¹Bhatia, G. K., ¹Sahijpal, S.
¹Department of Physics, Panjab University, Chandigarh, India

Hf-W isotopic systematics of Martian meteorites have provided evidence for the early accretion and rapid core formation of Mars. We present the results of numerical simulations performed to study the early thermal evolution and planetary scale differentiation of Mars. The simulations are confined to the initial 50 Myr (Ma) of the formation of solar system. The accretion energy produced during the growth of Mars and the decay energy due to the short-lived radio-nuclides 26Al, 60Fe, and the long-lived nuclides, 40K, 235U, 238U, and 232Th are incorporated as the heat sources for the thermal evolution of Mars. During the core-mantle differentiation of Mars, the molten metallic blobs were numerically moved using Stoke’s law toward the center with descent velocity that depends on the local acceleration due to gravity. Apart from the accretion and the radioactive heat energies, the gravitational energy produced during the differentiation of Mars and the associated heat transfer is also parametrically incorporated in the present work to make an assessment of its contribution to the early thermal evolution of Mars. We conclude that the accretion energy alone cannot produce widespread melting and differentiation of Mars even with an efficient consumption of the accretion energy. This makes 26Al the prime source for the heating and planetary scale differentiation of Mars. We demonstrate a rapid accretion and core-mantle differentiation of Mars within the initial ~1.5 Myr. This is consistent with the chronological records of Martian meteorites.

Reference
Bhatia GK, Sahijpal S (2015) The early thermal evolution of Mars. Meteoritics & Planetary Science (in Press)
Link to Article [doi: 10.1111/maps.12573]
Published by arrangement with John Wiley & Sons

¹Jens Hopp, ^1,2Mario Trieloff,^3,4Ulrich Ott
¹Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Im Neuenheimer Feld 234-236, D-69120 Heidelberg, Germany
³University of West Hungary, Károlyi Gáspár ter 4, H-9700 Szombathely, Hungary
⁴Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

In order to elucidate the early thermal history of enstatite chondrite parent bodies we determined 129I-129Xe whole rock ages of enstatite chondrites (5 EH, 2 EL, one EH impact melt) relative to the Shallowater reference meteorite (4562.3±0.4 Ma, all errors are 1σ). I-Xe ages of both EL6 chondrites (LON 94100: -4.38±0.60 Ma and Neuschwanstein: -3.87±0.73 Ma – negative sign indicates ages younger than Shallowater) agree well with data of other EL6 chondrites. LON 94100 displayed a second isochron at lower temperatures equivalent to a younger age of -5.25±1.17 Ma, perhaps reflecting different retention temperatures of respective carrier phases during sequential cooling. The enstatite chondrites Abee (EH4), Indarch (EH4), EET 96135 (EH4/5) and St. Marks (EH5) encompass a I-Xe age range of +0.57±1.05 Ma (EET 96135 #1) to -0.45±0.72 Ma (Abee), again in agreement with previously reported ages of EH chondrites. Only the age of St. Marks differs strongly from previously reported younger ages, now being more in accordance with other members of the EH clan. The EH3 chondrite Sahara 97096 showed the youngest I-Xe age of -7.87±0.46 Ma distinctly younger than other I-Xe ages of EH chondrites, including other EH3s. Due to the apparent high retention temperature of the I-Xe system in enstatite (estimated >800°C) this young age implies a later resetting of the I-Xe system by a severe thermal, likely impact-induced, event. The EH impact melt LAP 02225 records a similarly young thermal event. Though no isochron relationship could be established, the data fall within an apparent I-Xe age range of +5 to +15 Ma, similar to Sahara 97096. Overall, EH chondrite parent body experienced a thermal history determined by a complex interplay between impact disturbances and parent body metamorphism.

Reference
Hopp J, Trieloff M, Ott U (2015) I-Xe ages of enstatite chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.014]
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