Alex N. Halliday

Bio: Alex N. Halliday is an academic researcher from University of Oxford. The author has contributed to research in topics: Chondrite & Isotope fractionation. The author has an hindex of 104, co-authored 423 publications receiving 31802 citations. Previous affiliations of Alex N. Halliday include University of California, Los Angeles & Lamont–Doherty Earth Observatory.

Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons

Abstract: Continental crust forms from, and thus chemically depletes, the Earth's mantle. Evidence that the Earth's mantle was already chemically depleted by melting before the formation of today's oldest surviving crust has been presented in the form of Sm–Nd isotope studies of 3.8–4.0 billion years old rocks from Greenland1,2,3,4,5 and Canada5,6,7. But this interpretation has been questioned because of the possibility that subsequent perturbations may have re-equilibrated the neodymium-isotope compositions of these rocks8. Independent and more robust evidence for the origin of the earliest crust and depletion of the Archaean mantle can potentially be provided by hafnium-isotope compositions of zircon, a mineral whose age can be precisely determined by U–Pb dating, and which can survive metamorphisms4. But the amounts of hafnium in single zircon grains are too small for the isotopic composition to be precisely analysed by conventional methods. Here we report hafnium-isotope data, obtained using the new technique of multiple-collector plasma-source mass spectrometry9, for 37 individual grains of the oldest known terrestrial zircons (from the Narryer Gneiss Complex, Australia, with U–Pb ages of up to 4.14 Gyr (10–13). We find that none of the grains has a depleted mantle signature, but that many were derived from a source with a hafnium-isotope composition similar to that of chondritic meteorites. Furthermore, more than half of the analysed grains seem to have formed by remelting of significantly older crust, indicating that crustal preservation and subsequent reworking might have been important processes from earliest times.

Core formation on Mars and differentiated asteroids

Abstract: Meteorite chronometry based on the 182Hf–182W system can provide powerful constraints on the timing of planetary accretion and differentiation1,2,3,4, although the full potential of this method has yet to be realized. For example, no measurements have been made on the silicate-rich portions of planets and planetesimals other than the Earth and Moon. Here we report tungsten isotope compositions for two eucrites, thought to be derived from asteroid 4 Vesta, and from eight other basaltic achondritic meteorites that are widely considered to be from Mars. The eucrites, which are among the oldest differentiated meteorites, yield exceedingly radiogenic tungsten, indicating rapid accretion, differentiation and core formation on Vesta within the first 5–15 Myr of Solar System history, whereas the range of radiogenic tungsten measurements on the martian meteorites points towards tungsten depletion via melting and core formation within the first 30 Myr of the Solar System. The survival of tungsten isotope heterogeneity in the martian upper mantle implies that no giant impacts or large-scale convective mixing took place since this time. These results contrast with those obtained for the Earth–Moon system2,3 for which accretion and core formation related to giant impacts appears to have continued for at least an additional 20 Myr.

Thermal and Magmatic Evolution of the Moon

Abstract: As with all science, our continually developing concepts of lunar evolution are firmly tied to both new types of observations and the integration of these observations to the known pool of data. This process invigorates the intellectual foundation on which old models are tested and new concepts are built. Just as the application of new observational tools to lunar science in 1610 (Galileo’s telescope) and 1840 (photography) yielded breakthroughs concerning the true nature of the lunar surface, the computational and technological advances highlighted by the Apollo and post-Apollo missions and associated scientific investigations provided a new view of the thermal and magmatic evolution of the Moon. ### 1.1. Pre-Apollo view of the thermal and magmatic evolution of the Moon Many of the early views of the Moon manifested in mythology and art throughout the world were primarily tied to lunar and terrestrial cycles and the relationships between the Sun and the Moon. Prophetically, myths involving the lunar deities Mwuetsi from Zimbabwe and Coyolxauhqui from Mexico told of rather violent or catastrophic events in which the Moon was expunged from the Earth. Numerous ancient scientific observations were made about the nature of the Moon ranging from those uncovered in early Neolithic sites that correctly identified mare Crisium and mare Humorum to the insights made by Greek philosophers such as Anaxagoras (ca. 500-428 B.C.) and Democritus (ca. 460-370 B.C.), who attached terrestrial analogues to its character (stone, mountains). With the advent of the telescope (1610) and photography (1840) as scientific tools for lunar exploration, semiquantitative data could be collected that would provide an intellectual foundation for scientific interpretation. Initially, modern terrestrial geological analogs were extended to the Moon (lunar highlands, volcanic craters, seas). Combined with the rigors of computational modeling, these observational data were extended to predict the original thermal state of the Moon and its thermal and magmatic history. Its proximity to the Earth …

Composition of the Continental Crust

Abstract: This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.