April 2, 2012
Monday Geology Picture: Ferromanganese-Encrusted Boulder from the Ninetyeast Ridge, Indian Ocean
Posted by Evelyn Mervine
Two weeks ago I posted a picture of a fragment of boytroidal ferromanganese crust that was dredged from the Ninetyeast Ridge in the Indian Ocean. Today I thought I would share a few pictures of a ferromanganese-encrusted boulder that was recovered from the Ninetyeast Ridge during the same 2007 expedition.
Ferromanganese crusts actually caused some trouble for our rock descriptions. Before we could identify many rocks, we had to chip or saw off the ferromanganese crusts. Actually, due to alteration we had to break or saw apart many rocks in order to expose fresher surfaces that were easier to identify.
I’m afraid that rock-breaking became somewhat of a competitive sport on the expedition. Well, I guess there isn’t much else to do (other than describe and sort rocks) on a two month expedition in the middle of the Indian Ocean.
Someone sent us a chunk of ferromaganese thinking it was an iron meteorite. The stuff is quite heavy for its size. It now sits in our meteorite case under the meteorwrong section.
That’s great, Ryan! I wonder how that person obtained some ferromanganese…
From what I understand, it’s a byproduct of metal production. It’s called slag.
I should clarify that by saying that the stuff we receive is slag. I’m not saying that all ferromanganese is slag…
I have a question pertaining to the formation of ferromagnesian crusts at the site of mid-ocean ridges and possible mineralization associated with them.
In Physical chemistry of Atkins (2006) the standard potential of the following half-reactions are cited as follows:
Mn3+ + e- → Mn2+ + 1.51
Fe3+ + e- → Fe2+ +0.77
The data suggests that more oxidizing conditions are needed to oxidize Mn2+ to Mn3+ than oxidize Fe2+ to Fe3+. As I remember, I am told that magnesium deposits occur farthest onshore in contrast to Banded Iron Formations which occur deeper down where the conditions are more reducing in terms of oxygen availability.
Uranium precipitation from hydrothermal solutions are especially sensitive to oxidation-reduction reactions and U6+ is dissolved in fluids whereas U4+ precipitates.
Are there any contrasts between mid-ocean ridge hydrothermal vents precipitating Mn3+ or Fe3+ upon their entry into the cold-sea water in terms of the metals precipitated in the vent outlet? I know that hydrothermal fluids originating from mantle have meager contents of uranium. But since Mn-bearing fluids containing no Fe must be more oxidized than Fe-bearing fluids, the former must be able to carry more uranium than the latter. Since Th precipitation is independent of oxidation-reduction reactions, do Mn-crusts at the outlet of deep see hydrothermal vents have higher U/Th ratios than Fe-crusts?