Mineralogy and Geochemistry of Vent Deposits from Two Atypical Vent Sites on the Ultraslow Spreading Mid-Cayman Rise

Meg Tivey, Marine Chemistry and Geochemistry



A major goal of the Ocean Ridge Initiative is to make new discoveries along the dynamic mid-ocean ridge system, and investigate the impact of ridge processes on the global ocean.  A recent exciting discovery was made in 2010, using the deep-diving vehicle Nereus, under the direction of Chief Scientist Chris German.  Two sites of hydrothermal venting were discovered using a systematic surveying technique along the Mid-Cayman Rise (MCR).  The two hydrothermal systems are both atypical relative to known systems.  One is venting moderate temperature (≤226ºC) fluids and hosted in an oceanic core complex (a section of lower crust/mantle rocks exposed during seafloor spreading/faulting) at a depth of ~2350m (Von Damm Vent Field – VDVF); the other is hosted in basalt at nearly 5000m (Piccard Vent Field), making it the deepest active seafloor hydrothermal system sampled to date.  In January 2012 these two vent fields were mapped and sampled using Jason 2 and the RV Atlantis, and samples of vent fluids, plume particles, biota, and vent deposits were recovered.

The focus of this study is on understanding vent deposit formation under conditions that differ significantly from those at other seafloor vent sites.  The suite of collected seafloor vent deposit samples to be used include five hydrothermal deposit samples from the VDVF that include minor amounts of sulfide minerals, four recovered from the same locations where vent fluids were sampled (measured fluid temperatures ranged from 114 to 226ºC), and fifteen seafloor massive sulfide samples, including two black smoker chimneys that were taken from the same location as high quality fluid samples (measured fluid temperatures were 395 and 398ºC) from the Piccard Vent Field.  The objectives of the project are to 1) identify mineral content and geochemistry of these MCR vent deposits, and major differences relative to other seafloor massive sulfides; and 2) acquire preliminary data to submit a mature proposal to the NSF for study of MCR samples and partitioning of trace elements into sulfide minerals (e.g., pyrite, pyrrhotite, chalcopyrite, sphalerite).  Interpretation of all data (bulk geochemistry, mineralogy, trace element concentrations in specific minerals measured using both electron microprobe and ion microprobe) will be made with knowledge of the vent fluid chemistry.

Vent deposit mineral assemblages are records of current and past hydrothermal activity, and provide information about the conditions of deposit formation.  A major goal of studies of active seafloor hydrothermal systems has been to correlate mineral assemblages and trace element concentrations to known vent fluid concentrations and geologic setting.  While vent systems are ephemeral, the deposits remain and can be used to deduce past conditions.  The atypical nature of the two MCR vent systems offer an important opportunity to expand our understanding of the effects of extreme depth and source rock composition on hydrothermal circulation, metal transport, and formation of mineral deposits in the deep ocean.