Hannington Mark D.

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Hannington
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Mark D.
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  • Article
    Sulfide geochronology along the Endeavour Segment of the Juan de Fuca Ridge
    (John Wiley & Sons, 2013-07-08) Jamieson, John W. ; Hannington, Mark D. ; Clague, David A. ; Kelley, Deborah S. ; Delaney, John R. ; Holden, James F. ; Tivey, Margaret K. ; Kimpe, Linda E.
    Forty-nine hydrothermal sulfide-sulfate rock samples from the Endeavour Segment of the Juan de Fuca Ridge, northeastern Pacific Ocean, were dated by measuring the decay of 226Ra (half-life of 1600 years) in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years. This dating method is effective for samples ranging in age from ∼200 to 20,000 years old and effectively bridges an age gap between shorter- and longer-lived U-series dating techniques for hydrothermal deposits. Results show that hydrothermal venting at the active High Rise, Sasquatch, and Main Endeavour fields began at least 850, 1450, and 2300 years ago, respectively. Barite ages of other inactive deposits on the axial valley floor are between ∼1200 and ∼2200 years old, indicating past widespread hydrothermal venting outside of the currently active vent fields. Samples from the half-graben on the eastern slope of the axial valley range in age from ∼1700 to ∼2925 years, and a single sample from outside the axial valley, near the westernmost valley fault scarp is ∼5850 ± 205 years old. The spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship, with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift. These relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years.
  • Preprint
    Precipitation and growth of barite within hydrothermal vent deposits from the Endeavour Segment, Juan de Fuca Ridge
    ( 2015-10) Jamieson, John W. ; Hannington, Mark D. ; Tivey, Margaret K. ; Hansteen, Thor ; Williamson, Nicole M.-B. ; Stewart, Margaret ; Fietzke, Jan ; Butterfield, David A. ; Frische, Matthias ; Allen, Leigh ; Cousens, Brian ; Langer, Julia
    Hydrothermal vent deposits form on the seafloor as a result of cooling and mixing of hot hydrothermal fluids with cold seawater. Amongst the major sulfide and sulfate minerals that are preserved at vent sites, barite (BaSO4) is unique because it requires the direct mixing of Ba-rich hydrothermal fluid with sulfate-rich seawater in order for precipitation to occur. Because of its extremely low solubility, barite crystals preserve geochemical fingerprints associated with conditions of formation. Here, we present data from petrographic and geochemical analyses of hydrothermal barite from the Endeavour Segment of the Juan de Fuca Ridge, northeast Pacific Ocean, in order to determine the physical and chemical conditions under which barite precipitates within seafloor hydrothermal vent systems. Petrographic analyses of 22 barite-rich samples show a range of barite crystal morphologies: dendritic and acicular barite forms near the exterior vent walls, whereas larger bladed and tabular crystals occur within the interior of chimneys. A two component mixing model based on Sr concentrations and 87Sr/86Sr of both seawater and hydrothermal fluid, combined with 87Sr/86Sr data from whole rock and laser-ablation ICP-MS analyses of barite crystals indicate that barite precipitates from mixtures containing as low as 17% and as high as 88% hydrothermal fluid component, relative to seawater. Geochemical modelling of the relationship between aqueous species concentrations and degree of fluid mixing indicates that Ba2+ availability is the dominant control on mineral saturation. Observations combined with model results support that dendritic barite forms from fluids of less than 40% hydrothermal component and with a saturation index greater than ~0.6, whereas more euhedral crystals form at lower levels of supersaturation associated with greater contributions of hydrothermal fluid. Fluid inclusions within barite indicate formation temperatures of between ~120 and 240°C during barite crystallization. The comparison of fluid inclusion formation temperatures to modelled mixing temperatures indicates that conductive cooling of the vent fluid accounts for 60 – 120°C reduction in fluid temperature. Strontium zonation within individual barite crystals records fluctuations in the amount of conductive cooling within chimney walls that may result from cyclical oscillations in hydrothermal fluid flux. Barite chemistry and morphology can be used as a reliable indicator for past conditions of mineralization within both extinct seafloor hydrothermal deposits and ancient land-based volcanogenic massive sulfide deposits.
  • Article
    Should we mine the deep seafloor?
    (John Wiley & Sons, 2017-07-13) Beaulieu, Stace E. ; Graedel, Thomas E ; Hannington, Mark D.
    As land-based mineral resources become increasingly difficult and expensive to acquire, the potential for mining resources from the deep seafloor has become widely discussed and debated. Exploration leases are being granted, and technologies are under development. However, the quantity and quality of the resources are uncertain, and many worry about risks to vulnerable deep-sea ecosystems. Deep-sea mining has become part of the discussion of the United Nations Sustainable Development Goals. In this article we provide a summary of benefits, costs, and uncertainties that surround this potentially attractive but contentious topic.
  • Presentation
    Should We Mine the Seafloor? Presentations from the AAAS 2017 Annual Meeting, Boston, MA, U.S.A.
    ( 2017-02-18) Graedel, Thomas E ; Hannington, Mark D. ; Beaulieu, Stace E.
  • Preprint
    Hydrothermal exploration of mid-ocean ridges : where might the largest sulfide deposits be forming?
    ( 2015-11) German, Christopher R. ; Petersen, Sven ; Hannington, Mark D.
    Here, we review the relationship between the distribution of modern-day seafloor hydrothermal activity along the global mid-ocean ridge crest and the nature of the mineral deposits being formed at those sites. Since the first discovery of seafloor venting, a sustained body of exploration has now prospected for one form of hydrothermal activity in particular – high temperature “black smoker” venting - along >30% of the global mid ocean ridge crest. While that still leaves most of that ~60,000km continuous network to be explored, some important trends have already emerged. First, it is now known that submarine venting can occur along all mid-ocean ridges, regardless of spreading rate, and in all ocean basins. Further, to a first approximation, the abundance of currently active venting, as deduced from water column plume signals, can be scaled linearly with seafloor spreading rate (a simple proxy for magmatic heat-flux). What can also be recognized, however, is that there is an “excess” of high temperature venting along slow and ultra-slow spreading ridges when compared to what was originally predicted from seafloor spreading / magmatic heat-budget models. An examination of hydrothermal systems tracked to source on the slow spreading Mid Atlantic Ridge reveals that no more than half of the sites responsible for the “black smoker” plume signals observed in the overlying water column are associated with magmatic systems comparable to those known from fast- spreading ridges. The other half of all currently known active high-temperature submarine systems on the Mid-Atlantic Ridge are hosted under tectonic control. These systems appear both to be longer-lived than, and to give rise to much larger sulfide deposits than, their magmatic counterparts - presumably as a result of sustained fluid flow. A majority of these tectonic-hosted systems also involve water-rock interaction with ultramafic sources. Importantly, from a mineral resource perspective, this subset of tectonic-hosted vent-sites also represents the only actively-forming seafloor massive sulfide deposits on mid-ocean ridges that exhibit high concentrations of Cu and Au in their surface samples (>10wt.% average Cu content and >3ppm average Au). Along ultraslow-spreading ridges, first detailed examinations of hydrothermally active 74 sites suggest that sulfide deposit formation at those sites may depart even further from the spreading-rate model than slow-spreading ridges do. Hydrothermal plume distributions along ultraslow ridges follow the same (~50:50) distribution of “black smoker” plume signals between magmatic and tectonics settings as the slow spreading MAR. However, the first three “black smoker” sites tracked to source on any ultra-slow ridges have all revealed high temperature vent-sites that host large polymetallic sulfide deposits in both magmatic as well as tectonic settings. Further, deposits in both types of setting have now been revealed to exhibit moderate to high concentrations of Cu and Au, respectively. An important implication is that ultra- slow ridges may represent the strongest mineral resource potential for the global ridge crest, despite being host to the lowest magmatic heat budget.