ATOM PROBE TOMOGRAPHY OF ZIRCONIAN MINERALS FROM EARLY MARS AND EARTH INDICATE CRUSTAL REFUGIA ON EARLY BOMBARDMENT SURFACES

Zirconian accessory minerals are difficult to destroy through natural processes such that their crystallographic and chemical microstructure can bear witness to the occurrence and sometimes age of intense impact and thermal events [e.g. 1]. This physical record is therefore well suited to evaluating numerical models of the tempo and nature of early bombardment and ultimately the evolution of planetary crusts, hydrospheres and life. Recent models that rescale impactor flux according to a sawtooth decay in lunar impact frequency [2] deduce 100% to 600% resurfacing of the early Earth by impact-related mafic magmatism [3] and high temperature metamorphism of the outer 20km of crust. It is presumed that other bodies in the inner solar system experienced similar pervasive crustal modification due to gas giant planet migration (Nice Model). To test this we have conducted atom probe tomography of terrestrial zircons that survived extreme and prolonged post-impact heating at the center of the giant Vredefort impact crater for comparison with our data for 4.4 Ga grains from the martian regolith (NWA 7475) and recently published Hadean zircon data [4]. The Vredefort samples show nanoclustering of elements such as Pb and Al similar to zircons affected by high temperature tectonic metamorphism [4,5]. Conversely data for 4.4. Ga zircon and baddeleyite grains from Mars, together with EBSD analyses, reveal a homogenous trace element distribution at the atomic scale and other characteristics consistent with residence in crustal domain(s) that escaped severe metamorphism for most of martian history. These observations, together with the already established evidence for relatively cool and hydrous environments from some of Earth’s Hadean zircons, indicate that bombardment-driven melting and burial of early crusts was not as pervasive as recently modelled, and that islands of primitive ‘refugia’, crustal domains of potential habitability, persisted through the bombardment epoch roughly a billion years before the oldest known microfossils.

References: [1] Moser, D.E. et al., (2013) Nature 499, 454-457. [2] Morbidelli, A. et al., (2012) EPSL 355-6, 144-151. [3] Marchi, S. et al., (2014) Nature 511, 578-581. [4] Valley, J.W. et al. (2014) Nature Geoscience 7, 219-223. [5] Kusiak, M. et al., (2015) PNAS 112, 4958–4963.