The mineral chemistry and origin of inclusion matrix and meteorite matrix in the Allende CV3 chondrite |
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| Authors: | Alan S Kornacki John A Wood |
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| Institution: | Smithsonian Astrophysical Observatory, 60 Garden Street, and Department of Geological Sciences, Harvard University, 24 Oxford Street, Cambridge, MA 02138 USA |
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| Abstract: | The two textural varieties of olivine-rich Allende inclusions (rimmed and unrimmed olivine aggregates) consist primarily of a porous, fine-grained mafic constituent (inclusion matrix) that differs from the opaque meteorite matrix of CV3 chondrites by being relatively depleted in sulfides, metal grains, and (perhaps) carbonaceous material. Olivine is the most abundant mineral in Allende inclusion matrix; clinopyroxene, nepheline, sodalite, and Ti-Al-pyroxene occur in lesser amounts. Olivine in unrimmed olivine aggregates (Type 1A inclusions) is ferrous and has a narrow compositional range (Fo50–65). Olivine in rimmed olivine aggregates (Type 1B inclusions) is, on average, more magnesian, with a wider compositional range (Fo53–96). Olivine grains in the granular rims of Type 1B inclusions are zoned, with magnesian cores (Fo>80) and ferrous rinds (Fo<70). Ferrous olivines (Fo<65) in both varieties of inclusions commonly contain significant amounts of Al2O3 (as much as ~0.7 wt%), CaO (as much as ~0.4 wt%), and TiO2 (as much as ~0.2 wt%), refractory elements that probably occur in submicroscopic inclusions of Ca,Al,Ti-rich glass (rather than in the olivine crystal structure). Defocussed beam analyses of Allende matrix materials demonstrate that: (1) inclusion matrix in Type 1A inclusions is more enriched in olivine and FeO than inclusion matrix in the cores of Type 1B inclusions; (2) opaque matrix materials are depleted in feldspathoids and enriched in sulfides and metal grains relative to inclusion matrix; (3) the bulk compositions of Type 1A and Type 1B inclusions overlap; and (4) excluding sulfides and metal, the bulk compositions of Allende matrix materials cluster in a complementary pattern around the bulk composition of C1 chondrites.Inclusion matrix and meteorite matrix in Allende and other CV3 chondrites are probably relatively primitive nebular material, but a careful evaluation of the equilibrium condensation model suggests that these matrix materials do not consist of crystalline phases that formed under equilibrium conditions in a relatively cool gas of solar composition. Allende inclusion matrix is interpreted as an aggregate of condensates that formed under relatively oxidizing, non-equilibrium conditions from supercooled, supersaturated vapors produced during the vaporization of interstellar dust by aerodynamic drag heating in the solar nebula; CV3 meteorite matrix contains, in addition, a proportion of interstellar material that was heated (but not vaporized) in the nebula. Granular olivine in rimmed olivine aggregates may have formed during the recrystallization and incipient melting of aggregates of inclusion matrix in the nebula. The mineral chemistry of matrix olivine in Allende seems to have been established by three different processes: non-equilibrium vapor → solid condensation; recrystallization and partial melting in the nebula; and equilibration (without textural homogenization) in the meteorite parent body. |
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