Whole rock analytical techniques and glas analytical techniques of silicic magmas from the South East Rift, Manus Basin

There has been much recent interest in the origin of silicic magmas at spreading centres away from any possible influence of continental crust. Here we present major and trace element data for 29 glasses (and 55 whole-rocks) sampled from a 40 km segment of the South East Rift in the Manus Basin that span the full compositional continuum from basalt to rhyolite (50-75 wt % SiO2). The glass data are accompanied by Sr-Nd-Pb, O and U-Th-Ra isotope data for selected samples. These overlap the ranges for published data from this part of the Manus Basin. Limited increases in Cl/K ratios with increasing SiO2, La-SiO2 and Yb-SiO2 relationships, and the oxygen isotope data rule out models in which the more silicic lavas result from partial melting of altered oceanic crust or altered oceanic gabbros. Rather, the data form a coherent array that is suggestive of closed-system fractional crystallization and this is well simulated by MELTS models run at 0.2 GPa and QFM (quartz-fayalite-magnetite buffer) with 1 wt % H2O, using a parental magma chosen from the basaltic glasses. Although some assimilation of altered oceanic crust or gabbro cannot be completely ruled out, there is no evidence that this plays an important role in the origin of the silicic lavas. The U-series disequilibria are dominated by 238U and 226Ra excesses that limit the timescale of differentiation to less than a few millennia. Overall, the data point to rapid evolution in relatively small magma lenses located near the base of thick oceanic crust; we speculate that this was coupled with relatively low rates of basaltic recharge. A similar model may be applicable to the generation of silicic magmas elsewhere in the ocean basins.

Paleomagnetic and rock magnetic properties of ODP Leg 173 sites, west Iberia continental margin

We present detailed paleomagnetic and rock magnetic results of rock samples recovered during Leg 173. The Leg 173 cores display a multicomponent magnetization nature. Variations in magnetic properties correlate with changes in lithology that result from differences in the abundance and size of magnetic minerals. The combined investigation suggests that the magnetic properties of the "fresher" peridotite samples from Site 1070 are controlled mainly by titanomagnetite, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of titanomagnetites. These results are in excellent agreement with thermomagnetic characteristics where titanomagnetites with Curie temperature ~580°C were identified from the "fresher" peridotites. In contrast to the magnetic properties observed from the "fresher" peridotites, the low-temperature curves for the "altered" peridotites did not show any Verwey transition. Thermomagnetic analysis using the high-temperature vibrating sample magnetometer also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at ~420°C, probably maghemite. The field- and frequency-dependent relationships are also directly opposite to those in the reversal zone, with no signs of titanomagnetite characteristics. Altogether, these rock magnetic data seem to be sensitive indicators of alteration and support the contention that maghemite is responsible for the magnetic signatures displayed in the altered peridotites of the upper section. The magnetic minerals of the basement rocks from Sites 1068, 1069, and 1070 are of variable particle size but fall within the pseudo-single-domain size range (0.2-14 µm). The average natural remanent magnetization (NRM) intensity of recovered serpenitinized peridotite is typically on the order of 20 mA/m for samples from Site 1068, but ~120 mA/m for samples from Site 1070. The much stronger magnetization intensity of Site 1070 is apparently in excellent agreement with the observed magnetic anomaly high. Nearly half of the NRM intensity remained after 400°C demagnetization, suggesting that the remanence can contribute significantly to the marine magnetic anomaly.

Abundance of calcareous nannofossils across the K/T boundary at ODP Hole 113-690C (Table 1)

A biostratigraphically continuous, but intensely bioturbated, Cretaceous/Tertiary boundary sequence was cored during Ocean Drilling Program (ODP) Leg 113 on Maud Rise (65°S) in the Weddell Sea off East Antarctica. This interval is the first recovered by ODP/DSDP in the South Atlantic sector of the Southern Ocean and offers a unique opportunity to study the nannofossil sequences leading up to and beyond the terminal Cretaceous event at a high southern latitude. The K/T boundary lies just within Chron 29R and is placed at ODP Sample 113-690C-15X-4, 41.5 cm. An iridium anomaly was independently noted at about this level as well. Upper Maestrichtian-lower Paleocene sediments consist mostly of light-colored nannofossil chalks. Dark brown sediments at the base of the Danian (Zone CPla) are characterized by an increased clay content attributed to a drop in calcareous microplankton productivity following the terminal Cretaceous event. Although delineation of the boundary is hampered by intense bioturbation, the sharp color contrast between overlying clay-rich, dark brown chalks of the Tertiary and light cream colored chalks of the Cretaceous aids in the selection of the K/T horizon. Several dark colored burrows sampled at intervals as far as 1.3 m below the boundary and within the light colored Cretaceous chalk were found to contain up to 17% Tertiary nannofossils. Calcareous nannofossils from the boundary interval were divided into three groups for quantitative study. The three groups, "Cretaceous," "Tertiary," and "Survivor," exhibit a sequential change across the boundary with the Cretaceous forms giving way to a Survivor-dominated assemblage beginning at the boundary followed shortly thereafter by the appearance of the Tertiary taxa, Cruciplacolithus and Hornibrookina. The species, H. edwardsii, comprises nearly 50% of the assemblage just above the Zone CPla/CPlb boundary, an abundance not reported elsewhere at this level. Calculation of individual species abundances reveals several additional differences between this K/T boundary interval and those studied from middle and low latitude sections. The percentage of Thoracosphaera is much lower at the boundary in this section and a small form, Prediscosphaera stoveri, is extremely abundant in Cretaceous sediments just below the boundary.

Middle to Late Quaternary sedimentation and rock-magnetic of ODP Site 105-646

We examine rock-magnetic, carbonate, and planktonic foraminiferal fluxes to identify climatically controlled changes of terrigenous and pelagic sedimentation at Ocean Drilling Program (ODP) Site 646 (the Labrador Sea). Terrigenous sediments are brought to the site principally by bottom currents. We use a rock-magnetic parameter sensitive to changes in magnetic mineral grain size, the ratio of anhysteretic susceptibility to low-field magnetic susceptibility (XARM/X), to monitor changes in bottom-current intensity over time, with large values of XARM/X (finer-grained magnetic minerals) indicating weaker bottom currents. A second rock-magnetic parameter, magnetic mineral accumulation rate (KaT) was used to indicate variations in terrigenous flux. Planktonic foraminiferal and carbonate accumulation rates (Pfar and CaC03ar) are used as indicators of pelagic flux. Absolute age assignments are based on correlation between the planktonic foraminiferal oxygen-isotope variations for Site 646 and the SPECMAP master oxygen-isotope curve. Cross-correlation analyses of the parameters that we studied with respect to the SPECMAP curve suggest that from oxygen-isotope stages 21 to 11, sedimentation rate, KaT, X, CaCO3ar, and Pfar were at their maximums, whereas XARM/X was at its minimum during peak interglacials (i.e., 0 k.y. lag time with respect to minimum ice volume). However, all parameters we examined lag behind minimum ice volume from stages 11 to 1, indicating a change in timing of both pelagic and terrigenous fluxes at approximately 400 k.y. BP. The negative correlation coefficient between XARM/X and the SPECMAP curve further suggest that finer-grained magnetic minerals are deposited during glacial periods, which probably reflects weaker bottom currents. The shift observed in the lag times of parameters examined with respect to the SPECMAP record is attributed to a change in significance of orbital parameters. Spectral results exhibit strong power in eccentricity (about 100 k.y.) throughout the record. Kap X, CaCO3flr, and Pfar show significant power in obliquity (about 41 k.y.), whereas XARM/X shows significant power at 73 k.y. from stages 21 to 11. The 73-k.y. period in XARM/X is near the difference tone of obliquity and eccentricity: 1/43-1/102 = 1/69. Kar and XARM/X show power only in eccentricity from stages 11 to 1. X and Pfar show significant power in precession (about 18 and 22 k.y.) whereas CaC03ar has power at 34 k.y, which could be a combination of precession and obliquity. The shift in power of orbital parameters may by attributed to the effect of the about 413-k.y. signal of eccentricity.

Chemical composition and K-Ar age of rocks from the northern Sea of Japan

The Middle Paleozoic complex consists of terrigenous and volcanogenic materials metamorphized in greenschist facies. Clastic rocks have arkosic composition and are formed by alteration of basalts and metamorphic rocks. Metaeffusives were formed from basaltoid products of oceanic tholeiite magma indicating that underwater rise structures of the northern Sea of Japan were emplaced on the oceanic crust.

Petrology, geochemistry and K-Ar Age determination of De Gerlache Seamount basalts

The De Gerlache Seamounts are two topographic highs in the Bellingshausen Sea, southeastern Pacific. Petrological and geochemical studies together with K-Ar age determinations were carried out on four dredged basalt samples collected during a RV Polarstern expedition (ANT-XII/4) in 1995. Minor and trace element composition suggest alkaline basalt compositions. Compared to alkaline basalts of adjacent West Antarctica (the Jones Mountains) and of Peter I Island, the samples have lower mg-numbers, lower Ni and Cr contents and lower high field-strength elements (HFSE)/Nb and large-ion lithophile elements (LILE)/HFSE ratios. Three of the four samples have low K, Rb, and Cs concentrations relative to alkaline basalts. The K-depletion and other elemental concentrations may be explained by 1.1% melting of amphibole bearing mantle material. Additionally, low Rb and Ba values suggest low concentrations of these elements in the mantle source. K-Ar age determinations yield Miocene ages (20-23 Ma) that are similar in age to other alkaline basalts of West Antarctica (Thurston Island, the Jones Mountains, Antarctic Peninsula) and the suggested timing of onset of Peter I Island volcanism (~10-20 Ma). The occurrence of the DGS and Peter I Island volcanism along an older but reactivated tectonic lineation suggests that the extrusions exploited a zone of pre-existing lithospheric weakness. The alkaline nature and age of the DGS basalts support the assumption of plume activity in the Bellingshausen Sea.

(Table 1) K-Ar age and inert-gas-isotopes at DSDP Holes 68-501, 69-504B and 69-505B

Conventional K-Ar ages have been determined and inert-gas abundances have been measured on representative samples of altered rocks from Deep Sea Drilling Project Holes 501, 504B, and 505B in an attempt to correlate their degree of alteration with inert-gas and K-Ar data. Samples taken from the first 60 meters below the sediment/basalt interface give significantly higher ages than would be expected from the magnetic stratigraphy, though at greater depths the calculated ages are in broad agreement with the expected age. The inert gas ratios 20Ne/36Ar, 36Ar/84Kr, and 84Kr/130Xe also show a marked discontinuity at the 60-meter depth, and all these effects are interpreted as being a consequence of low-temperature alteration produced by burial metamorphism and by interaction with sea water (halmyrolysis).

Rock magnetic properties of ODP Leg 134 sites

During Leg 134, the influence of ridge collision and subduction on the structural evolution of island arcs was investigated by drilling at a series of sites in the collision zone between the d'Entrecasteaux Zone (DEZ) and the central New Hebrides Island Arc. The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain that extends from the northern New Caledonia Ridge to the New Hebrides Trench. High magnetic susceptibilities and intensities of magnetic remanence were measured in volcanic silts, sands, siltstones, and sandstones from collision zone sites. This chapter presents the preliminary results of studies of magnetic mineralogy, magnetic properties, and magnetic fabric of sediments and rocks from Sites 827 through 830 in the collision zone. The dominant carrier of remanence in the highly magnetic sediments and sedimentary rocks in the DEZ is low-titanium titanomagnetite of variable particle size. Changes in rock magnetic properties reflect variations in the abundance and size of titanomagnetite particles, which result from differences in volcanogenic contribution and the presence or absence of graded beds. Although the anisotropy of magnetic susceptibility results are difficult to interpret in terms of regional stresses because the cores were azimuthally unoriented, the shapes of the susceptibility ellipsoids provide information about deformation style. The magnetic fabric of most samples is oblate, dominated by foliation, as is the structural fabric. The variability of degree of anisotropy (P) and a factor that measures the shape of the ellipsoid (q) reflect the patchy nature of deformation, at a micrometer scale, that is elucidated by scanning electron microscope analysis. The nature of this patchiness implies that deformation in the shear zones is accomplished primarily by motion along bedding planes, whereas the material within the beds themselves remains relatively undeformed.

Determination of heavy metals on the Rock River through the analysis of sediments /

"HWRIC project 88-043."

Merit, red rock, and potomac : tomato varieties adapted to mechanical harvesting /

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