Oxygen isotope evidence for multi-stage hydrothermal alteration at a fossil slow-spreading center: the Silurian Trinity ophiolite (California, U.S.A. )

Abstract

The Trinity ophiolite consists of a widespread mantle unit (the Trinity Peridotite) and a thin crustal sequence ( ∼2 km) outcropping discontinuously. The well-exposed massifs of layered and isotropic gabbros, diabasic dikes and pillow lavas define a complete oceanic crustal sequence. The basalts, dikes and gabbros are characterized by retrograde metamorphic associations. Previous work has shown that the extent of alteration is directly correlated to the amount of circulating seawater and decreases downwards for increasing temperatures. Oxygen isotopic compositions were measured on whole-rock samples and a few mineral separates across the ophiolite section. Clearly, hydrothermal alteration in the presence of seawater has introduced large variations of '80/'60 ratios with respect to those of the starting rocks whose composition is assumed to be typical of mantle-derived materials. The evolution of δ 18O-values as a function of depth is similar to the Oman example but there are some significant differences: with respect to the mantle reference composition, the upper section (pillow lavas) is '80 enriched up to 10.1‰. Low δ 18O-values down to +4‰ are recorded by some sheeted dikes, isotropic and cumulate gabbros. However, the Trinity ophiolite strongly differs from the Oman ophiolite by the presence of 18O-enriched rocks at deep levels of the ophiolite section. Microthermometry on fluid inclusions from a few quartz-bearing samples and water/rock ratios inferred from previous Sr isotope data allow us to calculate a pattern of alteration temperatures consistent with a complex regime of water-rock interactions: low-temperature ( ∼ 110–170°C) exchange occurred pervasively through the pillow lava section but also irregularly affected deeper parts of the crust (isotropic and layered gabbros) in which a previous episode of high-temperature ( ∼ 300–390°C) interaction took place. Deep channelized penetration of seawater at low temperature could be related to the wider and deeper faulting pattern which characterizes slow-spreading centers. Such a low-temperature, off- axis interaction (pervasive in the upper section and heterogeneous at deeper levels) could destroy the δ 18) balance possibly established during the axial hydrothermal activity. The oxygen isotope composition of hydrothermally-altered rocks in the Trinity complex is consistent with a δ 18O-value of 0 ± 2‰ for Silurian seawater.