Chondritic meteorites are the most primitive rocks of our solar system and hence represent a privileged object for the study of its earliest evolution. Thermal metamorphism affecting these meteorites, testified by petrologic types 3 to 6 (in order of increasing metamorphic degree), is commonly attributed to heat generated by the decay of short-lived radioisotope, such as 26Al. According to some models, this heating produced concentric “onion-shell” structure in which the outer part, characterized by fast cooling, correspond to the less metamorphosed (lowest) petrologic types, whereas the inner part to the more metamorphosed, highest petrologic types. In this scenario an inverse correlation between cooling rate and metamorphic degree is to be expected as well as decreasing closure temperature of the Fe2+-Mg exchange reaction in pyroxene from lower to higher petrologic types. However, accurate investigation so far conducted on this subject (Molin et al., 1994; Folco et al., 1996; Folco et al., 1997) and recent results on Trenzano chondrite (Fioretti et al., 2007) yielded closure temperature of the Fe2+-Mg exchange in the range of 348±48 °C to 520±28 °C, independent on the petrologic type. A possible interpretation is that this inconsistence reflects secondary thermal effects, possibly due to shock, which could have induced a partial re-equilibration of pyroxene. By studying exceptionally undisturbed chondrites Trieloff et al. (2003), have indeed confirmed the expected correlation between cooling rate and petrologic type. Aim of our study is to measure the closure temperature of the Fe2+-Mg exchange reaction on orthopyroxene crystals extracted from exactly the same un-shocked chondrite samples investigated by Trieloff (2003) in order to evaluate the possible influence of shock heating on previous apparently inconsistent data. Two or three crystals from two different samples of H4 (Ste. Marguerite and Forest Vale), H5 (Nadiabondi and Richardton) and H6 (Kernouve and Guarena) chondrites were selected for the X-ray single crystal diffraction (XRD) study The same crystals used for XRD analysis were then analysed by electron microprobe operating in wavelength-dispersive spectrometry (WDS). To obtain the site partitioning structure refinements in Pbca sp. gr. were carried out with chemical constraints, taken from the electron microprobe analysis and assuming 1s as error. The closure temperature Tc of the Fe2+-Mg exchange reaction was measured using the equation lnkD = -2984(69)/T(K) + 0.747(0.078) obtained by Stimpfl (2005) for low-Fe pyroxenes (Fs14-Fs16). Tc values obtained from H5 and H6 orthopyroxene crystals all cluster around 53535 °C. They are identical within error and show no preferential relation with their petrologic type. Tc values calculated from H4 orthopyroxene are significantly higher and range from ca. 58033 °C (Forest Vale) to 67037 °C (Ste Marguerite). The highest closure temperatures recorded by the lower (H4) petrologic type is consistent with the onion-shell model, where the cooling rate of the more superficial, less metamorphosed H4 chondrites is faster. The indistinguishable Tc obtained from H5 and H6 orthopyroxene crystals is more puzzling. This is not what expected from the undisturbed cooling of the onion-shell parent body showed by thermochronometry (Trieloff et al., 2003). In this scenario our preliminary data request that the difference in cooling rate between H5 and H6 petrologic types (at a temperature near the Tc) is not large and is below the resolving power of the Fe2+-Mg exchange reaction method, whereas the cooling rate of the shallower H4 layer appears perceptively faster. REFERENCES: Fioretti et al. (2007) Met. Planetary Sci., 42, 2055-2066; Folco et al. (1996) Met.Planetary Sci., 31, 388-393; Folco et al. (1997) Met.Planetary Sci., 32, 567-575; Molin et al. (1994) Min. Mag., 58, 143-150

Reinvestigating unshocked H4-6 chondrites orthopyroxene closure temperature

CAMARA ARTIGAS, Fernando;
2009-01-01

Abstract

Chondritic meteorites are the most primitive rocks of our solar system and hence represent a privileged object for the study of its earliest evolution. Thermal metamorphism affecting these meteorites, testified by petrologic types 3 to 6 (in order of increasing metamorphic degree), is commonly attributed to heat generated by the decay of short-lived radioisotope, such as 26Al. According to some models, this heating produced concentric “onion-shell” structure in which the outer part, characterized by fast cooling, correspond to the less metamorphosed (lowest) petrologic types, whereas the inner part to the more metamorphosed, highest petrologic types. In this scenario an inverse correlation between cooling rate and metamorphic degree is to be expected as well as decreasing closure temperature of the Fe2+-Mg exchange reaction in pyroxene from lower to higher petrologic types. However, accurate investigation so far conducted on this subject (Molin et al., 1994; Folco et al., 1996; Folco et al., 1997) and recent results on Trenzano chondrite (Fioretti et al., 2007) yielded closure temperature of the Fe2+-Mg exchange in the range of 348±48 °C to 520±28 °C, independent on the petrologic type. A possible interpretation is that this inconsistence reflects secondary thermal effects, possibly due to shock, which could have induced a partial re-equilibration of pyroxene. By studying exceptionally undisturbed chondrites Trieloff et al. (2003), have indeed confirmed the expected correlation between cooling rate and petrologic type. Aim of our study is to measure the closure temperature of the Fe2+-Mg exchange reaction on orthopyroxene crystals extracted from exactly the same un-shocked chondrite samples investigated by Trieloff (2003) in order to evaluate the possible influence of shock heating on previous apparently inconsistent data. Two or three crystals from two different samples of H4 (Ste. Marguerite and Forest Vale), H5 (Nadiabondi and Richardton) and H6 (Kernouve and Guarena) chondrites were selected for the X-ray single crystal diffraction (XRD) study The same crystals used for XRD analysis were then analysed by electron microprobe operating in wavelength-dispersive spectrometry (WDS). To obtain the site partitioning structure refinements in Pbca sp. gr. were carried out with chemical constraints, taken from the electron microprobe analysis and assuming 1s as error. The closure temperature Tc of the Fe2+-Mg exchange reaction was measured using the equation lnkD = -2984(69)/T(K) + 0.747(0.078) obtained by Stimpfl (2005) for low-Fe pyroxenes (Fs14-Fs16). Tc values obtained from H5 and H6 orthopyroxene crystals all cluster around 53535 °C. They are identical within error and show no preferential relation with their petrologic type. Tc values calculated from H4 orthopyroxene are significantly higher and range from ca. 58033 °C (Forest Vale) to 67037 °C (Ste Marguerite). The highest closure temperatures recorded by the lower (H4) petrologic type is consistent with the onion-shell model, where the cooling rate of the more superficial, less metamorphosed H4 chondrites is faster. The indistinguishable Tc obtained from H5 and H6 orthopyroxene crystals is more puzzling. This is not what expected from the undisturbed cooling of the onion-shell parent body showed by thermochronometry (Trieloff et al., 2003). In this scenario our preliminary data request that the difference in cooling rate between H5 and H6 petrologic types (at a temperature near the Tc) is not large and is below the resolving power of the Fe2+-Mg exchange reaction method, whereas the cooling rate of the shallower H4 layer appears perceptively faster. REFERENCES: Fioretti et al. (2007) Met. Planetary Sci., 42, 2055-2066; Folco et al. (1996) Met.Planetary Sci., 31, 388-393; Folco et al. (1997) Met.Planetary Sci., 32, 567-575; Molin et al. (1994) Min. Mag., 58, 143-150
2009
GEOITALIA 2009. Settimo Forum Italiano di Scienze della Terra
Rimini, Italy
9-11 settembre 2009
35
707
708
orthopyroxene; chondrites; single crystal X-ray diffraction; closure temperature
Fioretti AM; Domeneghetti MC; Cámara F; Pistorino M; Carraro A
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/79190
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