McDonnell Distinguished Lecturer: Alex Halliday

Alex Halliday

Professor, Department of earth Sciences, University of Oxford

Abstract: Isotopic evidence and chemical compositions of meteorites demonstrate that Earth’s core grew both in absolute and relative terms during accretion.  The depletion of slightly siderophile elements such as vanadium is consistent with a primitive mantle that was more reduced during the earlier stages of accretion. This is confirmed by isotopic evidence that the core sequestered significant silicon before the Moon-forming Giant Impact.  There still is no consensus about how water and other highly volatile elements were added to Earth and how much may have been lost to the core.  The final stage of accretion is thought by some to be the addition of a late veneer that provided the (post core and Moon formation) mantle and crust inventory of highly siderophile elements (gold, platinum etc) and volatiles such as water, carbon and nitrogen.  The fact that this veneer appears to be a far smaller proportion of the Moon’s budget is hard to explain.  Isotopic and relative elemental abundances of Earth’s highly volatile elements (noble gases, H, C, N) provide evidence of a series of Solar, chondritic and cometary additions punctuated by major differential losses.  If chondritic materials are assumed to be the primary starting materials for most of the volatiles in the terrestrial planets then there has been extremely strong (up to 2 orders of magnitude) depletion in ¹H, ¹²C, ¹⁴N and ¹³⁰Xe relative to ²⁰Ne, ³⁶Ar and ⁸⁴Kr.  Indeed carbon and nitrogen are the most depleted elements in the silicate Earth reflecting core formation or losses to space.  It can be shown that a major fraction (>70%) of Earth’s hydrogen (water) budget is earlier than any putative late veneer.  Furthermore, the growing evidence of isotopic similarity between the Moon and silicate Earth appears inconsistent with the idea of a large (1 to 5%) veneer as has recently been proposed.