Stable-isotope evidence for the origin of secondary carbonate veins at DSDP Leg 58 Holes

The stable-isotope composition of carbonate minerals is a function of the temperature and isotopic composition of the materials from which they were precipitated or recrystallized. Because carbonates are among the most abundant secondary phases in oceanic volcanic rocks, information derived from their isotopic composition is useful in determining the environment(s) of seafloor alteration. Isotopic analyses of secondary carbonates in basalt recovered from numerous DSDP sites have been reported previously (Anderson and Lawrence, 1976; Brenneke, 1977; Lawrence et al., 1977; Seyfried et al., 1976; among others). These results are consistent with the formation of most secondary carbonates with sea water at low temperatures. The good recovery of basalts during DSDP Leg 58 provided the opportunity to extend the isotopic study of low-temperature alteration and vein formation to the crust of marginal ocean basins. The evidence for complex off-ridge volcanism and intrusive emplacement encountered at Leg 58 sites (Klein et al., 1978) suggested that modes of alteration at these sites might differ from those previously observed and described.

Conglomerate and sandstone petrography at DSDP Hole 58-445

Conglomerates and sandstones in lithologic unit V at DSDP Site 445 comprise lithic clasts, detrital minerals, bioclasts, and authigenic minerals. The lithic clasts are dominantly plagioclase-phyric basalt and microdolerite, followed by plagioclase-clinopyroxene-phyric basalt, aphyric basalt, chert, and limestone. A small amount of hornblende schist occurs. Detrital minerals are dominantly plagioclase, augite, titaniferous augite, olivine, green to pale-brown hornblende, and dark-brown hornblende, with subordinate chromian spinel, epidote, ilmenite, and magnetite, and minor amounts of diopside, enstatite, actinolite, and aegirine-augite. Bioclasts are Nummulites boninensis, Asterocyclina sp. cf. A. penuria, and some other larger foraminifers. Correlation of cored and dredged samples indicates that the Daito Ridge is mainly composed of igneous, metamorphic, ultramafic, and sedimentary rocks. The igneous rocks are mafic (probably tholeiitic) and alkalic. The metamorphic rocks are hornblende schist, tremolite schist, and diopside-chlorite schist. The ultramafic rocks are alpinetype peridotites. Mineralogical data suggest that there were two metamorphic events in the Daito Ridge. The older one was intermediate- to high-pressure metamorphism. The younger one was contact metamorphism caused by a Paleocene volcanic event, possibly related to the beginning of spreading of the west Philippine Basin. The ultramafic rocks suffered from the same contact metamorphism. During the Eocene, exposed volcanic and metamorphic rocks on the uplifted Daito Ridge may have supplied pebble clasts to the surrounding coast and shallow sea bottom. The steep slope offshore may have caused frequent slumping and transportation of the pebble clasts and shallow-water benthic organisms into deeper water, forming the conglomerates and sandstones treated here.