Sodic plagioclase thermometry of type 6 ordinary chondrites: Implications for the thermal histories of parent bodies

Abstract— The structural states of sodic plagioclase crystals of ∼50 μm in size from three H6, two L6, and one LL6 chondritic meteorites have been determined by measuring the Δ131 parameter with a Gandolfi camera after analyzing chemical compositions. The temperature for each sodic plagioclase crystal has been determined by plotting the Δ131 parameter, corrected for the influence of K, on the relation diagram between the Δ131 parameter and the temperature of synthesis of sodic plagioclase by Smith (1972). The temperature obtained is assigned to the crystallization temperature of sodic plagioclase, and the maximum plagioclase temperature for each meteorite can be assumed to correspond to the maximum temperature attained by each meteorite during metamorphism. The maximum metamorphic temperatures estimated are 725–742 °C for the H6 chondrites, 808–820 °C for the L6 chondrites, and 800 °C for the LL6 chondrite. These temperatures are lower than those based on Ca contents of clinopyroxenes (Dodd, 1981; McSween et al., 1988) but are consistent with those based on Ca contents of orthopyroxenes (McSween and Patchen, 1989; Langenhorst et al., 1995; Jones, 1997). The K content of sodic plagioclase correlates with the temperature obtained from the structural state. This positive correlation suggests that sodic plagioclase has formed in the course of equilibration processes of alkali elements in prograde metamorphism (i.e., during heating processes). The results of this study (i.e., the maximum metamorphic temperature of the H6 chondrites is lower than that of the L6 chondrites by ∼80 °C, and meteorites of the same chemical group show very similar maximum metamorphic temperatures) are in accordance with the predictions of calculations based on the 26Al heat source and the onion‐shell structure model of the parent bodies.

[1]  E. Scott,et al.  Constraints on the role of impact heating and melting in asteroids , 1997 .

[2]  H. McSween,et al.  Revised model calculations for the thermal histories of ordinary chondrite parent bodies , 1996 .

[3]  F. Langenhorst,et al.  Thermal and shock metamorphism of the Tenham chondrite: A TEM examination , 1995 .

[4]  G. Manhès,et al.  UPb systematics of phosphates from equilibrated ordinary chondrites , 1994 .

[5]  K. Keil,et al.  Shock metamorphism of ordinary chondrites , 1991 .

[6]  H. McSween,et al.  Pyroxene thermobarometry in LL-group chondrites and implications for parent body metamorphism , 1989 .

[7]  A. Lasaga,et al.  Crystal-growth kinetics of plagioclase in igneous systems; one-atmosphere experiments and application of a simplified growth model , 1987 .

[8]  J. R. Goldsmith.  Al/Si interdiffusion in albite: effect of pressure and the role of hydrogen , 1987 .

[9]  N. Morimoto,et al.  A transmission electron microscope study of pyroxene chondrules in equilibrated L-group chondrites , 1985 .

[10]  E. Olsen,et al.  Equilibration temperatures of the ordinary chondrites: A new evaluation , 1984 .

[11]  I. Parsons,et al.  Exsolution and coarsening mechanisms and kinetics in an ordered cryptoperthite series , 1984 .

[12]  J. R. Ashworth,et al.  Chondrite thermal histories constrained by experimental annealing of Quenggouk orthopyroxene , 1984, Nature.

[13]  R. Ostertag.  Shock experiments on feldspar crystals , 1983 .

[14]  D. Stöffler,et al.  Thermal annealing of experimentally shocked feldspar crystals , 1982 .

[15]  R. Young,et al.  Profile shape functions in Rietveld refinements , 1982 .

[16]  R. Kretz.  Transfer and exchange equilibria in a portion of the pyroxene quadrilateral as deduced from natural and experimental data , 1982 .

[17]  N. Fujii,et al.  Ordinary chondrite parent body - An internal heating model , 1982 .

[18]  P. Ribbe,et al.  Determinative diagrams for Al,Si order in plagioclases , 1980 .

[19]  J. R. Ashworth.  Chondrite thermal histories: Clues from electron microscopy of orthopyroxene , 1980 .

[20]  R. Schmitt,et al.  The petrogenesis of L-6 chondrites: insights from the chemistry of minerals , 1979 .

[21]  H. Takeda,et al.  Pyroxene geothermometry applied to a three-pyroxene achondrite from Allan Hills, Antarctica and ordinary chondrites , 1979 .

[22]  J. V. Heyse.  The metamorphic history of LL-group ordinary chondrites , 1978 .

[23]  D. J. Johnson,et al.  Crystallinity and crystallite size measurement in polyamide and polyester fibres , 1978 .

[24]  T. Bunch,et al.  Restudy of pyroxene-pyroxene equilibration temperatures for ordinary chrondrite meteorites , 1974 .

[25]  J. Smith.  Critical Review of Synthesis and Occurrence of Plagioclase Feldspars and a Possible Phase Diagram , 1972, The Journal of Geology.

[26]  R. Clayton,et al.  Oxygen isotope temperatures of 'equilibrated' ordinary chondrites. , 1972 .

[27]  W. R. Schmus,et al.  The composition and structural state of feldspar from chondritic meteorites , 1968 .

[28]  W. R. van Schmus,et al.  Equilibration Temperatures of Iron and Magnesium in Chondritic Meteorites , 1967, Science.

[29]  John A. Wood,et al.  A chemical-petrologic classification for the chondritic meteorites. , 1967 .

[30]  W. Mackenzie.  The crystalline modifications of NaAlSi 3 O 8 , 1957 .

[31]  H. S. Yoder,et al.  Variations in x-ray powder diffraction patterns of plagioclase feldspars , 1956 .

[32]  H. Urey.  THE COSMIC ABUNDANCES OF POTASSIUM, URANIUM, AND THORIUM AND THE HEAT BALANCES OF THE EARTH, THE MOON, AND MARS. , 1955, Proceedings of the National Academy of Sciences of the United States of America.

[33]  Julian R. Goldsmith.  Diffusion in Plagioclase Feldspars , 1952, The Journal of Geology.

[34]  O. F. Tuttle,et al.  High-Temperature Albite and Contiguous Feldspars , 1950, The Journal of Geology.