Drilling constraints on lithospheric accretion and evolution at Atlantis Massif, Mid-Atlantic Ridge 30°N

Abstract

[1] Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has B071031 of 25 produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45°rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises ∼70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.

Hydrogeologic Properties, Processes, and Alteration in the Igneous Ocean Crust

“…Interlayered with these are several olivine-rich rock types (∼5%; dunites, wehrlites, troctolites). A few thin mantle peridotite intervals are also present in the upper 180 msb (Blackman et al, 2011;Expedition 304/305 Scientists, 2006b). The gabbbroic rocks are mainly gabbros, including gabbronorite and orthopyroxene-bearing gabbro, making up 55.7% of the core.…”

“…Seawater-rock interactions manifest in core from Hole U1309D range from granulite to zeolite facies and alteration assemblages and vein fillings record the unroofing and uplift of the Atlantis Massif (Blackman et al, 2011;Nozaka, Fryer, & Andreani, 2008). Alteration intensity is highly variable, but generally decreases downhole.…”

Elastic and electrical properties and permeability of serpentinites from Atlantis Massif, Mid-Atlantic Ridge

“…Serpentinized peridotites co-exist with altered or fresh mafic rocks (e.g., Blackman et al, 2011) in a variety of marine environments (Bayrakci et al, 2016).…”

“…Massif is a young (<2Myr; Grimes et al (2008)) oceanic core complex typical of a slow spreading ocean lithospheric accretion environment, exposed by a major fault system (Blackman et al, 1998, Blackman et al, 2011, Schroeder and John, 2004. Near the top of the faulted escarpment of the southern ridge, the Lost City Hydrothermal Field (LCHF) lies on a down-dropped terrace at ~750-800 m water depth and represents a hydrothermal vent system associated with active serpentinization (Denny et al, 2015, Früh-Green et al, 2003, Kelley et al, 2001, Kelley et al, 2005.…”

Scientific Drilling

“…High fracture intensity in the detachment zone enabled circulation of seawater (McCaig et al, 2010), and associated alteration included formation of low-strength minerals that enhanced strain localization. Geophysical studies (Blackman et al, 2008;Canales et al, 2008;Collins et al, 2009;Blackman and Collins, 2010;Henig et al, 2012), seafloor mapping and sampling (Blackman et al, 2002;Schroeder and John, 2004;Boschi et al, 2006;Karson et al, 2006), and deep basement drilling and logging (Blackman et al, 2011;Ildefonse et al, Science Reports Science Reports Figure 1. Atlantis Massif tectonic setting and seismic structure.…”