Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Author Posting. © Macmillan Publishers, 2008. This is the author's version of the work. It is posted here by permission of Macmillan Publishers for personal use, not for redistribution. The definitive version was published in Nature 455 (2008): 790-794, doi:10.1038/nature07333.The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on seafloor formed along ridges spreading at less than 80 km/My1-4. These faults can accommodate extension for up to 1-3 Myrs5, and are associated with one of two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge (MAR). The first is symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and formation of abyssal hills on both flanks. The second is asymmetrical accretion involving an active detachment fault6 along one ridge flank. An examination of ~2500 km of the MAR between 12.5 and 35°N reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified to date in the study region is closely associated with asymmetrical accretion, which also exhibits high-levels of near continuous hydroacoustically and teleseismically recorded seismicity. Enhanced seismicity is probably generated along detachment faults accommodating a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which concentrates primarily at their ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths, or because the rising mantle is cooler. We suggest that much of the variability in seafloor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachments in oceanic lithospheric accretion.Supported by CNRS (JE), NSF (DKS, HS, JC, CL and SE), WHOI (JE, DKS, HS and JC), Harvard University (JE, CL and SE), Univ. of Leeds (JC), and MIT (JE)

Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge

Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (?365°C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al., Chemical Geology 184:37–48, 2002). Serpentinization has occurred under a low-temperature (<270°C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite—stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different 143Nd/144Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni <1 and subsurface and surface units have Co/Ni >1