There are several user-interface elements that could be enhanced by the use of tool-tips. None more so that the “Mark All Read” button.
While the icon is appropriate, it is not unequivocally clear what it will do when clicked, so a tool-tip would reinforce its purpose.
Unlike the other navigation buttons, Mark All Read is difficult or impossible to undo afterwards, so of all the buttons suffering from the Mystery Meat Navigation anti-pattern, this button has priority.