Atlantis Massif Oceanic Core Complex Research Articles

Link between melt-impregnation and metamorphism of Atlantis Massif peridotite (IODP Expedition 357)

IODP Expedition 357 drilled 17 shallow sites scattered over ~ 10 km in the west to east spreading direction across the Atlantis Massif oceanic core complex (OCC, MAR, 30 ºN). Mantle exposed in the footwall of the Atlantic Massif OCC is nearly wholly serpentinized (80–100%) harzburgite and subordinate dunite. A recent whole-rock chemistry study by Whattam et al. (Chemical Geology 594. 10.1016/j.chemgeo.2021.120681, 2022) subdivides Atlantis Massif peridotites into: Type I fluid–rock dominated serpentinite, which exhibits almost nil evidence of melt-impregnation or silica metasomatism; Type II melt–rock dominated, mafic melt-impregnated serpentinite; and Type III melt–rock dominated Si-metasomatized serpentinite. In this study, on the basis of EPMA, three kinds of Cr–spinel are distinguished in Expedition 357 serpentinite: (I) primary, unmetamorphosed mantle array, (II) low-Ti metamorphosed, and (III) high-Ti melt reacted. All Cr–spinel of western site Type I serpentinite is unmetamorphosed (n = 34) and comprises 68% of all unmetamorphosed Cr–spinel. Metamorphosed Cr–spinel (n = 100) are the most abundant and occur in the central and eastern site Type II and Type III serpentinite, whereas melt-reacted Cr–spinel and chromite are restricted to the central sites and occur predominantly in serpentinized dunite. Estimates of the degree of melt extraction of Type I serpentinite using F = 10ln(spinel Cr#) + 24 are ~ 9–17%. Fugacity calculations of primary, unmetamorphosed Cr–spinel yield Δlog(fO2)FMQ of − 1.7 to + 1.0 and calculations using olivine–spinel Mg–Fe exchange thermometry yield a mean closure temperature of 808 ± 39 °C. Mafic melt-impregnation resulted in Cr–spinel with anomalously high TiO2 of 0.27–0.68 wt.% and production of Ti-rich chromite (up to 1.23 wt.% TiO2). Greenschist facies metamorphism (< 500 °C) resulted in Mg–Fe2+ exchange between Cr–spinel and forsterite and anomalously low Cr–spinel Mg#; higher degrees of amphibolite facies metamorphism (~ 500–700 °C) also resulted in anomalously high Cr# due to Al–Cr exchange. As has been previously established, significant Al loss from chromite cores above 550 °C is the result of equilibration with fluids in equilibrium with chlorite, which may be valid for our samples. On the basis of Cr–spinel vs. whole-rock compositions, a clear relationship exists between melt-impregnation and metamorphism of central and eastern serpentinite, which we postulate to be the result of heat associated with magma injection and subsequent localized contact metamorphism. To our knowledge, such a relation between mafic melt-impregnation of peridotite and metamorphism (of peridotite) has not previously been established in general and specifically for the Atlantis Massif peridotite. Closure temperatures of 440–731 °C of metamorphosed Cr–spinel approximate greenschist to amphibolite facies metamorphic conditions.

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.

Deformation, CPO, and Elastic Anisotropy in Low‐Grade Metamorphic Serpentinites, Atlantis Massif Oceanic Core Complex

AbstractCrystallographic preferred orientation (CPO) and the associated seismic anisotropy of serpentinites are important factors for the understanding of tectonic settings involving hydrated Earth´s mantle, for example, at slow‐spreading mid‐ocean ridges. CPO of lizardite and magnetite in low‐grade metamorphic serpentinites from the Atlantis Massif oceanic core complex (Mid‐Atlantic Ridge, 30°N) were determined using synchrotron high energy X‐ray diffraction in combination with Rietveld texture analysis. Serpentinite mesh structures show weak CPO while deformed samples show a single (0001) maximum perpendicular to the foliation. Seismic anisotropies calculated from CPO show up to &gt;11% anisotropy for compressional waves (Vp) and shear wave splitting up to 0.38 km/s in the deformed samples. This indicates that deformation in shear zones controls elastic anisotropy and highlights its importance in defining the seismic signature of hydrated upper mantle.

Extensive decentralized hydrogen export from the Atlantis Massif

Abstract Hydrogen is an important energy source for subsurface microbial communities, but its availability beyond the flow focused through hydrothermal chimneys is largely unknown. We report the widespread export of H2 across the Atlantis Massif oceanic core complex (30°N, Mid-Atlantic Ridge; up to 44 nM), which is distinct from the circulation system feeding the Lost City Hydrothermal Field (LCHF) on the massif's southern wall. Methane (CH4) abundances are generally low to undetectable (&amp;lt;3 nM) in fluids that are not derived from the LCHF. Reducing fluids exit the seafloor over a wide geographical area and depth range, including the summit of the massif and along steep areas of mass wasting east of the field. The depth of the fluids in the water column and their H2/CH4 ratios indicate that some are sourced separately from the LCHF. We argue that extensive H2 export is the natural consequence of fluid flow pathways strongly influenced by tectonic features and the volume and density changes that occur when ultramafic rocks react to form serpentinites, producing H2 as a by-product. Furthermore, the circulation of H2-rich fluids through uplifted mantle rocks at moderate temperatures provides geographically expansive and stable environmental conditions for the early evolution of biochemical pathways. These results provide insight into the spatial extent of H2- and CH4-bearing fluids associated with serpentinization, independent of the focused flow emanating from the LCHF.

Velocity structure near IODP Hole U1309D, Atlantis Massif, from waveform inversion of streamer data and borehole measurements

AbstractSeismic full waveform inversion (FWI) is a promising method for determining the detailed velocity structure of the igneous oceanic crust, especially for locations such as the Mid‐Atlantic Ridge with significant lateral heterogeneity and seafloor topography. We examine the accuracy of FWI by inverting, after downward continuation to datum just above the seafloor, a multichannel seismic (MCS) profile from Atlantis Massif oceanic core complex at 30°N that passes close to Integrated Ocean Drilling Program (IODP) Hole U1309D and comparing the results against borehole measurements and existing on‐bottom refraction data. The comparisons include the results of IODP Expedition 340T, which extended the sonic logging and vertical seismic profiling to the bottom of the borehole at 1400 m below seafloor. Compared to travel time tomography, the refinement in velocity and velocity gradient produced by FWI significantly improves the overall match to the borehole measurements, and allows the multilevel pattern of deformation and alteration of the detachment footwall seen in Hole U1309D to be extrapolated across the Central Dome. Prestack depth migration of the profile using the FWI velocities reveals the top and edges of the high‐velocity, gabbroic core of the massif. It also indicates that the comparatively uniform gabbroic rocks drilled at Hole U1309D extend to ∼2.5 km below seafloor but overlie an extended, ∼2 km thick, mantle transition zone.

Nanoscale constraints on porosity generation and fluid flow during serpentinization

Research Article| February 01, 2016 Nanoscale constraints on porosity generation and fluid flow during serpentinization Benjamin M. Tutolo; Benjamin M. Tutolo * 1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA *E-mail: benjamin.tutolo@earth.ox.ac.uk Search for other works by this author on: GSW Google Scholar David F.R. Mildner; David F.R. Mildner 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA Search for other works by this author on: GSW Google Scholar Cedric V.L. Gagnon; Cedric V.L. Gagnon 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA Search for other works by this author on: GSW Google Scholar Martin O. Saar; Martin O. Saar 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA5Department of Earth Sciences, ETH-Zürich, 8006 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar William E. Seyfried, Jr. William E. Seyfried, Jr. 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Benjamin M. Tutolo * 1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA David F.R. Mildner 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA Cedric V.L. Gagnon 3National Institute of Standards and Technology Center for Neutron Research, Gaithersburg, Maryland 20899, USA4Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA Martin O. Saar 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA5Department of Earth Sciences, ETH-Zürich, 8006 Zurich, Switzerland William E. Seyfried, Jr. 2Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA *E-mail: benjamin.tutolo@earth.ox.ac.uk Publisher: Geological Society of America Received: 10 Sep 2015 Revision Received: 25 Nov 2015 Accepted: 30 Nov 2015 First Online: 09 Mar 2017 Online Issn: 1943-2682 Print Issn: 0091-7613 © 2016 Geological Society of America Geology (2016) 44 (2): 103–106. https://doi.org/10.1130/G37349.1 Article history Received: 10 Sep 2015 Revision Received: 25 Nov 2015 Accepted: 30 Nov 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Benjamin M. Tutolo, David F.R. Mildner, Cedric V.L. Gagnon, Martin O. Saar, William E. Seyfried; Nanoscale constraints on porosity generation and fluid flow during serpentinization. Geology 2016;; 44 (2): 103–106. doi: https://doi.org/10.1130/G37349.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Field samples of olivine-rich rocks are nearly always serpentinized—commonly to completion—but, paradoxically, their intrinsic porosity and permeability are diminishingly low. Serpentinization reactions occur through a coupled process of fluid infiltration, volumetric expansion, and reaction-driven fracturing. Pores and reactive surface area generated during this process are the primary pathways for fluid infiltration into and reaction with serpentinizing rocks, but the size and distribution of these pores and surface area have not yet been described. Here, we utilize neutron scattering techniques to present the first measurements of the evolution of pore size and specific surface area distribution in partially serpentinized rocks. Samples were obtained from the ca. 2 Ma Atlantis Massif oceanic core complex located off-axis of the Mid-Atlantic Ridge and an olivine-rich outcrop of the ca. 1.1 Ga Duluth Complex of the North American Mid-Continent Rift. Our measurements and analyses demonstrate that serpentine and accessory phases form with their own, inherent porosity, which accommodates the bulk of diffusive fluid flow during serpentinization and thereby permits continued serpentinization after voluminous serpentine minerals fill reaction-generated porosity. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Fluid evolution in an Oceanic Core Complex: A fluid inclusion study from IODP hole U1309 D—Atlantis Massif, 30°N, Mid‐Atlantic Ridge

In the detachment mode of slow seafloor spreading, convex-upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor. Large, long-lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30°N, Mid-Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase-separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400°C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low-salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.

Geophysical signatures of past and present hydration within a young oceanic core complex

AbstractBorehole logging at the Atlantis Massif oceanic core complex provides new information on the relationship between the physical properties and the lithospheric hydration of a slow‐spread intrusive crustal section. Integrated Ocean Drilling Program Hole U1309D penetrates 1.4 km into the footwall to an exposed detachment fault on the 1.2 Ma flank of the mid‐Atlantic Ridge, 30°N. Downhole variations in seismic velocity and resistivity show a strong correspondence to the degree of alteration, a recorder of past seawater circulation. Average velocity and resistivity are lower, and alteration is more pervasive above a fault around 750 m. Deeper, these properties have higher values except in heavily altered ultramafic zones that are several tens of meters thick. Present circulation inferred from temperature mimics this pattern: advective cooling persists above 750 m, but below, conductive cooling dominates except for small excursions within the ultramafic zones. These alteration‐related physical property signatures are probably a characteristic of gabbroic cores at oceanic core complexes.

The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex

Oceanic core complexes (OCCs) are domal exposures of oceanic crust and mantle interpreted to be denuded to the seafloor by large slip oceanic detachment faults. We combine previously reported U‐Pb zircon crystallization ages with (U‐Th)/He zircon thermochronometry and multicomponent magnetic remanence data to determine the cooling history of the footwall to the Atlantis Massif OCC (30°N, MAR) and help establish cooling rates, as well as depths of detachment faulting and gabbro emplacement. We present nine new (U‐Th)/He zircon ages for samples from IODP Hole U1309D ranging from 40 to 1415 m below seafloor. These data paired with U‐Pb zircon ages and magnetic remanence data constrain cooling rates of gabbroic rocks from the upper 800 m of the central dome at Atlantis Massif as 2895 (+1276/−1162) °C Myr−1 (from ∼780°C to ∼250°C); the lower 600 m of the borehole cooled more slowly at mean rates of ∼500 (+125/−102) °C Myr−1(from ∼780°C to present‐day temperatures). Rocks from the uppermost part of the hole also reveal a brief period of slow cooling at rates of ∼300°C Myr−1, possibly due to hydrothermal circulation to ∼4 km depth through the detachment fault zone. Assuming a fault slip rate of 20 mm/yr (from U‐Pb zircon ages of surface samples) and a rolling hinge model for the sub‐surface fault geometry, we predict that the 780°C isotherm lies at ∼7 km below the axial valley floor, likely corresponding both to the depth at which the semi‐brittle detachment fault roots and the probable upper limit of significant gabbro emplacement.

The internal structure of an oceanic core complex: An integrated analysis of oriented borehole imagery from IODP Hole U1309D (Atlantis Massif)

Oceanic core complexes at slow‐spreading ridges represent the uplifted footwalls of large‐offset ‘detachment’ faults that initiate at steep dips, and rotate to flatten via a ‘rolling hinge’ mechanism in response to flexural unloading. A key question is whether oceanic core complex development is accommodated entirely by displacement on the detachment fault zone, or if significant internal deformation of the footwall occurs during flexure and rotation. We investigate this by constraining the internal architecture of the Atlantis Massif oceanic core complex (Mid‐Atlantic Ridge, 30°N) using Formation MicroScanner borehole wall images and cores from the 1416 m‐deep Integrated Ocean Drilling Program Hole U1309D. Two distinct sets of structures are observed. N‐S‐striking, E‐dipping structures dominating the upper 385 m are interpreted as a brittle to semi‐brittle zone of fracturing in the footwall. Structures with this geometry occur down to 750 m below seafloor, suggesting that the detachment damage zone extends deep into the footwall. The nature of this deformation is, however, enigmatic: several cataclastic shear zones with reverse geometry in their current orientations may be rotated extensional faults or relate to shortening at the base of the flexing beam of a very weak footwall. By contrast, E‐W‐striking, N‐ and S‐dipping structures dominate the lowermost kilometer of the borehole. They likely represent conjugate fractures formed in the hanging wall of a late, E‐W normal fault zone, separating gabbroic rocks of the central dome of Atlantis Massif from serpentinized peridotite to the south, responsible for post‐detachment uplift of the southern margin of the massif.

Downward continued multichannel seismic refraction analysis of Atlantis Massif oceanic core complex, 30°N, Mid-Atlantic Ridge

[1] Detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the Atlantis Massif oceanic core complex (OCC), contrasting notably with its conjugate ridge flank. Multichannel seismic (MCS) data are downward continued using the Synthetic On Bottom Experiment (SOBE) method, providing unprecedented detail in tomographic models of the P-wave velocity structure to subseafloor depths of up to 1.5 km. Velocities can vary up to 3 km/s over several hundred meters and unusually high velocities (∼5 km/s) are found immediately beneath the seafloor in key regions. Correlation within situand dredged rock samples, video and records from submersible dives, and a 1.415 km drill core, allow us to infer dominant lithologies. A high velocity body(ies) found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the OCC, indicating a propagating magmatic source as the origin for this pluton(s). The western two-thirds of the Southern Ridge is capped in serpentinite that may extend nearly to the base of our ray coverage. The distribution of inferred serpentinite indicates that the gabbroic pluton(s) was emplaced into a dominantly peridotitic host rock. Presumably the mantle host rock was later altered via seawater penetration along the detachment zone, which controlled development of the OCC. The asymmetric distribution of seismic velocities and morphology of Atlantis Massif are consistent with a detachment fault with a component of dip to the southeast. The lowest velocities observed atop the eastern Central Dome and conjugate crust are most likely volcanics. Here, an updated model of the magmatic and extensional faulting processes at Atlantis Massif is deduced from the seismic results, contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow-rate spreading centers.

Lower crustal variability and the crust/mantle transition at the Atlantis Massif oceanic core complex

Seismic refraction data provide new constraints on the structure of the lower oceanic crust and its variability across the Atlantis Massif oceanic core complex, ∼30°N on the Mid‐Atlantic Ridge. A 40 km‐long spreading‐parallel profile constrains P‐wave velocities to depths of up to ∼7 km beneath the seafloor. Two shorter spreading‐perpendicular lines provide coverage to ∼2 km depth. The anomalous character of the massif's central dome crust is clear compared to the neighboring rift valley and similar‐age crust on the opposite ridge flank. The domal core of the massif, unroofed via detachment faulting, has velocities &gt;7.0 km/s at depths below ∼2.5 km sub‐seafloor, increasing to 7.5–7.8 km/s over the depth range 4.8–6.8 km. Within the core complex, the Moho does not appear to be sharp as no PmP arrivals are observed. Within the axial valley, velocities do not reach mantle‐transition zone values in the uppermost 6 km. We infer that crust there is of normal thickness but that a thinner than average mafic section is present in the central massif. Near IODP Hole U1309D, located on the central dome, there is a low velocity gradient interval at 1–3 km depth with velocities of 6.6–6.8 km/s, that coincides with a 3–5 km wide region where shallower velocities are highest. Given the predominantly gabbroic section recovered from the 1.4 km deep drillhole, this seismic structure suggests that the mafic body extends a few km both laterally and vertically.

Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples

Oceanic core complexes expose lower crustal and upper mantle rocks on the seafloor by tectonic unroofing in the footwalls of large-slip detachment faults. The common occurrence of these structures in slow and ultra-slow spread oceanic crust suggests that they accommodate a significant component of plate divergence. However, the subsurface geometry of detachment faults in oceanic core complexes remains unclear. Competing models involve either: (a) displacement on planar, low-angle faults with little tectonic rotation; or (b) progressive shallowing by rotation of initially steeply dipping faults as a result of flexural unloading (the “rolling-hinge” model). We address this debate using palaeomagnetic remanences as markers for tectonic rotation within a unique 1.4 km long footwall section of gabbroic rocks recovered by Integrated Ocean Drilling Program (IODP) sampling at Atlantis Massif oceanic core complex on the Mid-Atlantic Ridge (MAR). These rocks contain a complex record of multipolarity magnetizations that are unrelated to alteration and igneous stratigraphy in the sampled section and are inferred to result from progressive cooling of the footwall section over geomagnetic polarity chrons C1r.2r, C1r.1n (Jaramillo) and C1r.1r. For the first time we have independently reoriented drill-core samples of lower crustal gabbros, that were initially azimuthally unconstrained, to a true geographic reference frame by correlating structures in individual core pieces with those identified from oriented imagery of the borehole wall. This allows reorientation of the palaeomagnetic data, placing far more rigorous constraints on the tectonic history than those possible using only palaeomagnetic inclination data. Analysis of the reoriented high temperature reversed component of magnetization indicates a 46° ± 6° anticlockwise rotation of the footwall around a MAR-parallel horizontal axis trending 011° ± 6°. Reoriented lower temperature components of normal and reversed polarity suggest that much of this rotation occurred after the end of the Jaramillo chron (0.99 Ma). The data provide unequivocal confirmation of the key prediction of flexural, rolling-hinge models for oceanic core complexes, whereby oceanic detachment faults initiate at higher dips and rotate to their present day low-angle geometries as displacement increases.

Paleomagnetic and rock magnetic results from lower crustal rocks of IODP Site U1309: Implication for thermal and accretion history of the Atlantis Massif

Integrated Ocean Drilling Program (IODP) Expeditions 304/305 recovered a total of 1.4 km sequence of lower crustal gabbroic and minor ultramafic rocks from the Atlantis Massif oceanic core complex on the western flank of the Mid Atlantic Ridge (MAR) at 30°N. We conducted an integrated paleomagnetic and rock magnetic study on this sequence to help address the interplay between magmatism and detachment faulting. Detailed thermal and alternating-field demagnetization results demonstrate that stable components of magnetization of mainly reversed polarity with unblocking temperatures below the Curie temperature of magnetite are retained in gabbroic rocks at IODP Site U1309. Several samples also contain multicomponent remanences of both normal and reversed polarities that were acquired over sharply defined blocking temperature intervals, providing evidence for localized reheating of some intervals during both normal and reversed polarity periods. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier rocks recovered at Site U1309D. The overall magnetic inclination of Hole U1309D is -35°, implying significant (up to ~ 50° counterclockwise, viewed to the north) rotation of the footwall around a horizontal axis parallel to the rift axis (010°) may have occurred. The tectonic rotations inferred by the paleomagnetic data suggest that the original fault orientation dipped relative steeply toward the spreading axis and subsequently rotated to a shallower angle. Coupled with the newly published U–Pb zircon ages for Hole U1309D rocks [Grimes, C.B., John, B.E., Wooden, J.L., 2008. Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from 206Pb/ 238U zircon ages from Atlantis Massif and IODP Hole 1309D, (30°N, MAR). Geochem. Geophys. Geosyst. 9, Q08012. doi:1029/2008GC002063], the new paleomagnetic data provide temporal and thermal constraints on the accretion history of the Atlantis Massif.

Hydration due to high-T brittle failure within in situ oceanic crust, 30°N Mid-Atlantic Ridge

Analysis of an in situ fault zone within the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge) provides clues to the relevant deformation mechanisms and their temporal evolution within oceanic crust. IODP EXP304/305 drilled a succession of gabbroic lithologies to a final depth of 1415 m below the sea floor (mbsf), with very high recovery rates of up to 100% (generally ∼ 80%). We identified an intra-crustal fault zone between 720 and 780 mbsf in a section of massive gabbro, olivine gabbro, oxide gabbro units, and minor diabase intrusions. Of particular interest is the section between 744 and 750 mbsf, which unfortunately was marked by low recovery rates (17%). Electrical borehole-wall images show a 1-m-thick zone of east-dipping fractures within this interval, which is otherwise dominated by N–S dipping structures. Despite the high fracture density in this section, the hole walls are smooth, with rare breakouts, suggesting that the low recovery rate was due to a change in lithology rather than well conditions. The recovered rocks include ultracataclasite and possibly incohesive fault gouge that formed in the upper amphibolite regime, with mostly amphibole infill. Logging data suggest that the gabbroic rocks in this interval are rich in hydrous phases, consistent with increased amounts of amphibole found in the core. Equilibration temperature conditions of about 640 °C were obtained for plagioclase clasts and aluminous actinolite, assuming a pressure of 200 MPa. The permeability of the fault zone is in the range of 10 − 19 to 10 − 17 m 2. Although the permeability appears to be high within the fault zone relative to other parts of the section, it is no higher than that in typical lower crustal material. As a consequence, because brittle failure occurred at high temperatures, the fault zone was subsequently completely sealed by hydrous minerals, thereby preventing further fluid circulation and preserving water in the crust.

Structure, permeability, and strength of a fault zone in the footwall of an oceanic core complex, the Central Dome of the Atlantis Massif, Mid-Atlantic Ridge, 30°N

Fault-zone samples recovered from 159 to 174 m below the seafloor (mbsf) in Hole 1309D from the Integrated Ocean Drilling Program (IODP) at the Atlantis Massif, Mid-Atlantic Ridge, 30°N, include both cataclastic and mylonitic fault rocks, and their gabbroic and ultramafic host rocks. Laboratory experiments determined the strength and permeability of nine fault-zone and host-rock samples with a triaxial apparatus under conditions that simulate present-day, in situ pressure conditions. The permeabilities of cataclasites are ∼10 −18 m 2, while host-rock and mylonite permeabilities are <2 × 10 −19 m 2. When subjected to increasing differential stress, visible fractures increased the permeability of most rock types to >10 −17 m 2. The strength of cataclasites is 260–380 MPa, weaker than that of mylonites of ∼600 MPa. High resolution X-ray computed tomography (HRXCT) and optical microscopy shows that experimentally produced fractures preferentially form interconnected networks within cataclastic matrices. Thus, permeability and strength are a function of the fault-zone microstructure, which evolved during exhumation from upper mantle and lower crustal depths. Localization of cataclastic zones adjacent to altered ultramafic and mylonitic gabbroic rocks likely make the cataclastic portion of this fault a long-lived fluid conduit within the Atlantis Massif Oceanic core complex.

Thematic Section: Geology and geophysics of the Philippine Sea and adjacent areas in the Pacific Ocean

This volume can be regarded as a progress report on the latest survey efforts in the Philippine Sea and adjacent areas in the Pacific Ocean. The Philippine Sea occupies a large part of the western Pacific and the Philippine Sea Plate (PSP) forms the floor of this sea. The Izu-Ogasawara (Bonin), Mariana, Yap and Palau Trenches comprise a continuous, arcuate system which defines the eastern boundary of the PSP, and which is often referred to as the Izu-Bonin-Mariana (IBM) Arc system. The Pacific Plate subducts beneath the PSP along the IBM trenches. The PSP is composed of several large back-arc basins and this region evolved through several episodes of arc formation, rifting and back-arc spreading (Karig 1971). This region has been targeted by several international scientific programs. Early pioneering work of Murauchi et al. (1968) gave us a first glimpse of the crustal structure of the important components of the PSP. Japanese Geodynamics Project (GDP) 1973–78 gave us a first overview of the geology and geophysics of the northern PSP (Shiki 1985). Furthermore, Deep Sea Drilling Project (DSDP) Legs 6, 31, 58, 59 & 60 and Ocean Drilling Program (ODP) Legs 125, 126 & 195 drilled and cored the PSP, and established the current framework of our understanding of PSP structure and evolution (e.g. Karig 1975). Crustal structure of an oceanic island arc was determined for the first time in the IBM arc with improved seismic techniques (Suyehiro et al. 1996). More recently, the IBM arc has been a focus site of the NSF-MARGINS Subduction Factory experiment (http://www.nsf-margins.org/SF/SF.html). This thematic section primarily grew out of the community working for Japan's legal continental shelf survey project in the Philippine Sea and adjacent areas in the Pacific Ocean, aimed to gather information to help define Japan's continental shelf in the context of the United Nations Convention on the Law of the Sea (UNCLOS), which became effective in 1994 (see the latest summary by Hutchinson & Rowland 2007). Japan's legal continental shelf survey project is now (as of summer 2007 [June–August]) in the final stage of field survey, because Japan has to make a submission for extended continental shelves to the Commission on Limits of the Continental Shelf under UNCLOS by May 2009. The USA has also started its UNCLOS program in the Philippine Sea region, consisting of swath-mapping of the sea floor west of the West Mariana Ridge. Japan's continental shelf survey project consists of two major phases. The first phase started in 1983 with swath-mapping of the northern Philippine Sea and adjacent areas in the northwest Pacific Ocean by the Hydrographic and Oceanographic Department of Japan (HODJ). The project used survey vessels Takuyo and Shoyo to acquire narrow multibeam echo sounder SeaBeam swath bathymetry, single- and 24-channel seismic reflection, magnetic and gravity data and dredged bottom materials. Ship tracks were spaced at 9–11 km apart (Ohara et al. 1997). The first phase greatly increased our understanding of the PSP, allowing reconstruction of the first detailed evolutionary history of the Shikoku and Parece Vela Basins (Okino et al. 1994, 1998, 1999) and discovery of Godzilla Mullion (Ohara et al. 2001), the world's largest oceanic core complex (OCC) in the Parece Vela Basin, for example. Along with the HODJ's swath-mapping project, Japan Oil, Gas and Metals National Corporation (JOGMEC) in 1998 started a national project ‘Deep Sea Survey Technologies for Natural Resources in Japan’, sponsored by Ministry of Economy, Trade and Industry (METI). The project's main objective is to gather information about the natural resources in the Philippine Sea and adjacent areas in the Pacific Ocean, and to develop survey skills for studies of deep sea areas. The project acquireda large amount of state-of-the-art multichannel seismic reflection data and cored bottom materials with a wire-line rock drill, Deep-Sea Boring Machine System (D-BMS; Matsumoto & Sarata 1996) in the Philippine Sea and adjacent areas in the Pacific Ocean. After 20 years of HODJ's continued efforts, the Japanese government in 2003 started the second phase of the Law of the Sea-related continental shelf survey. In this phase, the project is directly supervised by the Prime Minister's cabinet office, involving all Japanese institutions that have capabilities of marine geological/geophysical research. The JOGMEC's project on natural resources has become a part of the legal continental shelf survey project in this phase. Along with HODJ and JOGMEC, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Geological Survey of Japan (GSJ-AIST) also took responsibility for the project. The principal structure of the project is as follows: HODJ is responsible for swath-mapping and seismic reflection/refraction surveys using ocean bottom seismometers (OBS). JOGMEC is responsible for multichannel seismic reflection surveys and bottom sampling mainly with D-BMS. JAMSTEC is responsible for seismic reflection/refraction surveys with OBS, especially in the IBM region. GSJ is responsible for geochemical analysis of bottom samples. Many scientists in universities are also working in sample analysis and geophysical interpretation. This thematic section presents parts of the scientific outcomes of Japan's legal continental shelf survey project. Ishihara and Koda compiled the gravity data of the northern Philippine Sea and calculated crustal thickness variations there, revealing very thin crust in the West Philippine Basin, for example. Yamashita et al. determined the deep crustal structure of the Parece Vela Basin related to initial arc rifting using pre-stack depth migration (PSDM) technique on the multichannel seismic reflection data. Ohara et al. found that, using seismic reflection/refraction data, the OCC in the Parece Vela Basin is gabbroic, sharing a common characteristic with the Atlantis Massif OCC in the Mid-Atlantic Ridge. Tsuji et al. applied a specific velocity analysis method to the JOGMEC's multichannel seismic reflection data of the eastern Ogasawara Plateau region in the northwest Pacific Ocean and determined the geometry of the Moho. Higuchi et al. established Cenozoic stratigraphy and sedimentation history of the northern Philippine Sea based on the JOGMEC's multichannel seismic reflection data. They identifed five sediment units, corresponding to the Early Eocene, Middle to Late Eocene, Oligocene, Miocene, and Pliocene-Pleistocene. Takayanagi et al. used cores obtained with D-BMS to determine lithology and age of carbonate deposits in the northern Philippine Sea and northwest Pacific Ocean, revealing that shallow-water carbonates in this region largely deposited during Oligocene time. Usui et al. found that the growth rate of ferromanganese deposits from the northern Philippine Sea is higher than those from the Pacific Ocean. This thematic section is the third Japanese monograph in the English language devoted to the geology and geophysics of the Philippine Sea and adjacent areas in the Pacific Ocean. The first was Tokuyama et al. (1995), published as a monograph of the 5th Japan-USSR symposium in 1985, entitled ‘Geology and geophysics of East Asian Marginal Seas’. The second was Stern and Arima (1998), published in Island Arc as a result of the first IBM Arc system workshop in 1996. It should be noted, however, that there are several other volumes that partly deal with the Philippine Sea: Uyeda et al. (1979), Hayes (1980, 1983), Hilde and Uyeda (1983), Nasu et al. (1985) Taylor and Natland (1995) and Eiler (2003). Our thematic section is the first result of Japan's legal continental shelf survey project and more papers will be published from this project in the future. We thus hope that this thematic issue stimulates interest and further research on the Philippine Sea and adjacent areas in the Pacific Ocean. We thank to the contributors to this thematic section for their time and effort, to the referees for their helpful reviews, and to the Editor-in-Chief, Professor Akira Ishiwatari, for his encouragement and support throughout the preparation of this thematic section.