Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust

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doi: 10.2475/06.2006.01
Authors:Coogan, L. A.; Howard, K. A.; Gillis, K. M.; Bickle, M. J.; Chapman, H.; Boyce, A. J.; Jenkin, G. R. T.; Wilson, R. N.
Author Affiliations:Primary:
University of Victoria, School of Earth and Ocean Sciences, Victoria, BC, Canada
University of Leicester, United Kingdom
Cambridge University, United Kingdom
SUERC, United Kingdom
Volume Title:American Journal of Science
Source:American Journal of Science, 306(6), p.389-427. Publisher: Yale University, Kline Geology Laboratory, New Haven, CT, United States. ISSN: 0002-9599
Publication Date:2006
Note:In English. 83 refs.; illus., incl. 2 tables, geol. sketch map
Summary:The mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems. We have investigated the role of zones of focussed fluid flow in the plutonic complex of the Oman Ophiolite in the near-axis cooling of the oceanic crust. Lineaments identified on aerial photographs, that occur at ∼1 km spacing, show evidence for extensive hydrothermal fluid flow through regions ∼10 to 50 m wide. Fluid flow is initiated in these regions at ∼800°C and continues at least into the lower greenschist facies. Strontium-isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the Moho. Computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are ∼1 × 108 kgm-2. Modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones. Heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the Sr-isotopic composition of samples from the focussed fluid flow zones. The combination of independent estimates of the fluid and heat fluxes, such as these, can provide more rigorous constraints on the thermal history than either approach used in isolation. Our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust. Significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid-ocean ridges. This is because fluids flowing through channels become chemically rock-buffered at smaller fluid fluxes than those flowing pervasively through a rock mass. Thus, if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought.
Sections:Petrology; Physical properties of rocks and minerals
Subjects:Alkaline earth metals; Cooling; Crust; Diffusion; Dikes; Experimental studies; Facies; Field studies; Fluid dynamics; Geochemical indicators; Geochemistry; Geologic thermometry; Heat flux; Hydrothermal conditions; Intrusions; Isotope ratios; Isotopes; Laboratory studies; Lineaments; Lower crust; Metals; Metamorphic rocks; Metaplutonic rocks; Metasomatism; Mid-ocean ridges; O-18/O-16; Oceanic crust; Ophiolite; Ophiolite complexes; Oxygen; Permeability; Petrology; Plutons; Semail Ophiolite; Sr-87/Sr-86; Stable isotopes; Strontium; Tectonics; Thermal history; Trace elements; Tracers; Arabian Peninsula; Asia; Oman
Coordinates:N230000 N240000 E0580000 E0570000
Abstract Numbers:06M/4282
Record ID:2006088301
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from the American Journal of Science, Yale University, Kline Geology Laboratory, New Haven, CT, United States
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