Fluid evolution during tectonic exhumation of oceanic crust at a slow-spreading paleoridge axis: evidence from the Lizard ophiolite U.K.

Abstract

Zoned quatz crystals within late syntectonic veins from brittle-ductile shear zones situated within the lowermost plutonic rocks of the Lizard ophiolite contain three groups of fluid inclusions. Type I are a phase separated primary and pseudosecondary group. Liquid-rich inclusions contain 3.2–8.0wt% NaCl equivalent with a mode at 5.7wt% NaCl equiv. Vapour-rich inclusions contain 0.0–6.1wt% NaCl equiv. with a mode at 2.6wt% NaCl equiv. Inclusions homogenize either by disappearance of the liquid or vapour phase, with a mode at 380°C. Type II inclusions contain 2.1–5.7wt% NaCl equiv. with a mode at 4.1 wt% NaCl equiv. Inclusions homogenize by disappearance of the vapour phase (L + V ⇄ L) with a mode at 230°C. Type III inclusions are a pseudosecondary and secondary group, with textures which are suggestive of underpressure-induced re-equilibration. They contain methane and a low salinity aqueous solution (0.3–2.2wt% NaCl equiv.). Liquid-rich satellite inclusions homogenize at 310-240°C (L + V ⇄ L). Vapour-rich inclusions homogenize by disappearance of the liquid phase (L + V ⇄ V) at 370-320°C. By assuming that the range in salinity is exclusively the product of phase separation of a 3.2wt% NaCl seawater solution, and by applying microthermometric data to the NaClH 2O system, analysis of the data suggests two-phase boundary constrained conditions of crystal growth, initiated within the super-critical field at around ∼410°C, with declining temperature passing into subcritical portions of the system, beyond which the fluid entered single-phase liquid conditions of crystal growth. Microthermometric and petrographic data, in comparison with present day ocean-ridge hydrothermal systems, is consistent with a reaction zone situated within an inferred decollement at or near the petrological moho, from which fluids were derived from and expelled along intersecting listric faults, at temperatures broadly comparable to shallow subsurface levels within present-day systems. This interpretation suggests that high vent temperatures were maintained as the heat flux waned, and that the hydrothermal system migrated downward through the uplifting gabbros, culminating in the reaction zone intersecting the upper mantle during the terminal pulses of tectonic exhumation.