Periodic Reporting for period 3 - SHRED (Survival of Hadean REmnants in a Dynamic mantle)
Reporting period: 2023-01-01 to 2024-06-30
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 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.