H-chondrite parent asteroid: A multistage cooling, fragmentation and re-accretion history constrained by thermometric studies, diffusion kinetic modeling and geochronological data

Abstract

We present a detailed thermometric study and cooling history analysis of selected H-chondrites from the petrologic types 4–6 on the basis of their mineralogical properties, and integrate these data with other available constraints on the cooling rates to develop a comprehensive model for the cooling, fragmentation and re-accretion history of the parent asteroid. Temperatures have been determined on the basis of two-pyroxene (2-Px) and spinel (Spnl)–orthopyroxene (Opx)/olivine (Ol) thermometers using the average of line scans and distributed spot analysis of coexisting pairs in each set. All of these minerals have been found to be compositionally homogeneous from ∼1 to 2μm from the interface within the resolution of microprobe analysis. The thermometric results for the H5 (Allegan and Richardton) and H6 (Guarena and Kernouvé) samples are very similar. Also, while the 2-Px temperature increases by ∼90°C from H4 to H5/6, a reverse trend is observed for the Spnl–Opx/Ol temperatures implying compositional resetting of these pairs during cooling. For the H4 sample (Forest Vale) all thermometric results are similar. The cooling rates calculated from numerical modeling of the compositional profiles in Opx–Cpx pairs in H5 and H6, corrected for the spatial averaging or convolution effect in microprobe analysis, are ∼25–100°C/ky, which are 3–4 orders of magnitude higher than the cooling rates implied by in situ cooling in an onion-shell parent body model. Similar numerical simulation of the compositional profile in Opx–Spnl pair in H4 yields a cooling rate ∼50°C/ky, which is in very good agreement with recent metallographic cooling rate of this sample and geochronological constraints on the cooling T–t path. Numerical simulation suggests that the slow cooling of the H5/6 samples at a rate of ∼15°C/My, as deduced by recent metallographic study, could not have commenced at a temperature above ∼700°C since, otherwise, the simulated compositional profile fails to match the observed profile. For the H5 samples, the T–t path constructed on the basis of Hf–W age of peak metamorphism and two stage cooling model satisfies the constraints imposed by the Pb–Pb ages of the phosphates and Ar–Ar ages of the feldspars vs. their respective closure temperatures, whereas that for H6 samples constructed using the same approach fails to satisfy these geochronological data. A second stage of even slower cooling at ∼3°C/My needs to be invoked to satisfy the geochronological age vs. closure temperature relations. We present a model of fragmentation and re-accretion history of the parent body that could lead to the reconstructed T–t paths of the H-chondrite samples studied in this work, and discuss some of their broader implications. The cooling rates retrieved from the available data on the Fe–Mg ordering states of orthopyroxenes of some other H5 and H6 samples are orders of magnitude faster than the metallographic cooling rates that are applicable to similar low temperature domain. It is, thus, likely that all samples of the same petrologic type did not share a common cooling history.