Researchers at Nanyang Technological University (NTU) in Singapore have unraveled a geological enigma that has puzzled scientists for over half a century: the 'missing lead paradox' in Earth's mantle. This breakthrough, detailed in a recent Nature Communications paper, reveals how ancient, unradiogenic lead—primordial material from 4.5 billion years ago—has been hidden deep within the planet's interior, preserved in high-pressure sulfide minerals.
The paradox stems from a mismatch in lead isotope ratios. Chondritic meteorites, thought to represent Earth's building blocks, show less radiogenic lead (produced by uranium and thorium decay) than rocks from the accessible mantle and crust. Where has the 'old' lead gone? Traditional explanations pointed to the core, but partition coefficients couldn't explain the full deficit. NTU's team, led by Simon Redfern, President's Chair in Earth Sciences, used computational simulations and high-pressure experiments to propose a silicate Earth solution: lead sulfide (PbS) phases stable under lower mantle conditions.
The Origins of the Lead Isotope Puzzle
Earth formed 4.5 billion years ago from chondritic material, rich in unradiogenic lead isotopes like 204Pb. Over time, uranium (238U to 206Pb, 235U to 207Pb) and thorium decay should produce a predictable mix. Yet, bulk silicate Earth (BSE) estimates from mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) appear too radiogenic, plotting right of the geochron on Pb-Pb diagrams. This 'future paradox' implies ~30-50% missing unradiogenic Pb, equivalent to 1-2 times the continental crust's Pb budget.
Early models suggested core sequestration via sulfide droplets during magma ocean crystallization, but metal-silicate and sulfide-silicate partition data (D_Pb <10) fall short. Lower crust xenoliths lack sufficient low-radiogenic Pb. Enriched mantle reservoirs (e.g., FOZO) show hints, but not enough volume. NTU's work shifts focus to sulfur-rich domains in the deep mantle, where Pb partitions strongly into sulfides (D_Pb >>1).
NTU Team's Innovative Approach
The multidisciplinary NTU team—Siyu Liu, Meng Guo, Shidong Yu, and Simon Redfern from the Asian School of the Environment and School of Materials Science and Engineering—combined first-principles calculations with experimental validation. Using CALYPSO for structure prediction, VASP for DFT optimizations, and PHONOPY for phonons, they explored Pb_xS_y stoichiometries up to 150 GPa (core-mantle boundary pressures).
High-pressure diamond anvil cell experiments up to 7 GPa confirmed PbS2 structures (SnS2-type, CuAl2-type). Ab initio molecular dynamics (AIMD) determined melting curves via NVE/NVT simulations. This rigorous methodology bridged theory and observation, revealing PbS stability to CMB conditions.
Discovery: Pb-S Phases as Lead Hideaways
Key finding: PbS (galena structure) is the most stable, undergoing B1→B33 (8.5 GPa) →B2 transitions, solid to CMB. PbS2 (I4/m) stable 4.5-14.5 GPa, PbS3 (P42_1m) 11.5-42.7 GPa. These phases feature Pb coordinated by 8-12 S atoms, with Bader charges showing ionic-covalent bonding. Band gaps confirm semiconducting nature, decreasing under pressure.
Crucially, PbS melting point exceeds the geotherm (~4000 K at CMB), while PbS3 melts near/above mantle T, and PbS2 higher. Sulfur-rich subducted slabs or late veneers form these, trapping primordial Pb isolated from U/Th (lithophile, incompatible in sulfides).
Redfern notes: "The missing lead paradox arises when we compare lead levels in surface rocks to ancient meteorites. Our simulations show PbS could store it deep in the mantle, crystallizing early and staying solid."
Sulfur Cycling and Mantle Dynamics
Under reducing conditions, sulfide melts segregate Pb downward. Upon cooling, PbS crystallizes, forming long-lived reservoirs. Polysulfides like PbS3 decompose upward (PbS3 → PbS + S), releasing Pb sporadically—explaining unradiogenic OIB signatures (e.g., Pitcairn, Samoa). This ties Pb evolution to redox state and sulfur subduction, refined by arc volcanism.
Quantitatively, a 0.1-1% sulfide layer at 100-150 GPa could hold the missing Pb, consistent with seismic low-velocity zones.
Implications for Earth's Early History
This resolves the paradox without non-chondritic models or oversized core Pb. It supports a sulfur-rich early magma ocean, with late sulfide addition from chondritic veneer (~0.5% Earth's mass). Links to Hadean eon dynamics, where sulfur controls metal-silicate partitioning.
For mantle convection, PbS piles could anchor plumes, influencing hotspot tracks. Redox-sensitive recycling explains Pb heterogeneity vs. uniform Sr-Nd-Hf.
NTU's Role in Global Geoscience
NTU's Earth Observatory of Singapore (EOS) and high-performance computing facilities enabled this. Redfern's team exemplifies Singapore's push in mineral physics, with facilities like laser-heated diamond anvils. This elevates NTU in planetary science rankings.
As Redfern states: "Singapore's investment in extreme-condition research positions us to tackle big questions like Earth's deep interior." Ties to careers in geochemistry: Explore research jobs or higher ed opportunities at NTU-like institutions.
Future Directions and Ongoing Research
Next: NanoSIMS on mantle xenoliths for Pb-S signatures; seismic correlations with sulfide layers. Broader: Apply to super-Earth exoplanets' interiors. Validates chondritic Earth model, impacts core formation timelines.
Challenges: Direct sampling deep sulfides; integrate with Urey ratio debates.
Photo by Michael Mwangi on Unsplash
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Read the full paper: Nature Communications. More: Phys.org coverage.
