V23B-0625 1340h
Magnetization Of Gabbroic Rocks and Peridotites Recovered From Mid Atlantic Ridge 14N - 16N, ODP Leg 209
ODP Leg 209 was performed from May to July, 2004 at Mid Atlantic Ridge 14N - 16N. The precruise site survey studies performed by Alvine and Shinkai 6500 submersibles indicated the existence of fresh mantle peridotites for the drilling sites. However, the recovered rocks were basalts, gabbroic rocks and peridotites few of which were unaltered. Natural remanent magnetization (NRM) intensities calculated from ODP Leg 209 gabbroic rocks and peridotites indicated very large variations from 0.001 A/m to 50 A/m. Many of the rocks recovered were observed to have drilling-induced remanent magnetizations (DIRM). The DIRM was in general easily removed by alternating field (AF) demagnetization up to 20mT, so for most of the rocks recovered, it was possible to determine stable characteristic remanent magnetization vector directions. The DIRM which was unstable against AF demagnetization showed nearly vertical direction (downward). In many cases, the DIRMs possess more than 90 percent of NRMs of the rocks recovered. The existence of the DIRM in the rocks recovered made it difficult to obtain original natunal remanent magnetization intensities not altered by drilling. So care must be taken in considering in-situ magnetization of ODP Leg 209 gabbroic rocks and peridotites before recovered from the sea-floor, specially as remanent magnetization source for overlying sea-floor spreading magnetic anomalies, though some of the ODP Leg 209 gabbroic rocks and peridotites that were not affected by DIRM indicated reasonably high magnetization of 1 - 2 A/m to contribute to lineated marine magnetic anomalies . Similar DIRMs have been found to exist among gabbroic rocks and peridotites recovered during Legs 118, 147, and 176. Non-magnetic drilling tools including drill pipes, core barrels, BHA and so on would be essential to further studies for magnetizations for deep ocean crustal rcoks and mantle peridotites that contain relatively coarse grain magnetites easily affected by DIRM of essentially isothermal remanent magnetization (IRM) origin.
V23B-0626 1340h
Lithium and strontium isotope compositions of serpentinite-hosted carbonate veins from the MAR (ODP Leg 209): Records of different stages of seafloor metamorphism
Geochemical investigations of carbonate veins hosted in serpentinized peridotites drilled from the Mid-Atlantic Ridge 14$\deg$-16$\deg$N (ODP Leg 209) were conducted to gain insights into temperature and composition of the alteration fluids from which carbonates were precipitated. We have examined carbonate veins that can be grouped into low-T ($<$20$\deg$C) aragonite veins hosted in serpentinite, low-T calcite veins hosted in troctolitic rocks, and high-T (90-220$\deg$C) calcite veins hosted in talc-serpentine schists. The geochemical and radiometric characteristics of these veins is presented in a companion paper (Bach, Paulick, and Rosner) in this session. Here we report on Li and Sr isotope compositions and their relations to age, temperature, and trace element composition. Strontium isotope compositions are similar to, or slightly lower than that of modern seawater. This is consistent with young radiocarbon ages ($<$55 kyrs) and limited Sr exchange between circulating seawater and basement at low temperatures.$\delta$$^{7}$Li values (relative to L-SVEC) of the majority of aragonite samples range between 14 and 20 $\permil$. At sites 1271 and 1274 on rift valley walls of inside corner highs north and south of the 15$\deg$20N fracture zone, the $\delta$$^{7}$Li values of aragonites increase systematically with depth. At site 1274 .$\delta$$^{7}$Li shows a weak positive correlation with 14-C formation ages. Our preliminary interpretation of the Li isotope data is a decrease in the Li isotope fractionation factor as temperatures increase with increasing basement depth. This interpretation would require a fractionation factor at 2$\deg$C of about 1.016 - significantly larger than $\alpha$ =1.011 proposed for biogenic aragonite formation at 25$\deg$C. The large fractionation observed in the upper part of the drilled cores could be due to lower temperature, different precipitation kinetics, variations in fluid chemistry (e.g., strongly variable Mg/Ca), or combination of these effects. Calcite veins from Sites 1271 and 1275 display at least two compositional groups. Low T calcites range $\delta$$^{7}$Li from 22 to 27 $\permil$ and precipitated at T$<$20$\deg$C from a fluid with seawater-like Sr isotope composition. Three high-T calcite veins record an earlier and deeper stage of metamorphism associated with detachment faulting. $\delta$$^{7}$Li values between 3 and 8 $\permil$ correlate with Sr isotope ratios (as low as 87Sr/86Sr=0.70385) and indicate a prominent contribution of Li and Sr leached from the peridotite basement.
V23B-0627 1340h
Extensional Faulting at 15\deg North on the Mid-Atlantic Ridge, ODP Leg 209
At slow spreading ridges, oceanic mantle can be uplifted and brought to the seafloor by extensional faulting, but the style of faulting and mechanisms by which this occurs remain poorly understood. We present data from faults observed in mantle peridotite and gabbro intrusions drilled at six sites on the Mid-Atlantic Ridge near the 15\deg20' Fracture Zone during ODP Leg 209. The data reveal that faulting and significant tectonic rotation can take place either via long-lived (detachment?) faults that are active through ductile and brittle regimes, or via successive series of short-lived (domino) faults. We recognize three styles of faults: a) High-temperature, mylonitic shear zones at Sites 1268, 1272 and 1274 that are overprinted by dominantly static greenschist facies alteration. These textures indicate that ductile faults were active at depth and later became inactive during denudation and are cut by brittle faults at shallow lithospheric levels. b) Brittle and semi-brittle faults that do not directly overprint ductile shear zones. These include partially- to non-cohesive serpentine mud fault gouge in zones ranging from 10 cm to several meters thick (Sites 1268, 1272 and 1274), and cohesive cataclasites and talc/tremolite schists (Site 1275). c) Ductile to brittle faults in peridotites from Sites 1270 and 1271. Strain in the peridotites at both sites appears to have been initially localized into gabbroic veins and dikes at granulite facies, and remained localized in these zones to sub-greenschist facies during long-lived faulting and denudation of peridotite. There is no apparent correlation between the faulting style and seafloor bathymetry. For example, bathymetry of Sites 1270 and 1275 indicate the presence of a detachment fault (oceanic core complex), yet 1270 displays type c faults and 1275 displays type b faults. Peridotite and gabbro from all six sites experienced significant tectonic rotation (50° to 90°) as interpreted from the inclination of the remnant magnetization vector (assuming horizontal axis rotations). This result is consistent with both rotation via long-lived (detachment) faulting, and/or multiple short-lived fault bounded domino systems. Variability in faulting styles and seafloor morphology along with significant tectonic rotations indicate that highly complex faulting may be typical at slow-spreading ridges.
V23B-0628 1340h
New insights into serpentinization at Atlantis Massif, 30$\deg$ N Mid-Atlantic Ridge, using wide-angle seismic method
The Atlantis Massif is an ultramafic core complex that was formed in the last 1.5-2.0 Myr at the intersection of the Atlantis Fracture Zone and the Mid-Atlantic Ridge near 30° N by tectonic extension along a long-lived oceanic detachment fault. The exhumation of deep crustal and upper mantle rocks in the footwall of the fault provides an excellent tectonic window into the oceanic lithosphere. The Atlantis Massif will be the subject of a deep-drilling investigation for upcoming IODP Legs 304 and 305 (November 2004 - February 2005). Near-offset seismic reflection data (offset up to 3 km) across the core complex imaged a reflection at 0.2-0.25 s below the seafloor, which has been interpreted as an older detachment fault [Canales et al., Earth Planet. Sci. Lett., 222, 543-560, 2004]. The application of a non-conventional multichannel seismic (MCS) imaging technique allows us to include wide-angle seismic reflection data (offset up to 6 km) in the imaging process. We find that this reflection is continuous along most of the profiles and is present beneath the exposed detachment surface over an area larger than previously estimated from the near-offset MCS sections. Complementary data from on-bottom shots and ocean bottom seismometers constrain both P and S-wave velocities down to 0.5-0.6 km below the seafloor, at approximately the depth of the widespread reflection. The combined seismic data suggest that the interval between the seafloor and the reflection contains serpentinized peridotite. We quantify the amount and distribution of alteration in this layer by using an effective medium theory, and we interpret the results to shed new light on serpentinization processes at Atlantis Massif. IODP drilling results this winter will allow us to compare our interpretation to ground-truth measurements.
V23B-0629 1340h
Influence of magma injection on faulting and topography at mid-ocean ridges
Fault development at mid-ocean ridges is controlled by a combination of axial thermal structure, the rheology of the crust and mantle, and the rate and distribution of magma injection in the crust. Specifically, dike intrusion influences faulting through its combined effects on crustal temperature and the local stress field. However, while many studies have investigated the sensitivity of faulting to thermal structure, few have examined the mechanical implications of dike injection on geologic time-scales. In this study, we develop a kinematic model for dike intrusion in an extending 2-D elastic-viscoplastic layer. Dike injection is simulated by pressurizing a vertical column of model elements located within the brittle layer. We test the sensitivity of fault development to the geometry of the magma injection zone and find that continual dike intrusion can lead to the formation of a steady-state rift valley, even in the absence of an across-axis variation in lithospheric thickness. In addition, we show that the steady-state graben width increases linearly with the depth to the top of the injection zone, while graben depth reaches a maximum when dike intrusion is isolated in the lower half of the brittle layer. Furthermore, we find that contrary to previous models, periods of magmatic accretion can result in long-lived extension on graben bounding normal faults. These results imply that both the geometry of the intrusion zone, as well as the partitioning of strain between periods of diking and magmatic quiescence, influence the development of topography at mid-ocean ridges. Applying our results to observations of axial morphology and crustal thermal structure inferred from seismic imaging along the Galßpagos Spreading Center, we hypothesize that the variations in axial relief are likely linked to changes in accretion zone geometry.
V23B-0630 1340h
The Geochemical Consequences of Serpentinization and Weathering of Oceanic Peridotites
A suite of partially to completely serpentinized and weathered abyssal peridotites dredged from rift valley walls along the SW Indian Ridge (11-$13\deg$N), the equatorial Mid-Atlantic Ridge, and the Mid-Cayman Rise, and from the Bullard, Islas Orcadas and Bouvet fracture zones on the westernmost SW Indian Ridge have been analyzed to determine chemical changes during peridotite-seawater reactions, their impact on seawater chemistry, and their consequences for subduction zone fluxes. The serpentinites are dominated by lizardite that, together with minor magnetite, replaces olivine and orthopyroxene. Chrysotile occurs in microscopic veins with very fine-grained magnetite or in wispy, white monomineralic veins. At times, orthopyroxene is replaced by tremolite and minor talc. With increasing H$_{2}$O$^{+}$ content (an indicator of extent of serpentinization), Si, Fe, Mg, Zn, Cu and Cr concentrations remain relatively constant, while Ca and Ba contents decrease and V and Ni increase. Comparison of these compositions with those of derived fresh rock precursors suggest mostly minor elemental exchange during serpentinization. Two different types of low-temperature seafloor weathering can be distinguished. Type 1 is dominated by weathering of relict olivine to iddingsite (clay+goethite+aragonite) and results in increases in Ca, Sr, and C, and losses of Mg and Si. Type 2 is characterized by weathering of serpentine and magnetite to talc+hematite$\pm$dolomite, with the chemical consequences being increases in Si, Fe, C, Mn, La, V, Y, Zr, Cu, and Zn and relative losses in Mg and H2O. V/Ti, La/Ce, and Y/Ho correlate well with each other, while Cu and Zn correlate with Zr/Hf (which ranges between 9 and 172). We suggest that scavenging of La, V, Y, and probably Cu, Zn, and Zr occurs during Type 2 slow weathering of serpentinite. Preliminary estimates of the magnitudes and directions of elemental exchange during both serpentinization and low-temperature weathering of peridotites have been derived both by derivation of volume change resulting from mineral reaction equations, and by identification of immobile components for use in calculation of a mass change term. These provide some of the first estimates of peridotite-seawater reactions on global geochemical fluxes.
V23B-0631 1340h
Poisson's Ratio Structure Through a Zone of Exhumed Mantle at the Goban Spur Rifted Margin, Southwest of the UK.
Zones of exhumed mantle have been identified at the west Iberia and Goban Spur rifted margins in the eastern North Atlantic where they form a transition zone up to 130 km wide between thinned continental crust and oceanic crust further seaward. P-wave velocities range from $\sim$4~km~s$^{-1}$ at top basement to 7.2-7.6~km~s$^{-1}$ at 4-6~km depth into basement and taken in isolation are consistent with a wide range of contrasting lithologies. Poisson's ratio may be used as a discriminator between possible compositions as, for P-wave velocities $<$6~km~s$^{-1}$, serpentinite can be clearly distinguised from basalt. We present here the Poisson's ratio structure through the zone of exhumed mantle at the Goban Spur margin up to 4~km into basement. Velocities are constrained by seven ocean-bottom hydrophones and six sonobuoys across this region at a separation of $\sim$15~km; S-wave arrivals are observed on five ocean-bottom hydrophones in this region as P-to-S conversions occurring at top basement. A regularised inversion with smoothing constraints was used to define the P- and S-wave velocity structures individually and the Poisson's ratio computed from these models.
V23B-0632 1340h
Fractionation of Mantle-Derived Melts in the Annieopsquotch Ophiolite, Newfoundland.
The Annieopsquotch ophiolite exposes a tectonically bounded section through c. 5.5 km tholeiitic gabbros, sheeted dykes and pillow basalts. The gabbro zone is divided into three parts. The lower 500 m comprises massive cumulate gabbros with enclaves ($<$50 m) of partly-reacted and digested layered troctolite/leucotroctolite. These are interpreted as relics of the substrate into which the gabbro-sheeted dyke-basalt sequence was emplaced. Overlying this is 1500 m of cumulate olivine gabbros and gabbros which form sills c. 30 m thick that are oriented parallel to the ophiolite pseudostratigraphy. Finer grain sizes at contacts and inward-growing crescumulates indicate cooling from both top and bottom. Gabbros in the sill complex are characterized by cumulate textures with minor intercumulus amphibole and oxides, and rarely show shape-preferred orientations. The upper 500 m of the gabbro zone is dominated by massive gabbros with more abundant interstitial Fe-Ti-oxides, and diabasic pods that grade up into sheeted dykes, suggesting it represents a level of frozen melt. Incompatible element contents of cumulate gabbros in the sill complex generally increase upwards, and modeling indicates that the cumulate sills crystallized from melts with compositions similar to those of the overlying sheeted dykes and basalts. Trapped melt fractions are estimated to be c. 20%, consistent with the absence of compaction structures in these gabbros. Models indicate that the parental magmas of the gabbros, as well as lavas and dykes, can be produced by an average of c. 40-45% fractionation of mantle-derived melts. Both field- and geochemical data thus suggests the Annieopsquotch lower crust records repeated in-situ intrusion and fractionation during upward migration of mantle-derived melts towards the surface, with localized ponding in an axial melt lens at the base of the dyke complex. The similarity in composition and degree of fractionation between the lower and upper crust suggest that the axial melt lens played a limited role in fractionating melt compositions.