My guess is your outlook mail icon was actually just a web page link, when chrome got set as the default browser it applied it's icon to all web related items like .url files which is what the outlook icon actually is.
You can manually change the icon, just right click on it, select properties and under web document tab you will see change icon, click that and then point it at a new icon file
like this or
any other that takes your fancy (download the .ico that is the one for Windows).