ALBERT

Description: Green biotite-like material occurring in Haast Schist at Brighton, eastern Otago, consists of biotite and a kaolin-group mineral interlayered on a 1 μm scale. Electron probe analyses of composite grains show K2O contents of 4.4 to 8.5 wt.%, in part reflecting the kaolin content, and in part, leaching of K+ ions. Phengitic muscovite shows similar but less extreme effects. Kaolin, siderite, calcite, and titanium oxides have formed in the rock during hydrothermal alteration of epidote, sphene, and biotite by carbonated waters. Potassium leaching due to surficial weathering processes appears to be widespread in biotites from the schists of eastern Otago, and biotites (hydrobiotites?) with K2O as low as 4 wt. % are reported. Similar material with 0.8 to 2.4% K2O and with Na2O about 1% is thought to be more highly degraded biotite. Microprobe analyses of kaolin and chlorites from associated rocks are given in addition to those of ‘biotite’ and phengitic muscovite. Analyses of sheet biotite and sheet muscovite from the Mataketake Range pegmatites in the highest grade parts of the Haast Schist terrane are presented for comparison. The study shows that biotite is more widespread in eastern Otago than was previously thought, and that almandine-rich garnet is not confirmed down-grade of the first appearance of biotite in eastern Otago.

Description: The significance of metastable and equilibrium pyroxene fractionation trends in tholeiitic magmas is discussed, and the development of sector zoning and skeletal growth are considered in relation to Nakamura's (1973) hypothesis of protosites on growing clinopyroxene crystal surfaces. At the Galapagos Spreading Centre (GSC) basalts and ferrobasalts investigated behave paradoxically in that the slower-cooled basalts follow the quench trend while the faster-cooled ferrobasalts define a much closer approach to the equilibrium trend.It is concluded that under metastable conditions fractionation trends in Ca-rich pyroxenes may be strongly influenced by textural features such as cotectic crystallization of plagioclase and the onset of liquid immiscibility, the latter leading to the development of strongly Fe-enriched ferroaugites. Even under plutonic conditions metastable crystallization can develop and severely reduce the pyroxene miscibility gap. A model for metastable crystallization is presented. These considerations are then addressed to the remarkable correspondence of the Skaergaard and Thingmuli pyroxene fractionation trends.

Description: Analyses of stable isotopes of monospecific planktonic foraminifers (G. quadrilobatus group) and monogeneric benthic foraminifers (Cibicidoides spp.) from late Neogene Atlantic Site 502 and Pacific Site 503 were conducted in order to determine the paleoceanographic changes resulting from the late Neogene uplift of the Panama Isthmus and from climatic cooling. In general, results at each site are similar to those from previous studies for the late Miocene and late Pliocene time interval, documenting the late Miocene (6 Ma) shift in carbon isotopes and the inferred growth of permanent Northern Hemisphere continental ice sheets beginning about 3.2 Ma. Comparison of Atlantic-Pacific planktonic-benthic isotope data for four stratigraphic intervals (~6-8, ~5-6, ~3-5, and ~2-3 Ma) suggests that increasing isolation of Atlantic and Pacific low-latitude waters may be related to the emergence of the Panama Isthmus. The contrast between Atlantic and Pacific benthic foraminiferal d13C increased in two steps from 0.60 per mil to 1 per mil (the modern contrast) at about 6 Ma and 3 Ma. The first increase (0.15 per mil) may represent the end of previously limited deep-water communication between the Atlantic and Pacific at the present location of Panama. The second increase (0.25 per mil) may be due to increased production of North Atlantic Deep Water. This probably reflects the development of modern deep-sea circulation. The d18O of planktonic foraminifers begins to increase in Atlantic Site 502 at 4.2 Ma and may reflect the increasing salinity of the North Atlantic Ocean arising from diminishing surface-water exchange across Panama. This increase is clearly shown by contrasting the d18O of Atlantic and Pacific planktonic foraminifers, as well as the d18O of planktonic and benthic foraminifers at Site 502. This inferred increase in surface-water salinity begins at the time of increasing provinciality of Atlantic and Pacific planktonic foraminifers.

Description: The usefulness of cosmogenic beryllium-10 (half life = 2.5 Ma) for studying the rates of accumulation of ferromanganese nodules is reported based on its measured depth distribution in the top 20 mm of these deposits. Accumulation rates have been obtained in the range of 1 to 4 mm/Ma, which are in good agreement with rates determined using the 230Th method on the same nodules. The use of 10Be offers promise in extending the dating to the outer few cm of the nodules. This contrasts with conventional methods using 230Th and 231Pa isotopes which, due to their comparatively short half lives, are limited to a few mm at the surface of the nodules. Detailed studies of 10Be in the manganese deposits coupled with other trace element analyses should prove valuable in understanding the processes of formation of these deposits and the chronology of events recorded by them.

Description: Deep-sea echinoderms of the Tongue of the Ocean, Bahama Islands, have been studied, using trawled collections made by the University of Miami together with observations from the deep submersible Alvin. Transect runs in the submersible permitted studies of population densities and behaviour of approximately 38 species of larger invertebrates, of which 27 were echinoderms. Several echinoderm species show a patchy distribution pattern which is apparently not related to available food resources. Some species are exclusively herbivores, feeding on fragments of turtle grass, Thalassia testudinata and sargassum weed, Sargassum spp. Feeding habits of some Tongue of the Ocean echinoderms are compared with those of the same species from further north, where supplies of plant material are not nearly so abundant.

Description: Scientists from the Woods Hole Oceanographic Institution, the U.S. Navy, the State University of New York at Albany, Wesleyan University, Nine dives in the research submersible ?Alvin? were made into Great Abaco Submarine Canyon to depths ranging from 1850 to 3666 m. Our observations indicate that the walls of this canyon are distinctly terraced, consisting of nearly vertical to overhanging rock cliffs and intervening, less steep sediment-covered slopes. The wall rock consists mostly of massive, shallow-water limestones and dolostones of Cretaceous age, coated on exposed surfaces with manganese oxides. These rocks are heavily jointed/fractured and thus very blocky to angular in appearance, with sponges and other sessile organisms commonly attached. Talus slopes and sedimentary breccia deposits containing angular boulders are present at the base of these steep escarpments. Short-term bottom current measurements in the axis of the eastern part of the canyon indicate that currents are relatively weak, reaching velocities of only 10 cm/sec. This relatively placid setting is further corroborated by the abundance of turtle grass (Thalassia) found along the canyon axis. However, abundant subdued, symmetrical ripple marks and large scour depressions at the base of boulders, indicate that high-energy events sporadically impact the canyon axis. Contemporary erosional activity along the axis of the western (headward) part of the canyon appears to be more significant, as evidenced by asymmetrical ripple marks, sand waves and bioerosion. Great Abaco Canyon has evolved with time via a variety of processes, including: (1) faulting: (2) subsidence; (3) defacement; and (4) erosional down-cutting. The location, orientation and initiation of this canyon appear to be structurally controlled by the Great Abaco Fracture Zone during pre-Santonian time. Regional subsidence during the Mesozoic allowed the walls of Great Abaco Canyon to build vertically by accretion of shallow-water limestones, whereas joint-controlled defacement has widened the canyon while maintaining steep walls. Erosional down-cutting in the canyon axis by carbonate sediment gravity flows also appears to have been important episodically, particularly during the Miocene and Pleistocene.