Hydrothermal alteration of an intact section of the upper oceanic crust formed at a superfast spreading rate
Lead Research Organisation:
University of Southampton
Department Name: Sch of Ocean and Earth Science
Abstract
The mid-ocean ridges form a chain of mountains in the oceans that circuit the Earth like seams on a baseball. These ridges are constructive plate boundaries where new ocean crust is formed by plate tectonic spreading and the eruption of magma from the mantle. This is the major process by which the Earth releases its internal heat, and ~60% of the Earth's surface was formed in the past 180 million years. Because magma is erupted onto and intruded into the ocean crust at ~1200 deg C, seawater that percolates into the crust becomes vigorously heated, commonly resulting in submarine geysers known as black smokers that disgorge >350 deg C, sulfide-rich fluids. There is a very close relationship between magma and hydrothermal 'hot water' circulation. Because the ocean ridges are mostly beneath >2000 m of water many of the processes related to their formation remain poorly understood. Scientists using submersibles can observe only the most recently erupted lavas. Geophysicists can make measurements of the ocean crust using seismic velocities (the speed that waves travel through rocks), but their results must be calibrated against actual rocks to be understood. Sampling the sub-surface of the ocean crust requires deep drilling and the recovery of basement rock cores has been a major goal of scientific ocean drilling since the 1960s. Ancient oceanic rocks preserved on land, known as ophiolites, suggest that the ocean crust is made of three basic layers: erupted lavas, commonly with pillowed shapes, that overly vertical intrusions known as sheeted dikes, which in turn overly, coarse-grained gabbroic rocks that are crystallized magma chambers. Until very recently, only one drill hole had penetrated the lavas into the dikes and no hole had sampled the dike-gabbro boundary even though that zone is the most critical in terms of heat exchange between magma and seawater, because those phases are separated by only a thin thermal contact layer. IODP Expeditions 309-312 recently returned to Hole 1256D, a deep drill hole in the eastern Pacific, and deepened that hole to >1500 m, completely through the lavas, dikes, and into the gabbros. Hole 1256D was drilled into crust that formed at a very fast spreading rate (>200 mm/yr), because gabbros were predicted to be at their shallowest there. Although only ~20% of the global ridge axis is spreading at fast spreading rates (>80 mm/yr full rate), 60% of the current ocean basins and ~30% of the Earth's surface was formed by fast spreading. Because crust formed at fast spreading rates should be relatively uniform, drilling at a single location can be reasonably extrapolated to describe a significant portion of the Earth's crust. The cores from Hole 1256D are a unique, hitherto unavailable, resource for understanding the igneous construction of the ocean crust and interactions between magma and seawater. By making careful descriptions linked with chemical analyses we will be able to work out the physical conditions of fluid-rock interaction and estimate the chemical and isotopic composition of the fluids. From the extent of chemical exchange we can make estimates of how much seawater passed through the crust. Using this new information for Hole 1256D we can compare our results with estimates of hydrothermal fluxes from global chemical budgets (such as poorly known estimates of river discharge), other less complete sections of ocean crust, and ophiolites. The amount of seawater that cools the ocean crust, and the temperatures to which it is heated, greatly influences the location and size of mid-ocean ridge magma chambers. Once we have worked out the conditions and fluxes of hydrothermal alteration, we will use these constraints in computer simulations that couple magma intrusion and crystallization with hydrothermal circulation. These computer models will allow us to establish what size and arrangements of magma chambers and hydrothermal fluid circulation are physically plausible.
Publications
Alt J
(2010)
Subsurface structure of a submarine hydrothermal system in ocean crust formed at the East Pacific Rise, ODP/IODP Site 1256
in Geochemistry, Geophysics, Geosystems
Boskabadi A
(2016)
Carbonate alteration of ophiolitic rocks in the Arabian-Nubian Shield of Egypt: sources and compositions of the carbonating fluid and implications for the formation of Au deposits
in International Geology Review
Brandstätter J
(2018)
The Origin of Carbonate Veins Within the Sedimentary Cover and Igneous Rocks of the Cocos Ridge: Results From IODP Hole U1414A.
in Geochemistry, geophysics, geosystems : G(3)
Busigny V
(2019)
A re-assessment of the nitrogen geochemical behavior in upper oceanic crust from Hole 504B: Implications for subduction budget in Central America
in Earth and Planetary Science Letters
Coggon R
(2011)
Hydrothermal calcium-carbonate veins reveal past ocean chemistry
in TrAC Trends in Analytical Chemistry
Coggon R
(2016)
Hydrothermal contributions to global biogeochemical cycles: Insights from the Macquarie Island ophiolite
in Lithos
Coggon R
(2014)
Controls on thallium uptake during hydrothermal alteration of the upper ocean crust
in Geochimica et Cosmochimica Acta
Coggon RM
(2010)
Reconstructing past seawater Mg/Ca and Sr/Ca from mid-ocean ridge flank calcium carbonate veins.
in Science (New York, N.Y.)
Gao Y
(2012)
Downhole variation of lithium and oxygen isotopic compositions of oceanic crust at East Pacific Rise, ODP Site 1256
in Geochemistry, Geophysics, Geosystems
Harris M
(2017)
Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D
in Earth and Planetary Science Letters
Harris M
(2015)
Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D
in Earth and Planetary Science Letters
Josso P
(2018)
Extraction and separation of rare earth elements from hydrothermal metalliferous sediments
in Minerals Engineering
Michibayashi K
(2019)
What Lies Beneath: The Formation and Evolution of Oceanic Lithosphere
in Oceanography
Patten C
(2017)
Hydrothermal mobilisation of Au and other metals in supra-subduction oceanic crust: Insights from the Troodos ophiolite
in Ore Geology Reviews
Patten C
(2015)
Mobility of Au and related elements during the hydrothermal alteration of the oceanic crust: implications for the sources of metals in VMS deposits
in Mineralium Deposita
Tominaga M
(2009)
Determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge: Electrofacies analyses of ODP/IODP Hole 1256D
in Geochemistry, Geophysics, Geosystems
Umino S
(2008)
Origin of the sheeted dike complex at superfast spread East Pacific Rise revealed by deep ocean crust drilling at Ocean Drilling Program Hole 1256D
in Geochemistry, Geophysics, Geosystems
Vance D
(2009)
Variable Quaternary chemical weathering fluxes and imbalances in marine geochemical budgets.
in Nature
Zihlmann B
(2018)
Hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite, Oman
in Lithos