O and H stable isotope compositions of oceanic crust and upper mantle rocks exposed in the Hess Deep near the Galapagos Triple Junction

A large variety of rocks, consisting of basalts, dolerites, gabbros and ultramafics, are exposed on the Intra-rift ridge in the Hess Deep, representing a section of the oceanic crust formed at the East Pacific Ridge [1]. We conducted a stable isotope study of O and H to assess the nature of the interaction between seawater and the oceanic crust in this region. basalt and dolerite samples have magmatic MORB values (δ18O ≈ 6.0, −105 ≤ δD ≤ −70) and low H2O+ content (< 0.3 wt%). With one exception, the metabasalts are enriched in 18O (δ18O ≈ 7) and have δD between −72 and −30 and H2O+ contents higher than 2 wt%. The metadolerites are depleted in 18O (δ18O ≈ 4.9), with high δD (> −57) and H2O+ contents (> 1.3%). abbros have δ18O ≈ 5.6, essentially identical to their initial magmatic values. Their δD values are generally higher than −57, suggesting that they have reacted with some seawater-derived fluids, as is also suggested by their mineralogy. In gabbros, displaying little or no visual evidence of alteration of their plagioclases and pyroxenes, Δ18Oplagioclase-pyroxene is small (≈ 0.3) and consistent with magmatic values. The calcic (an > 50) plagioclases have magmatic δ18O (≈ 5.8). Albitized, prehnitized plagioclases have variable δ18O values (4–8). It also appears that the 18O-modified gabbros are those in which plagioclases are transformed into albite and/or prehnite, which are characteristic of greenschist facies conditions. The lack of calcic (an > 50) plagioclases depleted in 18O and hornblende suggests that most of the gabbros did not react extensively under amphibolite facies conditions with seawater-derived fluids (T ≥ 400 °C) or that they were subsequently altered to greenschist facies gabbros. rpentinized peridotites were depleted in 18O (3.3–4.9). In one sample, the Δ18Oserpentine-magnetite (≈ 5.7) corresponds to a temperature of about 325 ± 50°C. These values are compatible with serpentinization by seawater-derived fluids at temperatures between 200° and 350°C. Another sample has a higher δ18O value of 10, corresponding to a serpentinization temperature of less than 50°C. O mass balance between seawater and the oceanic crust seems to have attained steady-state where enrichment in 18O of metabasalts is balanced by the depletion in 18O of metadolerites. The contribution of the gabbros is minor. The depletion in 18O of the serpentinized peridotites is not taken into account since we believe that the high-temperature serpentinization is characteristic of the Hess Deep and cannot be extrapolated to the normal oceanic crust.