Chemical composition of ocean-floor basalts and separated magnetites DSDP Legs 37, 45 and 46

Titanomagnetites separated from 15 different rock samples (including ocean-floor basalts from DSDP Legs 37, 45 and 46) were analyzed together with whole-rock samples by instrumental neutron-activation analysis for Sc, Cr, Co, Zn, Hf, Ta, Th and the REE La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Tm, Yb and Lu. In titanomagnetities from ocean-floor basalts and some other rocks, REE are enriched with respect to the whole-rock composition by factors of between 1.5 and 3 for light REE and between 1.0 and 1.9 for heavy REE; that is, REE with larger ionic radii are preferentially incorporated into the magnetite lattice. Three magnetite samples are REE depleted. Their whole-rock samples contain P in appreciable amounts, so apatite, an important REE-accumulating mineral, could have captured REE to some extent. All titanomagnetites show a marked negative Eu anomaly, this is most probably caused by discrimination of Eu(2+) from the magnetite lattice. Co, Zn, Hf and Ta are significantly enriched in magnetites. The distribution behaviour of Sc and Cr is masked chiefly by the crystallization of clinopyroxene and therefore is not easy to estimate. Ulvöspinel contents of about 70% for the titanomagnetites from ocean-floor basalts were estimated from qualitative microprobe analysis. Ulvöspinel contents of all other samples varied in a wide range from 20% to about 90%. No correlation could be observed between this and the REE contents of the magnetites. Ilmenite exsolution lamellae could only be observed in titanomagnetites from a doleritic basalt from Leg 45.

Chemical and mineral compositions of sedimentary deposits and magmatic rocks from DSDP Legs 41, 45, 48, and 49

This collective monography by a group of lithologists from the Geological Institute of the USSR Academy of Sciences summarizes materials of the Deep-Sea Drilling Project from the Atlantic Ocean. It gives results of processing materials on the sequences drilled during DSDP Legs 41, 45, 48 and 49. These studies were based on lithological-facial analysis combined with detailed mineralogical-petrographic description. Its chapters give a number of ideas on formation of the Earth sedimentary cover, which can be used for compilation of regional and global schemes of ocean paleogeography, reconstruction of history of some structures in the World Ocean, correlation between sedimentary processes on continents and in oceans, estimation of perspectives for oil and gas fields and ore formation.

(Table 1) Rare earth element contents in basalts and glasses of DSDP Legs 45 and 51

Rare-earth element (REE) distributions in altered basalts and glasses collected during some Legs of the Deep Sea Drilling Project show that a fractionation of these elements occurs during submarine weathering. When the alteration is well-marked, the REE distribution in altered glasses shows an enrichment in light rare-earths relative to the fresh glass. In particular, Ce is selectively enriched in palagonitized glasses that comprise, besides polymetallic nodules, another phase liable to explain the Ce depletion in seawater. Taking in account these processes of submarine weathering of the oceanic crust, a geochemical balance of Ce between authigenic phases of the marine environment is attempted.

Chemical composition of plagioclase from DSDP Legs 45 and 46, Atlantic Ocean

Phyric basalts recovered from DSDP Legs 45 and 46 contain abundant plagioclase phenocrysts which occur as either discrete single grains (megacrysts) or aggregates (glomerocrysts) and which are too abundant and too anorthitic to have crystallized from a liquid with the observed bulk rock composition. Almost all the plagioclase crystals are complexly zoned. In most cases two abrupt and relatively large compositional changes associated with continuous internal morphologic boundaries divide the plagioclase crystals into three parts: core, mantle and rim. The cores exhibit two major types of morphology: tabular, with a euhedral to slightly rounded outline; or a skeletal inner core wrapped by a slightly rounded homogeneous outer core. The mantle region is characterized by a zoning pattern composed of one to several spikes/plateaus superimposed on a gently zoned base line, with one large plateau always at the outside of the mantle, and by, in most cases, a rounded internal morphology. The inner rim is typically oscillatory zoned. The width of the outer rim can be correlated with the position of the individual crystal in the basalt pillow. The presence of a skeletal inner core and the concentration of glass inclusions in low-An zones in the mantle region suggest that the liquid in which these parts of the crystals were growing was undercooled some amount. The resorption features at the outer margins of low-An zones indicate superheating of the liquid with respect to the crystal. It is proposed that the plagioclase cores formed during injection of primitive magma into a previously existing magma chamber, that the mantle formed during mixing of a partially mixed magma and the remaining magma already in the chamber, and that the inner rim formed when the mixed magma was in a sheeted dike system. The large plateau at the outside of the mantle may have formed during the injection of the next batch of primitive magma into the main chamber, which may trigger an eruption. This model is consistent with fluid dynamic calculations and geochemically based magma mixing models, and is suggested to be the major mechanism for generating the disequilibrium conditions in the magma.

(Table 1) Basement penetration, recovery, and lithology of crustal DSDP holes in the North Atlantic Ocean

Oceanic crustal drilling by R. V. Glomar Challenger at 15 sites in the North Atlantic has led to a complex picture of the upper half kilometer of the crust. Elements of the picture include the absence of the source for linear magnetic anomalies, marked episodicity of volcanic activity, ubiquitous low temperature alteration and evidence for large scale tectonic disturbance. Comparison sections in the Pacific and much deeper crustal drilling are needed to attack problems arising from the North Atlantic results.