Tectono-magmatic controls on hydrothermal activity at the Mid-Atlantic Ridge vent fields Logatchev and 5°S

• Applicant: Professor Dr. Lars Helmuth Rüpke
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: from 2012 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 211158050

• Final Report Year: 2017

Final Report Abstract

The discovery of hydrothermal systems at mid-ocean ridges in the late 1970s was one of the most exciting achievements in oceanography. Hydrothermal systems efficiently mine heat from the young ocean crust, sustain unique ecosystems, and are related to the formation of commercially interesting ore deposits. Much has been learned from marine surveys of mid-ocean ridge segments and direct observations of vent sites at the ocean floor. The deep chemical and physical processes that control hydrothermalism remain, however, largely inaccessible to direct sampling and observations. Here we have used a joint model-data approach to elucidate the contrasting hydrothermal flow patterns beneath slow-spreading ridges that are in a tectonic phase and fastspreading ridges that are magma-dominated. At fault-controlled off-axis hydrothermal fields like Logatchev 1, hydrothermal upflow is channelized along permeable fault zones. Such long-lived preferential fluid pathways are also thought to be responsible for the larger massive sulphide ore deposits found along the Mid-Atlantic Ridge with respect to the smaller deposits of the fast-spreading East Pacific Rise. Efficient metal leaching and transport requires high-temperature fluid flow (>350°C). We found, maybe against intuition, that high-temperature upflow along highly permeable found zones only occurs for a relatively narrow range of fault widths and permeability contrasts. The fault zones that are just permeable enough to channelize upflow are the ones that result in the most efficient metal transport and in high temperature venting. Mid-ocean ridge segments dominated by magmatic crustal accretion do not show major deepreaching faults and hydrothermal flow has long been thought to either occur directly on-axis above the melt lens or along across-axis-oriented convection cells. We have designed a 3-D whole-crust convection model that revealed the existence of two interacting flow components that merge above the melt lens to feed ridge-centered vent sites. This new hybrid convection mode allowed reconciling previously incompatible models favoring either shallower on-axis or deeper off-axis hydrothermal circulation. In summary, we were able to constrain the contrasting styles of hydrothermal cooling at tectonic and magmatic mid-ocean ridge systems and to quantify the implications for the associated hydrothermal mass and energy fluxes.

Publications

• (2017) Faulting and off-axis submarine massive sulfide accumulation at slow spreading mid-ocean ridges: A numerical modeling perspective. Geochem. Geophys. Geosyst. (Geochemistry, Geophysics, Geosystems) 18 (6) 2305–2320
  Andersen, C.; Theissen-Krah, S.; Hannington, M.; Rüpke, L.; Petersen, S.
  
• (2014): Hybrid shallow on-axis and deep off-axis hydrothermal circulation at fastspreading ridges, Nature, Vol 508, Issue 7497
  Hasenclever J., Theissen-Krah S., Rüpke LH., Morgan JP., Iyer K., Petersen S., Devey CW.
  
• (2015): Fault geometry and permeability contrast control vent temperatures at the Logatchev 1 hydrothermal field, Mid-Atlantic Ridge, Geology
  Andersen C., Rüpke LH., Hasenclever J., Grevemeyer I., Petersen S.