Oxygen isotope concentrations of minerals from the upper kilometer of the ocean crust, DSDP Hole 504B

DSDP Hole 504B is the deepest basement hole in the oceanic crust, penetrating through a 571.5 m pillow section, a 209 m lithologic transition zone, and 295 m into a sheeted dike complex. An oxygen isotopic profile through the upper crust at Site 504 is similar to that in many ophiolite complexes, where the extrusive section is enriched in 18O relative to unaltered basalts, and the dike section is variably depleted and enriched. Basalts in the pillow section at Site 504 have delta 18O values generally ranging from +6.1 to +8.5? SMOW (mean= +7.0?), although minor zeolite-rich samples range up to 12.7?. Rocks depleted in 18O appear abruptly at 624 m sub-basement in the lithologic transition from 100% pillows to 100% dikes, coinciding with the appearance of greenschist facies minerals in the rocks. Whole-rock values range to as low as +3.6?, but the mean values for the lithologic transition zone and dike section are +5.8 and +5.4?, respectively. Oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone. The pillow section reacted with seawater at low temperatures (near 0°C up to a maximum of around 150°C) and relatively high water/rock mass ratios (10-100); water/rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section. The transition zone and dikes were altered at much higher temperatures (up to about 350°C) and generally low water/rock mass ratios (~1), and hydrothermal fluids probably contained mantle-derived CO2. Mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient (~2.5°C/m) across a 70 m interval at the top of the lithologic transition zone. Progressive reaction during axial hydrothermal metamorphism and later off-axis alteration led to the formation of albite- and Ca-zeolite-rich alteration halos around fractures. This enhanced the effects of cooling and 18O enrichment of fluids, resulting in local increases in delta 18O of rocks which had been previously depleted in 18O during prior axial metamorphism.

Major- and trace-element chemistry of DSDP Leg 70 Holes basalts and their alteration products

Major and trace element compositions of basalts from the lower part of Hole 504B indicate their cogenetic nature. The cored sequence of interlayered pillow lavas and massive lava flows was produced by eruption of lavas, slightly variable in composition. Plagioclase and olivine crystallization in a shallow magma chamber, followed by small-scale fractionation at higher levels, is responsible for these variations. Except in highly fractured zones within the basement, there are systematic variations in the style and degree of rock alteration with depth. Trace element characteristics of altered rocks and secondary minerals indicate that progressive changes in sea water composition occurred as it reacted with basaltic crust.

Strontium, oxygen and hydrogen isotope ratios of basalts from DSDP Hole 504B

DSDP Hole 504B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, Galapagos Spreading Center, penetrating 1.35 km into a section that can be divided into four zones-Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Zone IV consists of altered sheeted dikes. Isotopic geochemical signatures in relation to the alteration zones are recorded in Hole 504B, as follows: Zone Depth(m) Average87Sr/86Sr Average delta18O (?) Average deltaD (?) I 275-550 0.7032 7.3 -63 II 550-890 0.7029 6.5 -45 III 890-1050 0.7035 5.6 -31 IV 1050-1350 0.7032 5.5 -36 Alteration temperatures are as low as 10°C in Zones I and II based on oxygen isotope fractionation. Strontium isotopic data indicate that a circulation of seawater is much more restricted in Zone II than in Zone I. Fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 +/- 20°C in Zones III and IV, which is consistent with secondary mineral assemblages. The strontium, oxygen, and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in Zones III and IV are estimated to be 0.7037, 2?, and 3?, respectively. Strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water/rock ratios (in weight) of about 1.6. This value is close to that of the end-member hydrothermal fluids issuing at 21°N EPR. Basement rocks are not completely hydrothermally altered. About 32% of the greenstones in Zones III and IV have escaped alteration. Thus 1 g of fresh basalt including the 32% unaltered portion are required in order to make 1 g of end-member solution from fresh seawater in water-rock reactions.

Major- and trace-elements at DSDP Legs 68-70 Holes

The basalts recovered from the Costa Rica Rift by drilling at Deep Sea Drilling Project Sites 501, 504, and 505 during Legs 68, 69 and 70 of the Glomar Challenger are the most depleted in the most-hygromagmaphile elements (Th, Ta, Nb, and La) of all MORB recovered to date by the Glomar Challenger. The invariant ratios Nb/Ta, Zr/Hf, and Y/Tb show "chondritic values" (expected for Nb/Ta because of the very low concentrations in these elements). Four samples from a single unit are exceptions: they present a flat to slightly enriched, extended Coryell-Masuda plot, and at the same time their La/Ta ratio is 9 (normalized ratio = 1) instead of 19 (normalized ratio = 2), the value for all other samples. Only one of these two values of the La/Ta ratio had been found so far within a single hole, and moreover within large areas of the oceanic crust (several holes or dredges). The present result shows that local heterogeneity of the upper mantle with respect to the La/Ta ratio may exist.

(Table 1) Strontium concentrations and isotope ratios of anhydrite from DSDP/ODP Hole 504B

In samples from 1575 to 1982 mbsf anhydrite leached from whole-rock powders. 87Sr/86Sr and Sr/Ca determined following methods described in (Teagle et al., 1996, doi:10.2973/odp.proc.sr.148.113.1996, Teagle et al., 1998, doi:10.2973/odp.proc.sr.158.223.1998). For the period of the analysis (Jan. 1994-Dec. 1995) NIST SRM 987 gave 87Sr/86Srs0.710244 +/- 0.000018 (2 sigma, n = 115). Full analytical procedural blanks were <50 pg for Sr. delta18O following (Pickthorn and O´Neil, 1985). Repeated extractions and measurements of samples and standards were reproducible within +/- 0.5.