The depleted mantle is a principal component of the upper mantle and drives significant volcanism. However, our knowledge of the depleted mantle has remained limited due to its severely restricted accessibility, with only local exposure of abyssal peridotites along slow- and ultraslow-spreading mid-ocean ridges and transform faults. Although abyssal peridotites have yielded important insights into mantle composition and melting, the vast majority of these rocks have been obtained by dredging. Hence, they lack the context, spatial continuity, and orientation information critical for understanding a range of processes, including the spatial scale of compositional variations, melt migration and mantle flow. As a result, many properties of the depleted mantle have been inferred from the study of its melting products: mid-ocean ridge basalts. Here, we report the recovery of a long (1268 m) section of serpentinized abyssal mantle peridotite, interleaved with thin gabbroic intrusions, from Atlantis Massif (Mid-Atlantic Ridge) during IODP Expedition 399. The nearly continuous recovery in the principal hole (U1601C) provides an opportunity to obtain a robust and quantitative lithological, mineralogical, and structural inventory of the upper mantle. The peridotites are dominated by harzburgite (82%; Fig. 1), with significant dunite (18%); lherzolite is rare, as are ultramafic veins (wehrlite, orthopyroxenite, (olivine) websterite). Mineralogically and geochemically, the peridotites are depleted: harzburgites have low orthopyroxene content (average 15.9%) and often lack clinopyroxene altogether. Geochemically, the peridotites have high MgO/SiO2 and low Al2O3/SiO2, falling on the depleted end of the array defined by global mantle rocks. Orthopyroxene abundance varies significantly on scales ranging from the centimeter to hundreds of meters, forming a continuum from 0-30%. Dunite occurs as zones in harzburgite and, more frequently, in orthopyroxene-bearing dunite, and are typically tens of centimeters thick. Contacts between dunite and the surrounding peridotites are nearly always gradational, and well-defined dunite zones are an endmember of a continuum of variations in orthopyroxene content in the mantle rocks, with the full spectrum of harzburgite to dunite preserved in the core. We posit that the observed variations in mineralogy and therefore lithology are controlled by melt migration and the associated dissolution of orthopyroxene and precipitation of olivine, superimposed on the residues of relatively high degrees of melting. The geometry of melt flow is captured by the orientation of the dunite zones. Harzburgite-dunite contacts have predominantly intermediate dips and record a ~40° discordance with the mantle fabrics. This is consistent with the formation of a network of dunite channels with variable but predominantly intermediate dips oblique to mantle upwelling as a result of melt focusing toward the ridge axis. Going forward, the comprehensive rock record obtained during Expedition 399 provides a wealth of opportunities to make fundamental advances on our understanding of the oceanic upper mantle. This includes a determination of mantle heterogeneity on the centimeter to kilometer scale and investigations of the role of previous depletion in governing mantle composition and melting. The continuous nature of the core, and its re-orientation, will also be instrumental in studies of spatial variations in melting and melt transport, as well as research into mantle flow and the associated deformation mechanisms.

Comunicación oral presentada en The 7th Orogenic Lherzolite Meeting, del 30 de septiembre al 6 de octubre de 2024, celebrado en Oviedo (España)