Abstract
AbstractHydrothermal alteration of oceanic crust represents a globally significant geochemical exchange between the crust and oceans, introducing volatile elements into the oceanic lithosphere and removing latent heat from crystallization. The extent to which hydrothermal systems penetrate the lower oceanic crust is debated and is significant for competing models of lower oceanic crust formation. In this study, we apply halogen and noble gas geochemistry in order to better understand the fluid sources and conditions during a hydrothermal alteration in a suite of gabbros sampled from drill cores in the mid‐ (hole GT3A) and lower‐ (hole GT1A) crust of the Oman Ophiolite. Low Br/Cl in mid‐crustal samples is controlled by amphibole formed at relatively high fluid/rock ratios. Lower crustal samples, meanwhile, have Br/Cl ranging from seawater to much higher values (<5 × 10−3). These require alteration at highly variable fluid/rock ratios from ∼1 to <0.01 in the lower crust. Noble gas isotopes also suggest alteration at relatively high and low fluid/rock ratios in the mid‐ and lower‐crust, respectively. Indicators of alteration and proxies for fluid/rock ratios (loss on ignition, Pb/Ce, Br/Cl, 40Ar/36Ar96Ma) vary systematically with distance from major fault zones in the lower crust with the strongest correlation in the least altered minerals. Together with age constraints derived from K‐Ar decay, these data indicate faults were major conduits, supplying seawater to the lower crust roughly contemporaneous with the formation of the ophiolite, and commencing at the magmatic stage. These results suggest that hydrothermal alteration of the lower crust is primarily controlled by major fault structures.
Highlights
Hydrothermal alteration of the oceanic crust and its eventual subduction into the mantle exerts a key control on the long-term evolution of the surface and interior chemistry of Earth
We apply halogen and noble gas geochemistry in order to better understand the fluid sources and conditions during a hydrothermal alteration in a suite of gabbros sampled from drill cores in the mid- and lower- crust of the Oman Ophiolite
Combining the mineral abundances determined by X-Ray diffraction (XRD) with their Cl contents determined in situ by electron probe microanalysis (EPMA) allows us to assess the relative importance of each mineral to the whole rock Cl budget
Summary
Hydrothermal alteration of the oceanic crust and its eventual subduction into the mantle exerts a key control on the long-term evolution of the surface and interior chemistry of Earth. Subduction of volatile elements to the deep mantle may effectively remove them from the surface and sequester them at depth. This has important implications for Earth's deep carbon cycle and the long-term salinity of the oceans (Jarrard, 2003; Kelemen & Manning, 2015; Kendrick et al, 2013). Journal of Geophysical Research: Solid Earth (the “gabbro glacier”; e.g., Phipps Morgan & Chen, 1993) have been longstanding topics of discussion, with the former requiring deep hydrothermal circulation to remove latent heat from the lower crust. In recent years, increasing attention has focused on the role of major crustal faults as potential pathways for fluid delivery to the lower oceanic crust as a potential avenue to reconciling these conflicting observations (Abily et al, 2011; Coogan et al, 2006; Harris et al, 2017; Zihlmann et al, 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
