RADIOGENIC AND STABLE ISOTOPE EVIDENCE OF CONTRASTING EVOLUTION OF HYDROTHERMAL FLUIDS IN THE PACMANUS SYSTEM, MANUS BASIN (ODP LEG 193)

Ocean Drilling Program (ODP) Leg 193 investigated two sites of hydrothermal activity along the crest of the Pual Ridge in the eastern Manus Basin. A site of low-temperature diffuse venting, Snowcap (Site 1188), and a high-temperature black smoker site, Roman Ruins (Site 1189), were drilled to depths of 386 and 206 meters below sea floor (mbsf), respectively. Although the two sites are <1000 m apart, the ⁸⁷Sr/⁸⁶Sr and d³⁴S signatures of anhydrite recovered at both sites are very different.

Measured anhydrite ⁸⁷Sr/⁸⁶Sr ratios vary between 0.7050 and 0.7086; the most radiogenic values occur just beneath the variably altered dacitic cap at both sites. At Snowcap (Site 1188) there is a clear trend downhole to less radiogenic ⁸⁷Sr/⁸⁶Sr values, from 0.7086 beneath the dacite cap to values of 0.7060 at ~100 mbsf, and these low values are broadly maintained to the base of the hole.

The measured anhydrite d³⁴S values vary between 18.1‰ and 22.5‰. For Snowcap, most of the values are less than 20.99‰ and trend significantly toward lower values downhole. In contrast, anhydrite d³⁴S values at Roman Ruins are mostly equal to or greater than the seawater value, show no obvious downhole trends.

The data suggest a complex interplay among hydrothermal fluid, magmatic fluid, and seawater during alteration and mineralization of the PACMANUS system. These new results significantly expand the subsurface data on seafloor hydrothermal systems and may begin to explain the earliest processes of multistage mineralization and alteration history that typify ancient massive sulfide systems.