ALBERT

Description: The European Project for Ice Coring in Antarctica (EPICA) focuses on the drilling of two deep ice cores, the first at Dome C and the second at Kohnen station (75°00' S, 0°04' E) in Dronning Maud Land (DML). This paper deals with stable-isotope records from ice cores drilled in DML. In the first season, the deep EPICA DML core reached a depth of 450 m, recovering ice approximately 7000 years old. Generally, the d18O record indicates a stable Holocene climate and shows low variability. However, during the last 4000 years (based on a preliminary time-scale) the d18O values decrease continuously by about 0.6%, and the deuterium excess values increase by about 0.5%. The correlation between d18O and the deuterium excess d is investigated for a 50m long core section and the near-surface snow. High-pass filtered profiles are positively correlated, whereas the correlation between low-pass filtered profiles is negative. A post-depositional effect due to diffusion processes can be seen in a sub-annually resolved profile from snow-pit samples. Changes in the seasonality of the evolution of the snow cover and the consequences for stable-isotope content are demonstrated with data from ice core B31.

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.