Isotopic and petrological evidence of fluid-rock interaction at a Tethyan ocean-continent transition in the Alps:implications for tectonic processes and carbon transfer during early ocean formation

摘要

We report overprinting stable isotope evidence of fluid-rock interaction below two detachment faults along which mantle rocks were exhumed to the seafloor, between the respective landward and seaward limits of oceanic and continental crust, at a Tethyan ocean-continent transition (OCT). This OCT, which is presently exposed in the Tasna nappe (south-eastern Switzerland) is considered an on-land analogue of the well-studied Iberian OCT. We compare our results with the fault architecture (fault core-damage zone-protolith) described by Caine et al. [Geology (1996) Vol. 24, pp. 1025-1028]. We confirm the existence of a sharp boundary between the fault core and damage zone based on isotopic data, but the boundary between the damage zone and protolith is gradational. We identify evidence for: (1) pervasive isotopic modification to 8.4 ± 0.1 per thousand which accompanied or post-dated serpentinization of these mantle rocks at an estimated temperature of 67-109℃, (2) either (Ⅰ) partial isolation of some highly strained regions [fault core(s) and mylonite] from this pervasive isotopic modification, because of permeability reduction (Caine et al.) or (Ⅱ) subsequent isotopic modification caused by structurally channelled flow of warm fluids within these highly strained regions, because of permeability enhancement, and (3) isotopic modification, which is associated with extensive calcification at T = 54-100℃, primarily beneath the younger of the two detachment faults and post-dating initial serpentinization. By comparing the volumetric extent of calcification with an experimentally verified model for calcite precipitation in veins, we conclude that calcification could have occurred in response to seawater infiltration, with a calculated flux rate of 0.1-0.2 m year"1 and a minimum duration of 0.2-4.0 × 10~4 years. The associated time-averaged uptake flux of carbon during this period was 8-120 mol m~(-2) year~(-1). By comparison with the estimated area of exhumed mantle rocks at the Iberian OCT, we calculate a maximum annual uptake flux for carbon of 2-30 Tg year~(-1). This is an order of magnitude greater than that for carbon exchange at the mid-ocean ridges and 0.1-1.4% of the global oceanic uptake flux for carbon.