Abstract
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
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
| Originalsprog | Engelsk |
|---|
| Forlag | Natural History Museum of Denmark, Faculty of Science, University of Copenhagen |
|---|---|
| Status | Udgivet - 2017 |