Crustal accretion at fast spreading ridges and implications for hydrothermal circulation

Abstract:

Oceanic crust is continuously created at mid-ocean ridges, but the location of lower crust crystallization continues to be debated since the proposal of the gabbro glacier and many sills end-member models. Geophysical and geochemical studies find evidence for either of the models. The crust is cooled by a combination of heat diffusion and advection, and hydrothermal circulation is thought to play a key role in distinguishing between both models.

We use our numerical model for joint modeling of crustal accretion and hydrothermal circulation¹ to test different accretion and hydrothermal cooling scenarios. The results match the seismic and structural observations from the East Pacific Rise² and the Oman Ophiolite³, with a shallow melt lens at the correct location overlaying a narrow volume of partially molten rocks. Our results show that no more than 25-50% of the lower crust crystallizes in situ and that deep circulation is likely to occur at fast and intermediate spreading ridges. The occurrence of deep hydrothermal cooling however does not rule out that a major portion of the lower crust is formed in the shallow melt lens; our simulations rather suggest that it is necessary independent of where in the lower crust crystallization takes place.

1 Theissen-Krah, S., Iyer, K., Rupke, L. H. & Morgan, J. P. Coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges. Earth and Planetary Science Letters 311, 275-286, doi:10.1016/j.epsl.2011.09.018 (2011).

2 Dunn, R. A., Toomey, D. R. & Solomon, S. C. Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9 degrees 30'N. Journal of Geophysical Research-Solid Earth 105, 23537-23555 (2000).

3 Nicolas, A. & Boudier, F. Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise. Terra Nova 27, 77-96, doi:10.1111/ter.12137 (2015).