in Mg isotope composition of up to 11 %/amu between spinel and coexisting silicates. Equilibrium crystallization experiments performed to study the crystallization behavior of melts of POI compositions showed that the phase assemblages predicted on the basis of phase equilibria are generally consistent with those observed in POIs. Spinel is the liquidus phase for most of the inclusions that contain spinel with fractionated Mg isotopes, and therefore these spinels could be preserved if melting was below the liquidus temperature for the bulk composition of the inclusion. The presence of resorbed spinels in some inclusions whose bulk composition does not permit spinel in its phase assemblage requires that these spinels were relict. The petrographic and chemical properties of POIs indicate that they are not condensates or evaporative residues, but formed by melting or partial melting of pre-existing solids that had originated from isotopically distinct reservoirs. The presence of isotopically heterogeneous spinel in the POIs requires that the maximum temperature experienced by these objects was low enough, the duration of heating short enough, and the subsequent cooling rate fast enough to prevent homogenization of Mg isotopes by diffusive transport. The self-diffusion rate of Mg was measured in spinel and coexisting melt using isotope tracers. For spinel, the activation energy of 384±7 kJ for Mg and pre-exponential factor of 7791.9±1.3 cm^2/s is obtained. The temperatures of melting of POIs is estimated to be in the range of 1350-1500°C, the duration of the heating events were < 2 hr, and the initial cooling rate was > 25°/hr. The melting occurred after most ^(26)Al had decayed and possibly subsequent to the extensive oxygen isotopic exchange between solids and nebula gas. The melting of POIs is probably by flash heating or impact melting in a hot parcel or region of the nebula. These refractory objects (including CAIs) are believed to be the result of repetitive heating events that had frequently generated or reprocessed refractory residues over a prolonged timescale of very early solar system history.