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Hess Rise, in the western Pacific Ocean (Fig. 1), formed in the mid-Cretaceous south of the equator and moved north with the Pacific Plate (Lancelot and Larson, 1975; Lancelot, 1978; Valuer et al., 1979). Southern Hess Rise was a volcanic archipelago, at least until late Albian time, after which it subsided to become one of the major aseismic rises in the present western Pacific. A second pulse of volcanic activity apparently occurred in the Campanian-Maastrichtian interval, which may be related to tectonic uplift of Hess Rise (Valuer and Jefferson, this volume). Trachytic rocks underlie 412 meters of carbonate sediments at Site 465 on southern Hess Rise (Fig. 2). Twenty-four meters of trachyte were recovered from a 64-meter cored interval. The rocks are relatively homogeneous in texture, color, and composition, indicating that the cored sequence was probably part of only one magmatic event (Seifert et al., this volume). Large (> 5-mm) vesicles and oxidized parts of some flows suggest subaerial or shallow-water extrusions. The rocks are high in silica and relatively rich in Na2O, K2O, and light rare-earth elements. The upper part of the volcanic-rock sequence is a breccia, the fragments cemented by calcite, pyrite, and rare barite. Some of the resultant veins are more than 1 cm thick. In addition to the veins, many vesicles are also filled with these minerals. Brecciation and the number and thickness of veins decrease with depth in the hole. The degree of weathering, as indicated by water content, also decreases with depth. The abundance of the secondary minerals and their megascopic relationships suggest that their origins might be related to hydrothermal activity, and that a thermal event may have affected the flow rocks on southern Hess Rise after extrusion. Such a thermal event might be related to a second phase of volcanic activity, and thereby have some important geologic implications relative to the history of Hess Rise. Determination of stable-isotope ratios in the secondary minerals should provide adequate data for interpreting the origins of the minerals. This paper reports the results of isotopic analyses of sulfur, carbon, oxygen, and hydrogen and their bearing on the source materials and temperatures of secondary mineral formation. 40°N

Stable-Isotope Compositions and the Origin of Secondary Minerals in Altered Volcanic Rocks from Southern Hess Rise, Deep Sea Drilling Project Site 465