Hydrothermal activity at the ultraslow- to slow-spreading Red Sea Rift traced by chlorine in basalt

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

Geomorphology of the central Red Sea Rift: Determining spreading processes

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