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.

Contents of major and noble chemical elements in phosphorites and bottom sediments

Ag and Au are typically concentrated in phosphorites; they genetically related to organic matter of bottom sediments that extract these elements from seawater or interstitial water. Consequently, the phosphorites inherit Ag and Au from host sediments that are not always enriched in them. In contrast to other organic-rich sediments, analyzed sample of recent diatom ooze from the Namibian shelf is not enriched in Ag and Au, although some sediments from this region are enriched in Au. In addition to authigenic Au, allochthonous Au associated with quartz grains and micrograins can also be present in shelf phosphorites. This was observed in oceanic phosphorites of various types. Anomalous Au and Fe contents recorded in one seamount phosphorite sample can be related to extraction of Au and nonferrous metals by ferromanganese hydroxides from seawater. This process can serve as one of major mechanisms of Au supply to ferromanganese crusts on seamounts. Phosphorites and sediments are enriched in Ru simultaneously with U. Author's data show that U content varies from 17 (seamount phosphorite) to 887 ppm (Pleistocene phosphorite nodule from the Namibian shelf). This is probably caused by different types of behavior of light and heavy PGEs in the marine environment.

Geochemistry, trace elements and minerals at DSDP Holes 80-550B and 80-551

The Leg 80 basalts drilled on the Porcupine Abyssal Plain 10 km southwest of Goban Spur (Hole 550B) and on the western edge of Goban Spur (Hole 551), respectively, are typical light-rare-earth-element- (LREE-) depleted oceanic tholeiites. The basalts from the two holes are almost identical; most of their primary geochemical and mineralogical characteristics have been preserved, but they have undergone some low-temperature alteration by seawater, such as enrichment in K, Rb, and Cs and development of secondary potassic minerals of the "brownstone facies." K/Ar dating fail to give realistic emplacement ages; the apparent ages obtained become younger with alteration (causing an increase in K2O). Hole 551 basalts are clearly different from the continental tholeiites emplaced on the margins of oceanizing domains during the prerift and synrift stages.

Geochemistry and trace elements at DSDP Hole 78-543A

The upper part of the basaltic substratum of the Atlantic abyssal plain, approaching subduction beneath the Barbados Ridge and thus presumably beneath the Lesser Antilles island arc, is made of typical LREE-depleted oceanic tholeiites. Mineralogical (microprobe) and geochemical (X-ray fluorescence, neutron activation analyses) data are given for 12 samples from the bottom of Hole 543A, which is 3.5 km seaward of the deformation front of the Barbados Ridge complex. These basalts are overlain by a Quaternary to Maestrichtian-Campanian sedimentary sequence. Most of the basalts are relatively fresh (in spite of the alteration of olivine and development of some celadonite, clays, and chlorite in their groundmass), and their mineralogical and geochemical compositions are similar to those of LREE-depleted recent basalts from the Mid-Atlantic Ridge. The most altered samples occur at the top of the basaltic sequence, and show trends of enrichment in alkali metals typical of altered oceanic tholeiites.

Concentrations of major-element oxides and minor and trace elements at DSDP Legs 87, 57, 56 and 31

Sediments from Sites 582 (11 samples), 583 (19 samples), 584 (31 samples), 294 (1 sample), 296 (9 samples), 297 (3 samples), 436 (11 samples), and 439 (3 samples) were analyzed by X-ray fluorescence and/or instrumental neutron activation analysis. Ten major elements and 24 minor and trace elements (including 7 rare earth elements) were determined with these methods. Geochemistry varies systematically with both the site location and sediment age. Such variations are explained in terms of changes in sedimentation processes caused by plate motion and changes in ocean currents.