Where are the mantle's primordial helium and tungsten signatures stored?
In plain words
Some volcanic rocks carry helium and tungsten isotope ratios that date from Earth's first 100 million years. The hidden storage place that kept them unmixed for 4.5 billion years is unknown.
Precise statement
Ocean-island basalts show $3\mathrm{He}/4\mathrm{He}$ up to about 50 times the atmospheric ratio and negative mu-182W anomalies indicating reservoirs isolated since about $4.5\ \mathrm{Ga}$. Identify the host: LLSVPs, ultra-low-velocity zones, bridgmanite-enriched domains, or exchange with the core. An answer is a reservoir whose survival is shown dynamically and whose geochemical predictions match observed isotope correlations.
What would settle it
Isotope systematics (He, W, Ne, Xe) correlated with seismically imaged plume sources plus experimental core-mantle partitioning.