ALBERT

Description: Leg 49 of the International Phase of Ocean Drilling (IPOD) was designed with a wide spectrum of problems in mind. Topics investigated were (1) variation with time of mantle sources of spreading-center magmas, (2) aging of the volcanic layer, (3) degree of hydrothermal alteration and metamorphism of the volcanic layer, (4) lateral and vertical variation of the petrochemistry, magnetic properties, and structure of the volcanic layer, (5) north-south variability of spreading-center magmas through the North Atlantic, (6) initiation of glaciation during the late Pliocene, and (7) the influence of bottom currents originating at the Iceland-Faeroes Ridge on sediments of the ridge flank province of the western North Atlantic.

Description: The usefulness of cosmogenic beryllium-10 (half life = 2.5 Ma) for studying the rates of accumulation of ferromanganese nodules is reported based on its measured depth distribution in the top 20 mm of these deposits. Accumulation rates have been obtained in the range of 1 to 4 mm/Ma, which are in good agreement with rates determined using the 230Th method on the same nodules. The use of 10Be offers promise in extending the dating to the outer few cm of the nodules. This contrasts with conventional methods using 230Th and 231Pa isotopes which, due to their comparatively short half lives, are limited to a few mm at the surface of the nodules. Detailed studies of 10Be in the manganese deposits coupled with other trace element analyses should prove valuable in understanding the processes of formation of these deposits and the chronology of events recorded by them.

Description: Unlike many cruises of the Deep Sea Drilling Project, Leg 48 was scientifically planned to drill a number of sites to systematically address the problems of passive margin evolution. Site 400 was drilled at the foot of the Meriadzek Escarpment of North Biscay in 4399 meters depth. The site was located in a half-graben forming part of a succession of tilted and rotated fault blocks near the continent/ocean boundary. Site 401 was situated on the planated edge of a tilted fault-block underlying the southern edge of the Meriadzek Terrace on the north Biscay margin. Site 402 was located on the upper slope of the northern continental margin of the Bay of Biscay. The main objectives were to establish the presence or absence of shallow water Upper Cretaceous beds, and to penetrate pre-Aptian synrift sediments and the upslope equivalent of the deep water Albian-Aptian carbonaceous mudstones penetrated at Hole 400A.

Description: A submersible study has been conducted in February - March 1978 at the axis of the East Pacific Rise near 21°N. The expedition CYAMEX, the first submersible program to be conducted on the East Pacific Rise, is part of the French-American-Mexican project RITA (Rivera - Tamayo), a 3-year study devoted to detailed geological and geophysical investigations of the East Pacific Rise Crest. On the basis of the 15 dives made by CYANA in the axial area of the Rise, a morphological and tectonic zonation can be established for this moderately-fast spreading center. A narrow, 0.6 to 1.2 km wide zone of extrusion (zone 1), dominated by young lava flows, is flanked by a highly fissured and faulted zone of extension (zone 2) with a width of 1 to 2 km. Further out, zone 3 is dominated by outward tilted blocks bounded by inward-facing fault scarps. Active or recent faults extend up to 12 km from the axis of extrusion of the East Pacific Rise. This represents the first determination from direct field evidence of the width of active tectonism associated with an accreting plate boundary. Massive sulfide deposits, made principally of zinc, copper and iron, were found close to the axis of the Rise. Other signs of the intense hydrothermal activity included the discovery of benthic fauna of giant size similar to that found at the axis of the Galapagos Rift. We emphasize the cyclic character of the volcanicity. The main characteristics of the geology of this segment of the East Pacific Rise can be explained by the thermal structure at depth below this moderately-fast spreading center. The geological observations are compatible with the existence of a shallow magma reservoir centered at the axis of the Rise with a half-width of the order of 10 km.

Description: Forty basaltic rocks collected by submersible during the Cyamex expedition (1978) on the East PacifIc Rise at 21°N, a moderately fast spreading segment (6 cm/year opening rate) of the mid-ocean ridge, consist of angular pillow fragments and glass buds, sheet-flow slabs and samples of columnar pillars standing in collapsed fossillava pools. Most of the rocks are from the crestal are a of the Rise. The collection shows a striking petrographic homogeneity wh en compared with the range of basalts found on other segments of midocean ridges: olivine-phyric, or highly plagioclase-phyric rocks, so common in the slowspreading Famous are a in the Atlantic, are absent. All samples are typical lowpotassium oceanic tholeiites with a limited fractionation trend. Pillow-lavas, thin and thick sheet-flows cannot be distinguished by their major element compositions, as in the Galapagos rift which has the same spreading rate as the EPR at 21°N. Further, ferrobasalts have been described from the Galapagos rift, but do not appear in the Cyamex rocks. In the Cyamex area, olivine and plagioclase are the main silicate phases, and clinopyroxene is absent. In the pillows and sheet-flow samples, four generations of olivine and plagioclase crystals are distinguished. Samples from the fossillava pools are aphyric. The corresponding magma batches are presumed to have migrated rapidly through the magma chamber, and to have been extruded in large volumes, possibly during episodes ofhigh instantaneous opening rate. Fe-Ni and Fe-Cu-rich sulphide phases are common in an lava types as massive globules scatterred through the glass, or as microglobules decorating the walls of empty vesicles. Palagonite and Fe-Mn oxide thicknesses across the strike of the Rise indicate relative ages compatible with successive extrusions at the Rise axis.

Description: In Costa Rica the manganese deposits are an integral part of the Nicoya ophiolite complex. They contain both primary mineralization and, due to weathering, small residual concentrations. The most common manganese minerals in surface outcrops are psilomelane and pyrolusite. Most of them exist in jasper bodies often in a form of breccia in contact with underlaying basalt. In some instances stratiform manganese-oxide crusts and nodule deposits cap jasper bodies and stratabound deposits being overlain by radiolarites.

Description: Manganiferous soil concretions have been located in pineapple and sugar cane plantations of the Schofield Plateau, Oahu, and black manganiferous coatings on the surface of soils have been observed on the crests of embankments next to pineapple plantations on Oahu, Molokai, and Lanai. The concretions and coatings are found inoxisols derived from volcanic parent material. The moisture regime of the soils is characterized by alternate wet and dry periods. This alternation facilitates remobilization and reprecipitation of manganese and to a much lesser extent iron and associated trace metals in the soils. Optimum conditions for manganiferous soil concretion development are encountered on the Schofield Plateau, where mean annual rainfall exceeds 1000 mm/year but where there is a net evaporation loss from the soils during the dry period (May through October). Remobilization of manganese is less pronounced in the oxisols of Molokai and Lanai, where mean annual rainfall is lower. Thin manganiferous coatings rather than concretions therefore form in these islands. The contents of Mn, Fe, Cu, and Ni of the soils on which Hawaiian concretions form are higher than those of New Zealand soils in which manganese soil concretions are found. This is reflected by the much redder color of the Hawaiian concretion-bearing soils compared with their New Zealand counterparts. Rare earths show an enrichment sequence: parent rock ---+ soil ---+ soil concretion. A small positive Ce anomaly is noted in the soil and is more marked in the concretions. This may be due to the increasing oxidation of Ce in the concretions relative to the soil, although contamination of the soil by concretionary material may also playa role. The parent rock shows no Ce anomaly. Barium and Pb are enriched in concretions relative to the surrounding soil, whereas D, Th, Hf, Nb, Zr, and As are present in similar or higher concentrations in the soil relative to the concretions. Arsenic may follow Fe in the soils. Todorokite is the predominant manganese mineral in the Hawaiian soil concretions, not pyrolusite as previously reported. Scanning electron microscope studies show the crystalline nature of the manganese oxide minerals in the concretions.