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The mid-ocean rift in the Red Sea is recently regaining attention in the geosciences due to the possibility of investigating this young ocean in more detail than ever by state-of-the-art methods and modern deep-sea instrumentation. During the first AUV surveys of the Red Sea rift in Spring 2022, we collected multibeam bathymetry, backscatter, sub-bottom, and water column data over a 9 km long ridge segment in the Hadarba Deep between 22.49°N and 22.56°N to investigate the volcano-tectonic processes of this mid-ocean ridge. This area's total spreading rate of about 12 mm per year is defined as ultra-slow spreading. The high-resolution hydroacoustic data of the used Kongsberg Hugin Superior AUV (operated by Fugro) revealed more than 100 individual lava flows with different stages of sedimentation. The oldest lava flows are buried under 3-4 m of sediment, indicating ages of up to 28 ka. A dome volcano with a 2.5 km diameter and an average height of 300 m dominates the mapped area but has been inactive for at least ~8.4 ka. Several younger lava flows show recent episodes of volcanism along the rift axis. However, their sediment cover is below the vertical sub-bottom-profiler resolution of about 10 cm and thus might be only a few hundred years old or younger. We will present our geomorphological maps, analyses, and statistics that reveal a moderately faulted, ultra-slow spreading MOR segment in the Red Sea with a surprisingly large amount of magmatic extension and show implications for the formation history of this ridge segment.

The Red Sea is a unique place to study a young oceanic rift basin and the interplay between magma and tectonics at a young divergent plate boundary. The spreading rate of the Red Sea rift changes from ~17 mm/yr in the south to ~7 mm/yr in the north, and so does the morphology. The southern Red Sea is a continuous and well-developed oceanic rift, whereas the so-called deeps characterize the central portion with oceanic crust separated by shallower inter-trough zones, and the northern part contains more widely spaced deeps with extensive areas covered by sediments in between. While the central Red Sea morphology has been extensively studied, the structure of the northern Red Sea and its link to the central Red Sea are less clear. Indeed, the northern Red Sea rift, marked at its southern end by Mabahiss Deep, is offset by about 60 km to the central Red Sea axis by the still poorly understood Zabargad Fracture Zone.Here we aim to improve the understanding of the volcano-tectonic setting of the Mabahiss Deep area with new high-resolution bathymetric data from multiple multibeam surveys with R/V Thuwal and R/V Pelagia. Our results show that the 15 km long, 9 km wide, and 2250 m deep Mabahiss Deep, and the 800 m high and 5 km wide central volcano, are the most prominent structures of the area. The deep is bordered by a series of Red Sea parallel normal faults on both sides, forming a graben-like structure and thus suggesting a rift-like morphology. The central volcano has a 2 km wide summit caldera containing several volcanic cones. Several normal faults cut its southern flank, and radial fractures are present on its summit. In the multibeam backscatter data, several recent lava flows (Our new high-resolution bathymetric mapping allows a more precise structural and geomorphological analysis of the Mabahiss Deep area that represents a starting point for understanding the overall structure of the poorly studied northern Red Sea.

Basalts from high flux intra-plate volcanism (Iceland, Hawaii, Samoa) are characterised by 3He/4He that are significantly higher than those from the upper mantle sampled at mid-ocean ridges. The prevailing paradigm requires that a largely undegassed deep Earth is enriched in primordial noble gases (3He, 20Ne) relative to degassed convecting upper mantle. However, the He concentration and 3He/20Ne ratio of high 3He/4He oceanic basalts are generally lower than mid-ocean ridge basalts (MORB). This so called ‘He paradox’ has gained infamy and is used to argue against the conventional model of Earth structure and the existence of mantle plumes. While the paradox can be resolved by disequilibrium degassing of magmas it highlights the difficulty in reconstructing the primordial volatile inventory of the deep Earth from partially degassed oceanic basalts.Basalts from 26 to 11°N on the Red Sea spreading axis reveals a systematic southward increase in 3He/4He that tops out at 15 Ra in the Gulf of Tadjoura (GoT). The GoT 3He/4He overlaps the highest values of sub-aerial basalts from Afar and Main Ethiopian Rift and is arguably located over modern Afar plume. The along-rift 3He/4He variation is mirrored by a systematic change in incompatible trace element (ITE) ratios that appear to define two-component mixing between E-MORB and HIMU. Despite some complexity, hyperbolic mixing relationships are apparent in 3He/4He-K/Th-Rb/La space. Using established trace element concentrations in these mantle components we can calculate the concentration of He in the Afar plume mantle. Surprisingly it appears that the upwelling plume mantle has 5-20 times less He than the convecting asthenospheric mantle despite the high 3He/4He (and primordial Ne isotope composition). This contradicts the prevailing orthodoxy but can simply be explained if the Afar mantle plume is itself a mixture of primordial He-rich, high 3He/4He (55 Ra) deep mantle with a proportionally dominant mass of He-poor low 3He/4He HIMU mantle. This is consistent with the narrow range of Sr-Nd-Os isotopes and ITE ratios of the highest 3He/4He Afar plume basalts, and is in marked contrast to high 3He/4He plumes (e.g. Iceland) that do not have unique geochemical composition. The HIMU signature of the Afar plume basalts implies origin in recycled altered oceanic crust (RAOC). Assuming that no He is recycled and using established RAOC U and Th concentrations, the low He concentration (< 5 x 1013 atoms/g He) of the He-poor mantle implies that the slab was subducted no earlier than 70 Ma and reached no more than 700 km before being incorporated into the upwelling Afar plume. We suggest that the Afar plume acquired its chemical and isotopic fingerprint during large scale mixing at the 670 km transition zone with the Tethyan slab, not at the core-mantle boundary.This study implies that large domains of essentially He-poor mantle exist within the deep Earth, likely associated with the HIMU mantle compositions. Further, it implies that moderately high-3He/4He (< 30 Ra) mantle plumes (e.g. Reunion) need not contain a significant contribution of deep mantle, thus cannot be used a priori to define primitive Earth composition.

The Red Sea is a unique place to study the birth of an oceanic rift basin and the interplay between magma and tectonics at a young divergent plate boundary. The Red Sea is a NNW-SSE oriented and 2000 km long rift system with a spreading rate decreasing from ~16 mm/yr in the south to ~7 mm/yr in the north. The morphology also changes along the rift axis: the south portion is a continuous and well-developed rift, clearly exposing oceanic crust, the central portion is characterized by deeps made by oceanic crust separated by shallower inter-trough zones, and the northern part contains more widely spaced and less obvious deeps with the transition to the continental crust not well defined. While the central Red Sea morphology has been extensively studied, the structure of the northern Red Sea and its link to the central Red Sea are still unclear. Indeed, the northern Red Sea rift is offset by 100 km to the central Red Sea axis by the still poorly understood Zabargad fracture zone.Here we aim at improving the understanding of the volcano-tectonic structure of the axial part of the southern tip of the northern Red Sea that corresponds to the Mabahiss Deep. To this aim, we carried out multiple multibeam surveys with R/V Thuwal and R/V Kobi Ruegg to map the sea bottom to add to what had been done in earlier surveys. In addition, we obtained several sub-bottom profiling lines across and along the deep to better constrain the shallow sedimentary structure.Our results show that the 15 km long, 9 km wide and 2250 m deep Mabahiss Deep along with the 800 m high and 5 km wide central volcano are the key prominent structures of the area. The deep is bordered by a series of Red Sea parallel normal faults on two sides forming a graben-like structure and thus suggesting a rift-like morphology. The central volcano is well preserved and has a 2 km wide summit caldera containing several volcanic cones and thus suggesting a permanent magmatic source underneath of a relatively young age. The ocean floor outside the deep and the volcanic edifice is mostly covered by salt flows, limiting structural analysis of the surrounding areas.A comparison between the northern and central Red Sea suggests, although in both areas thick salt covers most of the ocean floor, that the axes have similar rift-like structures with stable axial volcanism. However, in the central Red Sea larger portions of the oceanic crust are free of salt and the deformation seems larger with more prominent faults that also affect the floor of the deeps and split apart volcanic edifices, enhancing the occurrence of diffused monogenic volcanic cones. Therefore, this might suggest, despite the central and northern Red Sea sharing the same structure and evolution, that the less volcanic and tectonic activity in the north probably reflects the decreasing spreading rate from south to north along the Red Sea.