Petrology and geochemistry of silicified upper Miocene chalk at DSDP Site 69-504

Chert, Porcellanite, and other silicified rocks formed in response to high heat flow in the lower 50 meters of 275 meters of sediments at Deep Sea Drilling Project Site 504, Costa Rica Rift. Chert and Porcellanite partly or completely replaced upper Miocene chalk and limestone. Silicified rock occurs as nodules, laminae, stringers, and casts of burrows, and consists of quartz and opal-CT in varying amounts, associated with secondary calcite. The secondary silica was derived from dissolution of opal-A (biogenic silica), mostly diatom frustules and radiolarian tests. Temperature data obtained at the site indicate that transformation of opal-A to opal-CT began at about 50°C, and transformation from opal-CT to quartz at about 55°C. Quartz is most abundant close to basement basalts. These silica transformations occurred over the past 1 m.y., and took place so rapidly that there was incomplete ordering of opal-CT before transformation to quartz; opal-CT formed initially with an uncommonly wide d spacing. Quartz shows poor crystallinity. Chemical data show that the extensively silicified rocks consist of over 96% SiO2; in these rocks, minor and trace elements decreased greatly, except for boron, which increased. Low Al2O3 and TiO2 contents in all studied rocks preclude the presence of significant volcanic or terrigenous detritus. Mn content increases with depth, perhaps reflecting contributions from basalts or hydrothermal solutions. Comparisons with cherts from oceanic plateaus in the central Pacific point to a more purely biogenic host sediment for the Costa Rica Rift cherts, more rapid precipitation of quartz, and formation nearer a spreading center. Despite being closer to continental sources of ash and terrigenous detritus, Costa Rica Rift cherts have lower Al2O3, Fe2O3, and Mn concentrations.

3He/4He and 4He/20Ne ratios in pore water and gas in Pacific deep-sea sediments

We have measured the 3He/3He and 3He/20 Ne ratios of thirty-nine pore water and gas samples in deep-sea sediments collected at twelve sites on the Pacific Ocean bottom during the cruises of Deep Sea Drilling Project Legs 87, 89, 90 and 92. The 3He/4He and 4He/20Ne ratios vary from 0.000000215 to 0.00000165 and from 0.29 to 20, respectively. He in the sample is composed of four components: (1) atmospheric He dissolved in seawater; (2) atmospheric He with mantle-derived He in Pacific bottom water; (3) in situ radiogenic He in the sediment; and (4) crustal He in the basement rock. Assuming that the 20Ne contents are constant with the value of seawater, the depth variations in the 4He/20Ne ratios at five Sites, 583D, 594, 597A, 598A and 504B, may provide useful information on 4He flux at the ocean bottom. The estimated 4He fluxes vary from 2000 to 40000 atoms cm**-2 s**-1 and are one to three orders of magnitude less than those calculated from the excess He in deep ocean water. An overall similarity between the geographical distribution of the 3He/4He ratios and heat flow data is found in the study area, between the East Pacific Rise across the Pacific Ocean and the Japanese Islands. The tendency is well explained by a conventional sea-floor spreading model.

Composition of sediment pore water and isotopes in carbonate and benthic foraminifera at DSDP Holes 68-501, 69-505 and Site 69-504

Two sites on the southern flank of the Costa Rica Rift were drilled on DSDP Legs 68 and 69, one on crust 3.9 m.y. old and the other on crust 5.9 m.y. old. The basement of the younger site is effectively cooled by the circulation of seawater. The basement of the older site has been sealed by sediment, and an interval in the uppermost 560 meters of basement recently reheated to temperatures of 60 to 120°C. Although the thickness of the sediments at the two sites is similar (150-240 m versus 270 m), the much rougher basement topography at the younger Site 505 produces occasional basement outcrops, through which 80 to 90% of the total heat loss apparently occurs by advection of warm seawater. This seawater has been heated only slightly, however; the temperature at the base of the sediments is only 9°C. Changes in its composition due to reaction with the basement basalts are negligible, as indicated by profiles of sediment pore water chemistry. Bacterial sulfate reduction in the sediments produces a decrease in SO4 (and Ca) and an increase in alkalinity (and Sr and NH3) as depth increases to an intermediate level, but at deeper levels these trends reverse, and all of these species plus Mg, K, Na, and chlorinity approach seawater values near basement. Si, however, is higher, and Li may be lower. At the older site, Site 501/504, where heat loss is entirely by conduction, the temperature at the sediment/basement contact is 59°C. Sediment pore water chemistry is heavily affected by reaction with the basaltic basement, as indicated by large decreases in d18O, Mg, alkalinity, Na, and K and an increase in Ca with increasing depth. The size of the changes in d18O, Mg, alkalinity, Ca, Sr, and SO4 varies laterally over 500 meters, indicating lateral gradients in pore water chemistry that are nearly as large as the vertical gradients. The lateral gradients are believed to result from similar lateral gradients in the composition of the basement formation water, which propagate upward through the sediments by diffusion. A model of the d18O profile suggests that the basement at Site 501/504 was sealed off from advection about 1 m.y. ago, so that reaction rates began to dominate the basement pore water chemistry. A limestone-chert diagenetic front began to move upward through the lower sediments less than 200,000 yr. ago.

(Table 1) Physical properties of basalts from DSDP Legs 69 and 70

Physical properties of basalts from Deep Sea Drilling Project Sites 504 and 505, south of the Costa Rica Rift, including wet-bulk density, water content, sonic velocity, and thermal conductivity, were measured on board D/V Glomar Challenger during Legs 69 and 70. The mean wet-bulk density is 2.90±0.06 g/cm**3, porosity 5.0±2.2%, sonic velocity 5.75±0.30 km/s, and thermal conductivity 1.67±0.09 W/m°K. Basalts from this young ocean crust (5.9 m.y.) have relatively low porosity and consequently high density and sonic velocity, as compared to average DSDP basalts. Some systematic trends in depth variation of physical properties were found: down to Core 40 in Hole 504B, grain densities were lower than those deeper in the hole, whereas porosity in the upper section was higher. This can be attributed either to differences in the flow type or in the nature of alteration of basalts at the different levels in the hole.

Geochemistry of lavas and dikes from the upper oceanic crust of Hess Deep Rift, eastern Pacific Ocean

Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87Sr/86Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle (1992, doi:10.1016/0012-821X(92)90221-G) gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods.