Periodic Reporting for period 3 - SHRED (Survival of Hadean REmnants in a Dynamic mantle)
Période du rapport: 2023-01-01 au 2024-06-30
Plate tectonics drives the formation and destruction of crust and introduces surface material into the deep Earth, while mantle convection mixes materials back together, erasing their diversity. Geochemical heterogeneities in modern volcanics indicate the survival of Hadean (≈ 4.5 Ga) remnants, and their mare existence raises first-order questions: What is the nature of the material carrying the odd geochemical signatures? How can Hadean material survive in an actively convecting mantle? What are the physical properties of material that can be preserved for billions of years, and yet that can be entrained in mantle plumes? Can Hadean remnants be stored in the structures seismically imaged in the lowermost mantle? Answering these questions is the challenging aim of SHRED.
Seismic studies have identified regions in the lowermost mantle called ‘Large Low-Shear-Velocity Provinces’ (LLSVPs) and their unique characteristics are attributed to remnants of a Hadean magma ocean, or to core-mantle interactions, or to the accumulation of subducted plate material. These very deep structures could therefore be the place where materials with unusual geochemical features are stored.
The project combines the acquisition of geochemical data at various scales (from an island chain to mineral inclusions) with large-scale geodynamical modelling tracing the source of plumes within the mantle, and local thermochemical plume modelling that will constrain the dimensions of the sites in the deep mantle where ancient material could be stored for a large portion of Earth history. The project is analytically very challenging because the detection of traces of ancient material in present-day plume basalts requires the acquisition of extremely accurate and precise isotopic data by thermal ionisation mass spectrometry. These combined high-precision analyses will be used to determine the age and geochemical characteristics of the source material. The scale of geochemical heterogeneities will be evaluated using in-situ analyses of minerals and whole-rock chemical and isotopic analyses. By applying such approach to single oceanic islands and island chains with different compositions, we will evaluate the scale of ancient materials in the deep mantle. In parallel, the project will numerically model the time scale of mixing during the early Earth. These data will then be connected to the constraints provided by the large-scale geodynamical modelling of the source of plumes.
Seismic studies have identified regions in the lowermost mantle called ‘Large Low-Shear-Velocity Provinces’ (LLSVPs) and their unique characteristics are attributed to remnants of a Hadean magma ocean, or to core-mantle interactions, or to the accumulation of subducted plate material. These very deep structures could therefore be the place where materials with unusual geochemical features are stored.
The project combines the acquisition of geochemical data at various scales (from an island chain to mineral inclusions) with large-scale geodynamical modelling tracing the source of plumes within the mantle, and local thermochemical plume modelling that will constrain the dimensions of the sites in the deep mantle where ancient material could be stored for a large portion of Earth history. The project is analytically very challenging because the detection of traces of ancient material in present-day plume basalts requires the acquisition of extremely accurate and precise isotopic data by thermal ionisation mass spectrometry. These combined high-precision analyses will be used to determine the age and geochemical characteristics of the source material. The scale of geochemical heterogeneities will be evaluated using in-situ analyses of minerals and whole-rock chemical and isotopic analyses. By applying such approach to single oceanic islands and island chains with different compositions, we will evaluate the scale of ancient materials in the deep mantle. In parallel, the project will numerically model the time scale of mixing during the early Earth. These data will then be connected to the constraints provided by the large-scale geodynamical modelling of the source of plumes.
The installation of a new thermal ionisation mass spectrometer and of a triple Q-ICPMS occurred at the beginning of the project. While it was demonstrated that the required performances were reached by the end of February, full installation of the triple Q-ICPMS was disturbed by the sanitary crisis in March 2020. The entire course of the project was significantly slowed down during a period of over a year. Nevertheless, we were able to set up the analytical procedures to run high-precision strontium and neodymium and we now achieve an external precision better than 5 ppm on both 87Sr/86Sr and 143Nd/144Nd. The first preliminary data were presented at the 2021 Goldschmidt conference and a publication presenting in details the results is now accepted at Chemical Geology. The same precision is also reached on 142Nd/144Nd on pure solutions and the chemical procedure to isolate pure neodymium from rock samples is now operational. The first results on rock samples and international standard materials are presently being acquired. The triple-ICPMS was quite intensively used to ensure that all the chemical procedures necessary for high-precision isotopic analyses were effective. It was also used to determine the trace element contents of rock samples and we started to set up the measurement of strontium isotopic ratios on rock solutions using the separating power of the reaction cell. The first results are promising but more development work is definitely necessary.
The first volcanic samples that we decided to focus on are the samples collected on the new volcano located next to Mayotte in the Indian ocean. This scientific target was not originally planned in the SHRED project but the opportunity of getting the first new geochemical results for a brand-new and unknown submarine volcano could not be missed. The first results were obtained during the spring of 2021 and they were presented at the virtual Goldschmidt conference that occurred in early July 2021. More complete geochemical and isotopic data acquired since were presented at the 2022 Goldschmidt conference in Hawaii. High-precision Nd and W measurements are planned in the near future.
Given how the sanitary situation prevented us from working normally over the past two years, we think that the achievements reached within the first two and a half year of the project are reasonable and we already anticipate that we will need a prolongation of the project after its original deadline.
The first volcanic samples that we decided to focus on are the samples collected on the new volcano located next to Mayotte in the Indian ocean. This scientific target was not originally planned in the SHRED project but the opportunity of getting the first new geochemical results for a brand-new and unknown submarine volcano could not be missed. The first results were obtained during the spring of 2021 and they were presented at the virtual Goldschmidt conference that occurred in early July 2021. More complete geochemical and isotopic data acquired since were presented at the 2022 Goldschmidt conference in Hawaii. High-precision Nd and W measurements are planned in the near future.
Given how the sanitary situation prevented us from working normally over the past two years, we think that the achievements reached within the first two and a half year of the project are reasonable and we already anticipate that we will need a prolongation of the project after its original deadline.
The project will provide the first high-precision isotopic data that include both 142Nd and 182W isotopic anomalies on the same samples. We are now operational for the Nd part of the setting up and on the way to reliable measurements for W isotopes. The combination of these two short-lived and extinct chronometers coupled to geochemical data and more classical radiogenic isotopes will allow us to provide a time frame on the age of material present in the plume sources. Such approach lies clearly beyond the state of the art because the coupled approach measured on the same samples has never been done up to today. In addition, by studying the scale of geochemical heterogeneities using both trace elements and classical radiogenic isotopes, we evaluate the size of heterogeneous material present in the mantle plume source. For example, the data acquired on the new volcano that erupted next to Mayotte demonstrate that the source has not changed over the period of eruption of the entire volcano allowing the calculation of the volume of mantle source involved in the melting event. The geodynamical modelling of mixing processes during the Early Earth magma ocean phase is now functional and provides some time scale for the survival of geochemical heterogeneities at the beginning of Earth history. These constraints will then be confronted to the isotopic constraints and we should be able to determine whether Hadean isotopic heterogeneities could or could not survive the early magma ocean stage of the Earth mantle. If isotopic heterogeneities cannot survive the magma ocean stage, the process responsible for the presence of early Earth isotopic anomalies will need to be re-evaluated with for example the possibility that they come from a core-mantle interaction at present. Coupling the 182W and 142Nd data will constrain such process since W can come from the core but not Nd. In summary, by the end of the project, we anticipate that we will be able to trace the location of mantle plumes sources. quantify the size of isotopic heterogeneities and determine their origin