IODP Expeditions 304 & 305 Characterize the Lithology , Structure , and Alteration of an Oceanic Core Complex

More than forty years after the Mohole Project (Bascom, 1961), the goal of drilling a complete section through in situ oceanic crust remains unachieved. Deep Sea Drilling Project – Ocean Drilling Program (DSDP-ODP) Hole 504B within the eastern Pacifi c (Alt et al., 1993) is the deepest hole ever drilled into ocean crust (2111 mbsf), but it failed to reach lower crustal plutonic rocks below the pillow basalts and sheeted dikes. IODP Expeditions 309 and 312 eventually recovered the long-sought transition from sheeted dikes into underlying gabbros by drilling into very fast-spreading Pacifi c crust (Wilson et al., 2006). The lithology and structure of oceanic crust produced at slow-spreading ridges are heterogeneous (e.g., Cannat et al., 1997) and offer unique drilling access to lower crust and upper mantle rocks. After ODP Hole 735B penetrated 1500 m of gabbro at the Southwest Indian Ridge (Dick et al., 2000), IODP Expeditions 304 and 305 recently recovered just over 1400 m of little-deformed, gabbroic lower crust from a tectonic window along the slowspreading Mid-Atlantic Ridge.


Introduction and Goals
More than forty years after the Mohole Project (Bascom, 1961), the goal of drilling a complete section through in situ oceanic crust remains unachieved.Deep Sea Drilling Project -Ocean Drilling Program (DSDP-ODP) Hole 504B within the eastern Pacifi c (Alt et al., 1993) is the deepest hole ever drilled into ocean crust (2111 mbsf), but it failed to reach lower crustal plutonic rocks below the pillow basalts and sheeted dikes.IODP Expeditions 309 and 312 eventually recovered the long-sought transition from sheeted dikes into underlying gabbros by drilling into very fast-spreading Pacifi c crust (Wilson et al., 2006).The lithology and structure of oceanic crust produced at slow-spreading ridges are heterogeneous (e.g., Cannat et al., 1997) and offer unique drilling access to lower crust and upper mantle rocks.After ODP Hole 735B penetrated 1500 m of gabbro at the Southwest Indian Ridge (Dick et al., 2000), IODP Expeditions 304 and 305 recently recovered just over 1400 m of little-deformed, gabbroic lower crust from a tectonic window along the slowspreading Mid-Atlantic Ridge.
IODP Expeditions 304 and 305 at the Atlantis Massif, Mid-Atlantic Ridge 30°N, were designed to investigate the processes that control oceanic core complex (OCC) formation and exposure of lower crust and upper mantle rocks in young (<2 Ma) oceanic lithosphere accreted at slow-spreading ridges.The corrugated, central portion of this domal massif (Fig. 1) displays morphologic and geophysical characteristics inferred to be representative of an OCC exposed via long-lived, low-angle, normal or detachment faulting (Cann et al., 1997;Blackman et al., 1998Blackman et al., , 2004)).Geophysical inter- velocities were inferred from the seismic refraction data.One interpretation of this "D refl ector" suggested that it marks an alteration front within the peridotite-dominated massif.The multi-channel seismic (MCS) processing employed by Canales et al. (2004) resulted in the D refl ector being quite continuous across the dome.A subsequent study by Singh et al. (2004) using different processing parameters to emphasize deep refl ectivity produced a less continuous, but still pervasive interval of refl ectivity whose top generally coincided with Canales' D refl ector.Modeling of sea-surface gravity and sparse seafl oor data (Blackman et al., 1998(Blackman et al., , 2004;;Nooner et al., 2003) suggests that rocks beneath the central and southern dome have densities 200-400 kg•m -3 greater than rock to either side.
Rock samples collected from the central dome by the manned submersible Alvin are dominated by angular talus and rubble of serpentinized peridotite, metabasalt, and limestone (Blackman et al., 2004).A few samples from the central dome show cataclastic deformation or are highly serpentinized or metasomatically altered peridotite.The protolith of most of the serpentinite sampled on the south wall (Fig. 1) is inferred to be harzburgite.Talc-rich fault rocks preserve textural and geochemical characteristics of their ultramafi c protoliths (Boschi et al., 2006).Microstructural analysis of samples from the south wall (Schroeder and John, 2004) indicates ductile deformation initially at granulite facies, overprinted by semi-brittle and brittle deformation down to subgreenschist facies.The observations and sample distribution suggest that strong semi-brittle and brittle deformation is concentrated at shallow structural levels (~100 m beneath the domal surface) along the Southern Ridge (Schroeder and John, 2004;Karson et al., 2006).Rocks sampled from the hanging wall by manned submersible are wholly basalt (Blackman et al., 2004).

Drilling at the Atlantis Massif Reveals Dominantly Gabbroic Core
Igneous rocks recovered from Site U1309 span a broad range in composition, from the most primitive crustal rocks ever cored in slow-spreading oceanic lithosphere (Mg# up to ~90) to highly evolved rock types (Fig. 2).To a fi rst order, the gabbroic section can be divided into two major igneous units (Johnson et al., 2005).The upper unit extends to ~600 mbsf and broadly shows an increase downhole in olivine-rich rock types, although the detailed lithostratigraphy is more complicated.There is a sharp decrease in whole-rock Mg# of the gabbros at ~600 mbsf (Fig. 2).Oxide-rich gabbros are concentrated in a zone between these upper and lower units.The lower unit also shows a general increase downhole in olivinerich rock types, with oxide-rich intervals near the base of the hole.At least two thin, mantle-peridotite intervals are recognized in the upper 180 m of the section, implying that the gabbroic section recovered in Hole U1309D was in part intruded into mantle peridotite.pretations suggested that unaltered mantle rock occurred <1 km below the seafl oor (Collins et al., 2003;Blackman et al., 2004;Canales et al., 2004), implying that drilling through an alteration front in ultramafi c rocks could be achieved and that fresh mantle peridotite could be recovered at moderate depths.Additional objectives of drilling at the footwall concerned the dominant mechanism(s) of footwall uplift and the interactions between tectonics and magmatism in OCCs.Drilling within the hanging wall and through the detachment fault system was aimed at sampling and further constraining the latter and at assessing petrogenetic relationships between volcanic rocks in the hanging wall and potential source rocks recovered in the footwall.
We attempted drilling at three sites (Fig. 1 [C]), one in the footwall through the inferred detachment fault and two in the hanging wall, but we did not succeed in drilling the fractured basalt in the hanging wall.In contrast, Hole U1309D, in the footwall of the detachment fault, penetrated 1415.5 m below the seafl oor, and recovery averaged 75% (IODP Expeditions 304-305 Scientists, 2005).Shallow holes on the footwall provided limited samples whose composition and structure support the hypothesis that the corrugated dome coincides with an exposed detachment fault.

Geological and Geophysical Background
The Atlantis Massif formed within the past 1.5-2My. and was exhumed along a detachment fault exposed over an 8-10-km-wide, 15-km-long area that forms the elongate, doubly plunging domal seafl oor morphology (Fig. 1).Adjacent basaltic rocks to the east are interpreted as part of a hanging-wall block above the detachment fault.Multichannel seismic data (Canales et al., 2004) suggest that the fault system may dip gently under the seafl oor at the base of the dome and continue at a shallow angle (<15°) beneath the eastern block toward the present-day ridge axis (Fig. 1C).The Southern Ridge is shallower than the central dome, shoaling to 700 m below sea level, and its corrugated surface extends eastward to the median valley wall.
Prior to IODP drilling, the core of the Atlantis Massif was inferred to comprise dominantly mantle peridotite.The peridotite-hosted, possibly serpentinization-driven Lost City hydrothermal vent fi eld (Früh-Green et al., 2003;Kelley et al., 2005) is located just below the summit of the Southern Ridge, approximately 5 km south of Site U1309.Analysis of seismic refraction data across the central dome of the Atlantis Massif (Collins et al., 2003) indicated, at least locally, P-wave velocities of ~8 km•s -1 within several hundred meters below the seafl oor, possibly indicative of pristine mantle rocks.Interpretation of multi-channel seismic refl ection data suggested a major difference in structure between the outside (conjugate) corner lithosphere versus that hosting the Atlantis Massif (Canales et al., 2004).A strong refl ector is visible at 0.2-0.5 s below much of the domal surface and coincides roughly with the depth below which mantle and grades locally into troctolitic gabbro; it is commonly spatially associated with troctolites.A series of olivine-rich rocks (~5% in hole U1309D; dunites, wehrlites, troctolites), grouped as olivine-rich troctolites, are interlayered with gabbroic rocks.In contrast to troctolite, olivine-rich troctolite displays subhedral to rounded medium-grained olivine and interstitial to poikilitic plagioclase and clinopyroxene in variable proportions.These rocks may represent primitive cumulates.
Fe-Ti oxide gabbro, resulting from pervasive, latemagmatic-stage infi ltration of evolved melt, composes 7% of the rocks recovered from Hole U1309D.The most common occurrence of oxide concentrations (~80% of the oxide gabbros) is seen as randomly dispersed patches in undeformed, generally coarse-grained gabbro.They are also found as discrete dikelets or layers cutting other rock types with either sharp or diffuse boundaries, and associated with intervals of ductile deformation.

Structure and Alteration
The core from Hole U1309D can be divided structurally into three major units (Fig. 2): • Structural Unit 1 (0-170 mbsf) is marked by a high but decreasing degree of cataclasis downhole; abundant, late, relatively undeformed diabase; a high degree of green-Holes U1309B and U1309D have interfi ngered lithologic units that vary in thickness from centimeters up to ~100-200 m where intrusive contacts are preserved.Contact relations where visible between gabbro and other rock types, except diabase at Site U1309, suggest that gabbro is generally intrusive into more olivine-rich rock types, such as olivine gabbro and troctolite, and in turn intruded by felsic ("leucocratic") dikes and oxide gabbro.These relationships are more common between 400 and 650 mbsf than in the lower part of the hole, where gabbro contacts are commonly more diffuse.The ~140-m-thick interval of olivine-rich troctolite and minor associated lithologic units from 1094 to 1236 mbsf in Hole U1309D forms an integral lithologic package that has been intruded by numerous crosscutting gabbroic dikes of variable thickness at temperatures below the troctolite solidus.Broader-scale contacts with adjacent olivine gabbro appear to be dominantly intrusive and formed under hypersolidus conditions.Late diabase intrusions are found in several places throughout Holes U1309B and U1309D.
The most abundant rock type recovered is gabbroic in composition and spans a wide range of modal composition, including minor (rarely exceeding a few percent) amounts of olivine, Fe-Ti oxides, and orthopyroxene.Gabbros and gabbronorites exhibit signifi cant variation in grain size (<1 mm to >10 cm), occasionally within a single section of core.Olivine gabbro, the second-most abundant rock type, has highly variable modal compositions on a submeter scale Overall, the section is moderately altered at conditions ranging from granulite to zeolite facies and dominated by static, greenschist facies assemblages.Pseudomorphs of igneous textures remain largely unmodifi ed.Magmatic deformation fabrics, as defi ned by the preferred orientation of plagioclase, were recorded in 22% of the recovered rocks.These fabrics are weak to very weak, except in local intervals, and tend to be developed more clearly in gabbroic rocks with fi ner initial grain size; foliation is also well developed in the rare layered intervals.In many places, weak crystal-plastic deformation seems to overprint magmatic foliations (Fig. 3).High-strain, crystal-plastic shear zones are rare (Figs. 2 and  3) and are typically restricted to clearly defi ned, mostly granulite-grade shear zones ranging in width from millimeters to a few meters.This contrasts markedly with the much larger number of high-strain shear zones recorded in the gabbroic complex at ODP Site 735B (Fig. 3A) on the Southwest Indian Ridge (Dick et al., 1999(Dick et al., , 2000)).Vein intensity in Hole 1309D decreases signifi cantly below 785 mbsf (Fig. 2) and tends to correlate with cataclastic deformation in general and with fault zones on a local scale.On the scale of the entire core, there is no systematic, lithologydependent distribution of vein type downhole.The amount of strain recorded by brittle fracture and cataclasis is limited overall, except for a few fault zones concentrated in the upper 50 m of Hole U1309D and in discrete intervals downhole, some of which correspond to the boundaries of the defi ned major structural units, in particular at ~750 mbsf (Fig. 2).Cataclasis is associated locally with oxide gabbro intervals and dikelets, leucocratic veins, and contact zones between diabase intrusions and their gabbroic host rocks.
The lack of signifi cant structures indicative of high displacement by either ductile or brittle processes severely limits the possible thickness of fault zones that could compose a detachment system over the central dome.Poor recovery of the upper 20 m of the footwall allows the possibility that this narrow zone accommodated very high strain along a dominantly brittle fault, as documented at the 15°45'N OCC (MacLeod et al., 2002;Escartín et al., 2003).If so, the fault zone thickness differs from the 50-100 m estimates at the Southern Ridge (Williams et al., 2003;Schroeder and John, 2004;Karson et al., 2006).This difference results either from a decreasing thickness of the fault from the south to the center of the dome or from an overestimated thickness at the Southern Ridge because of limitations in accurately determining the structural depth of submersible samples.Extensive amphibolite facies deformation is lacking, and high-strain ductile shear zones are rare.The absence of a thick zone of high-temperature ductile deformation in the footwall and the apparent tectonic history (less rotation in the upper 180 m and variable rotations between several distinct, few-hundred-meter sections downhole, Fig. 2) suggested by paleomagnetic inclination measurements indicate complexity in structural evolution that differs from a simple model of a deep-rooted detachment fault, predicted to be associated with high-temperature deformation, and with constant or monotonically varying footwall rotation with depth.Altered mineral assemblages in rocks from Site U1309 record cooling of mafi c plutonic rocks from submagmatic conditions (>1000°C) to zeolite facies temperatures (<200°C) during the unroofi ng and uplift of the Atlantis Massif.Individual samples generally display a range of superimposed metamorphic conditions, but no single sample records the entire cooling history of the site.Overall, alteration intensity is moderate, tends to decrease downhole, and is commonly related to vein intensity (Fig. 2).Local exceptions to this general downhole decrease in alteration intensity often correlate with an increase in modal abundance of olivine.Coarser-grained gabbro intervals are generally more altered than medium-to coarse-grained units.Intervals of olivine-rich troctolite show alteration restricted to heterogeneous serpentine networks, with strong alteration gradients from the contact with intensely veined intercalated gabbros to fresher cores.The latter contain local intervals of very fresh (as low as 1% serpentinization) olivine-rich (as much as >90%) rocks.
The metamorphic and alteration history recorded at Site U1309 is summarized as follows: 1. High-temperature, near-solidus, mylonitic deformation and recrystallization at granulite-to upper-amphibolitefacies conditions.
2. A widespread, largely static upper-greenschist-facies to lower-amphibolite-facies event (Figs. 2 and 4).The upper ~380 m of the core shows a beautiful alteration profi le with evidence for pervasive static infi ltration of seawater with decreasing temperature.In addition, these same rocks show localized alteration superimposed in some cases on pervasive alteration, presumably refl ecting localized fl uid fl ow and metasomatism.Below this depth, alteration is generally restricted to halos around veins, fractures, and igneous contacts.
3. Static, lower greenschist to subgreenschist metamorphism.Serpentinization above ~300 mbsf is restricted to rocks where olivine was in excess over plagioclase and was therefore still present after the corona-forming reaction between olivine and plagioclase (stage 2) went to completion.At deeper levels, serpentine, prehnite, and hydrogrossular are often localized on closely spaced, variably oriented fractures.

Correlations Between Alteration and Geophysical Signature
Downhole logging and physical property measurements on core samples do not correlate strongly with rock type (Fig. 2).Instead, alteration may play a stronger role in determining the geophysical signature of the upper 1.5 km of the central dome.Average sample density varies only slightly, from 2.800 g•cm -3 in the upper 380 mbsf to 2.900 g•cm -3 in the lower ~1 km (Fig. 2).Meter-scale intervals show more variability in both sample and logging measurements, with oxide gabbro sometimes reaching >3.200 g•cm -3 and highly altered olivine-rich zones dropping to <2.700 g•cm -3 .Average compressional velocity is 5.6 km•s -1 in the upper ~400 m and 5.8 km•s -1 for the interval ~500-800 mbsf.The mid-expedition checkshot experiment extended to 840 mbsf, the base of the hole at that stage.Instrument failure and poor weather precluded acquisition of seismic data in the fi nal logging run.Seismic velocity values obtained by sample and sonic log measurements range from ~5.1 to 6.8 km•s -1 .Values less than 5.5 km•s -1 are common in the upper 180 mbsf and in the olivinerich interval ~280-380 mbsf, below which values near 6 km•s -1 are most common.Only thin intervals have a velocity greater than 6.5 km•s -1 corresponding to olivine gabbro or olivine-rich troctolite units with little alteration.
Shipboard processing of the checkshot data revealed only fi rst-arrival information.Although the signal-to-noise ratio was generally good for the stacked data, considerable noise was present due to impacts of the pipe in the upper part of the hole.After the expedition, individual traces were analyzed, and particularly noisy records were removed prior to re-stacking and bandpass fi ltering (5-120 Hz).The processed data (Fig. 5) show a secondary arrival that trails the fi rst peak by a time that decreases from 41 ms to essentially zero between stations at 275 and 435 mbsf.Using the delays at the 275 and 345 mbsf stations and a velocity of 5.5 km•s -1 , a refl ector at ~390 mbsf could produce the secondary arrival that also corresponds to a strong refl ector on the MCS line closest to Hole U1309D.It is likely that this represents the D refl ector in this part of the dome.
The temperature-acceleration-pressure (TAP) tool recorded a temperature of 120°C at the bottom of the hole (Fig. 6).Temperature increased with depth as expected but was somewhat lower than predicted from a simple cooling-plate model of a spreading ridge fl ank with an age of ~2 Ma.The measured temperatures are a minimum estimate, however, because of hole cooling during coring.These initial results suggest that thermally driven fl ow is likely to occur in the hole.Several drops of a few degrees were recorded in narrow intervals (two of which coincide with documented fault zones at ~750 and 1110 mbsf) on repeated TAP runs (Fig. 6), perhaps indicating localized fl uid fl ow.

Concluding Remarks
Igneous rocks recovered at Site U1309 provide an exceptional record of magmatic accretion, tectonic exposure, and hydrothermal alteration in a slow-spreading ridge en vironment.The exposures of peridotite along the southern wall of the Atlantis Massif, the geophysical results suggesting that at least portions of the dome contain fresh olivinerich rock (possibly olivine-rich troctolites), and the downhole variability at Site U1309 all likely indicate signifi cant lateral lithological heterogeneity over distances as short as hundreds of meters across the footwall.
The Atlantis Massif is the fourth location where drilling an OCC (i.e., a corrugated massif, or an inside corner high capped by fault rocks) at a slow-spreading ridge has been attempted by ODP or IODP.A total of fourteen holes (>10 m deep) were cored at seven different sites in three different OCCs during ODP Legs 118, 153, 176, 179, and 209 (Robinson, et al., 1989;Cannat, et al., 1995;Dick, et al., 1999;Pettigrew et al., 1999;Kelemen, et al., 2004).Those efforts recovered only gabbroic sections, ranging from Fe-Ti oxide gabbros to troctolites, locally intruded by diabase.The latest result s of IODP Expeditions 304 and 305 point to the need for a new paradigm of crustal accretion in regions that are classically inferred to be representative of magma-starved portions of the ridge.One possible working model relates the development of OCCs to the occurrence of periodically larger gabbroic bodies beneath slow-spreading ridge segment ends (Ildefonse et al., 2006).Uplift and exhumation of these gabbroic bodies would be enabled by deformation that localized predominantly within the serpentinized peridotite that initially surrounded them.The northward extension, inside the core of the Atlantis Massif, of the serpentinites outcropping at the southern wall is presently unknown.These serpentinites could represent a relatively thin sheet of mantle rocks surrounding a dominantly gabbroic core.Alternatively, a signifi cant, but unknown part of the Atlantis Massif footwall may be composed primarily of serpentinite and peridotite.Further IODP operations in this region should combine deeper drilling in Hole U1309D and drilling the southeast shoulder of the Atlantis Massif.

Figure 1
Figure 1.[A] Location of Expeditions 304 and 305.[B] Base map of Atlantis Massif showing prior geological and geophysical data coverage and location of drilling sites.Seismic refl ection and refraction lines and seafl oor mapping and sampling sites are shown.MCS = multi-channel seismic, OBH = ocean-bottom hydrophone.[C] MCS line Meg-10, stacked section showing D refl ector and possible continuation of the detachment fault beneath the hanging wall (dashed red line).

Figure 5 .Figure 6 .
Figure 5. Filtered waveforms for checkshot experiment in upper part of Hole U1309D.Main arrival has strong signal-to-noise ratio.Removal of especially noisy traces brings out character of later arrivals.At upper stations, a secondary arrival (arrows) probably corresponds to D refl ector ofCanales et al., (2004).