TY - BOOK

T1 - Probing into the early history of Mars and the Earth

T2 - New insights into the accretion and differentiation history of the terrestrial planets from measurement of isotopic variations in terrestrial and extraterrestrial materials

AU - Bouvier, Laura Claudia

PY - 2022

Y1 - 2022

N2 - The early infancy of terrestrial planets began about 4.5 Gyr ago in a chaotic and turbulent environmentconsisting of dust and gas, the protoplanetary disk. The terrestrial planets grew by accretion of small dustparticles to form kilometer-sized planetesimals and then Moon to Mars-sized planetary embryos. A finalviolent collision between a Mars-sized body and the Earth lead to the formation of the Moon. The accretionprocess provided enough energy to entirely melt terrestrial planets and induced the so-called early planetarydifferentiation. This process resulted in the creation of multiple layers in terrestrial planets with a metallic core,a rocky primitive mantle, and an outer layer, the primitive crust. First, iron-rich droplets sunk through a globalmagma ocean into the center of the planet to form the metallic core. Then, the magma ocean solidified togenerate a rocky unstable primitive mantle. Due to gravitational instability, the mantle will partially meltresulting in the extraction of the outer layer, the primitive crust. These early differentiation events had a stronginfluence on the long-term evolution of the terrestrial planets, such as controlling the onset of plate tectonicregime on Earth. In addition, the establishment of a stable primordial crust is a requirement for the emergenceof life. It is thus crucial to understand the nature and the timing of such events on terrestrial planets.There is no preservation of rocky material at the surface of the Earth that formed in these earliestevolutionary stages due to their recycling during billions of years of plate tectonic regime. Mars could havepreserved such ancient crustal material because of the lack of plate tectonic regime for the majority of itshistory. Therefore, Mars represents an important analogue of Earth in its infancy. To characterize the natureand the timing of the early planetary differentiation events, we sought for variations at the atomic scale in rockor mineral samples from the oldest terranes on Earth and from martian meteorites. These so-called radiogenicvariations can be caused by the accumulation of a daughter isotope due to the radioactive decay of a parentisotope. The amount of the daughter isotope depends on the time and the relative initial abundances of thedaughter and parent isotopes. Interesting, these two factors are intimately related to the nature and/or the ageof the mantle or crustal source from which the rock or the mineral are derived from. For instance, we combinedthe Lu-Hf (Lutetium/Hafnium) isotope system with the U-Pb (Uranium/Lead) isotope systems of zirconminerals extracted from a martian meteorite, NWA 7034. This meteorite originates from the Southernhemisphere of Mars and preserves fragments as old as ~4.43 Gyr-old. We determined that these zircons derived from an ancient crust formed within 20 Myr after the beginning of the Solar System. A corollary is Marsformed extremely rapidly such as the accretion and the planetary differentiation were completed in less than20 Myr after Solar System formation.To further constrain the nature and the age of this primordial crust, we applied an additionalchronometer, the 92Nb92Zr (92-Niobium/92-Zirconium) isotope system. This geochronometer is extremelysensitive to early episodes of planetary differentiation given that the short-lived 92Nb nuclide survived in ourSolar System only during the first 180 Myr. Any radiogenic 92Zr variations are consequently related to eventsthat occurred when 92Nb was still extant, namely core formation or mantle-crust differentiation. We determinedthat two of the seven NWA 7034 zircons previously analyzed yield radiogenic 92Zr variations. Our resultsreflect the mechanism of formation of the crust from which the NWA 7034 zircons derived although additionalwork is required to decipher the nature of this mechanism.To track early differentiation events on Earth, we applied the 92Nb92Zr chronometer to the oldestterrestrial samples, the Jack Hills zircons from Western Australia and for two primitive meteorites. Thedetection of radiogenic 92Zr variations in >3.8 Gyr old terrestrial zircons could be attributed to early crustformation. We determined that these ancient zircons does not display any radiogenic 92Zr variations, whichconfirm a late formation of the crust from which they derived (> 70 Myr after Solar System formation). Thetwo primitive meteorites called Orgueil and Murchison are designated as carbonaceous chondrites andrepresent fragments of planetesimals that avoided differentiation. The detection of 92Zr variations in theseprimitive meteorites compared to Earth can be attributed to core formation. We determined that Earth displaysa deficit in 92Zr compared to Orgueil but not compared to Murchison or other primitive carbonaceouschondrites. This could be either attributed to the terrestrial core formation but also to some heterogeneousdistribution of 92Nb nuclide among the carbonaceous chondrites. As such, the 92Zr deficit in Earth comparedto Orgueil can be alternatively caused by the enrichment in 92Nb nuclide in this latter.

AB - The early infancy of terrestrial planets began about 4.5 Gyr ago in a chaotic and turbulent environmentconsisting of dust and gas, the protoplanetary disk. The terrestrial planets grew by accretion of small dustparticles to form kilometer-sized planetesimals and then Moon to Mars-sized planetary embryos. A finalviolent collision between a Mars-sized body and the Earth lead to the formation of the Moon. The accretionprocess provided enough energy to entirely melt terrestrial planets and induced the so-called early planetarydifferentiation. This process resulted in the creation of multiple layers in terrestrial planets with a metallic core,a rocky primitive mantle, and an outer layer, the primitive crust. First, iron-rich droplets sunk through a globalmagma ocean into the center of the planet to form the metallic core. Then, the magma ocean solidified togenerate a rocky unstable primitive mantle. Due to gravitational instability, the mantle will partially meltresulting in the extraction of the outer layer, the primitive crust. These early differentiation events had a stronginfluence on the long-term evolution of the terrestrial planets, such as controlling the onset of plate tectonicregime on Earth. In addition, the establishment of a stable primordial crust is a requirement for the emergenceof life. It is thus crucial to understand the nature and the timing of such events on terrestrial planets.There is no preservation of rocky material at the surface of the Earth that formed in these earliestevolutionary stages due to their recycling during billions of years of plate tectonic regime. Mars could havepreserved such ancient crustal material because of the lack of plate tectonic regime for the majority of itshistory. Therefore, Mars represents an important analogue of Earth in its infancy. To characterize the natureand the timing of the early planetary differentiation events, we sought for variations at the atomic scale in rockor mineral samples from the oldest terranes on Earth and from martian meteorites. These so-called radiogenicvariations can be caused by the accumulation of a daughter isotope due to the radioactive decay of a parentisotope. The amount of the daughter isotope depends on the time and the relative initial abundances of thedaughter and parent isotopes. Interesting, these two factors are intimately related to the nature and/or the ageof the mantle or crustal source from which the rock or the mineral are derived from. For instance, we combinedthe Lu-Hf (Lutetium/Hafnium) isotope system with the U-Pb (Uranium/Lead) isotope systems of zirconminerals extracted from a martian meteorite, NWA 7034. This meteorite originates from the Southernhemisphere of Mars and preserves fragments as old as ~4.43 Gyr-old. We determined that these zircons derived from an ancient crust formed within 20 Myr after the beginning of the Solar System. A corollary is Marsformed extremely rapidly such as the accretion and the planetary differentiation were completed in less than20 Myr after Solar System formation.To further constrain the nature and the age of this primordial crust, we applied an additionalchronometer, the 92Nb92Zr (92-Niobium/92-Zirconium) isotope system. This geochronometer is extremelysensitive to early episodes of planetary differentiation given that the short-lived 92Nb nuclide survived in ourSolar System only during the first 180 Myr. Any radiogenic 92Zr variations are consequently related to eventsthat occurred when 92Nb was still extant, namely core formation or mantle-crust differentiation. We determinedthat two of the seven NWA 7034 zircons previously analyzed yield radiogenic 92Zr variations. Our resultsreflect the mechanism of formation of the crust from which the NWA 7034 zircons derived although additionalwork is required to decipher the nature of this mechanism.To track early differentiation events on Earth, we applied the 92Nb92Zr chronometer to the oldestterrestrial samples, the Jack Hills zircons from Western Australia and for two primitive meteorites. Thedetection of radiogenic 92Zr variations in >3.8 Gyr old terrestrial zircons could be attributed to early crustformation. We determined that these ancient zircons does not display any radiogenic 92Zr variations, whichconfirm a late formation of the crust from which they derived (> 70 Myr after Solar System formation). Thetwo primitive meteorites called Orgueil and Murchison are designated as carbonaceous chondrites andrepresent fragments of planetesimals that avoided differentiation. The detection of 92Zr variations in theseprimitive meteorites compared to Earth can be attributed to core formation. We determined that Earth displaysa deficit in 92Zr compared to Orgueil but not compared to Murchison or other primitive carbonaceouschondrites. This could be either attributed to the terrestrial core formation but also to some heterogeneousdistribution of 92Nb nuclide among the carbonaceous chondrites. As such, the 92Zr deficit in Earth comparedto Orgueil can be alternatively caused by the enrichment in 92Nb nuclide in this latter.

M3 - Ph.D. thesis

BT - Probing into the early history of Mars and the Earth

PB - GLOBE Institute, Faculty of Health and Medical Sciences, University of Copenhagen

ER -