Stable isotope geochemistry of Alpine ophiolites: a window to ocean-floor hydrothermal alteration and constraints on fluid-rock interaction during high-pressure metamorphism

Abstract

The subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle; however, despite its potential importance, fluid-rock interaction during high-pressure metamorphism is relatively poorly understood. The stable isotope and major element geochemistry of Pennine ophiolite rocks from Italy and Switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites, suggesting that they interacted with little pervasive fluid during highpressure metamorphism. Cover sediments also have oxygen isotope ratios within the expected range of their protoliths. In the rocks that escaped late greenschistfacies retrogression, different styles of sub-ocean-floor alteration may be identified using oxygen isotopes, petrology, and major or trace element geochemistry. Within the basalts, zones that have undergone highand low-temperature sub-ocean-floor alteration as well as relatively unaltered rocks can be distinguished. Serpentinites have d 18 O and d 2 H values that suggest that they were formed by hydration on or below the ocean floor. The development of high-pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub-ocean-floor alteration and which contained hydrated, fine-grained, reactive assemblages. Given that the transformation of blueschist-facies metabasic rocks to eclogite-facies assemblages involves the breakdown of hydrous minerals (e.g. lawsonite, zoisite, and glaucophane), and will thus liberate considerable volumes of fluids, metamorphic fluid flow must have been strongly channelled.High-pressure (quartzccalciteBomphaciteBglaucophaneBtitanoclinohumite) veins that cut the ophiolite rocks represent one possible channel; however, stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids. Fluids were more likely channelled along faults and shear zones that were active during high-pressure metamorphism. Such strong fluid channelling may cause fluids to migrate toward the accretionary wedge, especially along the slab-mantle interface, which is probably a major shear zone. This may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting. Additionally, because fluids probably interact with relatively small volumes of rock in the channels, they cannot "scavenge" elements from the subducting slab efficiently.

Record of Jurassic mass transport processes through the orogenic cycle: Understanding chaotic rock units in the high-pressure Zermatt-Saas ophiolite (Western Alps)

“…Therefore, this evidence may suggest that the LMO represents a depositional setting located in the oceanward part of an ocean-continent transition setting, protected by the deposition of continental derived material, or an embryonic oceanic basin far from the continental margins. Although it is not within the scope of this work, the latter interpretation seems to better agree with (1) the geochemical and isotopic data from this region (e.g., see Cartwright and Barnicoat, 1999;Widmer et al, 2000;Martin et al, 2008), (2) the recent interpretation of the contact between ZSOs and continental units (i.e., Etirol-Levaz slice) as a product of the Alpine tectonics (e.g., Fassmer et al, 2016), and (3) the lack of any kind of continental-derived material within the CCU.…”

“…Based on scattered records of hydrothermal oceanic alteration (Cartwright and Barnicoat, 1999;Widmer et al, 2000;Martin et al, 2008) and geochemical mid-oceanic ridge basalt affinity (Bearth and Stern, 1979;Dal Piaz et al, 1981;Pfeifer et al, 1989), the ZSO was interpreted as consistent with a mid-ocean ridge setting of formation. However, the local occurrence and mixing and/or juxtaposition of oceanic-and continent-derived material and units allowed the different interpretation of the ZSO as either the remnant of an hyperextended continental margin in an ocean-continent transition zone (e.g., see Beltrando et al, 2014, and references therein) or the result of the Alpine tectonic juxtaposition between oceanic and continental margin units (e.g., Fassmer et al, 2016, and references therein).…”

Fenites associated with carbonatite complexes: A review

“…Migration of fluids in fracture or vein networks limits the volume of country rock interacting with the fluid (Cartwright and Barnicoat, 1999), therefore maintaining chemical gradients over larger distances. Better understanding of the fluid flow mechanism and dynamics in fenites has important implications for understanding element redistribution from source intrusions into surrounding fenites.…”

Tracing the effects of high-pressure metasomatic fluids and seawater alteration in blueschist-facies overprinted eclogites: Implications for subduction channel processes

“…Carbonates from ophiolites, ranging from -4 to +3‰ in  13 C, were also interpreted to reflect a marine C isotope signal, either by derivation from calcareous metasediments (Cartwright and Barnicoat, 1999) or inherited from early seafloor alteration (Miller et al, 2001). The  13 C signature of the carbonates that represent the C source of the fluid may have been slightly modified by metamorphism, because devolatilization produces a moderate decrease in  13 C and a significant decrease in  18 O in the residue due to the release of high- 18 O CO 2 by decarbonation reactions (Agrinier et al, 1985).…”