Major and trace elements at DSDP Leg 74 Holes

Deep Sea Drilling Project Leg 74 drilled basement on the Walvis Ridge at Sites 525, 527, and 528. These sites are located on the crest and flanks of the segment of the Ridge about 68 to 70 m.y. old in the central province of the Ridge. Each site has a number of distinct subaqueous flows separated by sediment layers. Although variation in geochemistry among units and sites is related in part to alteration or crystal fractionation, some is caused by small-scale compositional variation in the mantle source of the basalts. Leg 74 basalts are similar to other basalts recovered from the Walvis Ridge and the Rio Grande Rise. They show distinct compositional differences to mid-ocean ridge basalts in general, to those recovered from the South Atlantic at this latitude, and to basalts presently erupting in Tristan da Cunha. The composition of the Walvis Ridge basalts does not suggest simple mixtures of present-day MORB and Tristan da Cunha melts. If the Walvis Ridge represents the trace of the Tristan da Cunha hot spot as the plates separated, then the composition of the mantle source has differed at different times in the past, which suggests mantle heterogeneity.

Various ratios of C1-C5 hydrocarbons, gas composition and computed temperature at DSDP Leg 64 Holes

I have evaluated shipboard data and preliminary interpretations related to organic geochemistry in light of additional shore-based analyses. Data on interstitial gas, the C/N ratio, and fluorescence indicate that organic matter was altered by sills and that these were all single intrusions except the upper sill complex at Site 481, which was a multiple emplacement. Site 477 had the highest in situ temperature, estimated from interstitial gas composition to be 225°C.

Petrology of high-MgO bronzite andesite resembling boninite at DSDP Hole 60-458

A high-MgO andesite which is texturally similar to boninite and a variolitic basalt collected from Site 458, about 100 km west of the Mariana Trench, have been studied through microprobe analyses and melting experiments at high water pressures. The boninite-type andesite is very similar in composition and texture to a boninite from Bonin Islands, except that the former is more calcic than the latter. The variolitic basalt contains magnesian pigeonite (Ca12Mg74Fe14) in cores of augite microphenocrysts. This pigeonite crystallized at temperatures above 1200°C. In the melting experiments of the boninite-type rock, clinopyroxene crystallizes as a liquidus phase at pressures at least above 8 kbar. No olivine crystallizes near the liquidus temperatures, indicating that the magma of this rock cannot be in equilibrium with the upper mantle periodotite (lherzolite) at depths at least greater than 25 km. The boninite-type rock is probably a product of fractional crystallization of a more primitive magma (e.g., olivine-bearing boninite magma) by separation of olivine and orthopyroxene. The magma of the variolitic basalt also cannot be in equilibrium with the upper mantle peridotite, and may be a product of fractional crystallization of a more primitive basaltic magma.

Magnetic properties and geochemistry in DSDP Holes 69-504B an d 69-505B

More than 60 basalt samples from two Deep Sea Drilling Project holes on the Costa Rica Rift were studied for magnetic properties and were found to have no properties significantly different from other DSDP basalts. Opaque mineralogical and thermomagnetic properties of these samples, however, to some extent show differences from normal submarine basalts; a new type of thermomagnetic curve and wide range of chemical compositions were recognized. Oxidized samples possibly containing incipient ilmenite exsolution lamellae were reduced and re-equilibrated during heating. The Curie temperatures of the re-equilibrated titanomagnetites are interpreted to be those of the original crystallized phase before oxidation.

Interstitial water chemistry at DSDP Leg 67 Holes

Interstitial water chemistry has proved to be a sensitive indicator for early diagenetic reactions, particularly those related to organic matter oxidation. Downhole chemical variations in the pore waters from Deep Sea Drilling Project Holes 496 and 497 on the Middle America Trench slope off Guatemala are anomalous because both salinity and chlorinity show strong decreases to half the values of seawater, and d18O values become positive (maximum of about +2.5% at the bottom of the holes). These observations are explained in terms of dilution of pore waters after retrieval as a result of decomposition of the gas hydrates before removal of pore waters by shipboard squeezing techniques. In all holes, except Hole 495 (drilled in pelagic sediments), decomposition of organic matter leads to rapid sulfate depletion and subsequent methane generation. Associated with methane generation are large increases in alkalinity and dissolved ammonia. The latter component causes ion exchange reactions with clay minerals, which results in maxima in magnesium and perhaps potassium. At greater depths, as yet unidentified reactions cause the removal of magnesium. Especially in the deeper Trench Sites 499 and 500, rapid variations in calcium, magnesium, and alkalinity occur in turbidite sequences.

Geochemistry of carbon at DSDP Legs 56-57 Holes

Forty-three core sections from Sites 434, 435, 438, 439, and 440 on the landward side and six core sections from Site 436 on the seaward side of the Japan Trench were obtained through the JOIDES Organic Geochemistry Advisory Panel for study of the origin and state of genesis of the organic matter associated with these continental slope, accretionary wedge, and outer trench slope sediments of the Japan Trench. The lipid fraction of these sediments is derived primarily from terrigenous organic matter and thus is allochthonous to the area. The associated kerogen fraction is of mixed allochthonous and autochthonous origin. The total organic carbon content seaward of the trench is less than that on the landward side. The composition of this organic matter is similar but not identical to that found in the landward side sediments. The organic matter within these sediments is in a diagenetic state in which geopolymerization of biogenic organic matter is nearly complete, but microbial alteration is still occurring.

Jurassic sedimentation history at DSDP Hole 76-534A

Site 534 reflects a complex interplay of global, basinal, and local influences on sedimentation during the Callovian and Late Jurassic. Rifting and rapid subsidence of the continental margins of the North Atlantic-Tethys seaway occurred during the late Early Jurassic (Sinemurian-Pliensbachian), but rapid spreading between the North American margin (Blake Spur Ridge and magnetic lineation) and the northwest African margin did not commence until the Bathonian or earliest Callovian. Site 534, drilled on marine magnetic anomaly "M-28" of Bryan et al. (1980), was initially about 150 km from either continental margin. The ?middle Callovian basal sediments are dusky red silty marl. Callovian transgression led to active carbonate platforms on the margin, recorded at Site 534 as a rise in the CCD (carbonate compensation depth), then arrival of lime-rich turbidites from the Blake Plateau platform across the Blake Spur Ridge. The host pelagic sediment is greenish black, organic-rich, radiolarian-rich, silty claystone. Hydrothermal activity on the nearby spreading ridge enriched this lower unit in metals. In the Oxfordian, the input of terrestrial silt rapidly diminished; radiolarians or other bioclasts were not preserved. The dark variegated claystone has fine-grained marl and reddish claystone turbidite beds. The late Callovian-Oxfordian Western Tethys has radiolarian chert deposition, marine hiatuses, or organic-rich sediments. The Kimmeridgian and Tithonian had a stable or receding sea level. Near the end of the Jurassic many of the carbonate platforms of the margins were buried beneath prograding fan or alluvial deposits. Carbonate deposition shifted to the deep sea. Site 534 records the deepening of the CCD and ACD (aragonite compensation depth) during the Kimmeridgian and early Tithonian, then a rise of the ACD in the middle Tithonian. Similar trends occurred throughout the Western Tethys-Atlantic. High nannofossil productivity of the seaway led to deposition of very widespread white micritic limestone in the late Tithonian-Berriasian. The underlying sediment had a slower deposition rate of carbonate, therefore its higher clay and associated Fe content produced a red marl. A short sea-level incursion occurred on the Atlantic margins during the Kimmeridgian and is reflected in the Site 534 greenish gray marl unit by numerous turbidite beds of shallow-water carbonates.

40Ar/39Ar ages of boulders drilled at DSDP Hole 57-439

The 40Ar/39Ar stepheating dating method was applied to parts of three boulders recovered at Site 439, DSDP Leg 57. All the samples gave a well-defined isochron. The isochron ages agree with each other within the experimental uncertainties and give a mean value of 21.4 ± 1.0 Ma.

Secondary minerals and geochemistry at DSDP Legs 68 and 69

Basalt samples recovered during DSDP Legs 68, 69, and 70 from a 550-meter-thick section in two holes near the Costa Rica Rift (Holes 501 and 504B) were found to contain the following secondary minerals: trioctahedral and dioctahedral smectite, chlorite, mixed-layer clays, talc, hematite, pyrite, foujasite, phillipsite, analcime, natrolite, thomsonite, gyrolite, aragonite, calcite, anhydrite, chalcocite, Fe-hydrosilicate, okenite, apophyllite, actinolite, cristobalite, quartz, and magnesite. A less positive identification of bismutite was made. A mineral rich in Mn and minerals with strong reflections at 12.9 Å and 3.20 Å remain unidentified. Trioctahedral smectite replaces glass and olivine in the basalt groundmass. The other secondary minerals occur in veins. The distribution of the secondary minerals in the basalt section shows both hydrothermal and oxidizing-nonoxidizing zonation. Most of the secondary minerals formed under alkaline, nonoxidizing conditions at temperatures up to 120° C. An acidic regime probably existed in the lowest portion of basalt. Oxidative diagenesis followed nonoxidative diagenesis in the upper part of the section. Oxidative diagenesis is characterized by the absence of celadonite, rare occurrences of dioctahedral smectite, and widespread hematite and phillipsite.

(Table 1) Mineral composition at DSDP Leg 64 Holes

Mineral composition and compounds of sediments from the Guaymas Basin.

(Table 1) Sulfur isotopes and sulfur content at DSDP Leg 64 Holes

Hydrothermal pyrite samples from Holes 477, 477A, and 478 have sulfur isotope values ranging from about 10 to 11 per mil. Samples with negative sulfur isotope values generally have low sulfide sulfur contents and reflect mixed bacterial and hydrothermal sulfur sources. At higher sulfur contents, the hydrothermal component predominates, producing positive isotope values. Hydrothermal sulfide derives from reduction of seawater sulfate and may contain a significant basaltic component. Hydrothermal anhydrite is restricted to a narrow zone beneath a dolerite sill at Site 477 and, because of partial sulfate reduction in the circulating waters, has isotopic values (23.5-25 per mil), heavier than seawater.