Description, geochemistry and mineral composition of ODP Site 193-1189 jasperoids, PACMANUS field

Bright red "jasperoids" were recovered at three positions during Leg 193 drilling below Roman Ruins (Site 1189) in the PACMANUS hydrothermal field. These do not represent fossil exhalative oxide deposits equivalent to those associated with sulfide chimneys at the Roman Ruins seafloor. Rather, they constitute an integral, relatively early stage involving oxidized fluids in the development of veins and breccias that characterize the mostly sulfidic stockwork zone intersected below Roman Ruins in Hole 1189B. They formed by growth of quartz in open spaces created by hydrofracturing, the characteristic feature being mostly euhedral cores dusted by tiny hematite flakes. In one occurrence there are also frondlike aggregates and possible earlier cavity linings of hematite, overgrown by quartz, that potentially formed by maturation of ferruginous gels first deposited in the openings. The trace element geochemistry of the jasperoids, apart from minor enrichment in uranium, provides no indication that they represent subsurface conduits for fluids that deposit Fe-Mn-Si at the seafloor, though this remains a possibility for some such deposits.

Mineralogical composition, copper and d18O at DSDP Site 59-448

Alteration in a submarine remnant volcanic arc should leave an important record of (1) the mineralogy of sea water-volcanic arc rock interaction; (2) the chemistry of solid reaction products; (3) the isotopic characteristics of such reactions (Muehlenbachs and Clayton, 1972; Spooner, Beckinsale, et al , 1977; Spooner, Chapman, et al., 1977); (4) the metallogenesis within such a sequence (Mitchell and Bell, 1973); and (5) the geothermal gradient during the alteration. The volcaniclastic breccias, tuffs, and igneous units of Sites 448 (993 m) and 451 (930.5 m) on the Palau-Kyushu and West Mariana ridges, respectively, are particularly suited for such studies because the thick sequences have remained submarine throughout their history, seemingly unaffected by magmatic or hydrothermal events after cessation of volcanic activity. Also, shipboard observations indicated a change in alteration products with depth. At both sites the igneous units and volcaniclastic rocks were altered to brownish clays and zeolites near the top of the volcanic sequence; to bright blue green clays and zeolites at moderate depths; and to very dark, nearly opaque, forest green clays and zeolites at still greater depths. Native copper occurs both as disseminated pockets in the volcaniclastic breccias and vesicular basalts and as veins in the breccias; native copper is restricted to stratigraphic levels characterized by the absence of sulfides or oxides of copper and iron. Although some native copper is found in vesicles of basalts and may be orthomagmatic, most of it is clearly secondary. Near dikes and sills, higher sulfur fugacity conditions caused the precipitation of iron and copper sulfides with an absence of native copper (Garrels and Christ, 1965). The occurrence of native copper may be an initial stage of Cu metallogenesis that forms porphyry coppers in island arcs (Mitchell and Bell, 1973). This study will address primarily the possibility that hydrothermal sea water interaction with volcanic arc rocks has created the mineralogical and isotopic zonation in Leg 59 cores. Hydrothermal activity can be expected in a rapidly growing island arc and is probably the result of a high geothermal gradient prevalent during arc magmatic activity. The chemical character of the alteration is further discussed by Hajash (1981).

Strontium- and lead-isotope composition of some basalts at DSDP Hole 69-504B

Seventeen whole-rock samples, generally taken at 25- to 50-meter intervals from 5 to 560 meters sub-basement in Deep Sea Drilling Project Hole 504B, were analyzed for 87Sr/86Sr ratios, and rubidium and strontium concentrations. Ten of these samples also were analyzed for Pb-isotope composition. Strontium-isotope ratios for eight samples from the upper 260 meters of the hole range from 0.70287 to 0.70377, with a mean of 0.70320. In the interval 330 to 560 meters, five samples have a restricted range of 0.70259 to 0.70279, with a mean of 0.70266, almost identical to the average value of fresh mid-ocean-ridge basalts. In the interval 260 to 330 meters, approximately intermediate strontium- isotope ratios are found. The higher 87Sr/86Sr ratios in the upper part of the hole can be interpreted in terms of strontium-isotope alteration during basalt-sea-water interaction. Relative to average fresh mid-ocean ridge basalts, the upper 260 meters of basalts are enriched by an average of about 9% in sea-water strontium 87Sr/86Sr = 0.7091). This Sr presumably is located in the smectites, which, as the main secondary minerals throughout the hole, replace olivine and matrix glass and locally fill vesicles (analyzed samples contained no veins). The strontium-isotope data strongly suggest that the integrated flux of sea water through the upper part of the Hole 504B crust has been greater than through the lower part. This is also suggested by (1) the common occurrence of Feoxide- hydroxide minerals as alteration products above 270 meters, but their near absence below 320 meters, (2) the presence of vein calcite above 320 meters, but its near absence below this level, and (3) the occurrence of vein pyrite only below a depth of 270 meters. Sea-water circulation in the lower basalts may have been partly restricted by the greater number of relatively impermeable massive lava flows below 230 meters sub-basement. Although sufficient sea water was present within the lower part of the hole to produce smectitic alteration products, the overall water /rock ratio was low enough to prevent significant modification of strontium-isotope ratios. Lead-isotope ratios of Hole 504B basalts form approximately linear arrays in plots of 208Pb/204Pb and 207Pb/204Pb versus 206Pb/204Pb. The arrays are similar to those reported for basalts from other mid-ocean ridges. There is no trend in Hole 504B lead-isotope ratios with vertical position in the basement. The arrays indicate that the lead-isotope composition of the upper mantle from which the Hole 504B basaltic melts were derived was inhomogeneous.