Magnetic properties of gabbros from ODP Hole 118-735B

A total of 500.7 m of continuous, vertical, oceanic gabbroic section was recovered during Leg 118. The gabbros obtained exhibited various degrees of alteration and deformation, which gave us a good opportunity to study the magnetic properties of oceanic gabbros. Many of these gabbros, which are mainly Fe-Ti oxide gabbros, have strong and unstable secondary magnetic components that were acquired during drilling. Stable inclinations, which are probably in-situ magnetic directions, show a single polarity, with an average value of 66° (±5°), meaning that the studied 501-m oceanic gabbroic block may be a candidate for the source of the marine magnetic anomaly. This may also imply that the metamorphism of oceanic gabbros causing acquisition of magnetization probably occurred within one geomagnetic polarity chron (about 0.3 to 0.7 m.y.) after these gabbros formed at the ridge, leading us to conclude that oceanic gabbros record the so-called Vine-Matthews-Morley type of initial magnetization at the ridge. The average intensity value of stable magnetic components of individual samples, which may be a minimum estimate for remanent magnetizations, is 1.6 A/m. Assuming this magnetic intensity value and a uniform magnetization within an oceanic gabbroic layer having a thickness of 4.5 km (i.e., whole layer 3), it is possible to explain most of the marine magnetic anomaly. If magnetic properties of the samples obtained from Hole 735B are common to oceanic gabbros, layer 3 may contribute more significantly to seafloor spreading magnetic anomalies than previously thought.

Seismic velocities and elastic properties of gabbroic rocks from ODP Hole 118-735B

The nearly continuous recovery of 0.5 km of generally fresh, layer 3 gabbroic rocks at Hole 735B, especially near the bottom of the section, presents scientists an unusual opportunity to study the detailed elastic properties of the lower oceanic crust. Extending compressional-wave and density shipboard measurements at room pressure, Vp and Vs were measured at pressures from 20 to 200 MPa using the pulse transmission method. All of the rocks exhibit significant increases in velocity with increasing pressure up to about 150 MPa, a feature attributed to the closing of microcrack porosity. Measured velocities reflect the mineralogical makeup and microstructures acquired during the tectonic history of Hole 735B. Most of the undeformed and unaltered gabbros are approximately 65:35 plagioclase/clinopyroxene rocks plus olivine or oxide minerals, and the observed densities and velocities are fully consistent with the Voigt-Reuss-Hill (VRH) averages of the component minerals and their proportions. Depending on their olivine content, the predominant olivine gabbros at 200 MPa have average Vp = 7.1 ± 0.2 km/s, Vs = 3.9 ± 0.1 km/s, and grain densities of 2.95 ± 0.5 g/cm3. The less abundant iron-titanium (Fe-Ti) oxide gabbros average Vp = 6.75 ± 0.15 km/s, Vs = 3.70 ± 0.1 km/s, and grain densities of 3.22 ± 0.05 g/cm3, reflecting the higher densities and lower velocities of oxide minerals compared to olivine. About 30% of the core is plastically deformed, and the densities and directionally averaged velocities of these shear-zone tectonites are generally consistent with those of the gabbros, their protoliths. Three sets of observations indicate that the shear-zone metagabbros are elastically anisotropic: (1) directional variations in Vp, both vertical and horizontal and with respect to foliation and lineation; (2) discrepancies among Vp values for the horizontal cores and the VRH averages of the component minerals and their mineral proportions, suggesting preferred crystallographic orientations of anisotropic minerals; and (3) variations of Vs of up to 7%, with polarization directions parallel and perpendicular to foliation. Optical inspection of thin sections of the same samples indicates that plagioclase feldspar, clinopyroxene, and amphibole typically display crystallographic-preferred orientations, and this, plus the elastic anisotropy of these minerals, suggests that preferred orientations are responsible for much of the observed anisotropy, particularly at high pressure. Alteration tends to be localized to brittle faults and brecciated zones, and typical alteration minerals are amphibole and secondary plagioclase, which do not significantly change the velocity-density relationships.

Chemistry of gabbroic rocks from ODP Hole 118-735B

Gabbroic rocks and their late differentiates recovered at Site 735 represent 500 m of oceanic layer 3. The original cooling of a mid-ocean ridge magma chamber, its penetration by ductile shear zones and late intrusives, and the subsequent penetration of seawater through a network of cracks and into highly permeable magmatic hydrofracture horizons are recorded in the metamorphic stratigraphy of the core. Ductile shear zones are characterized by extensive dynamic recrystallization of primary phases, beginning in the granulite facies and continuing into the lower amphibolite facies. Increasing availability of seawater during dynamic recrystallization is reflected in depletions in 18O, increasing abundance of amphibole of variable composition and metamorphic plagioclase of intermediate composition, and more complete coronitic or pseudomorphous static replacement of magmatic minerals. Downcore correlation of synkinematic assemblages, bulk-rock oxygen isotopic compositions, and vein abundance suggest that seawater is introduced into the crust by way of small cracks and veins that mark the end of the ductile phase of deformation. This "deformation-enhanced" metamorphism dominates the upper 180 and the lower 100 m of the core. In the lower 300 m of the core, mineral assemblages of greenschist and zeolite facies are abundant within or adjacent to brecciated zones. Leucocratic veins found in these zones and adjacent host rock contain diopside, sodic plagioclase, epidote, chlorite, analcime, thomsonite, natrolite, albite, quartz, actinolite, sphene, brookite, and sulfides. The presence of zircon, Cl-apatite, sodic plagioclase, sulfides, and diopside in leucocratic veins having local magmatic textures suggests that some of the veins originated from late magmas or from hydrothermal fluids exsolved from such magmas that were subsequently replaced by (seawater-derived) hydrothermal assemblages. The frequent association of these late magmatic intrusive rocks within the brecciated zones suggests that they are both artifacts of magmatic hydrofracture. Such catastrophic fracture and hydrothermal circulation could produce episodic venting of hydrothermal fluids as well as the incorporation of a magmatically derived hydrothermal component. The enhanced permeability of the brecciated zones produced lower temperature assemblages because of larger volumes of seawater that penetrated the crust. The last fractures were sealed either by these hydrothermal minerals or by late carbonate-smectite veins, resulting in the observed low permeability of the core.

Geochemistry and isotopic composition of gabbros from ODP Hole 118-375B

Gabbros drilled from the shallow (720 m) east wall of the Atlantis II transform on the Southwest Indian Ridge (SWIR; 32°43.40', 57°16.00') provide the most complete record of the stratigraphy and composition of the oceanic lower crust recovered from the ocean basins to date. Lithologies recovered include gabbro, olivine gabbro, troctolite, trondhjemite, and unusual iron-titanium (FeTi) oxide-rich gabbro containing up to 30% FeTi oxides. The plutonic rock sequence represents a tholeiitic fractionation trend ranging from primitive magmas having Mg numbers of 67 to 69 that fractionated troctolites, to highly evolved liquids that crystallized two-pyroxene, FeTi oxide-rich gabbros and, ultimately, trondhjemite. Isotopic compositions of unaltered Leg 118 gabbros are distinct from Indian Ocean mid-ocean ridge basalts (MORB) in having higher 143Nd/144Nd (0.51301-0.51319) and lower 206Pb/204Pb values (17.35-17.67); 87Sr/86Sr values (0.7025-0.7030) overlap those of SWIR basalts, but are generally lower than MORBs from the Southeast Indian Ridge or the Rodrigues Triple Junction. More than one magma composition may have been introduced into the magma chamber during its crystallization history, as suggested by the higher 87Sr/86Sr, 206Pb/204Pb, and lower 143Nd/144Nd values of chromium-rich olivine gabbros from the bottom of Hole 735B. Whole-rock gabbro and plagioclase mineral separate 87Sr/86Sr values are uniformly low (0.7027-0.7030), irrespective of alteration and deformation. By contrast, 87Sr/86Sr values for clinopyroxene (0.7025-0.7039) in the upper half of Hole 735B are higher than coexisting plagioclase and reflect extensive replacement of clinopyroxene by amphibole. Hydrothermal veins and breccias have elevated 87Sr/86Sr values (0.7029-0.7035) and indicate enhanced local introduction of seawater strontium. Oxygen- and hydrogen-isotope results show that secondary amphiboles have uniform dD values of -49 to -54 per mil and felsic hydrothermal veins range from -46 to - 77 per mil. Oxygen-isotope data for secondary amphibole and visibly altered gabbros range to low values (+1.0-+5.5 per mil), and O-isotope disequilibrium between coexisting pyroxene and plagioclase pairs from throughout the stratigraphic column indicates that seawater interacted with much of the gabbro section, but at relatively low water/rock ratios. This is consistent with the persistence of low 87Sr/86Sr values, even in gabbros that were extensively deformed and altered.