LAUSR

Abstract The numerous chemical and isotopic studies of oceanic basalts have shaped our perception of mantle geochemistry over the last six decades. As partial melts of Earth’s mantle, basalts are indirect tracers of mantle composition. Because the scale of isotopic heterogeneity is smaller than the scale of melt production, melts from isotopically heterogeneous mantle ingredients mix into variable blends on their way to eruption. Basalts are therefore isotopically less variable than their mantle sources. Decrypting “the message from oceanic volcanism” thus necessitates developing strategies to see through this ubiquitous sampling bias, but also acknowledging the inherent limitations imposed by investigating mantle composition through basalts. Understanding how large the bias between melts and mantle actually is requires decoding process versus source-related causes for the isotopic variability of basalts. Ultimately, deciphering the effective range of isotopic variability in Earth’s mantle is crucial for connecting isotopic signals in basalts to different materials and thus, the geologic processes that govern silicate earth evolution. Invariably, however, basalts are weighted averages of melts from isotopically different mantle constituents. As such, their incompatible element and isotopic composition is inherently biased towards the incompatible element enriched source components. The incompatible element depleted components of Earth’s mantle must therefore range to more extreme compositions than the basalts. But although isotope data from peridotites and olivine-hosted melt inclusions have extended the bounds of mantle heterogeneity, the overall extent of incompatible element depletion and mass fraction of incompatible element depleted mantle is still elusive. Mantle depletion is driven by the rate of melt extraction, or mantle processing, and thus interconnects the geochemical and geodynamical evolution of Earth’s mantle. Better constraining mantle depletion is therefore at the root of understanding our planet’s principal mode of operation.