Publication Date:
2022-10-26
Description:
© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ma, Q., Dick, H. J. B., Urann, B., & Zhou, H. Silica-rich vein formation in an evolving stress field, Atlantis Bank Oceanic Core Complex. Geochemistry Geophysics Geosystems, 21(7), (2020): e2019GC008795, doi:10.1029/2019GC008795.
Description:
Drilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at 〉800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.
Description:
This study was supported by the Chinese National Key Basic Research Program (Grant 2012CB417300). H. Dick and B. Urann were supported by U.S. National Science Foundation (Grant OCE‐MG&G 8371300). Emmanuel Codillo provided numerous useful comments and moral support. We thank N. Chatterjee for assistance in analyzing major element mineral composition in the MIT Electron Microprobe Laboratory. The great contributions of 360 Scientific Party for their initial shipboard description and interpretations of the Hole U1473A cores made this work possible. Special thanks go to C. J. MacLeod, Expedition cochief scientist, and Peter Blum, staff scientist, Stephen Midgley, IODP operations superintendent, and Siem Offshore James Samuel McLelland, offshore installation manager, ship's master Terry Skinner, and the crew and drillers on the JOIDES Resolution.
Keywords:
Felsic veins
;
Magma chambers
;
Ocean ridge
;
Geothermometry
;
Flourine‐chlorine
;
Dynamics
Repository Name:
Woods Hole Open Access Server
Type:
Article
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