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ERC

ISOREE Report Summary

Project ID: 682778
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - ISOREE (New insight into the origin of the Earth, its bulk composition and its early evolution)

Reporting period: 2016-09-01 to 2018-02-28

Summary of the context and overall objectives of the project

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. Four tasks have been defined in this five-year research program with the aim of :

• Better constraining the Earth mantle’s composition
• Dating the successive magmatic ocean stages on Earth
• Defining precisely the age of the Moon’s formation
• Refining the giant impact model and the Earth-Moon relationship

Understanding early mantle evolution by direct observation of the material available on Earth’s surface is difficult because very rare samples dated from the first few hundred of Myrs of the Earth’history have been found. The analysis of short- and long-lived radiogenic systems on the oldest samples are able to provide some of the best constraints on the composition of the mantle and the extent of chemical differentiation in the Early Earth. On order to build a robust model for the Earth’evolution, the composition of the staring material (the Earth’s building blocks) must be known. The meteorite classification lists many samples from different groups with distinct isotope compositions (nucleosynthetic aonmalies). These samples are believed to have been formed in different locations of the solar nebula and are also characterized by different compositions in major and trace elements and have recorded different redox conditions. These conditions play a strong role in the behavior of chemical elements (preferential affinity for the silicate or metal phases).
The Earth is the only planet harbouring life in the Solar System for about 4 billion years. This unique evolution is due to a combination of several characteristics/processes produced during its primitive evolution. The results of the ISOREE project will make it possible to better understand the origin of the Earth and help finds Earth-like exoplanets.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Task 1. Define the Ce-Nd terrestrial correlation.
We have improved the analytical part (chemistry and mass spectrometry) in order to 1) measure the Ce isotope composition on smaller samples and 2) increase the precision on the determination of the 138Ce/142Ce ratio by using different amplifiers on the mass spectrometers. The main goal is to characterize the different terrestrial reservoirs (mantle-crust) for Ce-Nd isotopes. For determining the composition of the crust, terrestrial sample coming from the Archean crust (Superior Province, Barberton, Swaziland) and loess deposits from several continents have been analyzed. We have dissolved and collected the cerium fraction for a large number of ocean island basalts (Polynesia, Hawaii, Fangataufa, Réunion, Gough, Cap Verde, Island, San Miguel) and chondrites from different types. We have decided to add a purification step for the Nd fraction in order to measure both 146Sm-142Nd and 147Sm-143Nd systematics. The Nature paper published in March 2018 entitled “Hadean silicate differentiation preserved by anomalous 142Nd/144Nd ratios in the Réunion hotspot source” by Peters et al. shows that active volcanic hotspots can tap into domains in Earth’s deep interior that were formed in the first hundreds of million years after the beginning of Solar System formation. Archean samples may also preserve the chemical signatures of early-formed silicate reservoirs. Using coupled measurements of 146Sm-142Nd and 147Sm-143Nd systematics, we can obtain 1) age for the formation of these reservoirs and 2) bring further constrains on the early mantle dynamics. Samples from South Africa have been studied and these results have been published in EPSL.

Published manuscript:
Schneider K., Hoffmann E., Boyet M., Münker C., Kröner A. 2018. Coexistence of enriched and modern-like 142Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa. Earth Planet. Sci. Lett., 487, 54-66.

Submitted manuscript:
Bellot N., Boyet M., Doucelance R., Bonnand P., Savov I.P., Plank T., Elliott T., Origin of negative cerium anomalies in subduction-related volcanic samples: constraints from Ce and Nd isotopes. Submitted to Chemical Geology.

Manuscript in preparation:
Boyet M., Bonnand P., Israel C., Doucelance R., le Roex A.P., Origin of the Ce anomalies in Gough Island lavas.
Bonnand P., Israel C., Boyet M., Doucelance R. and Auclair D., Radiogenic and stable Ce isotopes measurements by Thermal ionisation Mass spectrometer.
Israel C., Bonnand P., Boyet M., Doucelance R. and Auclair D., Does the 138Ce/142Ce-143Nd/144Nd mantle array passes through the chondritic composition?


Task 2. Extra-terrestrial material: Age of the giant impact forming the Moon, and Ce isotope composition of meteorites and lunar samples.
We have not started yet the analysis of lunar samples. We have obtained the first Nd isotope measurement on a fall enstatite chondrites from the EL3 group. The aim of this study is to better constrain the Earth’s building blocks. Although enstatite chondrites appeared to be the closest in isotope composition to terrestrial samples, resolvable differences between enstatite chondrites and the Earth reported for Nd, Mo and Ru isotope compositions was used to exclude this material as representative of the Earth. We show that enstatite chondrites from the EL3 and EL6 subgroups may come from different parent bodies. Samples from the EL3 subgroup have Nd and Ru isotope ratios undistinguishable from that of the Bulk Silicate Earth and their Mo isotope composition has never been measured. Because enstatite chondrites are relatively small in size and number, they are usually not available for destructive isotopic measurements. We have requested new samples to NASA and several museums. We have already received several samples. We have also bought a new EL3 sample from Almahata Sitta (fall in northern Sudan on October 2008) containing fragments of the EL3 subgroup. Because these samples

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Progress beyond the state of the art and expected results until the end of the project

TASK 1. Ce-Nd isotope measurements on ocean island basalts and chondrites suggest that the bulk silicate Earth has a chondritic Sm/Nd and La/Ce composition. High-precision measurements of the 138Ce/142Ce ratio is obtained by TIMS using a combination of 1011-1012 Ω amplifiers.
TASK 2. Chondrites from the EL3 subgroup are the best candidates for the Earth’s building blocks.
TASK 3. REE are not incorporated into the core even in reducing conditions and La/Ce is not fractionated.
TASK 4. The development of a triple spike method for measurement for the Ce isotope composition improves the precision (2SD = 30 ppm) relative to sample/standard bracketing method.
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