Workshop report: Exploring deep oceanic crust off Hawai`i

Abstract. For more than half a century, exploring complete sequence of the oceanic crust from seafloor through Mohorovičić discontinuity (Moho) and into uppermost mantle has been one most challenging missions scientific ocean drilling. Such technological achievement would provide humankind with profound insights largest realm our planet expand fundamental understanding Earth's deep interior its geodynamic behavior. The formation new at mid-ocean ridges subsequent aging over millions years, leading to subduction, arc volcanism, recycling some components mantle, comprise dominant geological cycle matter energy on Earth. Although previous drilling cored drill holes old (> 110 Ma) young (< 20 crust, sampling remains relatively shallow 2 km intact crust) unrepresentative average crust. To date, no hole penetrates 100 m average-aged that records long-term history seawater–basalt exchange (60 90 Myr). In addition, nature, extent, evolution subseafloor biosphere within poorly unknown. address these fundamentally significant issues, an international workshop “Exploring Deep Oceanic Crust off Hawai`i” brought together 106 scientists engineers 16 countries represented entire spectrum disciplines, including petrologists, geophysicists, geochemists, microbiologists, modelers, drilling/logging engineers. aim was develop full International Ocean Discovery Program (IODP) proposal 2.5 North Arch Hawai`i research vessel Chikyu. This samples down cumulate gabbros mature (∼ 80 formed spreading rate ∼ 3.5 cm a−1. A Moho reflection observed 5.5 below this site, concluded proposed essential “pilot hole” inform design future