TY - BOOK

T1 - Accretion and early evolution of Earth

T2 - Insights from Nd-isotope systematics of meteorites and Archean terrestrial rocks

AU - Saji, Nikitha Susan

PY - 2017

Y1 - 2017

N2 - The formation of terrestrial planets like Earth represent the end-stages of evolution of a circumstellardisk that start out as a dense core of gas and dust in an interstellar molecular cloud andgravitationally collapse to form a central star and a surrounding protoplanetary disk. Gas andmicron-sized dust, of which the early protoplanetary disk is initially composed of, coalesce overthe course of several millions of years to form the precursors to planets that make up the solarsystem today. The final assembly of Earth-like planets is complete only after a protracted latestageevolution that extends over at least 100 Myr, characterized by violent collisions betweenMars- to Moon-sized planetary embryos.Evidence for the many details of solar system evolution - such as the diverse stellar sources thatcontributed material to solar system bodies to what role disk processes and late-stage impactshad to play in determining the bulk composition as well as pace of the chemical dierentiationand internal dynamics of terrestrial planets - is preserved in the form of isotopic signatures insome of the oldest terrestrial and extraterrestrial samples available to us. A potential means tounravel this is by the application of Nd-isotope systematics as the coupled 146;147Sm - 143;142Nddecay system enables the study of chronology of planetary silicate mantles while the stableNd-isotopes help track the origin and early transport of material. Deciphering this information,however, requires the analytical capability to precisely and accurately measure variationsin isotope ratios as little as few parts per million. In the first part of this thesis, a state-of-theartanalytical protocol for high-precision measurements of Nd-isotopes using MC-ICPMS isreported. The refined purification chemistry together with acquisition of mass-dependent dataenables the measurement of Nd isotope ratios with a far superior precision compared to existingTIMS methods while ensuring data accuracy.The second part of this thesis centers on the application of the new Nd-isotope analytical pro-tocol to investigate the long-standing issue of nucleosynthetic heterogeneity in Nd-isotopes betweensolar system reservoirs. By the high-precision analysis of a number of bulk primitive meteorites,it is shown that the non-radiogenic Nd-isotope composition of solar system materials isgoverned by the heterogeneous distribution of at least three distinct nuclosynthetic components- namely the classical main s-/r-process component, a tentative, hither-to unidentified anomalousr-process/nuclear-field shift component and pure p-process component. Analysis of theleachates and residue of Tagish Lake carbonaceous chondrite further confirm the presence ofthe main s-process component as well as that of a second anomalous r-process component, withthe latter having largely similar mineralogical/chemical properties as the former. Accountingfor the eect of nucleosynthetic heterogeneity obviates the need for the 142Nd excess on Earth’smantle relative to most chondritic meteorites to be explained by radiogenic ingrowth from avery early global silicate dierentiation on Earth or accretion of Earth from precursors with asuprachondritic Sm/Nd ratio. Furthermore, the heterogeneous distribution of the classical s-/rprocesscomponent as well as the pure p-process component in solar system materials is foundto be related to selective thermal processing of dust in the early nebula given the correlation observedfor these eects with Fe-peak neutron-rich isotope anomalies, whose origin is attributedto distinct nucleosnythetic sites other than classical s-, r- or p-process.The revision of bulk silicate Earth 142Nd parameters in the light of nucleosynthetic Nd-isotopeheterogeneity requires that the early chronology of silicate dierentiation on Earth be revisited.Moreover, the excessive scatter in the existing 142Nd data for terrestrial Archean rocks pointto the eect of possible analytical artifacts. Resampling and analysis using the new analyticalprotocol of Eoarchean and Paleoarchean rocks from Isua supracrustal belt, SW Greenland forwhich both positive and negative 142Nd anomalies have been previously reported respectively,reveal a rather homogeneous positive 142Nd composition for the Isua mantle source and a reductionin the magnitude of 142Nd anomaly with time, in contradiction to the earlier data and bestexplained by early onset of plate tectonic-like crustal recycling and mantle homogenization processes.Coupled 146;147Sm - 143;142Nd systematics define a formation age for the Isua depletedmantle reservoir that coincides with the estimated ages for Moon-forming giant, suggesting theformation of the Isua reservoir in the ensuing magma ocean and possibly representing the earliestinstance of crust extraction known on Earth. This early-formed crust-mantle reservoir isinferred to be a major reservoir that survived for a large part of the Hadean eon and supplied theother Archean terranes from which evidence exists for Hadean crustal remnants.8

AB - The formation of terrestrial planets like Earth represent the end-stages of evolution of a circumstellardisk that start out as a dense core of gas and dust in an interstellar molecular cloud andgravitationally collapse to form a central star and a surrounding protoplanetary disk. Gas andmicron-sized dust, of which the early protoplanetary disk is initially composed of, coalesce overthe course of several millions of years to form the precursors to planets that make up the solarsystem today. The final assembly of Earth-like planets is complete only after a protracted latestageevolution that extends over at least 100 Myr, characterized by violent collisions betweenMars- to Moon-sized planetary embryos.Evidence for the many details of solar system evolution - such as the diverse stellar sources thatcontributed material to solar system bodies to what role disk processes and late-stage impactshad to play in determining the bulk composition as well as pace of the chemical dierentiationand internal dynamics of terrestrial planets - is preserved in the form of isotopic signatures insome of the oldest terrestrial and extraterrestrial samples available to us. A potential means tounravel this is by the application of Nd-isotope systematics as the coupled 146;147Sm - 143;142Nddecay system enables the study of chronology of planetary silicate mantles while the stableNd-isotopes help track the origin and early transport of material. Deciphering this information,however, requires the analytical capability to precisely and accurately measure variationsin isotope ratios as little as few parts per million. In the first part of this thesis, a state-of-theartanalytical protocol for high-precision measurements of Nd-isotopes using MC-ICPMS isreported. The refined purification chemistry together with acquisition of mass-dependent dataenables the measurement of Nd isotope ratios with a far superior precision compared to existingTIMS methods while ensuring data accuracy.The second part of this thesis centers on the application of the new Nd-isotope analytical pro-tocol to investigate the long-standing issue of nucleosynthetic heterogeneity in Nd-isotopes betweensolar system reservoirs. By the high-precision analysis of a number of bulk primitive meteorites,it is shown that the non-radiogenic Nd-isotope composition of solar system materials isgoverned by the heterogeneous distribution of at least three distinct nuclosynthetic components- namely the classical main s-/r-process component, a tentative, hither-to unidentified anomalousr-process/nuclear-field shift component and pure p-process component. Analysis of theleachates and residue of Tagish Lake carbonaceous chondrite further confirm the presence ofthe main s-process component as well as that of a second anomalous r-process component, withthe latter having largely similar mineralogical/chemical properties as the former. Accountingfor the eect of nucleosynthetic heterogeneity obviates the need for the 142Nd excess on Earth’smantle relative to most chondritic meteorites to be explained by radiogenic ingrowth from avery early global silicate dierentiation on Earth or accretion of Earth from precursors with asuprachondritic Sm/Nd ratio. Furthermore, the heterogeneous distribution of the classical s-/rprocesscomponent as well as the pure p-process component in solar system materials is foundto be related to selective thermal processing of dust in the early nebula given the correlation observedfor these eects with Fe-peak neutron-rich isotope anomalies, whose origin is attributedto distinct nucleosnythetic sites other than classical s-, r- or p-process.The revision of bulk silicate Earth 142Nd parameters in the light of nucleosynthetic Nd-isotopeheterogeneity requires that the early chronology of silicate dierentiation on Earth be revisited.Moreover, the excessive scatter in the existing 142Nd data for terrestrial Archean rocks pointto the eect of possible analytical artifacts. Resampling and analysis using the new analyticalprotocol of Eoarchean and Paleoarchean rocks from Isua supracrustal belt, SW Greenland forwhich both positive and negative 142Nd anomalies have been previously reported respectively,reveal a rather homogeneous positive 142Nd composition for the Isua mantle source and a reductionin the magnitude of 142Nd anomaly with time, in contradiction to the earlier data and bestexplained by early onset of plate tectonic-like crustal recycling and mantle homogenization processes.Coupled 146;147Sm - 143;142Nd systematics define a formation age for the Isua depletedmantle reservoir that coincides with the estimated ages for Moon-forming giant, suggesting theformation of the Isua reservoir in the ensuing magma ocean and possibly representing the earliestinstance of crust extraction known on Earth. This early-formed crust-mantle reservoir isinferred to be a major reservoir that survived for a large part of the Hadean eon and supplied theother Archean terranes from which evidence exists for Hadean crustal remnants.8

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122333609505763

M3 - Ph.D. thesis

BT - Accretion and early evolution of Earth

PB - Natural History Museum of Denmark, Faculty of Science, University of Copenhagen

ER -