The Rainbow hydrothermal vent field is a system of ultramafic-hosted hydrothermal vents located at 36°14'N on the Mid-Atlantic Ridge (MAR). It was discovered in 1994 from temperature readings of ten high-temperature black smokers at a depth of approximately , where fluids can exceed . The site is shallower and larger in area than many other vent fields along the Azores section of the MAR with an area of . Located southeast of Faial Island, it is a popular geochemical sampling and modeling site due to close proximity to the Azores and definitive representation of serpentinization from hydrothermal circulation and synthesis.
Vent geology, biology, and fluid content make Rainbow comparable to other hot hydrothermal vents of the Azores such as Lucky Strike and Menez Gwen. However; chlorinity, metal concentration, and pH distinguish it from neighboring vent fields. As a hot, ultramafic-hosted vent field, pH levels of fluids are extremely low with much H2 and CH4 generated from water interactions with mafic igneous rocks.
Though not actively considered for development, Rainbow lies within the MoMAR (Monitoring of the Mid Atlantic Ridge) survey area for a marine observatory.
Discovery and expeditions
Rainbow has had a number of visits since its initial discovery in 1994. Remotely Operated Vehicles (ROVs), submersibles, and Conductivity Temperature Depth (CTD) probes have been deployed to sample, characterize, and explore the vent field.
Rainbow was first identified in 1994 from TOBI side-scan sonar and CTD data on the HEAT Cruise, which returned bathymetric geomorphology of the Rainbow Massif. Premier fluid sampling occurred during the FLORES cruise in 1997, also sampling Azores MAR sites Menez Gwen and Lucky Strike. The area was surveyed in greater detail in 2001 on the IRIS cruise, marking a debut in magnetism, gravimetry, and water sample data from the ROV Victor 6000. Some vent fluid and gas sampling was also performed on the IRIS cruise of 2001. In 2002, the SEAHMA 1 cruise was conducted to sample geology and biology at the Azores triple junction where the African, Eurasian, and North American plates meet. On the MoMARDREAM cruises of 2007 and 2008, the ROV Victor and submersible Nautile were used in collecting a variety of geological samples. During the MoMARDREAM cruises, a total of 29 dredges (14 from 2007, 15 in 2008) were collected. Fluid sampling was also performed on the KNOX18RR cruise in 2008, with samples collected by the ROV Jason. 2009 saw the first Portuguese journey to the Rainbow massif from the EMEPC/LUSO/2009 Expedition to study Azores vent biology. In 2012, trace metals were assessed from numerous field studies of the MAR using the R/V Knorr and ROV Jason II.
Geologic setting
Rainbow is located on a massif at depth, shared with two fossil (mostly inactive) vent sites Ghost City and Clamstone. As a slow-spreading ridge at approximately 2.2 cm/yr, extensive faulting has uplifted gabbro and peridotite and exposed ultramafic rock to cold seawater. Faulting may also be responsible for magnitude 3 - 3.5 earthquakes observed in hydroacoustic data, suggesting that the region is tectonically active. Unlike high-temperature basalt systems, this ultramafic setting is associated with a positive magnetic anomaly; postulated to come from magnetite precipitation. The active Rainbow site exhibits numerous active and inactive chimneys at serpentinized peridotite outcrops, distinguishable from sediment cover either by protruding from sediment or at a scarp. Chloride concentrations from vent fluids suggest a common heat source for the site, though the location and geometry of heat sources is unknown.
Hydrothermal circulation
Due to the extensive faulting at the Rainbow massif, cold ocean seawater is able to permeate deep into the seafloor. Water circulates down a fault line, actively reacting with various sediment and rock layers until it is warmed by an underlying heat source. When warmed, it can undergo a phase transition - often resulting in dramatic changes to fluid chemistry. Super-heated vent fluids then rise and are ejected from the seafloor, where a dramatic drop in temperature from cold-water mixing can cause some fluid chemicals to precipitate out and form chimneys.