Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36°14′N)

Abstract

Rainbow is a dome-shaped massif at the 36 14 0 N nontransform offset along the Mid-AtlanticRidge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (350 C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220 C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscalesulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at 160-260 C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

Three‐Dimensional Seismic Structure of the Mid‐Atlantic Ridge: An Investigation of Tectonic, Magmatic, and Hydrothermal Processes in the Rainbow Area

“…Along with these changes, the major faults that bound the axial valleys increase in throw and lateral separation approaching the NTD, culminating in core complex formation at the NTD itself. The now‐inactive detachment fault at Rainbow is thought to be located on the western side of the massif, where bathymetry data show a clear break in slope between the massif and the neighbor valley floor (e.g., Andreani et al, ). Consistent with that location, seismic reflection images from the MARINER experiment show evidence for the detachment from an unconformity of flat reflectors against material with a different reflectivity structure (Figure a).…”

Geologic implications of seafloor character and carbonate lithification imaged on the domal core of Atlantis Massif

“…Hydrothermal systems hosted by this formation could be related to the alteration of the gabbroic core and/or serpentinization of interfingered ultramafic formations (Andreani et al, 2014;McCaig et al, 2010;Blackman et al, 2014). The origin and extent of possible hydrothermal circulation processes in the subsurface of OCCs and their exact fluid pathways have been discussed extensively in previous studies; yet, the direct evidence remains very sparse except for fluid seepage found from seafloor observations (Andreani et al, 2014;Delacour et al, 2008aDelacour et al, , 2008bEscartín et al, 2008;Früh-Green et al, 2003;Karson et al, 2006;Kelley et al, 2001Kelley et al, , 2005Ludwig et al, 2006Ludwig et al, , 2011Macleod et al, 2011;Tucholke et al, 2013). It has been observed that nearly all ultramafic and gabbroic rocks recovered from the upper 800 m of IODP Hole U1309D on the Central Dome, located in close proximity to our prevalent lithified carbonate crust at Argo II track A39 (Fig.…”

“…Asymmetric spreading, where one flank exhibits detachment faulting, is estimated to occur along approximately 50% of the Mid-Atlantic Ridge (MAR) axis (Escartín et al, 2008). Since their discovery, OCC exposures have provided information on the formation and evolution of the slow-spreading axial environment, including detachment faulting (Blackman et al, 1998(Blackman et al, , 2002(Blackman et al, , 2011Dick et al, 2008;Canales et al, 2008;Henig et al, 2012;MacLeod et al, 2009;Mallows and Searle, 2012;Morris et al, 2009;Schroeder and John, 2004;Tucholke et al, 1998Tucholke et al, , 2001, subsurface fluid flow and associated serpentinization processes (Andreani et al, 2014;Blackman et al, 2002Blackman et al, , 2011Delacour et al, 2008aDelacour et al, , 2008bFrüh-Green et al, 2003Karson et al, 2006;Kelley et al, 2001;McCaig et al, 2010;Schroeder et al, 2002;Tucholke et al, 2013), and the accompanying biogeochemical activities (Andreani et al, 2014;Früh-Green et al, 2003Karson et al, 2006;Kelley et al, 2001Kelley et al, , 2005Schroeder et al, 2002;Tucholke et al, 2013).…”

Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR)

“…Many seafloor massive sulfides in the recent oceans, including the largest and those with the highest Cu grade (Petersen and Hein, 2013;Hein et al, 2013), are hosted in the peridotite-dominated lithosphere with a thin and heterogeneous crust (Gràcia et al, 2000;Kelley et al, 2001;Petersen et al, 2009;Pertsev et al, 2012;Kostitsyn et al, 2013;Tucholke et al, 2013;Andreani et al, 2014;Ciazela et al, 2015), where melt-mantle reaction is likely more enhanced (Ciazela et al, 2017b). In on-land prospecting, the search for chalcophile deposits seeks regions with both an appropriately enriched source rock, and a mechanism for concentrating those elements in a deposit.…”