Magnetic properties of igneous rocks recovered at ODP Leg 127 sites

Measurements of natural remanent magnetization (NRM), initial susceptibility (K), anisotropy of magnetic susceptibility, frequency dependent susceptibility (Xfd), and viscous remanent magnetization (VRM) are reported from volcanic rocks recovered during ODP Leg 127 in the Japan Sea. The results indicate a significant difference between the basalts drilled in the Yamato Basin (Site 794 and 797) and in the Japan Basin (Site 795). The Koenigsberger ratios (Q) show very low values in the Yamato Basin attesting that the remanence is not dominant over the induced magnetization. This evidence could explain why no magnetic anomaly pattern has been recognized in this basin. Experiments of VRM acquisition and decay show that both the processes are multistage with the acquisition process proceeding more rapidly and deviates more from a log (t) law than the corresponding decay. The sediments interlayered with the basalts in the acoustic basement of the Yamato Basin show processes of remagnetization related to the emplacement of the dikes. Temperatures of heating between 200° and 250°C were estimated from the different unblocking temperatures of the two components of magnetization.

Mineral an chemical composition of basalts from DSDP Sites 417 and 418, West Atlantic Ocean

Major element composition ranges of closely associated basalt glass-whole rock pairs from individual small cooling units approach the total known range of basalt glass and whole rock compositions at IPOD sites 417 and 418. The whole rock samples fall into two groups: one is depleted in MgO and distinctly enriched in plagioclase but has lost some olivine and/or pyroxene relative to its corresponding glass; and the other is enriched in MgO and in phenocrysts of olivine and pyroxene as well as plagioclase compared to its corresponding glass. By analogy with observed phenocryst distributions in lava pillows, tubes, and dikes, and with some theoretical studies, we infer that bulk rock compositions are strongly affected by phenocryst redistribution due to gravity settling, flotation, and dynamic sorting after eruption, although specific models are not well constrained by the one-dimensional geometry of drill core. Compositional trends or groupings in whole rock data resulting from such late-stage processes should not be confused with more fundamental compositional effects produced in deep chambers or during partial melting.

Geochemistry of silicate melt inclusions from ODP Holes 137-504B and 140-504B

Geochemical data from plagioclase-hosted silicate melt inclusions from Leg 140, Hole 504B diabase dikes are reported. Hand-picked plagioclase grains were heated to 1260°-1280°C to remelt the glass inclusions and to infer trapping temperatures. The samples were then polished to expose the inclusions, which were analyzed by electron and ion microprobes. Inclusion compositions are mainly in equilibrium with the host plagioclase and are more depleted in incompatible elements than the host rock. Simple crystal-liquid equilibrium calculations show that the melt inclusions could have been in equilibrium with depleted abyssal peridotite diopsides, whereas whole-rock basalt compositions generally could not have been. The melt inclusions are significantly more depleted than normal (N-type) mid-ocean-ridge basalt (MORB) and are consistent with being produced by 8%-16% incremental or open-system melting with 2% residual porosity in the peridotite source. These magmas were formed during pressure-release melting of the mantle over a range of depths between 30 and 15 km.

(Table T1) Geochemistry of calcium carbonate veins from ODP Holes 206-1256C and 206-1256D

Drilling a complete deep crustal section has been a primary yet elusive goal since the inception of scientific ocean drilling. In situ ocean crustal sections would contribute enormously to our understanding of the formation and subsequent evolution of the ocean crust, in particular the interplay between magmatic, hydrothermal, and tectonic processes. Ocean Drilling Program (ODP) Leg 206 was the first of a multileg project to drill an in situ crustal section that penetrated the gabbroic rocks of the Cocos plate (6°44.2'N, 91°56.1'W), which formed ~15 m.y. ago on the East Pacific Rise during a period of superfast spreading (>200 mm/yr) (Wilson, Teagle, Acton, et al., 2003, doi:10.2973/odp.proc.ir.206.2003). During Leg 206, the upper 500 m of basement was cored in Holes 1256C and 1256D with moderate to high recovery rates. The igneous rocks recovered are predominantly thin (10 cm to 3 m) basalt flows separated by chilled margins. There are also several massive flows (>3 m thick), although their abundance decreases with depth in Hole 1256D, as well as minor pillow basalts, hyaloclastites, and rare dikes. The lavas have been slightly (<10%) altered by low-temperature hydrothermal fluids, which resulted in pervasive dark gray background alteration and precipitation of saponite, pyrite, silica, celadonite, and calcium carbonate veins. Here we present a geochemical analysis of the CaCO3 recovered from cores. The compositions of ridge flank fluids within superfast spreading crust will be determined from these data, following the approach of Hart et al. (1994, doi:10.1029/93JB02035), Yatabe et al. (2000, doi:10.2973/odp.proc.sr.168.003.2000), and Coggon et al. (2004, doi:10.1016/S0012-821X(03)00697-6).

Geochemistry of lower sheeted dike complex samples of ODP Holes 137-504B and 140-504B

Rocks of the lower sheeted dike complex of Hole 504B sampled during Leg 140 were analyzed for major and trace element compositions to investigate the effects of igneous processes and hydrothermal alteration on the compositions of the rocks. The rocks are relatively uniform in composition and similar to the shallower dikes. They are moderately evolved mid-ocean-ridge basalts (MORB) with relatively high MgO (7.9-10 wt%) and Mg# (0.60-0.70), and have unusually low incompatible element contents (TiO2 = 0.42-1.1 wt%, Zr = 23-62 ppm). Discrete compositional intervals in the hole reflect varying degrees of differentiation, and olivine and plagioclase accumulation in the rocks, and may be related to injection of packets of dikes having similar compositions. Systematic depletions of total REE, Zr, Y, TiO2, and P2O5 in centimeter-size patches are most likely attributed to exclusion of highly differentiated, late-stage interstitial liquids from small portions of the rocks. The rocks exhibit increased H2O+ reflecting hydrothermal alteration. Replacement of primary plagioclase by albite and oligoclase led to local gains of Na2O, losses of CaO, and slightly positive Eu anomalies. Some mobility of P2O5 led to minor increases and decreases in P2O5 contents, and some local mobility of Ti may have occurred during alteration of titanomagnetite to titanite. Higher temperatures of alteration in the lower sheeted dikes led to breakdown of pyroxene and sulfide minerals and losses of Zn, Cu, and S to hydrothermal fluids. Later addition of anhydrite to the rocks in microfractures and replacing plagioclase caused local increases in sulfur contents. The lower sheeted dikes are a major source of metals to hydrothermal fluids for the formation of metal sulfide deposits on and within the seafloor.

Element analyses of DSDP/ODP Hole 504B basalt and breccia (Table 2)

Geochemical well logs were used to measure the dry weight percent oxide abundances of Si, Al, Ca, Mg, Fe, Ti, and K and the elemental abundances of Gd, S, Th, and U at 0.15-m intervals throughout the basement section of Hole 504B. These geochemical data are used to estimate the integrated chemical exchange resulting from hydrothermal alteration of the oceanic crust that has occurred over the last 5.9 Ma. A large increase in Si in the transition zone between pillows and dikes (Layers 2B and 2C) indicates that mixing of hot, upwelling hydrothermal fluids with cold, downwelling seawater occurred in the past at a permeability discontinuity at this level in the crust, even though the low-to-high permeability boundary in Hole 504B is now 500 m shallower (at the Layer 2A/2B boundary). The observations of extensive Ca loss and Mg gain agree with chemical exchanges recorded in the laboratory in experiments on the reactions that occur between basalt and seawater at high temperatures. The K budget requires significant addition to Layer 2A from both high-temperature depletion in Layers 2B and 2C and low-temperature alteration by seawater. Integrated water/rock ratios are derived for the mass of seawater required to add enriched elements and for the mass of hydrothermal fluid required to remove depleted elements in the crust at Hole 504B.

(Table 1) Sulfur contents and isotope composition in rocks and minerals from DSDP/ODP Hole 504B

DSDP Hole 504B is the only hole in oceanic crust to penetrate through the volcanic section and into hydrothermally altered sheeted dikes. We have carried out petrologic and sulfur isotopic analyses of sulfide and sulfate minerals and whole rocks from the core in order to place constraints on the geochemistry of sulfur during hydrothermal alteration of ocean crust. The nearly 600 m-thick pillow section has lost sulfur to seawater and has net d34S = -1.8 per mil due to degassing of SO2 during crystallization and subsequent low temperature interaction with seawater. Hydrothermally altered rocks in the 200 m-thick transition zone are enriched in S and 34S (4300 ppm and +3.0 +/-1.2 per mil, respectively), whereas the more than 500 m of sheeted dikes contain 720 ppm S with d34S = +0.6 +/-1.4 per mil. These data are consistent with the presence of predominantly basaltic sulfur in hydrothermal fluids deep in the crust: following precipitation of anhydrite during seawater recharge, small amounts of seawater sulfate were reduced at temperatures >250°C through conversion of igneous pyrrhotite to secondary pyrite and minor oxidation of ferrous iron in the crust. The S- and 34S-enrichments of the transition zone are the results of seawater sulfate reduction and sulfide deposition during subsurface mixing between upwelling hot (up to 350°C) hydrothermal fluids and seawater. Seawater sulfate was probably reduced through oxidation of ferrous iron in hydrothermal fluids and in the transition zone rocks. Alteration of the upper crust resulted in loss of basaltic sulfur to seawater, fixation of minor seawater sulfur in the crust and redistribution of magmatic sulfur within the crust. This caused net increases in sulfur content and d34S of the upper 1.8 km of the oceanic crust.

(Table 2) Chlorine geochemistry and mineralogy of altered oceanic crust samples

Bulk chlorine concentrations and chlorine stable isotope compositions were determined for hydrothermally altered basalt (extrusive lavas and sheeted dikes) and gabbro samples (n = 50) from seven DSDP/ODP/IODP drill sites. These altered oceanic crust (AOC) samples span a range of crustal ages, tectonic settings, alteration type, and crustal depth. Bulk chlorine concentrations range from < 0.01 wt.% to 0.09 wt.%. In general, higher chlorine concentrations coincide with an increase in temperature of alteration and amphibole content. d37Cl values of whole rock AOC samples range from -1.4 to +1.8 per mil. High d37Cl values (>=~0.5 per mil) are associated with areas of higher amphibole content. This observation is consistent with theoretical calculations that estimate amphibole should be enriched in 37Cl compared to co-existing fluid. Negative to near zero d37Cl values are found in areas dominated by clay minerals. Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. AOC is a major Cl host in the subducting oceanic lithospheric slab. Here we show that bulk chlorine concentrations are ~3 times higher than previous estimates resulting in a greater contribution of Cl to the mantle.