Maud Boyet

Chondrite Barium, Neodymium, and Samarium Isotopic Heterogeneity and Early Earth Differentiation

Isotopic variability in barium, neodymium, and samarium in carbonaceous chondrites reflects the distinct stellar nucleosynthetic contributions to the early solar system. We used 148Nd/144Nd to correct for the observed s-process deficiency, which produced a chondrite 146Sm-142Nd isochron consistent with previous estimates of the initial solar system abundance of 146Sm and a 142Nd/144Nd at average chondrite Sm/Nd ratio that is lower than that measured in terrestrial rocks by 21 ± 3 parts per million. This result strengthens the conclusion that the deficiency in 142Nd in chondrites relative to terrestrial rocks reflects 146Sm decayand earlyplanetary differentiation processes.

147Sm–143Nd and 176Lu–176Hf in eucrites and the differentiation of the HED parent body

The 147Sm–143Nd and 176Lu–176Hf systematics of 21 whole-rock eucrites, including five cumulates, have been investigated by MC-ICP-MS. A statistically significant Sm–Nd isochron was obtained on 18 samples with an age of 4464±75 Ma (MSWD=1.26) and an intercept of 0.50680±0.00010. This age clearly is controlled by the cumulate eucrites. The 21 basaltic and cumulate eucrites together do not form a statistically significant Lu–Hf isochron. Basaltic eucrites, however, form an errorchron with an age of 4604±39 Ma (MSWD=4.52), which becomes an acceptable isochron if the error on the Lu/Hf ratio is doubled with respect to the laboratory estimate. The initial 176Hf/177Hf is 0.27966±0.00002. Three of the cumulate eucrites regressed together further yield a statistically significant age of 4470±22 Ma, indistinguishable from their Sm–Nd age. We therefore conclude that cumulate eucrites are ∼100 Myr younger than basaltic eucrites and basaltic eucrites are close in age to the Solar System. There is no evidence in the present data to support a decay constant of 176Lu significantly different from 1.93 10−11 yr−1 [Sguigna, Can. J. Phys., 60 (1982) 361–364]. The broad range of variation and strong correlation of Lu/Hf and Sm/Nd ratios require that eucrites are partial melts and not residual magmas. The relative Lu/Hf and Sm/Nd fractionation is controlled by the plagioclase to mafic mineral ratio in the residue. The steep correlation between these two ratios is explained by the enhancement of plagioclase saturation in the low-gravity field of the eucrite parent body. Basaltic eucrites probably formed as large-degree melts of a chondritic source. The strong Lu/Hf and Sm/Hf fractionation observed in cumulates clearly reflects the presence of residual ilmenite during melting: our preferred interpretation is that cumulates were impregnated by small-degree melts of ilmenite-bearing gabbros. Since pressure on the eucrite parent body never reaches the garnet stability field, this observation questions the ubiquitous presence of residual garnet in the mantle sources of magmas formed on larger planets such as the Moon, Mars, and possibly Earth.

142Nd evidence for early Earth differentiation

Abstract We measured 142Nd/144Nd in metabasalts and metagabbros from the western and southern parts of the Isua Supracrustal Belt, West Greenland. The samples were selected based on field and thin section criteria so as to minimize metamorphic effects on the isotope systematics. We developed a new multi-stage chemical separation scheme and a new isotope measurement technique using multiple-collector inductively coupled plasma mass spectrometry and, upon replication of measurements, achieved a precision better than 20 ppm. Out of eight samples, three show a positive 142Nd anomaly of 30 ppm consistent with a previous measurement by Harper and Jacobsen [Nature 360 (1992) 728–732]. These samples also define a 147Sm–143Nd isochron age that agrees with the commonly accepted U–Pb zircon age of 3.78 Ga for Isua, while the non-anomalous samples show substantial dispersion. The latter lack of coherence is interpreted as indicating either isotopic heterogeneities in the Isua upper mantle or redistribution of Nd by fluids during late Archean metamorphic events. Scatter among the non-anomalous samples is also observed for the Lu–Hf isotope systematics and confirms the severity of metamorphic remobilization. If the 147Sm–143Nd and 146Sm–142Nd systems are explained by a two-stage model, it is required that the upper mantle went through a major differentiation event within a few tens of millions of years after planetary accretion. If early Archean convection decoupled the two Nd isotope systems, this necessitates that the episode of differentiation took place within no more than 150 Myr of planetary accretion. One interpretation of the high-Sm/Nd reservoir is that it represents the remnants of a stagnant lithospheric lid overlying a magma ocean. Because the lid is shattered by impacts, it is unable to sink. Alternative explanations, such as an ultra-deep magma ocean at the core–mantle boundary or massive subduction of early crust, call for the early burial of enriched material and the ubiquitous presence of a 142Nd anomaly in the early Archean mantle.

Chronological evidence that the Moon is either young or did not have a global magma ocean

Chemical evolution of planetary bodies, ranging from asteroids to the large rocky planets, is thought to begin with differentiation through solidification of magma oceans many hundreds of kilometres in depth. The Earth’s Moon is the archetypical example of this type of differentiation. Evidence for a lunar magma ocean is derived largely from the widespread distribution, compositional and mineralogical characteristics, and ancient ages inferred for the ferroan anorthosite (FAN) suite of lunar crustal rocks. The FANs are considered to be primary lunar flotation-cumulate crust that crystallized in the latter stages of magma ocean solidification. According to this theory, FANs represent the oldest lunar crustal rock type. Attempts to date this rock suite have yielded ambiguous results, however, because individual isochron measurements are typically incompatible with the geochemical make-up of the samples, and have not been confirmed by additional isotopic systems. By making improvements to the standard isotopic techniques, we report here the age of crystallization of FAN 60025 using the 207Pb–206Pb, 147Sm–143Nd and 146Sm–142Nd isotopic systems to be 4,360 ± 3 million years. This extraordinarily young age requires that either the Moon solidified significantly later than most previous estimates or the long-held assumption that FANs are flotation cumulates of a primordial magma ocean is incorrect. If the latter is correct, then much of the lunar crust may have been produced by non-magma-ocean processes, such as serial magmatism.

Composition of the Earth's interior: the importance of early events

The detection of excess 142Nd caused by the decay of 103 Ma half-life 146Sm in all terrestrial rocks compared with chondrites shows that the chondrite analogue compositional model cannot be strictly correct, at least for the accessible portion of the Earth. Both the continental crust (CC) and the mantle source of mid-ocean ridge basalts (MORB) originate from the material characterized by superchondritic 142Nd/144Nd. Thus, the mass balance of CC plus mantle depleted by crust extraction (the MORB-source mantle) does not sum back to chondritic compositions, but instead to a composition with Sm/Nd ratio sufficiently high to explain the superchondritic 142Nd/144Nd. This requires that the mass of mantle depleted by CC extraction expand to 75–100 per cent of the mantle depending on the composition assumed for average CC. If the bulk silicate Earth has chondritic relative abundances of the refractory lithophile elements, then there must exist within the Earths interior an incompatible-element-enriched reservoir that contains roughly 40 per cent of the Earths 40Ar and heat-producing radioactive elements. The existence of this enriched reservoir is demonstrated by time-varying 142Nd/144Nd in Archaean crustal rocks. Calculations of the mass of the enriched reservoir along with seismically determined properties of the D″ layer at the base of the mantle allow the speculation that this enriched reservoir formed by the sinking of dense melts deep in a terrestrial magma ocean. The enriched reservoir may now be confined to the base of the mantle owing to a combination of compositionally induced high density and low viscosity, both of which allow only minimal entrainment into the overlying convecting mantle.

The elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archaean rocks

The first indisputable evidence for very early differentiation of the silicate Earth came from the extinct 146Sm–142Nd chronometer. 142Nd excesses measured in 3.7-billion-year (Gyr)-old rocks from Isua (southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean eon (about 4,570–4,000 Gyr ago). As dictated by mass balance, the differentiation event responsible for the formation of the Isua early-depleted reservoir must also have formed a complementary enriched component. However, considerable efforts to find early-enriched mantle components in Isua have so far been unsuccessful. Here we show that the signature of the Hadean enriched reservoir, complementary to the depleted reservoir in Isua, is recorded in 3.4-Gyr-old mafic dykes intruding into the Early Archaean rocks. Five out of seven dykes carry 142Nd deficits compared to the terrestrial Nd standard, with three samples yielding resolvable deficits down to −10.6 parts per million. The enriched component that we report here could have been a mantle reservoir that differentiated owing to the crystallization of a magma ocean, or could represent a mafic proto-crust that separated from the mantle more than 4.47 Gyr ago. Our results testify to the existence of an enriched component in the Hadean, and may suggest that the southwest Greenland mantle preserved early-formed heterogeneities until at least 3.4 Gyr ago.

Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation

New Rb-Sr, 146,147Sm-142,143Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, 147Sm-143Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial 146Sm/144Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for 146Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd—142Nd/144Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga.

Coupled 182W-142Nd constraint for early Earth differentiation

Recent high precision 142Nd isotope measurements showed that global silicate differentiation may have occurred as early as 30–75 Myr after the Solar System formation [Bennett V, et al. (2007) Science 318:1907–1910]. This time scale is almost contemporaneous with Earth’s core formation at ∼30 Myr [Yin Q, et al. (2002) Nature 418:949–952]. The 182Hf-182W system provides a powerful complement to the 142Nd results for early silicate differentiation, because both core formation and silicate differentiation fractionate Hf from W. Here we show that eleven terrestrial samples from diverse tectonic settings, including five early Archean samples from Isua, Greenland, of which three have been previously shown with 142Nd anomalies, all have a homogeneous W isotopic composition, which is ∼2ε-unit more radiogenic than the chondritic value. By using a 3-stage model calculation that describes the isotopic evolution in chondritic reservoir and core segregation, as well as silicate differentiation, we show that the W isotopic composition of terrestrial samples provides the most stringent time constraint for early core formation (27.5–38 Myr) followed by early terrestrial silicate differentiation (38–75 Myr) that is consistent with the terrestrial 142Nd anomalies.

Evidence for Nb2+ and Ta3+ in silicate melts under highly reducing conditions: A XANES study

Abstract Niobium (Nb) K-edge and tantalum (Ta) LIII-edge XANES spectra were acquired at the part-permillion concentration level in silicate glasses quenched from chondritic melts equilibrated at 5 GPa and under moderately to highly reducing conditions (IW-1, IW-4.5, IW-7.9). Standard materials have also been analyzed for Nb and Ta, and the data were used to construct the calibration curves of E0 (threshold energy) vs. valence. Under moderately reducing conditions our results are consistent with niobium and tantalum being mainly pentavalent in the silicate melts as also suggested by previous studies. We do not exclude that at IW-1, a small fraction of Nb and Ta could be reduced, leading to a mean formal valence slightly lower than five. At IW-4.5, Ta is mainly in the form Ta3+, and at IW-7.9, Ta appears to be Ta1+, whereas Nb is divalent (Nb2+). The possibility for Nb and Ta to be present in reduced forms has implications for the behavior of the two elements during the processes of differentiation on planetary bodies formed in the reduced parts of the early Solar System. Element partitioning is a function of size and valence, and our results show that high field strength elements could be reduced, which could change their chemical affinity. This may also be important for the Earth and Moon formation and early differentiation, as exemplified by the “Nb paradox.”

Application of radiogenic isotopes in Geosciences: Overview and perspectives

Wider use of radiogenic isotopes in geosciences has been enabled by developments in massspectrometry at the beginning of the 21st century. Nowadays, radiogenic isotope geochemistry forms an integrated part of geosciences in a range of applications, starting from formation of planetary systems, genesis, and the evolution of Earths lithosphere and associated mineral and oil deposits, as well as environmental tracers. Two primary types of information are available from radiogenic isotopes studies: age determination and isotopic source tracing. In this chapter, the range of isotope systematics currently used in geosciences and their applications are reviewed, together with progress in analytical technologies. The chapter brings together internationally recognised researchers whohave been at the forefront of analytical technologies in the field of geochemistry of radiogenic isotopes.