Chemical element contents and accumulation rates in metalliferous sediments from DSDP Hole 34-319, Bauer Deep, Southeast Pacific

Marked variations in the chemical and mineralogical composition of sediments at Site 319 have occurred during the 15 My history of sedimentation at this site. The change in composition through time parallels the variability observed in surface sediments from various parts of the Nazca Plate and can be related to variations in the proportion of hydrothermal, hydrogenous, detrital and biogenous phases reaching this site at different times. Metal accumulation rates at Site 319 reach a maximum near the basement for most elements, suggesting a strong hydrothermal contribution during the early history of this site. The hydrothermal contribution decreased rapidly as Site 319 moved away from the spreading center, although a subtle increase in this source is detectable about the time spreading began on the East Pacific Rise. The most recent sedimentation exhibits a strong detritalhydrogenous influence. Post-depositional diagenesis of amorphous phases has converted them to ironrich smectite and well-crystallized goethite without significantly altering the bulk composition of the sediment.

Chemical composition of DSDP cherts and associated host sediments

Chert and associated host sediments from Monterey Formation and Deep Sea Drilling Project (DSDP) sequences were analyzed in order to assess chemical behavior during diagenesis of biogenic sediments. The primary compositional contrast between chert and host sediment is a greater absolute SiO2 concentration in chert, often with final SiO2 >=98 wt%. This contrast in SiO2 (and Si/Al) potentially reflects precursor sediment heterogeneity, diagenetic chemical fractionation, or both. SiO2 concentrations and Si/Al ratios in chert are far greater than in modern siliceous oozes, however and often exceed values in acid-cleaned diatom tests. Compositional contrasts between chert and host sediment are also orders-of-magnitude greater than between multiple samples of the host sediment. Calculations based on the initial composition of adjacent host, observed porosity reductions from host to chert and a postulated influx of pure SiO2, construct a chert composition which is essentially identical to observed SiO2 values in chert. Thus, precursor heterogeneity does not seem to be the dominant factor influencing the current chert composition for the key elements of interest. In order to assess the extent of chemical fractionation during diagenesis, we approximate the precursor composition by analyzing host sediments adjacent to the chert. The SiO2 concentration contrast seems caused by biogenic SiO2 dissolution and transport from the local adjacent host sediment and subsequent SiO2 reprecipitation in the chert. Along with SiO2, other elements are often added (with respect to Al) to Monterey and DSDP chert during silicification, although absolute concentrations decrease. The two Monterey quartz chert nodules investigated, in contrast to the opal-CT and quartz chert lenses, formed primarily by extreme removal of carbonate and phosphate, thereby increasing relative SiO2 concentrations. DSDP chert formed by both carbonate/phosphate dissolution and SiO2 addition from the host. Manganese is fractionated during chert formation, resulting in MnO/Al2O3 ratios that no longer record the depositional signal of the precursor sediment. REE data indicate only subtle diagenetic fractionation across the rare earth series. Ce/Ce* values do not change significantly during diagenesis of either Monterey or DSDP chert. Eu/Eu* decreases slightly during formation of DSDP chert. Normative La/Yb is affected only minimally as well. During formation of one Monterey opal-CT chert lens, REE/Al ratios show subtle distribution changes at Gd and to a lesser extent near Nd and Ho. REE compositional contrasts between diagenetic states of siliceous sediment and chert are of a vastly smaller scale than has been noted between different depositional environments of marine sediment, indicating that the paleoenvironmental REE signature is not obscured by diagenetic overprinting.

Chemical composition of basalts from ODP holes at the Kerguelen Plateau

Petrographic and geochemical study of basalts in the Kerguelen Plateau basement revealed changes in composition and character of volcanism during development of this tectonovolcanic structure. The Kerguelen Plateau is one of the largest intraplate rises in the World Ocean. It started to form about 120 Ma ago. Age of basalts and overlying sediments shows that plateau formation was in the northwest direction. Basalts of the Kerguelen Plateau basement are products of tholeiitic melts in terms of geochemistry, but differ from mid-ocean ridge basalt (MORB). They are enriched in incompatible trace elements and rare earth elements (REE) relative to MORB, and degree of enrichment varies in basalts from different segments of the plateau. Composition of basalts does not directly depend on their age. Specific features of plateau magmatism are commonly explained in terms of a long-living deep magma plume, which variously interacted with a depleted upper mantle source at different stages of plateau formation. However, taking into account block morphology and deep structure of the plateau, one can suggest that plateau volcanism was initiated by a large fault. As the volcanism prograded to the northwest, depth of fault penetration into the mantle changed. Composition of basalts in the plateau basement was also governed by formation depth of primary melts.

Chemical and isotope compositions of interstitial water in sediments from Mediterranean basin at DSDP Sites 42-372 and 42-374

The Br/Cl, Li/Cl and B/Cl ratios and boron isotope compositions of hypersaline pore fluids from DSDP Sites 372 and 374 were measured in an attempt to evaluate the origin of the brines. In Site 374 the relationships between the Cl concentrations (up to 5000 mM) and Br/Cl (~0.012), Na/Cl (as low as 0.1), B/Cl (0.0025), and d11B values (43-55?) of the deep pore water between 380 and 405 mbsf, located within the Messinian sediments, reflect remnants of ~65-fold evaporated sea water. The original evaporated sea water was modified by: (1) dilution with overlying or less saline water by about 30%; and (2) slight dissolution of NaCl evaporites. The variations in d11B show a continuous increase in d11B values with depth in Site 374, up to 66.7? at a depth of 300 mbsf (Upper Pliocene marl sediments). The conspicuous 11B enrichment trend is consistent with elemental boron depletion, which was calculated from the expected boron concentrations of evaporated sea water with corresponding Br/Cl and Na/Cl ratios. Li/Cl variations also show a depletion of Li relative to evaporated sea water. The apparent depletions of B and Li, as well as the 11B enrichment, reflect uptake of these elements by clay minerals at low water/sediment ratios.

Major and trace elements and minerals at DSDP Hole 79-544A

Fifty m of basement rocks underlying 185 m of Neogene and Mesozoic sediments were drilled seaward of the Mazagan Slope about 100 km west of Casablanca during Leg 79. These rocks are metagranites with mylonitic textures consisting dominantly of quartz, plagioclase, and potassium feldspar. Chemically, they are strongly peraluminous. This along with the absence of hornblende suggest that these rocks are similar to the S-type granites. Petrographic and chemical data suggest the possible existence of a former weathering surface on top of the Mazagan metagranite.

Major, trace and rare earth elements and minerals at DSDP Legs 69-70 Holes

We obtained major and trace element data on 113 samples from basalts drilled during DSDP Legs 69 and 70 in the Costa Rica Rift area. The majority have major and trace element characteristics typical of ocean-ridge tholeiities. Most of the basalts are relatively MgO rich (MgO > 8 wt.%) and have Mg values (MgO/MgO + 0.85FeO x 100) of about 53, characteristics that clearly indicate that the various magmas underwent only a small amount of crystal fractionation before being erupted onto the seafloor. According to their normative mineralogies, the rocks are olivine tholeiites. A few samples plot close to the diopside-hypersthene join of the projected basalt tetrahedron. Except for basalts from two thin intervals in Hole 504B, which differ significantly from all the other basalts of the hole, practically no chemical downhole variation could be established. In the two exceptional intervals, both TiO2 and P2O5 contents are markedly enriched among the major oxides. The trace elements in these intervals are distinguished by relatively high contents of magmatophile elements and have flat to enriched chondrite-normalized distribution patterns of light rare earth elements (LREE). Most of the rocks outside these intervals are strongly depleted in large-ionlithophile (LIL) elements and LREE. We offer no satisfactory hypothesis for the origin of these basalts at this time. They might have originated within pockets of mantle materials that were more primitive than the LIL-element-depleted magmas that were the source of the other basalts. A significant change with depth in the type of alteration occurs in the 561 meters of basalt cored in Hole 504B. According to the behavior of such alteration-sensitive species as K2O, H2O-, CO2, S, Tl, and the iron oxidation ratio, the alteration is oxidative in the upper part and nonoxidative or even reducing in the lower part. The oxidative alteration may have resulted from low temperature basalt/seawater interaction, whereas hydrothermal solutions may be responsible for the nonoxidative alteration.