ALBERT

Description: We present results of a microprobe investigation of fresh and least-deformed and metamorphosed gabbroic rocks from Leg 118, Hole 735B, drilled on the east side of the Atlantis II Fracture Zone, Southwest Indian Ridge. This rock collection comprises cumulates ranging from troctolites to olivine-gabbro and olivine-gabbronorite to ilmenite-rich ferrogabbros and ferrogabbronorites. As expected, the mineral chemistry is variable and considerably expands the usual oceanic reference spectrum. Olivine, plagioclase, and clinopyroxene are present in all the studied samples. Orthopyroxene and ilmenite, although not rare, are not ubiquitous. Olivine compositions range from Fo85 to Fo30, while plagioclase compositions vary from An70 to An27. Mg/(Mg + Fe2+) of clinopyroxene (mostly diopside to augite) varies from 0.88 to 0.54. Mg/(Mg + Fe2+) of orthopyroxene varies from 0.84 to 0.50. These minerals are not significantly zoned. All mineralogical data indicate that fractional crystallization is an important factor for the formation of cumulates. However, sharp contacts, interpreted as layering boundaries or intrusion margins, suggest polycyclic fractionation of several magma batches of limited volumes. Calculated compositions of magmas in equilibrium with the most magnesian mineral samples at the bottom of the hole represent fractionated liquids through separation of olivine, plagioclase, and clinopyroxene at moderate to low pressures (less than 9 kb). Crystallization of orthopyroxene and ilmenite occurs in the most differentiated liquids. Mixing of magmas having various compositions before entering the cumulate zone is another mechanism necessary to explain extremely differentiated iron-rich gabbros formed in this slow-spreading ridge environment.

Description: Sulfide mineralogy, sulfur contents, and sulfur isotopic compositions were determined for samples from the 500-m gabbroic section of Ocean Drilling Program Hole 735B in the southwest Indian Ocean. Igneous sulfides (pyrrhotite, chalcopyrite, pentlandite, and troilite) formed by accumulation of immiscible sulfide droplets and crystallization from intercumulus liquids. Primary sulfur contents average around 600 ppm, with a mean sulfide d34S value near 0 per mil, similar to the isotopic composition of sulfur in mid-ocean ridge basalt glass. Rocks from a 48-m interval of oxide gabbros have much higher sulfur contents (1090-2530 ppm S) due to the increased solubility of sulfur in Fe-rich melts. Rocks that were locally affected by early dynamothermal metamorphism (e.g., the upper 40 m of the core) have lost sulfur, averaging only 90 ppm S. Samples from the upper 200 m of the core, which underwent subsequent hydrothermal alteration, also lost sulfur and contain an average of 300 ppm S. Monosulfide minerals in some of the latter have elevated d34S values (up to +6.9 per mil), suggesting local incorporation of seawater-derived sulfur. Secondary sulfides (pyrrhotite, chalcopyrite, pentlandite, troilite, and pyrite) are ubiquitous in trace amounts throughout the core, particularly in altered olivine and in green amphibole. Pyrite also locally replaces igneous pyrrhotite. Rocks containing secondary pyrite associated with late low-temperature smectitic alteration have low d34S values for pyrite sulfur (to - 16.6 per mil). These low values are attributed to isotopic fractionation produced during partial oxidation of igneous sulfides by cold seawater. The rocks contain small amounts of soluble sulfate (6% of total S), which is composed of variable proportions of seawater sulfate and oxidized igneous sulfur. The ultimate effect of secondary processes on layer 3 gabbros is a loss of sulfur to hydrothermal fluids, with little or no net change in d34S.

Description: Extended high-iron differentiation occurred while the gabbroic rocks of Hole 735B were undergoing intense ductile and brittle deformation beneath a spreading ridge segment near Atlantis II Fracture Zone, Southwest Indian Ridge. Within the partially molten mass, the deformation formed fissures, cracks, and porphyroclastic to gneissic shear zones with fine-scale porosity structure into which dense, iron-rich liquids or crystal mushes could migrate. The iron-rich liquids differentiated from melts squeezed during the deformation from interstitial spaces in adjacent or nearby olivine gabbros and troctolites, most of which retain a porosity of less than 3%, based on low abundances of TiO2, P2O5, and Zr. Oxide minerals formed at a very late stage from the squeezed liquids and were left in places as extensive ilmenite-rich concentrates, following compaction of the partially molten surrounding rock and continued filter-pressing of residual liquids. The oxide concentrates contain abundant undeformed globular aggregates of pyrite, pyrrhotite, and chalcopyrite and thus crystallized after most ductile deformation had taken place. Most of the content of potassium, phosphorus, zirconium, and other excluded elements squeezed from the rocks was reincorporated into intruding basalt magmas, producing enhancements of the abundances of these elements in drilled basalts and dredged basalt glasses. A semiquantitative liquid line of descent has been estimated for FeO*, TiO2, P2O5, MnO, and sulfur abundances, based on starting glass compositions from basalts dredged from the Atlantis II Fracture Zone and gabbro bulk compositions and mineralogy. Parental melts were sodic and titanium-rich abyssal tholeiites, typical of the region. Four somewhat different parental magma types were involved, based on strontium compositions of the gabbros. These produced variably differentiated gabbros that alternate throughout the section. The oxide gabbros were derived from the two more Sr-rich parental lineages. Progressive iron enrichment is presumed to have taken place to the point of immiscible separation of siliceous and very iron-rich liquids, as indicated by the mineral data and comparisons to experimental analogs. The siliceous component at Hole 735B is represented by late trondhjemitic dikelets in oxide ferrogabbros, whereas the iron-rich liquids probably were the sources of many of the oxide concentrates. Both silicic and iron-rich segregations locally penetrated porosity space in more primitive crystallizing gabbros, reacting with minerals and intercumulus liquids already present. Liquid density calculations indicate that the iron-rich liquids should have sunk through crystal cumulates until porosity-limiting horizons were reached, whereas the siliceous liquids were buoyant. The iron-rich liquids left from immiscible segregation of trondhjemite had high abundances of sulfur (〉3000 ppm) and MnO (〉0.6%), accounting for the consistently high abundances of globular sulfides in the oxide concentrates and the high MnO contents of ilmenites. Deformation accelerated subsolidus recrystallization of the gabbro mass and carried it to virtually every rock. Plagioclase, pyroxenes, and oxide minerals consequently have modified compositions. Pyroxene and two-oxide thermometers indicate that the transition between ductile and brittle deformation took place below about 900°C. Static recrystallization of oxides proceeded in the presence of hydrous fluids until brown amphibole became stable at about 600°C.

Description: 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.

Description: Abundant iron-titanium (Fe-Ti) oxide gabbro, olivine gabbro, and troctolite were drilled at Hole 735B adjacent to the Atlantis II Fracture Zone of the Southwest Indian Ridge during Leg 118. The Fe-Ti oxide gabbro occurs as intrusive bodies into olivine gabbro with very sharp intrusive contacts. The size of the intrusive bodies varies from a millimeter to a few tens of meters. Mineralogical parameters, such as anorthite content of plagioclase and Mg/(Mg+Fe) ratios of mafic minerals exhibit bimodal distributions corresponding to olivine and Fe-Ti oxide gabbros, respectively. When the two major gabbro types are looked at separately, several downhole mineralogical cycles are recognized. The Fe-Ti oxide gabbros exhibit two such cycles with plagioclase becoming more sodic and mafic minerals becoming more iron-rich downward in the drill core. The olivine gabbros and troctolites, however, exhibit two cycles showing an upward increase in sodium in plagioclase and iron in mafic minerals. The mineralogical variations of these gabbros and the intrusive contact relationships probably resulted from downward intrusion of evolved magma into underlying solid or almost solidified olivine gabbros and troctolite. The dense evolved melt at the top of the cumulus pile probably formed from the crystallization of olivine gabbro cumulates followed by extreme fractional crystallization of residual melt in an isolated, ephemeral magma chamber. The interlayered occurrence of evolved and primitive gabbros from Hole 735B represents a typical section of lower ocean crust formed at a very slow spreading ridge.

Description: 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.

Description: 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.

Description: 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.

Description: Rock samples from Hole 735B, Southwest Indian Ridge, were examined to determine the principal vein-related types of alteration that occurred, the nature of fluids that were present, and the temperatures and pressures of these fluids. Samples studied included veined metagabbro, veined mylonitic metagabbro, felsic trondhjemite, and late-stage leucocratic diopside-bearing veins. The methods used were standard petrographic analysis, mineral chemical analysis by electron microprobe, fluid inclusion petrography and analysis by heating/freezing techniques and laser Raman microspectroscopy, and oxygen isotopic analyses of mineral separates. Alteration in lithologic Units I and II (above the level of Core 118-735B-3OR; approximately 140 meters below the seafloor) is dominated by hydration by seawater-derived fluids at high temperature, up to about 700°C, and low water/rock ratio, during and immediately after pervasive ductile deformation. Below Core 118-735B-30R, pervasive deformation is less common, and brittle veining and brecciation are the major alteration styles. Leucocratic centimeter-scale veins, often containing diopside and plagioclase, were produced by interaction of hot (about 500°C) seawater-derived fluid and gabbro. The water/rock ratio was locally high at the veins and breccia zones, but the integrated water/rock ratio for the lower part of the hole is probably low. Accessory hydrous magmatic or deuteric phases formed from magmatic volatiles in some gabbro and in trondhjemite. Most subsequent alteration was affected by fluids that were seawater-derived, based on isotopic and chemical analyses of minerals and analyses of fluid inclusions. Many early-generation fluid inclusions, associated with high-temperature veining, contain appreciable methane as well as saline water. The source of methane is unclear, but it may have formed as seawater was reduced during low water/rock interaction with ultramafic upper mantle or ultramafic and mafic layer 3. Temperatures of alteration were calculated on the basis of coexisting mineral chemistry and isotopic values. Hydrothermal metamorphism commenced at about 720°C and continued to about 550°C. Leucocratic veining took place at about 500°C. Alteration within brecciated horizons was also at about 500° to less than 400°C, and the trondhjemite was altered at about 550° to below 490°C. Pressures calculated from a diopside-bearing vein, based on a combination of fluid inclusion and isotopic analysis, were 90 to 100 MPa. This pressure places the sample, from Core 118-735B-70R in Unit V, at about 2 km below the seafloor.

Description: We redescribed the ~0.5-km gabbro section drilled in Hole 735B at the Ocean Drilling Program Gulf Coast Repository. Included in this work was a redivision and clarification of the location and nature of the major lithologic boundaries and a division of the major units into subunits. In all, we found 495 distinct lithologic intervals in the core. Most of the section consists of a single olivine gabbro body having only minor cryptic variations, which we think represents a small intrusion. At the top of the section, the olivine gabbro is intercalated with a medium- to coarse-grained gabbronorite, which we postulate was intruded by the olivine gabbro. The base of the olivine gabbro has been intruded by troctolites and troctolitic gabbros, which may be the precursors of a major troctolite intrusive body immediately below the base of the hole. This section is variously crosscut by small microgabbro bodies, which are the products of crystallization and wall-rock reaction of small magma bodies that migrated through the olivine gabbro prior to complete solidification. Overall, the plutonic section drilled in Hole 735B is unlike those found at layered intrusions as it lacks evidence for extensive magmatic sedimentation. Rather, it appears to represent a plutonic basement composed of small, relatively short-lived, rapidly crystallized intrusions. This is consistent with the ephemeral volcanism and low rates of magma supply postulated for very slow-spreading ocean ridges. This whole section underwent "syntectonic differentiation": a process in which deformation and compaction of a rigid, partially molten gabbro drove intercumulus melt out of the olivine gabbro into ductile shear zones. Chemical exchange, precipitation of oxides, and trapping of the migrating melt at the end of deformation altered the gabbro in the shear zones to ferrogabbro. These oxide-rich horizons have the potential to be major shallow-dipping seismic reflectors. The largest such zone is 103 m thick and consists of foliated disseminated oxide olivine and oxide olivine gabbros of lithologic Units III and IV. The last igneous event was back-intrusion of trondhjemite veins that formed either by fractional crystallization from the interstitial melt and/or by wall rock anatexis of intruded amphibolites. Alteration and relatively rapid cooling of the gabbro body occurred by penetration and circulation of seawater into the plutonic section caused by thermal contraction and cracking under tensile stress, much as envisaged by Lister (1970). Initially, this circulation was greatly enhanced tectonically by the tensile component provided by lithospheric necking and the formation of brittle-ductile faults beneath the median valley. This circulation was sufficiently pervasive to alter about 25% of all the matrix pyroxene in the body, mostly to amphibole, in the amphibolite facies. Alteration was heaviest in the vicinity of the brittle-ductile faults, where formation of crack networks, cataclasis, and granulation were ongoing processes continuously creating porosity and permeability during deformation. At the end of the brittle-ductile deformation phase, the brittle-ductile fault zones became the most impermeable horizons in the core and suffered little additional alteration. This was due to the extensive syntectonic recrystallization of the matrix mineralogy, which effectively reset the stored elastic thermal strain to zero. In the relatively undeformed horizons, where the stored elastic thermal strain remained substantial, cracking and alteration continued under static conditions as the gabbro cooled, though at lower rates of seawater circulation, following a similar pattern to layered intrusions such as the Skaergaard Complex (e.g., Bird, 1986). Alteration of the massif nearly stopped within the middle amphibolite facies with the cessation of brittle-ductile deformation. Significant lower amphibolite facies diopside-bearing vein networks occur only within the undeformed olivine gabbros in Unit V. Only minor amounts of greenschist and zeolite facies mineralization are found, primarily overprinting early higher-temperature vein and crack networks in the undeformed gabbros. The sharp decrease in alteration below middle amphibolite facies is thought to result from reduced circulation of seawater that accompanied a sharp drop in the available tensile stress for cracking. This probably reflected the transfer of the gabbro body out of the zone of brittle-ductile deformation and lithospheric necking by the formation of a new set of master faults in the median valley closer to the axis of volcanism. Following this, alteration continued under static conditions and accompanying lower rates of seawater circulation with initiation of block uplift of the gabbro massif into the transverse ridge of the Atlantis II Fracture Zone. The last alteration/tectonic event evident within the core is a set of vertically oriented, irregular cracks, frequently covered with smectite. These cracks probably formed during unloading of the gabbros by erosion to sea level after its initial uplift to form an island. They are largely absent from the brittle-ductile deformation zones, indicating that insufficient stored thermal strain was available there (even after cooling from near 500°C to ambient temperature) to overcome the internal strength of the rock under lithostatic load.