Geochemistry at DSDP Hole 69-504B

Cores from Deep Sea Drilling Project Holes 501, 504B and 505B have an unusual near-vein zonation in basalts. Megascopically, zonation occurs as differently colored strips and zones whose typical thickness does not exceed 6 to 7 cm. Microscopically, the color of zones depends on variably colored clay minerals which are the products of low-temperature hydrothermal alteration in basalt. These differently colored zones form the so called "oxidative" type of alteration of basalts. Another "background," or, less precisely termed, "non-oxidative," type of alteration in basalts is characterized by large-scale, homogeneous replacement of olivine, and filling of vesicles and cracks by an olive-brown or olive-green clay mineral. The compositions of clay minerals of the "background" type of alteration, as well as the composition of co-existing titanomagnetites, were determined with an electron microprobe. There are sharp maxima in potassium and iron content, and minima in alumina, silica, and magnesia in clay minerals in the colored zones near veins. Coloring of clay and rock-forming minerals by iron hydroxides and a decrease of the amount of titanomagnetite, which apparently was the source of redeposited iron, occur frequently in colored zones. We assume that the large-scale "background" alteration in the basalts occurred under the effect of pore waters slowly penetrating through bottom sediments. Faulting can facilitate access of fresh sea water to basalts; thus above the general homogeneous background arise zones of "oxidative" alteration along fractures in basalts. The main factors controlling these processes are time (age of basalt), grain size, temperature, thickness of sedimentary cover, and heat flow.

Chemical composition and sulfur-isotope ratios in DSDP Legs 69-and 70

As part of the geochemical-petrological study of basalts recovered from DSDP Hole 504B (Leg 70) on the southern flank of the Costa Rica Rift, we investigated specially the relationships between the distribution and isotopic composition of sulfur of scattered and vein sulfides on the one hand, and the observed pattern and processes of secondary alterations on the other. The following groups of observations are essential: (1) variations in the contents and isotopic composition of sulfur of different forms of sulfides are clearly interrelated and are observed solely in porous horizons established on the basis of detailed geophysical experiments; (2) the enrichment of sulfides in the light sulfur isotope decreases from the upper to the lower horizons, and within horizons in the direction of the less-altered rock; (3) the increase of d34S values of scattered sulfides in individual permeable zones parallels a decrease in the degree of iron oxidation in the contents of crystallization water, and in the concentrations of Mg, K, and Li in the rock.

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.

d18O, lithologies, mineralogy and determination of temperature and depth of formation of DSDP Site 69-504 siliceous rocks

Seven opal-CT-rich and five quartz-rich porcellanites and cherts from Site 504 have a range in oxygen-isotope values of 24.4 and 29.4 per mil. In opal-CT rocks, d18O becomes larger with sub-bottom depth and with age. Quartz-rich rocks do not show these trends. Boron, in general, increases with decreasing d18O for porcellanites and cherts considered together, supporting the conclusion that boron is incorporated within the quartz crystal structure during precipitation of the SiO2. Silicification of the chalks at Site 504 began 1 m.y. ago - that is, 5 m.y. after sedimentation commenced on the oceanic crust. Temperatures of chert formation determined from oxygen-isotope compositions reflect diagenetic temperatures rather than bottom-water temperatures, and are comparable to temperatures of formation determined by down-hole measurements. Opal-A in the chalks began conversion to opal-CT when a temperature of 50°C was reached in the sediment column. Conversion of opal-CT to quartz started at 55 °C. Silicification occurred over a stratigraphic thickness of about 10 meters when the temperature at the top of the 10 meters reached about 50°C. It took about 250,000 years to complete the silica transformation within each 10-meter interval of sediment at Site 504. Quartz formed over a stratigraphic range of at least 30 meters, at temperatures of about 54 to 60°C. The time and temperatures of silicification of Site 504 rocks are more like those at continental margins than those in deep-sea, open-ocean deposits.

Magnetic properties of basalts from DSDP Hole 69-504B

The NRM intensity, AF demagnetization characteristics, hysteresis parameters, initial susceptibility, and thermomagnetic characteristics of 18 basalt specimens from Deep Sea Drilling Project Hole 504B were determined. In six samples, the grain size was large enough to allow microprobe analysis. We conclude (1) that the dominant magnetic mineral is titanomagnetite/titanomaghemite; (2) that, except for the upper few meters of the core where the grains are in the stable monodomain state, the grain size of the magnetic mineral lies in the pseudo-single-domain range (< 10 µm); (3) that maghemitization (i.e., low-temperature (< 350°C) oxidation) has taken place. We discuss possible geological histories.