Distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry: examples from Tethyan ophiolites in the Western Alps
Ja. Miler et I. Cartwright, Distinguishing between seafloor alteration and fluid flow during subduction using stable isotope geochemistry: examples from Tethyan ophiolites in the Western Alps, J METAMORPH, 18(5), 2000, pp. 467-482
Large amounts of fluid, bound up in the hydrated upper layers of the ocean
crust, are consumed at convergent margins and released in subduction zones
through devolatilization. The liberated fluids may play an integral role in
subduction zone processes, including the generation of arc-magmas. However
, exhumed subduction zone rocks often record little evidence of large-scale
fluid flow, especially at deeper levels within the subduction zone. Basalt
ic pillows from the high-pressure Corsican and Zermatt-Saas ophiolites show
a range of delta(18)O values that overall reflect seafloor alteration prio
r to subduction However, comparison between the delta(18)O values of the co
res and rims of the pillows suggests that the delta(18)O values of the pill
ow rims at least have been modified during subduction and high-pressure met
amorphism. Pillows that have not undergone high-pressure metamorphism gener
ally have rims with higher delta(18)O values than their cores, whereas the
converse is the case in pillows that have undergone high-pressure metamorph
ism. This reversal in the core to rim oxygen isotope relationship between u
nmetamorphosed and metamorphosed pillows is strong evidence for fluid-rock
interaction occurring during subduction and high-pressure metamorphism. How
ever, the preservation of different delta(18)O values in the cores and rims
of individual pillows and within and between different pillows suggests th
at fluid flow within the subduction zone was strongly channelled. Resetting
of the delta(18)O values in the pillow rims was probably due to fluid-host
ed diffusion that occurred over relatively short time-scales (<1 Myr).