David M. Christie

Petrography of the dike‐gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes

[1]xa0The Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) three-leg campaign at Site 1256 (Leg 206, Expeditions 309 and 312) provides the first continuous in situ sampling of fast spread ocean crust from the extrusive lavas, through the sheeted dikes, and down into the uppermost gabbros (Cocos plate; East Pacific Rise; eastern equatorial Pacific). The lowest ∼60 m of the dikes above the gabbros were transformed to “granoblastic dikes” through a metamorphic overprint characterized by two-pyroxene domains formed under granulite-facies conditions. Equilibrium temperatures estimated by the two-pyroxene thermometer range between 930°C and 1050°C, implying that conditions within the granoblastic zone were appropriate for hydrous anatexis, with the potential to generate partial melts of trondhjemitic composition. The downhole evolution of the granoblastic overprint is expressed by systematic changes in texture, phase composition, and calculated equilibrium temperature, consistent with thermal metamorphism by a deeper heat source. Thermal modeling implies a long-lasting heat source located beneath the granoblastic dikes, providing thermal energy over several thousands of years. The most likely such source is a steady state, high-level axial magma chamber (AMC) located at the base of the sheeted dike section. We interpret the interval of granoblastic dikes as part of a dynamic conductive boundary overlying the AMC.

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

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 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 degrees 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 similar to 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.

Mechanisms of geochemical and geophysical variations along the western Galapagos Spreading Center

(1) Improved insights into the processes of hot spot-ridge interaction along the Galapagos Spreading Center (GSC) are revealed by geochemical data between � 91Wand 98W. Principal components analysis reveals that >87% of the total isotopic variability can be explained with only two mantle source components. The depleted component has lower ratios of highly to moderately incompatible elements, higher Nd isotopic ratios, and lower Sr and Pb isotopic ratios. The second component is relatively enriched in incompatible elements, has more radiogenic Pb and Sr and less radiogenic Nd, and is comparable to the C or common mantle component observed at many locations in the Pacific. The enriched components signature is strongest nearest the hot spot at � 92W and diminishes with distance from the hot spot to 95.5W. Near 95.5W, lava compositions change sharply, becoming dominated by the depleted component and remaining so farther west, to 98W. Thus, the Galapagos hot spot most clearly influences the composition of the GSC between 91W and 95.5W. The depleted component between 91W and 98W differs from that evident at the Galapagos Archipelago, along the GSC east of 91W, and along the East Pacific Rise. This suggests some form of compositional zoning in the regional mantle. If the depleted materials are intrinsic to the Galapagos mantle plume, then the plume is bilaterally zoned and feeds a depleted component to the GSC at 91W-98W that is distinct from the depleted material elsewhere in the region. This possibility is supported by melting models in which the Galapagos plume is a uniform mixture of a depleted matrix and fine-scale enriched veins. The expression of the more fusible veins is predicted to be enhanced nearest the hot spot (� 92W), where plume-like upwelling drives rapid flow and melting deeper in the melting zone (where the veins are melting). With increasing westward distance from the hot spot, the deep, plume-driven flow is predicted to decrease, as does the expression of the enriched veins in lava compositions. The model therefore adequately explains the compositional and crustal variations from 92W to 95.5W. The average model composition of the plume in this region does not differ significantly from that of the ambient mantle beneath other ridges not influenced by hot spots.

Thermal regime of the Southeast Indian Ridge between 88°E and 140°E: Remarks on the subsidence of the ridge flanks

[1]xa0The flanks of the Southeast Indian Ridge are characterized by anomalously low subsidence rates for the 0–25 Ma period: less than 300 m Ma−1/2 between 101°E and 120°E and less than 260 m Ma−1/2 within the Australian-Antarctic Discordance (AAD), between 120°E and 128°E. The expected along-axis variation in mantle temperature (∼50°C) is too small to explain this observation, even when the temperature dependence of the mantle physical properties is accounted for. We successively analyze the effect on subsidence of different factors, such as variations in crustal thickness; the dynamic contribution of an old, detached slab supposedly present within the mantle below the AAD; and depletion in ϕm, a parameter here defined as the “ubiquitously distributed melt fraction” within the asthenosphere. These effects may all contribute to the observed, anomalously low subsidence rate of the ridge flanks, with the most significant contribution being probably related to the depletion in ϕm. However, these effects have a deep-seated origin that cannot explain the abruptness of the transition across the fracture zones that delineate the boundaries of the AAD, near 120°E and near 128°E, respectively.

Mission Moho: Formation and evolution of oceanic lithosphere

The formation and evolution of the oceanic lithosphere is the dominant process in the chemical differentiation and physical evolution of our planet. Plate tectonic processes completely repave the ocean basins every 100–200 million years. Lithosphere formation encompasses the transfer and transformation of material and energy from Earths mantle to the crust and from the crust to the ocean and atmosphere. Independent of sunlight, the evolving ocean crust supports life in unique seafloor and subseafloor habitats that may resemble Earths earliest ecosystems. From its formation until its return to the mantle by subduction, the evolving oceanic lithosphere interacts with seawater, sequesters water and other materials, and ultimately recycles them back into the mantle.

Geological, Biological, Chemical, and Physical Interactions in Back‐Arc Spreading Systems—An Introduction

Each chapter of this volume is based on a presentation at the international Theoretical Institute, Interactions among Physical, Chemical, Biological, and Geological Processes in Back-arc Spreading Systems, which was jointly sponsored by Ridge 2000 and InterRidge, on Jeju Island, South Korea, in May 2004. The volume is divided into two sections. Papers in the first section synthesize the state of knowledge of back-arc basins in general, from the perspectives of a broad range of disciplines. The papers are arranged according to their subject matter, roughly following the flow of energy and material from their origins in the mantle, through the magmatic systems that create and modify the oceanic crust, to the hydrothermal and biological systems that derive their existence from magmatic heat and chemicals. Papers in the second section are case studies, focused on the geology and geophysics of individual back-arc basins. These include two back-arc systems that are more than usually complex, as a result of their more complex tectonic settings.