Stellar populations and dust in the galaxy NGC 2976, a low-luminosity member of the M81 group

Photographic UBV photometry of NGC 2976, a low-luminosity member of the central M81 group of galaxies, is presented. Young stars in the central disk determine the optical view and the classification of this Sc(pec) galaxy. It is surrounded by a halo of an old population which contains nearly all the mass and half the luminosity of the system. This halo has some properties typical of spheroidal dwarf galaxies: an exponential brightness profile, an ellipticity trend of the isophotes typical of low-mass systems, and mass and luminosity near the upper limit of typical dwarfs. In the central population I disk, star formation proceeds in dense associations scattered irregularly in a broad ringlike region of 1.2 kpc radius just inside the turnover of the rotation curve. This star formation episode may last since some 10⁸ a; it is possibly triggered by gas infall from the interstellar cloud generated during encounters between other group members in the central M81 group. A direct triggering by recent encounters is excluded since NGC 2976 is undisturbed in its outer parts.     

A revised identification of the oldest sea-floor spreading anomalies between Australia and Antarctica

We propose that magnetic anomalies south of Australia and along the conjugate margin of Antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34. The original anomaly identification has two troublesome aspects: (1) it does not account for an “extra” anomaly between anomalies 20 and 21, and (2) it provides no explanation for the rough topography comprising the Diamantina Zone. With our revised identification there is no “extra” anomaly and the Diamantina Zone is attributed to a period of very slow spreading (～4.5mm/yr half rate) between 90 and 43 m.y. The ages bounding the interval of slow spreading (90 and 43 m.y.) correspond to times of global plate reorganizations. Our revised identification opens up the possibility that part of the magnetic quiet zone south of Australia formed during the Cretaceous long normal polarity interval. Breakup of Australia and Antarctica probably occurred sometime between 110 and 90 m.y. B.P. The “breakup unconformity” identified by Falvey in the Otway Basin may correspond to a eustastic sea level change.     

Origin of the Willaumez-Manus Rise,Papua New Guinea

The aseismic Willaumez-Manus Rise on the Bismarck Sea floor separates the Manus Basin from the New Guinea Basin. The rise does not appear to be an extinct spreading axis, or a remnant arc, but may be the result of excess magmatism possibly related to an inferred mantle hot spot beneath St. Andrew Strait. A preferred interpretation is that the rise is the raised edge of the New Guinea Basin, formed in response to a thermal anomaly beneath the extensional Manus Basin which formed later than the New Guinea Basin.     

Element signatures of subduction-zone fluids. An experimental study of the element partitioning (Dfluid/rock) of natural partly altered igneous rocks from the ODP drilling site 1,256

The trace element signatures of fluids were investigated by leaching experiments on natural samples of partly altered mafic igneous rocks recovered from the drilling site 1,256 of ODP Leg 206 on the Cocos plate (Central America). Experiments with ultrapure water were performed at 400 °C/0.4 GPa and 500 °C/0.7 GPa. Both fluids and residual solids were examined to obtain the partition coefficients (D^fluid/rock) of various trace elements. Element partition coefficients (D^fluid/rock) obtained at 500 °C/0.7 GPa are significantly lower compared to results obtained at 400 °C/0.4 GPa, which is in contrast to observations at higher pressures (2.2–6 GPa) and temperatures between 700 and 1,400 °C (Kessel et al. in Earth Planet Sci Lett 237: 873–892, 2005a; Spandler et al. in Chem Geol 239: 228–249, 2007). This finding may indicate a considerable pressure effect on the leaching processes and strongly divergent fluid–rock interactions in the upper part of a subduction zone at 0.4–0.7 GPa compared to deeper subduction areas with higher pressures. Furthermore, this may be interpreted as one of the earliest fractionation processes during the subduction of crustal material.     

The origin of the Naturaliste Plateau,SE Indian Ocean: Implications from dredged basalts

Although a 1972 dredging by USNS Eltanin from the submarine Naturaliste Plateau was reported to yield rocks of continental origin, a re‐examination of the dredge haul shows that the rock clasts are in fact altered tholeiitic basalts. They have affinities both with MOR basalts and, especially, within‐plate basalts. Petrographically they correlate most closely with the Bunbury Tholeiitic Suite on the Australian mainland to the east. The basalts are reworked cobbles in a manganiferous Quaternary slump mass, which contains a quartz‐rich, felsic, detrital mineral suite with a granite‐gneiss provenance. The basalt cobbles were part of a basal conglomerate, which covered large areas of the Plateau. It is suggested that this was laid down from nearby elevated volcanic structures formed during the inception of seafloor spreading and the separation of Greater India from Australia at about 122 Ma BP.     

UBV observation and analysis of VV Orionis

TheUBV photoelectric observation of the eclipsing binary star VV Ori were made at Al-Battani Observatory, Baghdad, Iraq, during nine nights in November and December, 1984 and January, 1985. As a result, three light curves were completed.An analysis of theB andV normal points light curves using the optimization method shows the existence of a slight shift in the primary mid-minimum epoch and the temperature of the secondary component is consistent with B5-type star. A new set of geometrical and physical elements is also presented.     

Ultramafic blocks from the ocean floor southwest of Australia

Samples dredged from the ocean floor near the junction of the Naturaliste Fracture Zone and the Diamantina Zone, 300 km southwest of Cape Leeuwin, Western Australia, contain 3- to 10-cm blocks of Cr-spinel lherzolites and Al-spinel-olivine clinopyroxenites. Both assemblages show textural evidence for deformation and recrystallization, with extensive development of kaersutite in one clinopyroxenite. The blocks are enclosed by clay and Fe-Mn oxide-rich fragmental material, which also contains a diverse detrital mineral suite and microfossils.Diopsides in the lherzolites are Na- and Cr-rich, with marked similarities to those of lherzolite nodules in alkaline basaltic suites. The Al- and Ti-rich diopsides of the clinopyroxenites are more magnesium than those of common pyroxene-rich nodules. The lherzolites are interpreted as upper mantle residues, while the clinopyroxenites probably represent partly recrystallized cumulates from high-temperature basaltic magmas traversing the lherzolite mantle. Both are thought to have been derived from an ultramafic body emplaced into the shallow crust near the Cretaceous/early Tertiary continent-ocean boundary off Australia.