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HT P2/c-C2/c phase transition and kinetics of Fe-Mg order-disorder of an Fe-poor pigeonite: Implications for the cooling history of ureilites

Contributions to Mineralogy and Petrology, ISSN: 0010-7999, Vol: 162, Issue: 3, Page: 599-613
2011
  • 27
    Citations
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  • 12
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Metrics Details

  • Citations
    27
    • Citation Indexes
      27
  • Captures
    12

Article Description

A natural Ca-poor pigeonite (WoEnFs) from the ureilite meteorite sample PCA82506-3, free of exsolved augite, was studied by in situ high-temperature single-crystal X-ray diffraction. The sample, monoclinic P2/c, was annealed up to 1,093°C to induce a phase transition from P2/c to C2/c symmetry. The variation with increasing temperature of the lattice parameters and of the intensity of the b-type reflections (h + k = 2n + 1, present only in the P2/c phase) showed a displacive phase transition P2/c to C2/c at a transition temperature T = 944°C, first order in character. The Fe-Mg exchange kinetics was studied by ex situ single-crystal X-ray diffraction in a range of temperatures between the closure temperature of the Fe-Mg exchange reaction and the transition temperature. Isothermal disordering annealing experiments, using the IW buffer, were performed on three crystals at 790, 840 and 865°C. Linear regression of ln k versus 1/T yielded the following equation: In K = -3717(±416)/T(K)+1.290(±0.378); (R = 0.988). The closure temperature (T) calculated using this equation was ~740(±30)°C. Analysis of the kinetic data carried out taking into account the e. s. d.'s of the atomic fractions used to define the Fe-Mg degree of order, performed according to Mueller's model, allowed us to retrieve the disordering rate constants CK for all three temperatures yielding the following Arrhenius relation: In(CK) = In K - Q/(RT) = 20.99(±3.74) -26406 (±4165)/T(K); (R = 0.988) An activation energy of 52. 5(±4) kcal/mol for the Fe-Mg exchange process was obtained. The above relation was used to calculate the following Arrhenius relation modified as a function of X (in the range of The cooling time constant, η = 6 × 10 K year calculated on the PCA82506-3 sample, provided a cooling rate of the order of 1°C/min consistent with the extremely fast late cooling history of the ureilite parent body after impact excavation. © 2011 Springer-Verlag.

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