ALBERT

Description: Uranium and thorium are concentrated in Cretaceous limestone, chert, ash, basalt, and other rock types at Deep Sea Drilling Project Site 465 located on the southern Hess Rise in the central North Pacific. U concentrations, up to 194 ppm on a carbonate-free basis, are among the highest recorded for any deep-sea deposits. U was initially derived from seawater and concentrated by absorption on terrigenous (humic) organic matter in limestone in a shallow marine environment. U and Th were probably concentrated further by low-temperature hydrothermal fluids emanating from the basaltic basement. Mainly montmorillonite, an alteration product of basalt and ash, and organic matter in sedimentary rocks acted as hosts for U and Th. The unique combination of sediments rich in humic organic matter, abundant smectite in altered ash and basalt, and warm hydrothermal solutions provided the necessary conditions for migration and concentration of U and Th. To better understand the conditions limiting the migrating and concentration of U and Th, other rocks deposited during the ocean-wide Cretaceous anoxic events should be analyzed for these elements.

Description: 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.

Description: During R/V T. Thompson cruise TT-192 around the the Mariana and Volcano Volcanic Arcs 60% of the dredge recoveries contained ferromanganese deposits. Substrate rocks consisted of intermediate to mafic volcanic rocks, volcaniclastic rocks, volcanic breccia, and minor limestone. Volcaniclastic rocks were dominantly sandstone but ranged from mudstone to conglomerate. Ferromanganese deposits occur in five ways: Yellow-brown to black ferromanganese encrustations on rocks; brown, grey, and black ferromanganese oxides cementing volcaniclastic sandstone; stratiform, stratabound Mn oxide rich layers that are pale to dark grey, rarely brownish-grey, with a submetallic luster; cobbles of dark grey submetallic, irregularly shaped masses of manganese (Mn nodules ?) and weatehered black cobbles rich in magnetite. A selection of these deposits was chemically analysed including major and minor elements, rare earth elements, and platinum-group elements. The chemistry determination was done by Induction Coupled Plasma analyses done by J. G. Crock, and K. Kennedy at the U.S. Geological Survey laboratories in Reston, Denver and Menlo Park respectively. Water and CO2 determinations were done by wet chemical techniques.