Geochemistry of the oceanic crust from ocean drilling samples

This book presents new data on chemical and mineral compositions and on density of altered and fresh igneous rocks from key DSDP and ODP holes drilled on the following main tectonomagmatic structures of the ocean floor: 1. Mid-ocean ridges and abyssal plains and basins (DSDP Legs 37, 61, 63, 64, 65, 69, 70, 83, and 91 and ODP Legs 106, 111, 123, 129, 137, 139, 140, 148, and 169); 2. Seamounts and guyots (DSDP Legs 19, 55, and 62 and ODP Legs 143 and 144); 3. Intraplate rises (DSDP Legs 26, 33, 51, 52, 53, 72, and 74 and ODP Legs 104, 115, 120, 121, and 183); and 4. Marginal seas (DSDP Legs 19, 59, and 60 and ODP Legs 124, 125, 126, 127, 128, and 135). Study results of altered gabbro from the Southwest Indian Ridge (ODP Leg 118) and serpentinized ultramafic rocks from the Galicia margin (ODP Leg 103) are also presented. Samples were collected by the authors from the DSDP/ODP repositories, as well as during some Glomar Challenger and JOIDES Resolution legs. The book also includes descriptions of thin sections, geochemical diagrams, data on secondary mineral assemblages, and recalculated results of chemical analyses with corrections for rock density. Atomic content of each element can be quantified in grams per standard volume (g/1000 cm**3). The suite of results can be used to estimate mass balance, but parts of the data need additional work, which depends on locating fresh analogs of altered rocks studied here. Results of quantitative estimation of element mobility in recovered sections of the upper oceanic crust as a whole are shown for certain cases: Hole 504B (Costa Rica Rift) and Holes 856H, 857C, and 857D (Middle Valley, Juan de Fuca Ridge).

Chemistry of igneous rocks from, the Celebes Sea basement

Major and trace elements, mineral chemistry, and Sr-Nd isotope ratios are reported for representative igneous rocks of Ocean Drilling Program Sites 767 and 770. The basaltic basement underlying middle Eocene radiolarianbearing red clays was reached at 786.7 mbsf and about 421 mbsf at Sites 767 and 770, respectively. At Site 770 the basement was drilled for about 106 m. Eight basaltic units were identified on the basis of mineralogical, petrographical, and geochemical data. They mainly consist of pillow lavas and pillow breccias (Units A, B, D, and H), intercalated with massive amygdaloidal lavas (Units Cl and C2) or relatively thin massive flows (Unit E). Two dolerite sills were also recognized (Units F and G). All the rocks studied show the effect of low-temperature seafloor alteration, causing almost total replacement of olivine and glass. Calcite, clays, and Fe-hydroxides are the most abundant secondary phases. Chemical mobilization due to the alteration processes has been evaluated by comparing elements that are widely considered mobile during halmyrolysis (such as low-field strength elements) with those insensitive to seafloor alteration (such as Nb). In general, MgO is removed and P2O5 occasionally enriched during the alteration of pillow lavas. Ti, Cs, Li, Rb, and K, which are the most sensitive indicators of rock/seawater interaction, are generally enriched. The most crystalline samples appear the least affected by chemical changes. Plagioclase and olivine are continuously present as phenocrysts, and clinopyroxene is confined in the groundmass. Textural and mineralogical features as well as crystallization sequences of Site 770 rocks are, in all, analogous to typical mid-ocean-ridge basalts (MORBs). Relatively high content of compatible trace elements, such as Ni and Cr, indicate that these rocks represent nearly primitive or weakly fractionated MORBs. All the studied rocks are geochemically within the spectrum of normal MORB compositional variation. Their Sr/Nd isotopic ratios plot on the mantle array (87Sr/87Sr 0.70324-0.70348 with 143Nd/144Nd 0.51298-0.51291) outside the field of Atlantic and Pacific MORBs. However, Sr and Nd isotopes are typical of both Indian Ocean MORBs and of some back-arc basalts, such as those of Lau Basin. The mantle source of Celebes basement basalts does not show a detectable influence of a subduction-related component. The geochemical and isotopic data so far obtained on the Celebes basement rocks do not allow a clear discrimination between mid-ocean ridge and back-arc settings.

Cenozoic plaktonic foraminfers from the Celebes and Sulu Seas

This paper presents the planktonic foraminifer biostratigraphy of the sites drilled during Ocean Drilling Program Leg 124 in the Celebes and Sulu Seas. It discusses preservation of foraminifers in pelagic sediments and in calcareous turbidites. In the Celebes Sea, pelagic carbonates are only found in the Eocene and Oligocene at Site 770. The faunas are poorly preserved due to severe dissolution and offer little biostratigraphic detail. In the Sulu Sea, pelagic carbonates are found in the upper Pliocene and Pleistocene at Sites 768 and 769 and throughout the recovered sequence at the shallower Site 771. The foraminifer faunas from these sediments allow for recognition of most standard zones. Variations in preservation of pelagic foraminifer faunas with time are due to changes in the depth of the lysocline. Shifts to improved preservation at Sites 768 and 769 are synchronous in the upper Pliocene/Pleistocene and may be related to global sea-level cycles. Planktonic foraminifers are also abundant in calcareous turbidites, which were deposited in both basins from the late Miocene onward. However, the turbidites are fine-grained, and biostratigraphic marker species are absent as a result of size-sorting during transport. In the Celebes Sea, shelf-derived material was a major component of early-late Miocene and middle Pliocene to early Pleistocene turbidites. Changes in the composition of the turbidites may correspond to global sea-level changes. In the Sulu Sea, a shift from shelf-derived material in Pliocene calcareous turbidites to a pelagic source in the Pleistocene may be related to subsidence of the Cagayan Ridge.

Electrical properties of basalts of ODP Sites 124-768 and 124-770

Conductivity of 54 basalt samples from ODP Sites 768 and 770 was measured as a function of temperature and fluid salinity. Porosity was also measured for all samples, and cation exchange capacity was measured for 46 of the samples. Porosity measurements indicated that porosity is underestimated for basalts like these, unless one uses extensive drying at high vacuum. At salinities greater than 29 ppt, and throughout the range of salinity and temperatures likely in situ, sample conductivity (Co) is controlled by porosity (phi) according to the Archie relation Co = 0.22*Cw phi*1-3 (orFF = 4.5/f1.3), where Cw is conductivity of the pore fluids and FF = Cw/CO is the formation factor. At lower salinity, clay-surface conduction or microcrack conduction may dominate. We are unable to distinguish reliably between the two mechanisms, but we do detect their effects subtly at high salinity and strongly at low salinity.

Geochemistry of Neogene volcanic ashs in ODP Leg 124 holes

Mineralogical and geochemical analyses were performed on 40 ash layers of Pleistocene to late Miocene age, recovered during Leg 124 in the Celebes and Sulu Seas (Sites 767, 768, and 769). They provide information on alteration processes related to burial diagenesis. The zonal distribution of secondary volcanic products emphasizes a major diagenetic change, characterized by the complete replacement of volcanic glass by an authigenic smectite-phillipsite assemblage, in tephra layers dated at 3.5-4 Ma. This diagenetic "event" occurs simultaneously in the two basins, and, on the basis of isotopic data, under low-temperature conditions. It is independent of distinct sedimentation rates and related to a relative quiescence of on-land volcanic activity. This period suggests a more uniform paleooceanographic situation having tectonic significance, and probably reflects a kinetic and environmental control of diagenetic reactions.