ALBERT

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: 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: A novel strip-sampling technique has been applied to the 500-m gabbroic section drilled at site 735 during Leg 118. Twenty-two continuous strips of 1.1- to 4.5-m length were cut longitudinally from the core, allowing for a more representative sampling of this section of the deep ocean crust. A full suite of trace element and isotopic (Sr, Nd, Pb, Os, d18O) analyses were conducted on these strip samples; for comparison, analyses were conducted on a small suite of protolith samples, selected for their fresh and unaltered appearance. Amphibole, diopside, and plagioclase from 18 vein samples were also analyzed for Sr and Nd isotopes. Although the evidence for a seawater component in these gabbros is clear (87/86 Sr up to 0.70316; 206/204 Pb up to 19.3; d18O down to 2.0‰; 187/188 Os up to 0.44), the trace element signatures are dominated by magmatic effects (infiltration and impregnation by late-stage melts derived locally or from deeper levels of the crust). The average upper 500 m 735B gabbro section is somewhat lower than average N-MORB in trace elements such as Ba (30%), Nb (50%), U (40%), and heavy REE (Yb and Lu, 30%), but somewhat enriched in others such as La (23%), Ce (24%), Pb (23%), and Sr (40%). Although the section is largely comprised of cumulate gabbros (Natland et al., 1991), and many of the strip samples show marked Sr and Eu anomalies (plagioclase cumulation), the average composition of the total 500 m section shows no Sr or Eu anomalies (〈1%). This implies that there has been local separation of melt and solids, but no large scale removal of melts from this 500-m gabbro section.

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.