Temporal evolution of 142Nd signatures in SW Greenland from high precision MC-ICP-MS measurements

Measurements of 142Nd isotope signatures in Archean rocks are a powerful tool to investigate the earliest silicate differentiation events on Earth. Here, we introduce a new analytical protocol that allows high precision radiogenic and mass-independent Nd isotope measurements by MC-ICP-MS. To validate our method, we have measured well-characterized ∼3.72 to ∼3.8 Ga samples from the Eoarchean Itsaq Gneiss Complex and associated supracrustal belts, as well as Mesoarchean greenstones and a Proterozoic dike in SW Greenland, including lithostratigraphic units that were previously analyzed for 142-143Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ142Nd values for ∼3.72 to ∼3.8 Ga rocks from the Isua region range from +9.2 ± 2.6 to +13.2 ± 1.1 ppm and are in good agreement with previous studies. Using coupled 142,143Nd/144Nd isotope systematics from our data for ∼3.8 Ga mafic-ultramafic successions from the Isua region, we can confirm previous age constraints on the earliest silicate differentiation events with differentiation age of 4.390−0.060+0.045 Ga. Moreover, we can resolve a statistically significant decrease of 142Nd/144Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ∼3.8 Ga (μ142Nd = +13.0 ± 1.1) and ∼ 3.72 Ga (μ142Nd = +9.8 ± 1.0). Even lower but homogeneous μ142Nd values of +3.8 ± 1.1 are found in ∼3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε143Nd(i) and εHf(i) values of Isua rocks scatter more than it would be expected from a single stage differentiation event as implied from nearly uniform μ142Nd values, suggesting that the previously described decoupling of Hf and Nd isotopes is not a primordial magma ocean signature. Instead, we conclude that some of second stage processes like younger mantle depletion events or recycling of subducted material affected the 147Smsingle bond143Nd isotope systematics. The preservation of pristine whole-rock isochrons largely rules out a significant disturbance by younger alteration events. Based on isotope and trace element modelling, we argue that the temporal evolution of coupled 142,143Nd/144Nd isotope compositions in the ambient mantle beneath the Isua rocks is best explained by the progressive admixture of material to the Isua mantle source that must have had present-day-like μ142Nd compositions. In contrast, Mesoarchean mafic rocks from the ∼3.08 Ga Ivisaartoq greenstone belt and the 2.97 Ga inner Ameralik Fjord region as well as a 2.0 Ga Proterozoic dike within that region all have higher μ142Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ142Nd compositions

In search of the Earth-forming reservoir: Mineralogical, chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites

High-precision isotope data of meteorites show that the long-standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth-forming reservoir, we performed a comprehensive petrographic, elemental, and multi-isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent-body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium-tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent-body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth

Hochsiderophile Elemente in planetaren Materialien als Indikatoren für späte Akkretion

In the course of this thesis different planetary materials have been studied in order to evaluate the origin of the excess abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) in the Earth’s mantle and lunar crustal rocks. Concentrations of HSE and 187Os/188Os isotope ratios have been determined for chondritic meteorites, terrestrial peridotites and lunar impact melt rocks from Apollo 14, 16 and 17 landing sites. In the case of chondrites the obtained data reveal differences in the HSE abundance patterns and ratios such as Re/Os, 187Os/188Os, Pd/Ir, Rh/Ir and Au/Ir among chondrite classes. Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and possibly dilution by silicates. The HSE carrier phases responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior the formation of the host minerals in chondrite components. These correlations also suggest that metamorphism, alteration or igneous processes had negligible influence on relative HSE abundances on scales larger than bulk rocks. New HSE data on peridotites were used to further constrain HSE abundances in the Earth’s mantle and to place constraints on the distribution processes accounting for observed HSE variations between fertile and depleted mantle lithologies. Non- systematic variation of Rh abundances and constant Rh/Ir displayed by fertile lherzolites indicate a compatible behaviour of Rh during partial melting. In contrast, Au and Au/Ir correlate with peridotite fertility, indicating incompatible behaviour of Au during magmatic processes in the mantle. Correlations displayed by Pd/Ir, Re/Ir and Au/Ir with Al2O3 suggest HSE fractionation during partial melting, or may reflect refertilization of previously melt depleted peridotites due to reactive infiltration of silicate melts. Relative abundances of Rh and Au for the primitive mantle HSE model composition are similar to values of ordinary and enstatite chondrites. HSE abundances and 187Os/188Os in lunar impact melt rocks serve as important tracers to place constraints on the late accreted meteoritic materials during the late accretion period in the Earth-Moon system. Excellent linear correlations displayed among HSE abundances of subsamples from a given impact melt rock are explained by dilution processes or binary mixing between a high HSE end-member composition of the meteoritic impactor and a low-HSE end-member composition corresponding to the lunar target rocks. Some of the impactor end- member compositions identified in lunar impact melts are similar to chondrites, while others show suprachondritic HSE ratios. The strongly fractionated HSE abundance pattern of some Apollo 16 samples is similar to magmatic iron meteorites. This study reports the first sufficiently precise Re-Os isochron for a lunar impact melt rock. The Re-Os age of 4.11 ± 0.12 Ga obtained for Apollo 16 sample 67935 indicates that the Nectaris basin may be as old as 4.1 Ga. Excess HSE abundances of the Earth’s mantle and suprachondritic Pd/Ir and Ru/Ir inferred for the primitive mantle may be explained by binary mixing of chondritic material with some fractionated HSE component similar to that recorded in Apollo 16 impact melt rocks.Im Rahmen dieser Arbeit wurden verschiedene planetare Materialien analysiert, um den Ursprung der Exzessgehalte hochsiderophiler Elemente (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) im Erdmantel und in Krustengesteinen des Mondes zu erforschen. Dazu wurden präzise HSE Konzentrationen und 187Os/188Os Isotopenverhältnisse für chondritische Meteorite, terrestrische Peridotite und lunare Impaktschmelzgesteine von Apollo 14, 16 und 17 Missionen bestimmt. Im Fall der Chondrite weisen die neu erhobenen Daten Unterschiede im HSE Vorkommen von Re/Os, 187Os/188Os, Pd/Ir, Rh/Ir und Au/Ir zwischen verschiedenen Chondritklassen auf. Gut definierte lineare Korrelationen der hochsiderophilen Elemente, insbesondere für Gesamtgesteinsproben von gewöhnlichen Chondriten und EL Enstatitchondriten, werden als binäre Mischung und möglicherweise Verdünnung durch Silikatphasen interpretiert. Die HSE Trägerphasen, welche diese Korrelationen erzeugen, sind durch eine gleichmäßige chemische Zusammensetzung gekennzeichnet und deuten auf eine effiziente Homogenisierung von lokalen Nebelheterogenitäten während oder vor der Bildung der meisten Mineralphasen der Chondritkomponenten hin. Die beobachteten Korrelationen zeigen außerdem, dass Metamorphose, Alteration und magmatische Prozesse nur geringen Einfluss auf die relativen HSE Häufigkeiten im Maßstab größer als Gesamtgesteinsproben der Chondrite gehabt haben. Neue HSE Konzentrationsdaten für Peridotite wurden erhoben um das Vorkommen dieser Elemente im Erdmantel präziser zu bestimmen. Weiterhin wurde das Verteilungsverhalten und der für die beobachteten HSE Variationen zwischen fertilen und verarmten Peridotiten verantwortliche Fraktionierungsprozess genauer charakterisiert. Fertile Peridotite weisen eine unsystematische Variation der Rhodiumkonzentrationen und konstante Rh/Ir Verhältnisse auf. Diese Beobachtungen deuten auf ein kompatibles Verteilungsverhalten von Rh während partieller Schmelzbildung im Mantel hin. Im Gegensatz dazu zeigen Goldkonzentrationen und das Au/Ir Verhältnis eine Korrelation mit der Fertilität und deuten somit auf ein inkompatibles Verhalten von Gold während magmatischer Prozesse im Mantel hin. Korrelationen von Pd/Ir, Re/Ir und Au/Ir mit Al2O3 lassen sich durch Fraktionierung der HSE während partieller Schmelzextraktion erklären. Alternativ können die beobachteten Korrelationen durch Refertilisierungsprozesse erzeugt werden, bei denen eine Wiederanreicherung der HSE in zuvor schmelzverarmten Peridotiten durch den Prozess einer reaktiven Infiltration silikatischer Schmelzen erfolgt. Die für den primitiven Erdmantel extrapolierten Rhodium- und Goldgehalte sind vergleichbar mit Werten der gewöhnlichen Chondrite und der Enstatitchondrite. HSE Konzentrationen und 187Os/188Os Isotopenverhältnisse in lunaren Impaktschmelzgesteinen sind wichtige Indikatoren zur Herkunftsbestimmung des während der späten Akkretionsphase dem Erde-Mond System zugeführten Materials. Die HSE Konzentrationen für Subprobenaliquote eines Impaktschmelzgesteins sind linear miteinander korreliert und werden als Verdünnungen oder binäre Mischungen verschiedener Komponenten interpretiert. Dabei entspricht ein Mischungsendglied der Zusammensetzung der HSE-reichen meteoritischen Impaktorkomponente und das andere Endglied der HSE-verarmten Zielgesteine der Mondkruste. Die in den Impaktschmelzgesteinen identifizierten Impaktorkomponenten weisen sowohl chondritische als auch suprachondritische HSE Kompositionen auf. Stark fraktionierte HSE Muster einiger Apollo 16 Proben zeigen Ähnlichkeit zu den HSE Mustern von magmatischen Eisenmeteoriten. In dieser Studie wird die bis dahin erste hinreichend präzise Re-Os Isochrone für ein Impaktschmelzgestein präsentiert. Das Re-Os Alter von 4.11 ± 0.12 Ga für die Apollo 16 Probe 67935 deutet darauf hin, dass das Nectarisbecken bis zu 4.1 Ga alt sein könnte. Exzessvorkommen von HSE im Erdmantel zusammen mit suprachondritischen Pd/Ir und Ru/Ir Verhältnissen des primitiven Erdmantels können durch eine binäre Mischung von chondritischem Impaktormaterial und einer fraktionierten HSE Komponente, ähnlich der in Apollo 16 Proben nachgewiesenen Eisenmeteoritkomponente, erzeugt werden

Replication Data for: Ruthenium isotope fractionation in protoplanetary cores

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores

Replication Data for: Ruthenium isotope fractionation in protoplanetary cores

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores

Replication Data for: Ruthenium isotope fractionation in protoplanetary cores

Mass-dependent Ru isotope variations (δ102/99Ru) and Ru concentrations were determined for 35 magmatic iron meteorites from the five major chemical groups (IIAB, IID, IIIAB, IVA, IVB). In addition, four equilibrated ordinary chondrites were analyzed. The IIAB, IIIAB and IVB iron meteorites display increasingly heavier Ru isotopic compositions with decreasing Ru content. Modeling demonstrates that the trends for these three iron groups can be reproduced by the incremental extraction of isotopically lighter Ru into solids, which leads to progressively heavier δ102/99Ru in the remaining melt. The modeling further shows that the Ru isotopic variations of the IIAB and IIIAB irons are consistent with derivation from parental melts with an ordinary chondrite-like δ102/99Ru, whereas the IVB irons more likely derive from a melt with heavier δ102/99Ru. This heavy Ru isotopic composition of the IVB parental melt probably results from high-temperature processing of the IVB precursor material. The Ru isotope systematics of the IID and IVA irons are more complex and show no correlation between δ102/99Ru and Ru content. Although most samples exhibit heavy Ru isotopic compositions, especially the late-crystallized irons of these groups deviate from the expected fractional crystallization trends. This deviation most likely results from mixing and re-equilibration of early-crystallized solids and late-stage liquids, followed by further fractional crystallization. The mixing might be related to the migration of liquids through a complex network of dendrites or to the overturn of a cumulate inner core, and bears testimony to the complex solidification history of at least some protoplanetary cores

Neodymium and hafnium boundary contributions to seawater along the West Antarctic continental margin

Neodymium and hafnium isotopes and elemental concentrations (Sm, Nd, Hf, Zr) have been measured in three water column profiles south of the Antarctic Circumpolar Current in, and to the east of the Ross Sea, in conjunction with five bottom water samples from the Amundsen Sea Embayment. Neodymium and hafnium both appear to be released from sediments in the Embayment. In the case of Nd, this is reflected in radiogenic isotope compositions (εNd up to −5.4) and highly elevated concentrations (up to 41 pmol/kg). Hafnium isotopes, on the other hand, are only very slightly altered relative to the open ocean sites, and boundary release is most prominently indicated by elevated concentrations (>1 pmol/kg versus ∼0.7 pmol/kg). There is also a local input of both Hf and Nd at the Marie Byrd Seamounts, which leads to Nd isotope compositions as radiogenic as −3.1, and hafnium shifted to less radiogenic compositions in local bottom water. A compilation of the new data with literature data reveals a consistent view of the influence of Antarctica on the Nd isotope composition in Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW). Sector specific Nd addition shifts AABW formed in the Atlantic sector to less radiogenic isotope compositions (average εNd=−9) relative to LCDW (average εNd=−8.4), whereas AABW in the Pacific sector is shifted to more radiogenic values (average εNd=−7). The evolution towards more radiogenic εNd with depth in LCDW in the Pacific sector is likely to reflect admixture of AABW but, in addition, is also controlled by boundary exchange with the slope as observed at the Marie Byrd Seamounts. Hafnium isotopes are relatively homogeneous in the data set, ranging between εHf=+2 and +3.8 for most samples, excluding less radiogenic compositions in deep waters close to the Marie Byrd Seamounts. The Hf isotope composition in the Pacific sector is, however, slightly less radiogenic than in the Atlantic, corresponding to an average of +3 relative to an average of +3.8. This probably reflects unradiogenic Hf inputs from Antarctica to the Pacific sector, which are vertically homogenized by reversible scavenging. The Hf isotope heterogeneity in LCDW between both sectors is likely to indicate a shorter seawater residence time for Hf than for Nd, which is consistent with the dissolved – particulate phase partitioning of both elements

Neutron capture on Pt isotopes in iron meteorites and the Hf–W chronology of core formation in planetesimals

The short-lived 182Hf–182W isotope system can provide powerful constraints on the timescales of planetary core formation, but its application to iron meteorites is hampered by neutron capture reactions on W isotopes resulting from exposure to galactic cosmic rays. Here we show that Pt isotopes in magmatic iron meteorites are also affected by capture of (epi)thermal neutrons and that the Pt isotope variations are correlated with variations in 182W/184W. This makes Pt isotopes a sensitive neutron dosimeter for correcting cosmic ray-induced W isotope shifts. The pre-exposure 182W/184W derived from the Pt–W isotope correlations of the IID, IVA and IVB iron meteorites are higher than most previous estimates and are more radiogenic than the initial 182W/184W of Ca–Al-rich inclusions (CAI). The Hf–W model ages for core formation range from +1.6±1.0 million years (Ma; for the IVA irons) to +2.7±1.3 Ma after CAI formation (for the IID irons), indicating that there was a time gap of at least ∼1 Ma between CAI formation and metal segregation in the parent bodies of some iron meteorites. From the Hf–W ages a time limit of <1.5–2 Ma after CAI formation can be inferred for the accretion of the IID, IVA and IVB iron meteorite parent bodies, consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies

Replication Data for: Tellurium isotope cosmochemistry: Implications for volatile fractionation in chondrite parent bodies and origin of the late veneer.

Tellurium stable isotope compositions and abundances (δ128/126Te relative to SRM 3156) are reported for 43 ordinary, enstatite, and Rumuruti chondrites, which together with results from a companion study on carbonaceous chondrites are used to assess the origin of volatile element fractionations in chondrites. Whereas Te isotope variations among carbonaceous chondrites predominantly reflect mixing between isotopically light chondrules/chondrule precursors and CI-like matrix, Te isotope variations among non-carbonaceous chondrites mainly result from Te redistribution during parent body thermal metamorphism. The enstatite chondrites in particular display increasingly heavy Te isotopic compositions and decreasing Te concentrations with increasing degree of metamorphism, indicating migration of isotopically light Te from the strongly metamorphosed inner parts towards the cooler outer regions of the parent bodies. By contrast, ordinary and Rumuruti chondrites display less systematic Te isotope variations, implying more localized redistribution of Te during parent body thermal metamorphism

Replication Data for: Tellurium isotope cosmochemistry: Implications for volatile fractionation in chondrite parent bodies and origin of the late veneer.

Tellurium stable isotope compositions and abundances (δ128/126Te relative to SRM 3156) are reported for 43 ordinary, enstatite, and Rumuruti chondrites, which together with results from a companion study on carbonaceous chondrites are used to assess the origin of volatile element fractionations in chondrites. Whereas Te isotope variations among carbonaceous chondrites predominantly reflect mixing between isotopically light chondrules/chondrule precursors and CI-like matrix, Te isotope variations among non-carbonaceous chondrites mainly result from Te redistribution during parent body thermal metamorphism. The enstatite chondrites in particular display increasingly heavy Te isotopic compositions and decreasing Te concentrations with increasing degree of metamorphism, indicating migration of isotopically light Te from the strongly metamorphosed inner parts towards the cooler outer regions of the parent bodies. By contrast, ordinary and Rumuruti chondrites display less systematic Te isotope variations, implying more localized redistribution of Te during parent body thermal metamorphism