Abstract Details
(2020) Chemical Mass Balance, Depositional Efficiency, and Rates of Formation of Seafloor Massive Sulfide Deposits
Jamieson J, Sanchez Mora D, Peterkin B, Barreyre T, Escartin J & Cannat M
https://doi.org/10.46427/gold2020.1168
08m: Room 3, Tuesday 23rd June 22:06 - 22:09
John Jamieson
View all 4 abstracts at Goldschmidt2020
View abstracts at 9 conferences in series
Dennis Sanchez Mora
Ben Peterkin
Thibaut Barreyre View abstracts at 4 conferences in series
Javier Escartin View abstracts at 9 conferences in series
Mathilde Cannat View abstracts at 11 conferences in series
Dennis Sanchez Mora
Ben Peterkin
Thibaut Barreyre View abstracts at 4 conferences in series
Javier Escartin View abstracts at 9 conferences in series
Mathilde Cannat View abstracts at 11 conferences in series
Listed below are questions that have been submitted by the community that the author will try and cover in their presentation. To submit a question, ensure you are signed in to the website. Authors or session conveners approve questions before they are displayed here.
Submitted by Daniel Gregory on Thursday 18th June 22:42
Thanks John, great talk. Can you talk a bit about what factors are most important for getting high depositional efficiency? Is redox of the water column an important factor?
Hi Dan, Excellent questions. The depositional efficiency is ultimately a product of the degree of cool and mixing with local seawater that takes place in the subseafloor. There are many variables that can influence mixing and cooling (e.g., fluid temperature, flux, and composition; deposit size, age, and mineralogy; composition and permeability of the substrate). However, the inter-dependence of these different variables is difficult to evaluate. For example, increased hydrothermal sealing of conduits over time from the precipitation of minerals along fluid flow pathways could decrease depositional efficiency by reducing mixing or cooling of the hydrothermal fluids in the subsurface. However, hydrothermal sealing could also enhance depositional efficiency by forming silica or barite caps at the seafloor that reduce the potential for focused discharge at the seafloor. Overall, the numbers of variables that need to be considered when evaluating depositional efficiency suggest that there is no generalized efficiency that can be applied to seafloor hydrothermal systems in general. Instead there is likely a significant range of values and each deposit should be assessed independently. Hopefully, as we perform more of these types of investigations, trends associated with specific geological factors (substrate, tectonic setting, age) will emerge. With respect to the influence of the redox state of the overlying water column, I do not think that it has a significant direct influence on depositional efficiency. However, the amount of dissolved oxygen in the local seawater could influence the rate of dissolution or replacement of sulfide minerals within the deposit, which, over time, could influence the permeability of the hydrothermal deposit and thus depositional efficiency.
Thanks John, great talk. Can you talk a bit about what factors are most important for getting high depositional efficiency? Is redox of the water column an important factor?
Hi Dan, Excellent questions. The depositional efficiency is ultimately a product of the degree of cool and mixing with local seawater that takes place in the subseafloor. There are many variables that can influence mixing and cooling (e.g., fluid temperature, flux, and composition; deposit size, age, and mineralogy; composition and permeability of the substrate). However, the inter-dependence of these different variables is difficult to evaluate. For example, increased hydrothermal sealing of conduits over time from the precipitation of minerals along fluid flow pathways could decrease depositional efficiency by reducing mixing or cooling of the hydrothermal fluids in the subsurface. However, hydrothermal sealing could also enhance depositional efficiency by forming silica or barite caps at the seafloor that reduce the potential for focused discharge at the seafloor. Overall, the numbers of variables that need to be considered when evaluating depositional efficiency suggest that there is no generalized efficiency that can be applied to seafloor hydrothermal systems in general. Instead there is likely a significant range of values and each deposit should be assessed independently. Hopefully, as we perform more of these types of investigations, trends associated with specific geological factors (substrate, tectonic setting, age) will emerge. With respect to the influence of the redox state of the overlying water column, I do not think that it has a significant direct influence on depositional efficiency. However, the amount of dissolved oxygen in the local seawater could influence the rate of dissolution or replacement of sulfide minerals within the deposit, which, over time, could influence the permeability of the hydrothermal deposit and thus depositional efficiency.
Submitted by Melissa Anderson on Tuesday 23rd June 18:53
Hi John, I really enjoyed this talk. For your chart with the size vs. age, the fact that the one TAG sample is an outlier is probably why your R^2 is so high (it acts as an influential point). What mass accumulation rate do you get if you don't include that point?
Right. Good question. So, if the TAG Field data are excluded, the R^2 value decreases to 0.79, which is still high, but not spectacular for 4 data points, and, like I said in the recorded talk, I'm not sure if that correlation is meaningful. But it is certainly intriguing. The large range of values that we calculate for depositional efficiency would indicate that size and age should not necessarily correlate (i.e. not represent a consistent mass accumulation rate). However, our calculated depositional efficiencies also seem to be scale dependent, so I think that understanding the size/age correlation will be dependent on first understanding why our calculated efficiencies decrease with increasing scale of observation. Considering the idea that observations and calculations at a vent field, ridge segment, and ultimately global scale would include increasing proportions of diffuse, lower temperature discharge, one would expect that the efficiencies of deposition should increase, not decrease.
Hi John, I really enjoyed this talk. For your chart with the size vs. age, the fact that the one TAG sample is an outlier is probably why your R^2 is so high (it acts as an influential point). What mass accumulation rate do you get if you don't include that point?
Right. Good question. So, if the TAG Field data are excluded, the R^2 value decreases to 0.79, which is still high, but not spectacular for 4 data points, and, like I said in the recorded talk, I'm not sure if that correlation is meaningful. But it is certainly intriguing. The large range of values that we calculate for depositional efficiency would indicate that size and age should not necessarily correlate (i.e. not represent a consistent mass accumulation rate). However, our calculated depositional efficiencies also seem to be scale dependent, so I think that understanding the size/age correlation will be dependent on first understanding why our calculated efficiencies decrease with increasing scale of observation. Considering the idea that observations and calculations at a vent field, ridge segment, and ultimately global scale would include increasing proportions of diffuse, lower temperature discharge, one would expect that the efficiencies of deposition should increase, not decrease.
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