176Lu–176Hf and 147Sm–143Nd ages of the Martian shergottites: Evaluation of the shock-resetting hypothesis through diffusion kinetic experiments and modeling, and petrological observations

Abstract

We address the age controversy of the shergottite suite of Martian meteorites (∼200 Ma dates determined by 176Lu–176Hf, 147Sm–143Nd, and various other decay systems as opposed to a ∼4 Ga whole rock Pb–Pb date) by calculating the timescales needed to reset Hf and Nd isotopes in clinopyroxene, which is the primary host of rare earth elements amongst the minerals used to compose the 176Lu–176Hf and 147Sm–143Nd isochrons, at the peak- and post-shock P–T conditions likely to have been experienced by the shergottites. It is concluded that impact heating is highly unlikely to have significantly reset the 176Lu–176Hf and/or 147Sm–143Nd mineral isochrons of these meteorites. This conclusion is bolstered by the nature of measured concentration profiles of Cr and Fe across olivine–maskelynite boundaries, and Ti profiles across clinopyroxene–maskelynite/melt interfaces in the shergottite RBT 04262. The lack of any evidence of diffusion in the measured concentration profiles, coupled with the qualitative incompatibility of the measured crystal–melt fractionation trends at these interfaces with the nature of fractionation expected from equilibrium partitioning, provides strong evidence that no substantial chemical exchange took place between the solid and melt phases during peak-shock P–T conditions. Furthermore, we find that the compositional profiles of major elements in coexisting augite and pigeonite are homogenous and reflect a magmatic temperature of 1143±9 °C. We argue that the pyroxene compositions, and consequently the 176Lu–176Hf and 147Sm–143Nd geochronometers, were highly unlikely to have been reset by the thermal effect of a shock event. X-ray mapping of a selected section of RBT 04262 shows no evidence of enhanced grain boundary diffusion. We thus conclude that the ∼200 Ma dates recorded by the 176Lu–176Hf and 147Sm–143Nd geochronometers most likely record the crystallization age of the shergottites.