Quantifying the buffering of oceanic oxygen isotopes at ancient midocean ridges

Autor: Yoshiki Kanzaki
Rok vydání: 2020
Předmět:
Zdroj: Solid Earth, Vol 11, Pp 1475-1488 (2020)
ISSN: 1869-9529
Popis: To quantify the intensity of oceanic oxygen isotope buffering through hydrothermal alteration of the oceanic crust, a 2D hydrothermal circulation model was coupled with a 2D reactive transport model of oxygen isotopes. The coupled model calculates steady-state distributions of temperature, water flow and oxygen isotopes of solid rock and porewater given the physicochemical conditions of oceanic crust alteration and seawater δ18O. Using the present-day seawater δ18O under plausible modern alteration conditions, the model yields δ18O profiles for solid rock and porewater and fluxes of heat, water and 18O that are consistent with modern observations, confirming the model's validity. The model was then run with different assumed seawater δ18O values to evaluate oxygen isotopic buffering at the midocean ridges. The buffering intensity shown by the model is significantly weaker than previously assumed, and calculated δ18O profiles of oceanic crust are consistently relatively insensitive to seawater δ18O. These results are attributed to the fact that isotope exchange at shallow depths does not reach equilibrium due to the relatively low temperatures, and 18O supply via spreading solid rocks overwhelms that through water flow at deeper depths. Further model simulations under plausible alteration conditions during the Precambrian showed essentially the same results. Therefore, δ18O records of ophiolites that are invariant at different Earth ages can be explained by the relative insensitivity of oceanic rocks to seawater δ18O and do not require constant seawater δ18O through time.
Databáze: OpenAIRE
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