TY - JOUR
T1 - Cooling rates of LL, L and H chondrites and constraints on the duration of peak thermal conditions
T2 - Diffusion kinetic modeling and implications for fragmentation of asteroids and impact resetting of petrologic types
AU - Ganguly, Jibamitra
AU - Tirone, Massimiliano
AU - Domanik, Kenneth
N1 - Funding Information:
This research was supported by a NASA grant NNX14AG28G to JG. We are grateful to the curatorial staffs for meteorite collections of Smithsonian Institution, Johnson Space Center and Arizona State University for lending the meteorite samples used in this study. Constructive reviews by Bruce Watson, Fred Ciesla and an anonymous reviewer have led to substantial improvement of the manuscript. The editorial handling by Mike Toplis that not only involved a thorough and thoughtful review of the manuscript but also discussions through several e-mail exchanges is greatly appreciated.
Publisher Copyright:
© 2016 Elsevier Ltd
PY - 2016/11/1
Y1 - 2016/11/1
N2 - We have carried out detailed thermometric and cooling history studies of several LL-, L- and H-chondrites of petrologic types 5 and 6. Among the selected samples, the low-temperature cooling of St. Séverin (LL6) has been constrained in an earlier study by thermochronological data to an average rate of ∼2.6 °C/My below 500 °C. However, numerical simulations of the development of Fe–Mg profiles in Opx–Cpx pairs using this cooling rate grossly misfit the measured compositional profiles. Satisfactory simulation of the latter and low temperature thermochronological constraints requires a two-stage cooling model with a cooling rate of ∼50–200 °C/ky from the peak metamorphic temperature of ∼875 °C down to 450 °C, and then transitioning to very slow cooling with an average rate of ∼2.6 °C/My. Similar rapid high temperature cooling rates (200–600 °C/ky) are also required to successfully model the compositional profiles in the Opx–Cpx pairs in the other samples of L5, L6 chondrites. For the H-chondrite samples, the low temperature cooling rates were determined earlier to be 10–20 °C/My by metallographic method. As in St. Séverin, these cooling rates grossly misfit the compositional profiles in the Opx–Cpx pairs. Modeling of these profiles requires very rapid cooling, ∼200–400 °C/ky, from the peak temperatures (∼810–830 °C), transitioning to the metallographic rates at ∼450–500 °C. We interpret the rapid high temperature cooling rates to the exposure of the samples to surface or near surface conditions as a result of fragmentation of the parent body by asteroidal impacts. Using the thermochronological data, the timing of the presumed impact is constrained to be ∼4555–4560 My before present for St. Séverin. We also deduced similar two stage cooling models in earlier studies of H-chondrites and mesosiderites that could be explained, using the available geochronological data, by impact induced fragmentation at around the same time. Diffusion kinetic analysis shows that if a lower petrological type got transformed by the thermal effect of shock impacts to reflect higher metamorphic temperature, as has been suggested as a possibility, then the peak temperatures would have had to be sustained for at least 10 ky and 80 ky, respectively, for transformation to the petrologic types 6 and 4. Finally, we present a model that reconciles textural data supporting an onion-shell parent body of H-chondrites with rapid cooling rate at high temperature caused by impact induced disturbance, and also discuss alternatives to the onion shell parent body model.
AB - We have carried out detailed thermometric and cooling history studies of several LL-, L- and H-chondrites of petrologic types 5 and 6. Among the selected samples, the low-temperature cooling of St. Séverin (LL6) has been constrained in an earlier study by thermochronological data to an average rate of ∼2.6 °C/My below 500 °C. However, numerical simulations of the development of Fe–Mg profiles in Opx–Cpx pairs using this cooling rate grossly misfit the measured compositional profiles. Satisfactory simulation of the latter and low temperature thermochronological constraints requires a two-stage cooling model with a cooling rate of ∼50–200 °C/ky from the peak metamorphic temperature of ∼875 °C down to 450 °C, and then transitioning to very slow cooling with an average rate of ∼2.6 °C/My. Similar rapid high temperature cooling rates (200–600 °C/ky) are also required to successfully model the compositional profiles in the Opx–Cpx pairs in the other samples of L5, L6 chondrites. For the H-chondrite samples, the low temperature cooling rates were determined earlier to be 10–20 °C/My by metallographic method. As in St. Séverin, these cooling rates grossly misfit the compositional profiles in the Opx–Cpx pairs. Modeling of these profiles requires very rapid cooling, ∼200–400 °C/ky, from the peak temperatures (∼810–830 °C), transitioning to the metallographic rates at ∼450–500 °C. We interpret the rapid high temperature cooling rates to the exposure of the samples to surface or near surface conditions as a result of fragmentation of the parent body by asteroidal impacts. Using the thermochronological data, the timing of the presumed impact is constrained to be ∼4555–4560 My before present for St. Séverin. We also deduced similar two stage cooling models in earlier studies of H-chondrites and mesosiderites that could be explained, using the available geochronological data, by impact induced fragmentation at around the same time. Diffusion kinetic analysis shows that if a lower petrological type got transformed by the thermal effect of shock impacts to reflect higher metamorphic temperature, as has been suggested as a possibility, then the peak temperatures would have had to be sustained for at least 10 ky and 80 ky, respectively, for transformation to the petrologic types 6 and 4. Finally, we present a model that reconciles textural data supporting an onion-shell parent body of H-chondrites with rapid cooling rate at high temperature caused by impact induced disturbance, and also discuss alternatives to the onion shell parent body model.
KW - Chondrites
KW - Cooling rates
KW - Impact
KW - Metamorphism
KW - Parent body
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U2 - 10.1016/j.gca.2016.07.030
DO - 10.1016/j.gca.2016.07.030
M3 - Article
AN - SCOPUS:84983542216
SN - 0016-7037
VL - 192
SP - 135
EP - 148
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -