Accretion and early evolution of Earth:Insights from Nd-isotope systematics of meteorites and Archean terrestrial rocks

• Main Author: Saji, Nikitha Susan
• Format: Book
• Language: English
• Published: Natural History Museum of Denmark, Faculty of Science, University of Copenhagen 2017
• Subjects: Greenland; Tagish; Tagish Lake
• Online Access: https://curis.ku.dk/portal/da/publications/accretion-and-early-evolution-of-earth(13d0dd36-107a-4f9f-a6ad-562bf2f97f78).html; https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122333609505763

The formation of terrestrial planets like Earth represent the end-stages of evolution of a circumstellar disk that start out as a dense core of gas and dust in an interstellar molecular cloud and gravitationally collapse to form a central star and a surrounding protoplanetary disk. Gas and micron-sized dust, of which the early protoplanetary disk is initially composed of, coalesce over the course of several millions of years to form the precursors to planets that make up the solar system today. The final assembly of Earth-like planets is complete only after a protracted latestage evolution that extends over at least 100 Myr, characterized by violent collisions between Mars- to Moon-sized planetary embryos. Evidence for the many details of solar system evolution - such as the diverse stellar sources that contributed material to solar system bodies to what role disk processes and late-stage impacts had to play in determining the bulk composition as well as pace of the chemical dierentiation and internal dynamics of terrestrial planets - is preserved in the form of isotopic signatures in some of the oldest terrestrial and extraterrestrial samples available to us. A potential means to unravel this is by the application of Nd-isotope systematics as the coupled 146;147Sm - 143;142Nd decay system enables the study of chronology of planetary silicate mantles while the stable Nd-isotopes help track the origin and early transport of material. Deciphering this information, however, requires the analytical capability to precisely and accurately measure variations in isotope ratios as little as few parts per million. In the first part of this thesis, a state-of-theart analytical protocol for high-precision measurements of Nd-isotopes using MC-ICPMS is reported. The refined purification chemistry together with acquisition of mass-dependent data enables the measurement of Nd isotope ratios with a far superior precision compared to existing TIMS 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 between solar 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 is governed by the heterogeneous distribution of at least three distinct nuclosynthetic components - namely the classical main s-/r-process component, a tentative, hither-to unidentified anomalous r-process/nuclear-field shift component and pure p-process component. Analysis of the leachates and residue of Tagish Lake carbonaceous chondrite further confirm the presence of the main s-process component as well as that of a second anomalous r-process component, with the latter having largely similar mineralogical/chemical properties as the former. Accounting for the eect of nucleosynthetic heterogeneity obviates the need for the 142Nd excess on Earth’s mantle relative to most chondritic meteorites to be explained by radiogenic ingrowth from a very early global silicate dierentiation on Earth or accretion of Earth from precursors with a suprachondritic Sm/Nd ratio. Furthermore, the heterogeneous distribution of the classical s-/rprocess component as well as the pure p-process component in solar system materials is found to be related to selective thermal processing of dust in the early nebula given the correlation observed for these eects with Fe-peak neutron-rich isotope anomalies, whose origin is attributed to 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-isotope heterogeneity 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 point to the eect of possible analytical artifacts. Resampling and analysis using the new analytical protocol of Eoarchean and Paleoarchean rocks from Isua supracrustal belt, SW Greenland for which 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 reduction in the magnitude of 142Nd anomaly with time, in contradiction to the earlier data and best explained 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 depleted mantle reservoir that coincides with the estimated ages for Moon-forming giant, suggesting the formation of the Isua reservoir in the ensuing magma ocean and possibly representing the earliest instance of crust extraction known on Earth. This early-formed crust-mantle reservoir is inferred to be a major reservoir that survived for a large part of the Hadean eon and supplied the other Archean terranes from which evidence exists for Hadean crustal remnants. 8