The Hess Deep Rift Valley exposes a young (~1 Ma) section of lower oceanic crust generated at the East Pacific Rise (EPR). The drillers recovered 154 m of plutonic rocks from Hole 894G. The mineralogy and textures of these rocks suggest that they represent the roof of a magma chamber. These high-level gabbros, olivine-gabbros, and gabbronorites are slightly hydrated (average LOI = 0.75%, N = 27) and crosscut by a few olivine basaltic dikes. The dikes were altered under greenschist facies conditions, and the gabbros were amphibolitized. Most of the gabbros have Sr-isotopic ratios that range from 0.70247 to 0.70309, indicative of low water-rock ratios (<1), with a circulating fluid having a ⁸⁷Sr/⁸⁶Sr ratio calculated at 0.7032. The delta¹⁸O values of the gabbros range from 2.2 to 6.5 and are generally much lower (average delta¹⁸O = 4.8, N = 31) than the mantle reference value (5.7 ± 0.2), whereas olivine basalts do not show any ¹⁸O depletion. The d18O values of clinopyroxenes are often anomalously low (5.1–5.3). Clinopyroxene-plagioclase fractionation values reach 1.5 and reveal strong isotopic disequilibrium attributed to high-temperature isotopic exchange with a discrete aqueous fluid. Clinopyroxenes with delta¹⁸O = ~5 contain minute Mg-rich amphibole lamellae (1–5 mm) that represent a very early stage of high-temperature alteration before altering totally into amphibole. Except for a few samples with isotopic compositions that were reequilibrated at low temperatures (<200°C), the gabbros contain low-delta¹⁸O plagioclase (delta¹⁸O = 3–5) that recrystallized at high temperatures (400°–600°C) without exchanging major cations (An_50–65). The Hess Deep gabbros record mechanisms of isotopic exchanges governed by percolation of fluids along grain boundaries and self-diffusion of oxygen through plagioclase without involving macroscopic brittle deformation. These water-rock interactions produced a sequence of low-¹⁸O oceanic rocks early in the spreading history of the EPR that can be compared with the gabbro sequence of the Oman ophiolite.

Date of initial receipt: 26 July 1994
Date of acceptance: 12 January 1995

Return to Contents of Leg 147
Return to Contents of Scientific Results