Diamonds as the key to unravel the origin of Earth's water

Project Information

INHERIT

Grant agreement ID: 101041620

DOI

10.3030/101041620

EC signature date 27 January 2022

Start date 1 January 2023

End date 31 December 2027

Funded under

• European Research Council (ERC)

Total cost

€ 1 499 758,00

EU contribution

€ 1 499 758,00

Coordinated by

UNIVERSITA DEGLI STUDI DI PADOVA
Italy

Periodic Reporting for period 1 - INHERIT (Diamonds as the key to unravel the origin of Earth's water)

Reporting period: 2023-01-01 to 2025-06-30

Summary of the context and overall objectives of the project

Water makes Earth unique in the inner Solar System because of its abundance. One of the major unresolved questions in planetary sciences is how and when Earth acquired its water. Comparing Earth’s original deuterium-to-hydrogen ratio (D/H) with those of Solar System objects can constrain the provenance of water. However, this is a difficult task as Earth is an active planet and its D/H value has significantly changed since its formation. To explain the origin of Earth’s water, we need to determine its primordial D/H ratio by finding a deep mantle reservoir unaffected by geodynamic processes that modify D/H, but most importantly, we need to constrain how Earth’s D/H evolved through time.
To solve this mystery, INHERIT will use diamonds to study the Earth’s interior as they contain hydrogen and are inert and robust time capsules resisting breakdown for over billions of years. The INHERIT project has three main objectives: i) determine for the first time how the D/H ratio of Earth changed through time; ii) determine the primordial source of water in the mantle; iii) understand the physical behaviour of H in diamonds.
To complete these objectives, the H isotopic composition of a diverse set of natural diamonds will be determined by developing a protocol to precisely measure the D/H of diamonds. Geochemical analyses will be coupled with different diamond ages, depths, parageneses, and localities to establish the sources of water in the mantle through time. Atomistic state-of-the-art ab initio simulations will be performed to understand the atomic and diffusive behaviour of H in diamonds at realistic mantle conditions.
By studying H in diamonds with ages 3.5 to 0.1 Ga, we will provide the first geological record of the D/H evolution of Earth’s interior. Although the upper age of diamonds limits our knowledge to the early moments of our Planet’s formation, establishing how D/H evolved over time may reveal if the deep source of H had a constant D/H ratio or if there is a reliable trend back to when Earth formed. This ratio will be compared with those of the current ocean values and planetary bodies to evaluate potential common origins.
The new results will be fundamental in pinpointing the origin of Earth’s water, advancing our knowledge of planetary accretion and evolution, with long-term implications for understanding planetary habitability. This new knowledge will provide fundamental insights in the geological and biological evolution of Solar System planets and beyond.

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

The research team made the following advancements towards completing the INHERIT objectives:
-Tracking the evolution of D/H through time requires understanding the processes that govern H isotopic modification in the mantle. Samples from different localities, ages and depths have therefore been chosen for spectroscopic, petrographic, compositional and isotopic analysis. We are developing a new protocol for reliable H isotope analyses of diamonds that addresses the challenges associated with isotopic analyses using EA IRMS. Thus far, extensive testing has been done to effectively remove contaminant H on the surface of samples. After achieving optimal working conditions, the validity of the isotopic data has been tested using mantle minerals, before analysing diamonds. We are also developing a protocol to quantify H which may result in more accurate H contents compared to other techniques.
-Identification of mineral inclusions in diamonds via Raman spectroscopy is a crucial part of the sample analysis, allowing determination of diamond paragenesis and the P/T conditions of formation. To this end, a new program, RamanCrystalHunter, was developed which contains many useful tools for Earth Scientists but also material scientists, chemists and the cultural heritage community.
A large-scale FTIR study of the H2O contents of inclusions in diamonds was completed showing that inclusions reflect the H2O content of the diamond-forming media. Our results revealed that significant H loss from the inclusion is unlikely and that the trapping of H by N defects mitigates diffusive loss of H from the inclusion and the diamond to the mantle, reinforcing our findings (see below).
-To ensure that the D/H of diamond reflects a primordial source, the atomistic behaviour of H in diamond, from the moment of incorporation to the end of mantle residence, must be understood. A comprehensive literature review of FTIR data from diamonds revealed that H-defects form earlier on during diamond formation than previously thought. We showed that once trapped by N, H cannot disassociate from N-defects and instead aggregates to form different N/H- defects with continued mantle residence.
To understand how N- and H-defects form, we investigated rare Type Ib diamonds and indentified a new type of N-defect (Y-center) that forms during early N-aggregation and three previously unobserved, IR peaks that correspond to new N/H-defects. Using novel ab initio and molecular dynamics calculations, we show that these new defects are the first to form in diamond and act to “lock-in” hydrogen such that the D/H ratio of diamonds reflects that of the parental fluid.

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)

- Measurement of the D/H ratio of diamonds and other mantle minerals has only been attempted a handful of times, often leading to contentious results. We are developing a new procedure to ensure reproducible D/H data. The D/H ratios of different mantle samples, from different mantle environments, will allow identification of processes responsible for mantle heterogeneities and recycled components in the mantle. The deep cycle of volatiles represents a crucial topic nowadays, and these results will add an essential piece to the puzzle.
- The behaviour of H in diamonds was largely unknown until now as only few “late forming” H-defects had been identified. Our results revolutionized what we know about H in diamonds, particularly the formation and early aggregation of N/H-defects opening several new research directions. Our work shows that substitutional N-defects trap interstitial H in diamond. With continued mantle annealing, the number of H-defects decreases as they aggregate to form fewer, more thermodynamically stable defects. The identification of the “first-formed N/H-defects in diamond” proves that H is trapped during, or shortly after, diamond formation preventing H re-equilibration with the mantle. We propose that the H isotopic composition in N-bearing diamonds is preserved after formation and thus reflects that of the diamond-forming fluid. This has crucial implications for relating the H isotopic composition of diamond to primordial sources of H in the mantle and thus to the origin of Earth’s water.
-A previously unidentified N-defect, the Y-center, is shown to be common in diamonds with low N aggregation state and that this defect also plays in important role in the production of N-defects capable of trapping H. We developed the first deconvolution routine for diamonds in which Y-centers are included, producing higher quality fits of the N-region, compared to traditional routines, and more accurate N-defect contents and mantle residence time/temperatures. This new routine will be highly used in the diamond research community.

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