Paleomagnetic of ODP Hole 197-1203A

On the basis of studies of Holocene samples,submarine basaltic glass (SBG) is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield Thellier paleointensity data of exceptional quality. To be useful as a recorder of the long-term geomagnetic field, older SBG must retain these optimal properties. Here, we examine this issue through rock magnetic and transmission electron microscope (TEM) analyses of Cretaceous SBG recovered at Ocean Drilling Program Site 1203 (northwestern Pacific Ocean). These SBG samples have very low natural remanent magnetization intensities (NRM <50 nAm**2/g) and TEM analyses indicate a correspondingly low concentration of crystalline inclusions. Thellier experiments on samples with the strongest NRM intensity (>5*10**-11 Am**2) show a rapid acquisition of thermoremanent magnetization (TRM) with respect to NRM demagnetization. Taken at face value,this behavior implies magnetization in a very weak (617 WT) ambient field. But monitoring of magnetic hysteresis properties during the Thellier experiments (on subsamples of the SBG samples used for paleointensity determinations) indicates systematic variations in values over the same temperature range where the rapid TRM acquisition is observed. A similar change in properties during heating is observed on monitor SBG specimens using low-temperature data: with progressive heatings the Verwey transition becomes more distinct. We suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in SBG during the thermal treatments required by the Thellier method,resulting in paleointensity values biased to low values. We further propose that this process is pronounced in Cretaceous and Jurassic SBG (relative to Holocene SBG) because devitrification on geologic time scales (i.e., tens of millions of years) lowers the transition temperature at which the neocrystallization can commence. Magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in SBG during Thellier experiments.

Platinum-group, major, and trace element abundances in ODP Leg 183 basalts

Seventeen basalts from Ocean Drilling Program (ODP) Leg 183 to the Kerguelen Plateau (KP) were analyzed for the platinum-group elements (PGEs: Ir, Ru, Rh, Pt, and Pd), and 15 were analyzed for trace elements. Relative concentrations of the PGEs ranged from ~0.1 (Ir, Ru) to ~5 (Pt) times primitive mantle. These relatively high PGE abundances and fractionated patterns are not accounted for by the presence of sulfide minerals; there are only trace sulfides present in thin-section. Sulfur saturation models applied to the KP basalts suggest that the parental magmas may have never reached sulfide saturation, despite large degrees of partial melting (~30%) and fractional crystallization (~45%). First order approximations of the fractionation required to produce the KP basalts from an ~30% partial melt of a spinel peridotite were determined using the PELE program. The model was adapted to better fit the physical and chemical observations from the KP basalts, and requires an initial crystal fractionation stage of at least 30% olivine plus Cr-spinel (49:1), followed by magma replenishment and fractional crystallization (RFC) that included clinopyroxene, plagioclase, and titanomagnetite (15:9:1). The low Pd values ([Pd/Pt]_pm < 1.7) for these samples are not predicted by currently available Kd values. These Pd values are lowest in samples with relatively higher degrees of alteration as indicated by petrographic observations. Positive anomalies are a function of the behavior of the PGEs; they can be reproduced by Cr-spinel, and titanomagnetite crystallization, followed by titanomagnetite resorption during the final stages of crystallization. Our modeling shows that it is difficult to reproduce the PGE abundances by either depleted upper or even primitive mantle sources. Crustal contamination, while indicated at certain sites by the isotopic compositions of the basalts, appears to have had a minimal affect on the PGEs. The PGE abundances measured in the Kerguelen Plateau basalts are best modeled by melting a primitive mantle source to which was added up to 1% of outer core material, followed by fractional crystallization of the melt produced. This reproduces both the abundances and patterns of the PGEs in the Kerguelen Plateau basalts. An alternative model for outer core PGE abundances requires only 0.3% of outer core material to be mixed into the primitive mantle source. While our results are clearly model dependent, they indicate that an outer core component may be present in the Kerguelen plume source.

Geochemistry and petrology of basalts from Celebes Sea

Major-, trace-, and rare-earth element analyses are presented from a suite of basaltic rocks from the basement of the Celebes Sea. The major elements and trace-elements were determined by X-ray fluorescence techniques, and the rare-earth elements were analyzed by instrumental neutron activation analysis. Compositionally the Celebes Sea basalts are very similar to typical normal mid-ocean ridge basalts, such as those described from the Indian Ocean triple junction. Petrogenetic modeling shows that all of the basalts analyzed can be formed by 10% to 20% partial melting of a light rare-earth element-depleted spinel lherzolite followed by fractional crystallization of mixtures of olivine, Plagioclase, and iron oxide. The Celebes Sea is interpreted as a fragment of the basement of the Jurassic Argo abyssal plain trapped during the Eocene to the north of Australia.

Geochemistry of peridotites from ODP Leg 125 diapiric serpentinites

Refractory spinel peridotites were drilled during Leg 125 from two diapiric serpentinite seamounts: Conical Seamount in the Mariana forearc (Sites 778-780) and Torishima Forearc Seamount (Sites 783-784) in the Izu-Ogasawara forearc. Harzburgite is the predominant rock type in the recovered samples, with subordinate dunite; no lherzolite was found. The harzburgite is diopside-free to sparsely diopside-bearing, with modal percentages of diopside that range from 0% to 2%. Spinels in the harzburgites are chrome-rich (Cr/[Cr + Al] = 0.38-0.83; Fe3+/[Fe3+ + Cr + Al] = 0.01-0.07). Olivine and orthopyroxene are magnesian (Mg# = 0.92). Discrete diopsides reveal extreme depletion of light rare earth elements. Primary hornblende is rare. The bulk major-element chemistry shows low average values of TiO2 (trace), Al2O3 (0.55%) and CaO (0.60%), but high Mg# (0.90). These rocks are more depleted than the abyssal peridotites from the mid-oceanic ridge. They are interpreted as residues of extensive partial melting (= 30%), of which the last episode was in the mantle wedge, probably associated with the generation of incipient island-arc magma, including boninite and/or arc-tholeiite. These depleted peridotites probably represent the residues of melting within mantle diapirs that developed within the mantle wedge.

Mineral composition of mid-ocean-ridge basalts from ODP Leg 187 sites

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°-46°S, 126°-135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a "maximum olivine fractionation" model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth).

Geochemistry of Ontong Java Plateau basalts

Basement rocks from the Ontong Java Plateau are tholeiitic basalts that appear to record very high degrees of partial melting, much like those found today in the vicinity of Iceland. They display a limited range of incompatible element and isotopic variation, but small differences are apparent between sampled sites and between upper and lower groups of flows at Ocean Drilling Program Site 807.40Ar-39Ar ages of lavas from Site 807 and Deep Sea Drilling Project Site 289 are indistinguishable about an early Aptian mean of 122 Ma (as are preliminary data for the island of Malaita at the southern edge of the plateau), indicating that plateau-building eruptions ended more or less simultaneously at widely separated locations. Pb-Nd-Sr isotopes for lavas from Sites 289, 803, and 807, as well as southern Malaita, reflect a hotspot-like source with epsilon-Nd(T) = +4.0 to +6.3, (87Sr/86Sr)T = 0.70423-0.70339, and 206Pb/204Pb = 18.245-18.709 and possessing consistently greater 208Pb/204Pb for a given 206Pb/204Pb than Pacific MORB. The combination of hotspot-like mantle source, very high degrees of melting, and lack of a discernible age progression is best explained if the bulk of the plateau was constructed rapidly above a surfacing plume head, possibly that of the Louisville hotspot. Basalt and feldspar separates indicate a substantially younger age of ~90 Ma for basement at Site 803; in addition, volcaniclastic layers of mid-Cenomanian through Coniacian age occur at DSDP Site 288, and beds of late Aptian-Albian age are found at Site 289. Therefore, at least some volcanism continued on the plateau for 30 m.y. or more. The basalts at Site 803 are chemically and isotopically very similar to those at the ~122 Ma sites, suggesting that hot plume-type mantle was present beneath the plateau for an extended period or at two different times. Surviving seamounts of the Louisville Ridge formed between 70 and 0 Ma have much higher 206Pb/204Pb than any of the plateau basalts. Thus, assuming the Louisville hotspot was the source of the plateau lavas, a change in the hotspot's isotopic composition may have occurred between roughly 70 and 90 Ma; such a change may have accompanied the plume-head to plume-tail transition. Similar shifts from early, lower 206Pb/204Pb to subsequently higher 206Pb/204Pb values are found in several other oceanic plateau-hotspot and continental flood basalt-hotspot systems, and could reflect either a reduction in the supply of low 206Pb/204Pb mantle or an inability of some off-ridge plume-tails to melt refractory low 206Pb/204Pb material.

Composition and structural formulas of ODP Site 200-1224 Eocene ferrobasalt minerals

The ~46-m.y.-old igneous basement cored during Leg 200 in the North Pacific represents one of the few cross sections of Pacific oceanic crust with a total penetration into basalt of >100 m. The rocks, emplaced during the Eocene at a fast-spreading rate (~14 cm/yr; full rate) are strongly differentiated tholeiitic basalts (ferrobasalts) with 7-4.5 wt% MgO, relatively high TiO2 (2-3.5 wt%), and total iron as Fe2O3 (9.1-16.8 wt%). The differentiated character of these lavas is related to unusually large amounts of crystallization differentiation of plagioclase, clinopyroxene, and olivine. The lithostratigraphy of the basement (cored to ~170 meters below seafloor) is divided into three units. The deepest unit (lithologic Unit 3), is a succession of lava flows of no more that a few meters thickness each. The intermediate unit (lithologic Unit 2) is represented by intermixed thin flows and pillows, whereas the shallowest unit (lithologic Unit 1), comprises two massive flows. The rocks range from aphyric to sparsely clinopyroxene-plagioclase-phyric (phenocryst content = <3 vol%) and from holocrystalline to hypohyaline. Chilled margins of pillow fragments show holohyaline to sparsely vitrophyric textures. Site 1224 oxide minerals present a type of alteration not previously seen, where titanomagnetite is only partially destroyed and the pure magnetite component is partially removed from the mineral, leaving, in the most extreme case, a nearly pure ulvöspinel residuum. As a result of this dissolution, iron, mainly in the oxidized state, is added to the circulating solvent fluids. This means that a considerable metal source can result from low-temperature reactions throughout the upper ocean crust. The coarsest-grained lithologic Unit 1 rocks have interstitial myrmekitic intergrowths of quartz and sodic plagioclase (~An12), roughly similar in mineralogy and bulk composition to tonalite/trondhjemite veinlets in abyssal gabbros from the southwest Indian Ocean and Hess Deep, eastern equatorial Pacific. Based on idiomorphic relationships and projections into the simplified Q-Ab-Or-H2O granite ternary system, the myrmekitic intergrowths formed at the same time as, or just after, the oxide minerals coprecipitated and at low water vapor pressure (~0.5 kbar). Their compositions correspond to SiO2-oligoclase intergrowths that are considerably less potassic than dacitic glasses that erupt, although rarely, along the East Pacific Rise or that have been produced experimentally by partial melting of gabbro. Based on the crystallization history and comparison to experimental data, the original interstitial siliceous liquids resulted from late-stage immiscible separation of siliceous and iron-rich liquids. The rare andesitic lavas found along the East Pacific Rise may be hybrid rocks formed by mixing of these immiscible siliceous melts with basaltic magma.

Chemical and isotopic compositions of rocks and minerals from the Ashadze and Logachev hydrothermal fields, Mid-Atlantic Ridge

This study was aimed at reconstructing a sequence of events in the magmatic and metamorphic evolution of peridotites, gabbroids, and trondhjemites from internal oceanic complexes of the Ashadze and Logachev hydrothermal vent fields. Collections of plutonic rocks from Cruises 22 and 26 of R/V "Professor Logachev", Cruise 41 of R/V "Akademik Mstislav Keldysh", and from the Serpentine Russian-French expedition aboard R/V "Pourquoi pas?" were objects of this study. Data reported here suggest that the internal oceanic complexes of the Ashadze and Logachev fields formed via the same scenario in these two regions of the Mid-Atlantic Ridge. On the other hand, an analysis of petrological and geochemical characteristics of the rocks indicated that the internal oceanic complexes of the MAR axial zone between 12°58'N and 14°45'N show pronounced petrological and geochemical heterogeneity manifested in variations in degree of depletion of mantle residues and in Nd isotopic compositions of rocks from the gabbro-peridotite association. Trondhjemites from the Ashadze hydrothermal field can be considered as partial melting products of gabbroids under influence of hydrothermal fluids. It was supposed that presence of trondhjemites in internal oceanic complexes of MAR can be used as a marker for the highest temperature deep-rooted hydrothermal systems. Perhaps, the region of the MAR axial zone, in which petrologically and geochemically contrasting internal oceanic complexes are spatially superimposed, serves as an area for development of large hydrothermal clusters with considerable ore-forming potential.