¹Paolo A. Sossi, ^1,2Oliver Nebel, ^3,4Mahesh Anand, ⁵Franck Poitrasson
¹Research School of Earth Sciences, Australian National University, Canberra 2601, ACT, Australia
²School of Earth, Atmosphere and Environment, Monash University, Melbourne 3800, VIC, Australia
³Department of Physical Sciences, Open University, Milton Keynes, MK76AA, UK
⁴Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
⁵Laboratoire Géosciences Environnement Toulouse, CNRS UMR 5563 – UPS – IRD, 14-16, Avenue Edouard Belin, 31400, Toulouse, France

Iron is the most abundant multivalent element in planetary reservoirs, meaning its isotope composition (expressed as δ57Fe) may record signatures of processes that occurred during the formation and subsequent differentiation of the terrestrial planets. Chondritic meteorites, putative constituents of the planets and remnants of undifferentiated inner solar system bodies, have View the MathML sourceδFe57≈0‰; an isotopic signature shared with the Martian Shergottite–Nakhlite–Chassignite (SNC) suite of meteorites. The silicate Earth and Moon, as represented by basaltic rocks, are distinctly heavier, View the MathML sourceδFe57≈+0.1‰. However, some authors have recently argued, on the basis of iron isotope measurements of abyssal peridotites, that the composition of the Earth’s mantle is View the MathML sourceδFe57=+0.04±0.04‰, indistinguishable from the mean Martian value. To provide a more robust estimate for Mars, we present new high-precision iron isotope data on 17 SNC meteorites and 5 mineral separates. We find that the iron isotope compositions of Martian meteorites reflect igneous processes, with nakhlites and evolved shergottites displaying heavier View the MathML sourceδFe57(+0.05±0.03‰), whereas MgO-rich rocks are lighter (View the MathML sourceδFe57≈−0.01±0.02‰). These systematics are controlled by the fractionation of olivine and pyroxene, attested to by the lighter isotope composition of pyroxene compared to whole rock nakhlites. Extrapolation of the View the MathML sourceδFe57 SNC liquid line of descent to a putative Martian mantle yields a δ57Fe value lighter than its terrestrial counterpart, but indistinguishable from chondrites. Iron isotopes in planetary basalts of the inner solar system correlate positively with Fe/Mn and silicon isotopes. While Mars and IV-Vesta are undepleted in iron and accordingly have chondritic δ57Fe, the Earth experienced volatile depletion at low (1300 K) temperatures, likely at an early stage in the solar nebula, whereas additional post-nebular Fe loss is possible for the Moon and angrites.

Reference
Sossi PA, Nebel O, Anand A, Poitrasson F (2016) On the iron isotope composition of Mars and volatile depletion in the terrestrial planets. Earth and Planetary Science Letters (in Press)
Link to Article [doi:10.1016/j.epsl.2016.05.030]
Copyright Elsevier

¹S. Marchi, ²B.A. Black, ³L.T. Elkins-Tanton, ¹W.F. Bottke
¹Southwest Research Institute, Boulder, CO, United States
²City College, City University of New York, New York, NY, United States
³Arizona State University, Tempe, AZ, United States

Recent revisions to our understanding of the collisional history of the Hadean and early-Archean Earth indicate that large collisions may have been an important geophysical process. In this work we show that the early bombardment flux of large impactors (>100 km) facilitated the atmospheric release of greenhouse gases (particularly CO2) from Earth’s mantle. Depending on the timescale for the drawdown of atmospheric CO2, the Earth’s surface could have been subject to prolonged clement surface conditions or multiple freeze-thaw cycles. The bombardment also delivered and redistributed to the surface large quantities of sulfur, one of the most important elements for life. The stochastic occurrence of large collisions could provide insights on why the Earth and Venus, considered Earth’s twin planet, exhibit radically different atmospheres.

Reference
Marchi S, Black BA, Elkins-Tanton LT, Bottke WF (2016) Massive impact-induced release of carbon and sulfur gases in the early Earth’s atmosphere. Earth and Planetary Science Letters 449, 96–104.
Link to Article [doi:10.1016/j.epsl.2016.05.032]
Copyright Elsevier