ISOREE is a multi-disciplinary research program combining different approaches (isotope geochemistry, experimental petrology, spectroscopy) in order to bring further constrain on the origin of the Earth and the major events that occurred very early on in its history. The analysis of the oldest samples provides some of the best constraints on the composition of the mantle and the extent of chemical differentiation in the Early Earth and during this project we studied samples from South Africa and Antarctica using both short- and long-lived radiogenic systems. Isotope measurements prove that early-formed chemical heterogeneities were produced during the crystallization of the magma ocean around 4.35-4.4 Ga and have been preserved in the convective mantle during at least 1 Ga. This age is consistent with the timing of the giant impact forming the Moon and would correspond to the last magma ocean stage on Earth.
Many extra-terrestrial samples have been studied and measured for different isotopes in this project using very high precision techniques. To better understand the origin of the Earth, the nature of its building blocks and its bulk composition, we have performed many Nd and Sm isotope measurements on bulk meteorites, mineral separates and using leaching experiments. The Cerium isotopic composition of the BSE has been defined by measuring chondrites from different groups. We showed the importance of impact for changing the bulk Sm/Nd ratio of planetesimals during accretion. We studied the oldest known andesitic meteorite (EC 002) for determining the half-life and initial Solar System abundance of 146Sm.
The main goal was to determine the behaviour of rare earth elements during the first stage of solar system formation. The nucleosynthetic composition of meteorites compared to the Earth suggest that the Earth formed from NC (non-carbonaceous) material. Meteorites that are the closest to the Earth are enstatite chondrites formed in very reduced conditions. During this project, enstatite chondrites have been studied in detail to better understand the distribution of chemical elements at low fO2. Then we have compared the REE behavior in natural extra-terrestrial samples with those obtained by experimental petrology. Change of valences for Sm and Yb have been identified using XANES measurements in experiments for fO2 conditions like those defined in enstatite chondrites. Moreover, high pressure-temperature metal-silicate experiments have been realized for investigating the partitioning of lithophile elements, including rare earth element, during core formation. REE are not fractionated during this process.
The measurement of high precision Ce isotopic ratios has been developed to address several questions linked to redox conditions. High La/Ce fractionations are identified in sediments formed in oxic marine conditions due to the redox sensitive nature of Cerium. The measurement of the long-lived 138La-138Ce systematics has the potential to identify the presence of pelagic sediments in mantle sources and discuss the timing of the recycling because no cerium anomalies are expected in water columns before the great oxygenation event dated at 2.4 Ga. In this project we have measured a large quantity of ocean island basalts and mid-ocean ridge basalts with the aim of better characterising the nature of the recycled material and discuss the nature of the different isotopic endmembers (EM1, EM2).
The radiogenic 138La-138Ce system can be used to date the formation of cerium anomalies in sedimentary rocks and then identify if they are primary features or were later formed during secondary events. Banded iron formations from the Moodies have been analysed for trace elements concentrations and few samples are characterised by large negative Ce anomaly. Using a La-Ce isochron, we dated the cerium anomalies at 60 Ma, much younger than the expected 3200 Ma. This result proves that the presence of the negative Ce anomaly cannot be used to constrain the redox conditions during the deposition time. This work highlights the need for careful consideration of the potential for post-depositional alteration to modify elemental and isotopic redox proxies. Laboratory experiments have been performed to investigate the behaviour of Ce during oxidation reaction. Isotopic fractionations have been measured and they are correlated to Ce(III)-Ce(IV) ratios. These results confirm the redox control on the Ce isotopes behaviour.
