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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.

Plutonic rocks such as gabbros provide information on magmatic and tectonic processes which occur beneath a mid-ocean rift axis as well as on the formation of the oceanic crust. Igneous rocks, reported from the Red Sea Rift valley, have been limited to extrusive basalts so far. The only deeper crustal rocks found in the Red Sea area are from the rift flanks and are interpreted as late-stage continental rift magmatism. Here, we present the geochemistry of the first recovered gabbro fragments from the axis of the Red Sea Rift, sampled from a crater structure within the brine-filled Discovery Deep at the axis of the Red Sea Rift. Petrology and geochemistry show characteristics of a typical mid-ocean ridge gabbro formed at shallow crystallization depth. Clinopyroxene core mineral data fall within two groups, thus pointing to a multiphased magmatic history, including different magma batches and a joint late-stage fractional crystallization. Geobarometry, based on clinopyroxene cores, suggests lower crystallization pressures than similar geobarometric data reported for gabbroic samples from Zabargad (8–9 kbar) and Brother’s Islands (2.5–3.5 kbar) at the rift flanks. However, based on the evolved whole rock composition, its multiphase history, the thickness of the crust, the current location of the samples, and the uncertainties in the barometer, geobarometric estimates for the samples are likely overestimated. Instead, we propose that these rock fragments originate from the upper part of a fully developed oceanic crust in the central Red Sea Rift. High-resolution bathymetry and sparker seismic data reveal that the Discovery Deep is characterized by a significant normal fault and a strong reflector near the rift axis, which we interpret as a potential sill intrusion in an approximate depth of 400 m. Based on the lack of progressive alteration and the sampling location within a sediment-free crater structure, we interpret that the emplacement of the gabbros has to be geologically recent. We interpret the gabbro either as a xenolith transported by the eruptive volcanism that formed the crater, potentially related to the sill intrusion visible at depth, or as intrusive gabbro, which was uplifted and deposited in a talus fan by the adjacent normal fault, exposed by the formation of the volcanic crater.

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.

Highlights • Debunscha Maar magmas mixed and fractional crystallised at upper mantle depths • Its main magma source is peridotite with a minor pyroxenite component • Amphibole signal and high olivine Ca/Al indicate a metasomatised peridotite mantle • Mantle potential temperatures give no sign of an anomalous hot mantle Abstract Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77–87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9–11.7 wt.%), and are generally more primitive than the matrix glasses (MgO 5.0–5.5 wt.%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830 ± 170 MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis.

Continental rifting and ocean basin formation can be observed at the present day in the Red Sea, which is used as the modern analogue for the formation of mid-ocean ridges. Competing theories for how spreading begins—either by quasi-instantaneous formation of a whole spreading segment or by initiation of spreading at multiple discrete “nodes” separated by thinned continental lithosphere—have been put forward based, until recently, on the observations that many seafloor features and geophysical anomalies (gravity, magnetics) along the axis of the Red Sea appeared anomalous compared to ancient and modern examples of ocean basins in other parts of the world. The latest research shows, however, that most of the differences between the Red Sea Rift (RSR) and other (ultra)slow-spreading mid-ocean ridges can be related to its relatively young age and the presence and movement of giant submarine salt flows that blanket large portions of the rift valley. In addition, the geophysical data that was previously used to support the presence of continental crust between the axial basins with outcropping oceanic crust (formerly named “spreading nodes”) can be equally well explained by processes related to the sedimentary blanketing and hydrothermal alteration. The observed spreading nodes are not separated from one another by tectonic boundaries but rather represent “windows” onto a continuous spreading axis which is locally inundated and masked by massive slumping of sediments or evaporites from the rift flanks. Volcanic and tectonic morphologies are comparable to those observed along slow and ultra-slow spreading ridges elsewhere and regional systematics of volcanic occurrences are related to variations in volcanic activity and mantle heat flow. Melt-salt interaction due to salt flows, that locally cover the active spreading segments, and the absence of large detachment faults as a result of the nearby Afar plume are unique features of the RSR. The differences and anomalies seen in the Red Sea still may be applicable to all young oceanic rifts, associated with plumes and/or evaporites, which makes the Red Sea a unique but highly relevant type example for the initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

Hydrothermal circulation at mid-ocean ridges and assimilation of hydrothermally altered crust or hydrothermal fluids by rising magma can be traced by measuring chlorine (Cl) excess in erupted lavas. The Red Sea Rift provides a unique opportunity to study assimilation of hydrothermally altered crust at an ultra-slow spreading ridge (maximum 1.6 cm yr−1 full spreading rate) by Cl, due to its saline seawater (40–42‰, cf. 35‰ in open ocean water), the presence of (hot) brine pools (up to 270‰ salinity and 68 °C) and the thick evaporite sequences that flank the young rift. Absolute chlorine concentrations (up to 1300 ppm) and Cl concentrations relative to minor or trace elements of similar mantle incompatibility (e.g., K, Nb) are much higher in Red Sea basalts than in basalts from average slow spreading ridges. Mantle Cl/Nb concentrations can be used to calculate the Cl-excess, above the magmatic Cl, that is present in the samples. Homogeneous within-sample Cl concentrations, high Cl/H2O, the decoupling of Cl-excess from other trace elements and its independence of the presence of highly saline seafloor brines at the site of eruption indicate that Cl is not enriched at the seafloor. Instead we find basaltic Cl-excess to be spatially closely correlated with evidence of hydrothermal activity, suggesting that deeper assimilation of hydrothermal Cl is the dominant Cl-enrichment process. A proximity of samples to both evaporite outcrops and bathymetric signs of volcanism on the seafloor enhance Cl-excess in basalts. The basaltic Cl-excess can be used as a tracer together with new bathymetric maps as well as indications of hydrothermal venting (hot brine pools, metalliferous sediments) to predict where hydrothermal venting or now inactive hydrothermal vent fields can be expected. Sites of particular interest for future hydrothermal research are the Mabahiss Deep, the Thetis-Hadarba-Hatiba Deeps and Shagara-Aswad-Erba Deeps (especially their large axial domes), and Poseidon Deep. Older hydrothermal vent fields may be present at the Nereus and Suakin Deeps. These sites significantly increase the potential of hydrothermal vent field prospection in the Red Sea.

Unlike the well-studied narrow hotspot tracks, the origin of broadly distributed seamount provinces remains a topic of conjecture. Here we present major and trace element and Sr-Nd-Hf-Pb double spike isotope data of a comprehensive sample suite from the Bathymetrists Seamount Province, a broad belt of submarine volcanoes in the eastern equatorial Atlantic, and from the neighboring Cape Verde Ridge, a topographic high on the shoulder of a local fracture zone. The major and trace element results are consistent with the Bathymetrists Seamount Province having formed in an intraplate setting. The isotopic composition of the seamount lavas resemble a HIMU-like signature (206Pb/204Pbin = 19.23–20.35) similar to the nearby St. Helena hotspot composition. Based on plate tectonic reconstructions, a formation of the Bathymetrists Seamount Province by the postulated Sierra Leone plume, believed to be responsible for the geochemical anomaly at the mid ocean ridge at 1.7°N and the nearby St. Peter and Pauls rocks, is not supported. An alternative model that the Bathymetrists Seamount Province was created by edge driven convection in the upper mantle along the boundary of the neighboring Sierra Leone Rise plateau is also not supported by the available data. Plate tectonic reconstructions, however, are consistent with a hotspot origin for the Bathymetrists Seamount Province, as is the presence of a seismic tomographic anomaly at the southwest end of the seamount belt.

International Ocean Discovery Program Expedition 368 is the second of two consecutive cruises that form the South China Sea Rifted Margin program. Expeditions 367 and 368 share the common key objectives of testing scientific hypotheses of breakup of the northern South China Sea (SCS) margin and comparing its rifting style and history to other nonvolcanic or magma-poor rifted margins. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C. They are located within the continent-ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include 1. Determining the nature of the basement within crustal units across the COT of the SCS that are critical to constrain style of rifting, 2. Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, 3. Constraining vertical crustal movements during breakup, and 4. Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively. However, based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In total, the expedition completed operations at four sites (U1501, U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing to 990 m because of mechanical problems with the drilling equipment that limited the expedition from 25 May 2017 to the end of the expedition to operate with a drill string not longer than 3400 m. New alternate Site U1504 proposed during Expedition 367 met this condition. Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already drilled Site U1501. At Site U1501, we cored to 697.1 m in 9.4 days, with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m, set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10¾ inch casing. At Site U1503, we penetrated 995.1 m, setting 991.5 m of 10¾ inch casing, but no cores were taken. At Site U1504, we took 40 rotary core barrel (RCB) cores over two holes. The cored interval between both holes was 277.3 m with 26.8% recovery. An 88.2 m interval was drilled in Hole U1504B. At Site U1505, we cored 668.0 m with 101.1% recovery. Logging data was collected from 80.1 to 341.2 m. Operations at this site covered 6.1 days. Except for Site U1505, we drilled to acoustic basement, which prior to the expedition, except for Site U1501, had been interpreted to be crystalline basement. A total of 6.65 days were lost due to mechanical breakdown or waiting on spare supplies for repair of drilling equipment. At Site U1501 on the OMH, coring ~45 m into the acoustic basement sampled highly lithified sandstone to conglomerate of presumed Mesozoic age overlain by siliciclastic Eocene pre- to synrift sediments of Oligocene age and topped by primarily carbonaceous postrift sediments of early Miocene to Pleistocene age. Site U1502 on Ridge A was cased to 723.7 m. At this site, we recovered 180 m of hydrothermally altered brecciated basalts comprising sheet and pillow lavas below deep-marine sediments of Oligocene to late Miocene age. Coring was not performed within the upper 380 m (~Pliocene-Pleistocene) at Site U1502. At Site U1503 on Ridge C, 991.5 m of casing was installed in preparation for the planned deep drilling to ~1800 m, but no coring was performed due to mechanical failures, and the site was abandoned without further activity. Coring at Site U1504 on the OMH ~45 km east of Site U1501 recovered metamorphic schist to gneiss (greenschist facies) below late Eocene (?) carbonate rocks (partly reef debris) and early Miocene to Pleistocene sediments. At Site U1505, we cored to 480.15 m through Pleistocene to late Oligocene mainly carbonaceous ooze followed at depth by early Oligocene to late Eocene siliciclastic sediments. Efforts were made at every drill site to correlate the core with the seismic data and seismic stratigraphic unconformities interpreted within the Eocene to Plio-Pleistocene sedimentary sequence prior to drilling. The predrilling interpretation of ages of these unconformities was in general confirmed by drilling results. As a result of the constraints on the length of drill string that could be deployed during the later part of Expedition 368, the secondary expedition objectives addressing the environmental history of the SCS and Southeast Asia received more focus than planned because these sites are located in shallower water depths and required less penetration depth. This forced change in emphasis, however, was without fatal consequences for the primary tectonic objectives. The two expeditions together provided solid evidence for a process of breakup that included vigorous synrift magmatism as opposed to the often-favored interpretation of the SCS margin as a magma-starved margin.

Highlights • Magmatic chlorine-excess is a highly sensitive tracer of hydrothermal contamination. • Red Sea magmas show Cl-excess more extreme than even fast spreading ridges. • High-Cl Red Sea environment produces high-Cl hydrothermally altered crust and magmatic Cl-excess. • (Ultra)slow-spreading ridge magmas can assimilate hydrothermally altered crust. • Magmatic Cl-excess can guide hydrothermal prospection in the Red Sea. Abstract Newly formed oceanic crust is initially cooled by circulating seawater, although where this occurs and over what regions fluids enter the crust is still unclear. Differences in the chlorine (Cl) concentrations between mid-ocean ridge basalt and seawater potentially make Cl a sensitive tracer for this hydrothermal circulation, allowing assimilation of hydrothermal fluids or hydrothermally altered crust by rising magma to be traced by measuring excess Cl in erupted lavas. Such excess Cl has been found in basalts from fast-spreading ridges (Cl concentrations up to 1200 ppm), but not so far on ultraslow- and slow-spreading ridges, where lower Cl values in the basalts (~ 50–200 ppm) make variations harder to measure. The Red Sea, with its relatively saline bottom water (40–42‰, cf. 35‰ salinity in open ocean water), the presence of axial brine pools (up to 270‰ salinity) and thick evaporite sequences flanking the young rift provides an ideal opportunity to study the incorporation of hydrothermal Cl at an ultraslow- to slow-spreading ridge (max. 1.6 cm/yr). Both absolute Cl concentrations (up to 1300 ppm) and ratios of Cl to elements of similar mantle incompatibility (e.g. K, Nb) are much higher in Red Sea basalts than for average ultraslow- and slow-spreading ridges. An origin of these Cl-excesses by seafloor weathering or syn-eruptive contamination can be excluded, as can mineral/melt fractionation during melting or crystallisation, based on trace element data. Instead, the incorporation of Cl at depth derived from hydrothermal circulation either by direct assimilation of hydrothermal fluids or through mixing of magma with partial melts of the hydrothermally altered crust is indicated. We see no influence of local spreading rate, the intensity of seafloor fracturing or the calculated depth of last crystal fractionation on Cl-excess. Seafloor areas with clear evidence of present or recent hydrothermal activity (brine pool temperatures above ambient, presence of hydrothermal sediments) always show Cl-excess in the local basalts and there is a positive correlation between Cl-excess and intensity of local volcanism (as determined by the percentage of local seafloor showing volcanic bathymetric forms). From this we conclude that Cl-excess in basalts is related to high crustal temperatures and hydrothermal circulation and so can be used to prospect for active or recently extinct hydrothermal systems. Samples recovered within 5 km of a seafloor evaporite outcrop show particularly high Cl-excesses, suggesting addition of Cl from the evaporites to the inflow fluids and that this may be the length scale over which hydrothermal recharge occurs.

Hydrothermal circulation at slow-spreading ridges is important for cooling the newly formed lithosphere, but the depth to which it occurs is uncertain. Magmas which stagnate and partially crystallize during their rise from the mantle provide a means to constrain the depth of circulation because assimilation of hydrothermal fluids or hydrothermally altered country rock will raise their chlorine (Cl) contents. Here we present Cl concentrations in combination with chemical thermobarometry data on glassy basaltic rocks and melt inclusions from the Southern Mid-Atlantic Ridge (SMAR; ~ 3 cm year−1 full spreading rate) and the Gakkel Ridge (max. 1.5 cm year−1 full spreading rate) in order to define the depth and extent of chlorine contamination. Basaltic glasses show Cl-contents ranging from ca. 50–430 ppm and ca. 40–700 ppm for the SMAR and Gakkel Ridge, respectively, whereas SMAR melt inclusions contain between 20 and 460 ppm Cl. Compared to elements of similar mantle incompatibility (e.g. K, Nb), Cl-excess (Cl/Nb or Cl/K higher than normal mantle values) of up to 250 ppm in glasses and melt inclusions are found in 75% of the samples from both ridges. Cl-excess is interpreted to indicate assimilation of hydrothermal brines (as opposed to bulk altered rock or seawater) based on the large range of Cl/K ratios in samples showing a limited spread in H2O contents. Resorption and disequilibrium textures of olivine, plagioclase and clinopyroxene phenocrysts and an abundance of xenocrysts and gabbroic fragments in the SMAR lavas suggest multiple generations of crystallization and assimilation of hydrothermally altered rocks that contain these brines. Calculated pressures of last equilibration based on the major element compositions of melts cannot provide reliable estimates of the depths at which this crystallization/assimilation occurred as the assimilation negates the assumption of crystallization under equilibrium conditions implicit in such calculations. Clinopyroxene–melt thermobarometry on rare clinopyroxene phenocrysts present in the SMAR magmas yield lower crustal crystallization/assimilation depths (10–13 km in the segment containing clinopyroxene). The Cl-excesses in SMAR melt inclusions indicate that assimilation occurred before crystallization, while also homogeneous Cl in melts from Gakkel Ridge indicate Cl addition during magma chamber processes. Combined, these observations imply that hydrothermal circulation reaches the lower crust at slow-spreading ridges, and thereby promotes cooling of the lower crust. The generally lower Cl-excess at slow-spreading ridges (compared to fast-spreading ridges) is probably related to them having few if any permanent magma chambers. Magmas therefore do not fractionate as extensively in the crust, providing less heat for assimilation (on average, slow-spreading ridge magmas have higher Mg#), and hydrothermal systems are ephemeral, leading to lower total degrees of crustal alteration and more variation in the amount of Cl contamination. Hydrothermal plumes and vent fields have samples in close vicinity that display Cl-excess, mostly of > 25 ppm, which thus can aid as a guide for the exploration of (active or extinct) hydrothermal vent fields on the axis.

Highlights • The Red Sea Rift (RSR) comprises the typical terrain of slow MOR axes seen elsewhere. • Submarine salt glaciers occur extensively along the RSR and blanket parts of the RSR. • Inter-trough zones are not continental, but oceanic crust covered by evaporite flows. • We see a global mechanism for spreading initiation and no need for a “multi node” model. • We see prospects for large mineral deposits at passive margins that host evaporites. Abstract The transition from continental rifting to seafloor spreading is presently occurring at only a few places on Earth, such as the Red Sea or the Woodlark Basin. Competing theories for how spreading begins (either by quasi-instantaneous formation of a whole spreading segment or by initiation of spreading at multiple discrete “nodes” separated by thinned continental lithosphere) have been put forward. The major evidence for the nodes theory comes from the Red Sea and geophysical surveys carried out there in the “multi-deeps region” during the 1970's and 1980's. We present new high-resolution multibeam bathymetric information over the same region, which, when combined with acoustic backscatter data, seafloor sampling and magmatic geochemical information appears to provide no support for the nodes model. We show that, although the discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent “windows” onto a continuous spreading axis which is locally inundated and masked by massive slumping of sediments and evaporites from the rift flanks. The geophysical data that was previously used to support the presence of continental crust between the “nodes” can be equally well explained by processes related to the sedimentary blanketing and sub-sedimentary hydrothermal alteration. A single, “quasi-instantaneous segment formation” model would appear to be all that is required to explain observations from present-day rifting/spreading transitions globally.

Mt. Merapi in Central Java is one of the most active stratovolcanoes on Earth and is underlain by a multistage plumbing system. Crystal size distribution analyses (CSD) were carried out on recent Merapi basaltic-andesites and co-eruptive magmatic and plutonic inclusions to characterise the crystallisation processes that operate during storage and ascent and to obtain information on respective time scales. The basaltic-andesites exhibit log-linear, kinked-upwards CSD curves for plagioclase and clinopyroxene that can be separated into two main textural populations. Large plagioclase phenocrysts (≥1.6 mm) make up one population, but correspond to crystals with variable geochemical composition and reflect a period of crystal growth at deep to mid-crustal levels. This population was subsequently influenced by crystal accumulation and the onset of crustal assimilation, including the incorporation of high-Ca skarn-derived xenocrysts. Textural re-equilibration is required for these crystals to form a single population in CSD. A second episode of crystal growth at shallower levels is represented by chemically homogenous plagioclase crystals

Highlights • The Red Sea Rift overall morphology is typical for (ultra)slow-spreading ridges. • Distribution of various volcanic morphotypes correlates with mantle temperatures. • Spreading perpendicular ridges indicate stable magma focusing over 8–12 My. • Warm mantle under the RSR prohibits the occurrence of oceanic core complexes. • Specific characteristics of the RSR morphology are related to its young age. Abstract Continental rifting and ocean basin formation is occurring today in the Red Sea, providing a possible modern analogue for the creation of mid-ocean ridges. Yet many of the seafloor features observed along the axis of the Red Sea appear anomalous compared to ancient and modern examples of mid-ocean ridges in other parts of the world, making it unclear, until recently, whether the Red Sea is truly analogous. Recent work suggests that the main morphological differences between the Red Sea Rift (RSR) and other mid-ocean ridges are due to the presence and movement of giant, submarine salt flows, which blanket large portions of the rift valley and thereby the oceanic crust. Using ship-based, high-resolution multibeam bathymetry of the central RSR between 16.5°N and 23°N we focus here on the RSR volcanic terrains not covered by salt and sediments and compare their morphologies to those observed along slow and ultra-slow spreading ridges elsewhere. Regional variations in style and intensity of volcanism can be related to variations in volcanic activity and mantle heat flow. The Red Sea oceanic seafloor shows typical features of mature (ultra)slow-spreading mid-ocean ridges, such as 2nd order discontinuities (overlapping spreading centres) and magma focussing in the segment centres (forming spreading-perpendicular volcanic ridges of thick oceanic crust). The occurrence of melt-salt interaction at locations where salt glaciers blanket the neovolcanic zone, and the absence of large detachment faults are unique features of the central RSR. These features can be related to the young character of the Red Sea and may be applicable to all young oceanic rifts, associated with plumes and/or evaporites. Thus, the RSR falls in line with (ultra)slow-spreading mid-ocean ridges globally, which makes the Red Sea a unique but highly important type example for initiation of slow rifting and seafloor spreading and one of the most interesting targets for future ocean research.

There is a 2–3 order of magnitude difference between the chlorine concentrations of seawater and the magmas making up the oceanic crust. For this reason, chlorine is a key element for tracing present and past water–rock interactions. In order to trace processes such as assimilation of hydrothermally altered oceanic crust by mid-ocean ridge magmas, a high precision method with low detection limits is required, since conventional methods are not able to reveal variations in the intrinsically low chlorine content of these basalts. We present a new method to measure chlorine in basaltic glass by electron microprobe, using a combination of mapping and standard-sample bracketing techniques. Due to the high environmental background of chlorine, special emphasis is put on the influence of different preparation and cleaning procedures, as well as other possible effects on the measurements (e.g. sample size). Calibration of the method is performed by informational values of international standards and by minerals with a fixed chlorine content. Finally, new information on chlorine concentration values for commonly used standard reference materials (SRM) is provided. With this new method chlorine can be measured at very low detection limits (10’s of ppm’s) with a precision of 1–2 ppm standard deviation for low chlorine concentrations.