Geochemistry at DSDP Leg 92 Holes

The major-element and most of the trace-element data from the different laboratories that contributed to the study of samples recovered during Leg 82 are presented in the following tables. The different basalt groups, identified on the basis of their chemical properties (major and trace elements), were defined from the data available on board the Glomar Challenger as the cruise progressed (see site chapters, all sites, this volume). Most of the data obtained since the end of the cruise and presented in these tables confirm the classification that was proposed by the shipboard party (see site chapters, all sites, this volume). Nevertheless, special mention should be made about Site 564. The shipboard party proposed a single chemical group at this site but noticed significant variations down the hole, mainly in trace-element data. However, the range of variation was small compared to the precision of the measurements. These variations were confirmed by the onshore studies (see papers in Part IV of this volume, especially Brannon's paper, partly devoted to this topic).

Mineralogy and geochemistry of altered ash, pelagic clay, and a clay vein in basalt from DSDP Leg 62 Holes

Volcanic ash was recovered from lower Aptian to Albian deposits from DSDP Sites 463, 465, and 466; pelagic clay of the upper Pleistocene to Upper Cretaceous was recovered mainly from Site 464, with minor amounts at Sites 465 and 466. We present X-ray-mineralogy data on pelagic clay and altered volcanic ash recovered from the four Leg 62 sites. In addition, two ash samples from Sites 463 and 465, a pelagic clay from Site 464, and a clay vein from the basaltic basement at Site 464 each were analyzed for major, minor, and trace elements. Our purpose is to describe the mineralogy and chemistry of altered ash and pelagic clays, to determine the sources of their parent material, and to delineate the diagenetic history of these clay-rich deposits. Correlation of chemistry and mineralogy of ash and pelagic clay with volcanic rocks suspected to be their parent material is not always straightforward, because weathering and diagenetic alteration caused depletion or enrichment of many elements.

Description, geochemistry, mineralogy and isotopes of sulfides from ODP Site 193-1189, PACMANUS field

Chemical and isotopic data for rare massive and semimassive sulfide samples cored at Site 1189 (Roman Ruins, PACMANUS) suggest their genetic relationship with sulfide chimneys at the seafloor. Sand collected from the hammer drill after commencement of Hole 1189B indicates that at least the lower section of the cased interval was occupied by material similar to the stockwork zone cored from 31 to ~100 meters below seafloor (mbsf) in this hole, but with increased content of barite, sphalerite, and lead-bearing minerals. Fractional crystallization of ascending hydrothermal fluid involving early precipitation of pyrite may explain vertical mineralogical and chemical zoning within the stockwork conduit and the high base and precious metal contents of Roman Ruins chimneys. A mineralized volcaniclastic unit cored deep in Hole 1189A possibly represents the lateral fringe of the conduit system. Lead isotope ratios in the sulfides differ slightly but significantly from those of fresh lavas from Pual Ridge, implying that at least some of the Pb within the Roman Ruins hydrothermal system derived from a deeper, more radiogenic source than the enclosing altered volcanic rocks.

Chemical and isotopic compositions of basaltic glasses and crusts from the Mid-Atlantic Ridge rift valley

Data presented in the paper suggest significant differences between thermodynamic conditions, under which magmatic complexes were formed in MAR at 29°-34°N and 12°-18°N. Melts occurring at 29°-34°N were derived by melting of a mantle source with homogeneous distribution of volatile components and arrived at the surface without significant fractionation, likely, due to their rapid ascent. The MAR segments between 12° and 18°N combine contrasting geodynamic environments of magmatism, which predetermined development of a large plume region with widespread mixing of melting products of geochemically distinct mantle sources. At the same time, this region is characterized by conditions favorable for origin of localized zones of anomalous plume magmatism. These sporadic magmatic sources were spatially restricted to MAR fragments with the Hess crust, whose compositional and mechanical properties were, perhaps, favorable for focusing and localization of plume magmatism. The plume source between 12° and 18°N beneath MAR may be geochemically heterogeneous.

Physical properties and geochemistry at DSDP Leg 70 Holes

Ashes occurring both as distinct layers and mixed with pelagic sediments of the hydrothermal mounds lying south of the Galapagos Rift are mainly rhyolitic and basaltic. The ashes, of rhyolitic to intermediate composition, appear to belong to a calc-alkalic series and were probably derived from Plinian eruptions in Ecuador or Colombia. Basaltic ashes are made of nonvesicular sideromelane spalling shards and are of tholeiitic composition. They probably were derived locally from fault scarps. Most rhyolitic and basaltic glass shards studied are fresh except for hydration of the rhyolitic shards. Some shards are severely altered, however. Basaltic ash may be more common in pelagic sediments deposited near accretion zones and may be a source of silica and other elements released during diagenesis

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.

Whole-rock geochemical analyses of serpentinized harzburgites from ODP Site 209-1270

Small-scale shear zones are present in drillcore samples of abyssal peridotites from the Mid-Atlantic ridge at 15°20'N (Ocean Drilling Program Leg 209). The shear zones act as pathways for both evolved melts and hydrothermal fluids. We examined serpentinites directly adjacent to such zones to evaluate chemical changes resulting from melt-rock and fluid-rock interaction and their influence on the mineralogy. Compared to fresh harzburgite and melt-unaffected serpentinites, serpentinites adjacent to melt-bearing veins show a marked enrichment in rare earth elements (REE), strontium and high field strength elements (HFSE) zirconium and niobium. From comparison with published chemical data of variably serpentinized and melt-unaffected harzburgites, one possible interpretation is that interaction with the adjacent melt veins caused the enrichment in HFSE, whereas the REE contents might also be enriched due to hydrothermal processes. Enrichment in alumina during serpentinization is corroborated by reaction path models for interaction of seawater with harzburgite-plagiogranite mixtures. These models explain both increased amounts of alumina in the serpentinizing fluid for increasing amounts of plagiogranitic material mixed with harzburgite, and the absence of brucite from the secondary mineralogy due to elevated silica activity. By destabilizing brucite, nearby melt veins might fundamentally influence the low-temperature alteration behaviour of serpentinites. Although observations and model results are in general agreement, due to absence of any unaltered protolith a quantification of element transport during serpentinization is not straightforward.

Geochemistry and K-Ar dates of the Mazagan granodiorite at V30-225, VA79-96KD and DSDP Hole 79-544A, off Moroccan coast

Gneissic granodiorite was recovered by drilling at the base of the Mazagan escarpment, 100 km west of the Casablanca, Morocco, at 4000 m water depth. Coarse, predeformative muscovite yielded dates of -515 Ma, fine-grained muscovite of -455 Ma, biotite -360 and 335 Ma, and feldspar -315 Ma. These dates are tentatively correlated with the microscopic results. We assume a minimum age of middle Cambrian for the granodiorite, an Ordovician deformation and mylonitization, and a Late Carboniferous overprint under upper greenschist facies conditions.

Inorganic and organic geochemistry and sediment descriptions at DSDP Hole 93-603B

About one hundred samples of sediments and rocks recovered in Hole 603B were analyzed for type, abundance, and isotopic composition of organic matter, using a combination of Rock-Eval pyrolysis, C-H-N-S elemental analysis, and isotope-ratio mass spectrometry. Concentrations of major, minor, and trace inorganic elements were determined with a combination of X-ray fluorescence and induction-coupled plasma spectrometry. The oldest strata recovered in Hole 603B (lithologic Unit V) consist of interbedded light-colored limestones and marlstones, and black calcareous claystones of Neocomian age. The inorganic and organic geochemical results suggest a very terrigenous aspect to the black claystones. The organic geochemical results indicate that the limestones and marlstones contain a mixture of highly degraded marine and terrestrial organic matter. Comparison of the Neocomian carbonates at Site 603 with those on the other side of the North Atlantic, off Northwest Africa at Site 367, shows that the organic matter at Site 367 contains more marine organic matter, as indicated by higher pyrolysis hydrogen indices and lighter values of d13C. Comparison of inorganic geochemical results for the carbonate lithologies at Site 603 with those for carbonate lithologies at Site 367 suggests that the Site 603 carbonates may contain clastic material from both North American and African sources. The black claystones at Site 603, on the other hand, probably were derived almost entirely from North American clastic sources. Lithologic Unit IV overlying the Neocomian carbonates, consists of interbedded red, green, and black claystones. The black claystones at Site 603 contain more than ten times the organic carbon concentration of the interbedded green claystones. The average concentration of organic carbon in the black claystones (2.8%), however, is low relative to most mid-Cretaceous black claystones and shales in the Atlantic, particularly those found off Northwest Africa. The geochemical data all suggest that the organic matter in the black claystones is more abundant but generally more degraded than the organic matter in the green claystones, and that it was derived mainly from terrestrial sources and deposited in oxygenated bottom waters. The increased percentage of black claystone beds in the upper Cenomanian section, and the presence of more hydrogen-rich organic matter in this part of the section, probably resulted from the increased production and accumulation of marine organic matter that is represented worldwide near the Cenomanian/Turonian boundary in deep-sea and land sections. A few upper Cenomanian black claystone samples that have hydrogen indices > 150 also contain particularly high concentrations of V and Zn. Most samples of black claystone, however, are not particularly metal-rich compared with other black claystones and shales. Compared with red claystones from lithologic Unit IV, the green and black claystones are enriched in many trace transition elements, especially V, Zn, Cu, Co, and Pb. The main difference between the "carbonaceous" claystones of lithologic Unit IV and "variegated" or "multicolored" claystones of the overlying Upper Cretaceous to lower Tertiary Unit III is the absence of black claystone beds. As observed at several other sites (105 and 386), the multicolored claystones at Site 603 are somewhat enriched in several trace transition elements-especially Cu, Ni, and Cr-relative to most deep-sea clays. The multicolored claystones are not enriched in Fe and Mn, and therefore are not "metalliferous" sediments in the sense of those found at several locations in the eastern Pacific. The source of the slightly elevated concentrations of transition metals in the multicolored claystones probably is upward advection and diffusion of metals from the black claystones of the underlying Hatteras Formation. The red, orange, and green claystone beds of lithologic Unit II (Eocene), like those of Unit III, really represent a continuation of deposition of multicolored claystone that began after the deposition of the Neocomian carbonates. The color of the few black beds that occur within this unit results from high concentrations of manganese oxide rather than high concentrations of organic matter.

Geochemistry and fluxes at DSDP Hole 83-504B

This chapter documents the chemical changes produced by hydrothermal alteration of basalts drilled on Leg 83, in Hole 504B. It interprets these chemical changes in terms of mineralogical changes and alteration processes and discusses implications for geochemical cycling. Alteration of Leg 83 basalts is characterized by nonequilibrium and is heterogeneous on a scale of centimeters to tens or hundreds of meters. The basalts exhibit trends toward losses of SiO2, CaO, TiO2; decreases in density; gains of MnO, Na2O, CO2, H2O+ , S; slight gains of MgO; increased oxidation of Fe; and variable changes in A12O3. Some mobility of rare earth elements (REE) also occurred, especially the light REE and Eu. The basalts have lost Ca in excess of Mg + Na gains. Variations in chemical trends are due to differing water/rock ratios, substrate control of secondary mineralogy, and superimposition of greenschist and zeolite facies mineralogies. Zeolitization resulted in uptake of Ca and H2O and losses of Si, Al, and Na. These effects are different from the Na uptake observed in other altered basalts from the seafloor attributed to the zeolite facies and are probably due to higher temperatures of alteration of Leg 83 basalts. Basalts from the transition zone are enriched in Mn, S, and CO2 relative to the pillow and dike sections and contain a metal-sulfide-rich stockwork zone, suggesting that they once were located within or near a hydrothermal upflow zone. Samples from the bottom of the dike section are extensively fractured and recrystallized indicating that alteration was significantly affected by local variations in permeability.

Geochemistry of ODP Site 123-765 sediments

Drilling at Site 765 in the Argo Abyssal Plain sampled sediments and oceanic crust adjacent to the Australian margin. Some day, this site will be consumed in the Java Trench. An intensive analytical program was conducted to establish this site as a geochemical reference section forcrustal recycling calculations. About 150 sediment samples from Site 765 were analyzed for major and trace elements. Downhole trends in the sediment analyses agree well with trends in sediment mineralogy, as well as in Al and K logs. The primary signal in the geochemical variability is dilution of a detrital component by both biogenic silica and calcium carbonate. Although significant variations in the nonbiogenic component occur through time, its overall character is similar to nearby Canning Basin shales, which are typical of average post-Archean Australian shales (PAAS). The bulk composition of the hole is calculated using core descriptions to weight the analyses appropriately. However, a remarkably accurate estimate of the bulk composition of the hole can be made simply from PAAS and the average calcium carbonate and aluminum contents of the hole. Most elements can be estimated within 30% in this way. This means that estimating the bulk composition of other sections dominated by detrital and biogenic components may require little analytical effort: calcium carbonate contents, average Al contents, and average shale values can be taken from core descriptions, geochemical logs, and the literature, respectively. Some of the geochemical systematics developed at Site 765 can be extrapolated along the entire Sunda Trench. However, results are general, and Site 765 should serve as a useful reference for estimating the compositions of other continental margin sections approaching trenches around the world (e.g., outboard of the Lesser Antilles, Aegean, and Eolian arcs).

Geochemical compositions of minerals from chilled dike margins of ODP Hole 140-504B

Mineral and whole-rock geochemical data are presented for chilled dike margins from the lower sheeted dike complex of Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B. Compositions of phenocrystic plagioclase (An80-89); olivine (Fo82-86); clinopyroxene (Wo52En40Fs8, with Cr2O3 up to 1.2%); and rare chromian spinel (Cr# 43) are consistent with those from the lavas and the upper dike complex recovered previously (DSDP Legs 69, 70, 83, and ODP Leg 111). Major and trace element compositions fall in group D of Autio and Rhodes (1983) and have high CaO/Na2O, and low TiO2, K2O, and (La/Sm)N values consistent with previous analyses from this site.