This shouldn't be an answer but without knowing more information I can give you a heads up. Do you have a landscape maintenance company employed? This doesn't look like a willow to me...still could be I guess when you send pictures of the entire tree we'll know more for the ID.
You chose a super leaf specimen as it is partly alive and partly dead. When margins and tips of otherwise healthy material/leaves are browned or blackened that is a strong indication of over fertilization. High salts. This looks quite bad in fact. Don't really care that much about the ID, although that might help to figure out what is going wrong. Those leaves are showing severe problems with salt, usually fertilizer is the cause.
I am giving you a quick answer as we need more information and a picture of the entire tree and environment to be sure. But if you have a company responsible for fertilization or any chemical treatments near this tree you need to get enough information soon enough with which to start up a discussion. They would most certainly be responsible.
If this tree is near a lawn we need to know the last fertilization or pesticide application done for your lawn with what and when. Has anything been done to the plant bed in terms of pesticide, fertilizer...especially preemergent pesticide (herbicide is listed as a pesticide along with pesticides).
This is not a disease or insect. Well, fairly almost positively sure. Something has taken place within the last few weeks. Can you think of anything to add?
I've seen this before, of course, and had to go to court a few times for clients where their landscape maintenance company either screwed up with the chemicals, or the client had actually hired a separate fertilizing company to come in and fertilize a landscape after it had already been fertilized. Ugh! Many times it is the client over fertilizing or...well. Just some pre warnings. If I am at all talking sense this gives you time to document the damage, take pictures, get witnesses verified.
It might seem and I hope it is that I am blowing this out of proportion, but it just looks so similar to what people had to take to court and we all had to slug through. So, this is total supposition. Try not to get too worried. I certainly hope I am way off base but, this looks like chemical/salt damage to me.
Looking forward to more information and pictures. I apologize if I am wrong! A few clients waited far too long so I am a bit sensitized for getting the documentation in place IF this is truly even close to your problem, okay?
This type of leaf is more of a willow shrub, not a willow tree which is more long and narrow. Definitely need a picture of the entire tree. But fried tips and margins are a big indicator of high salts (fertilizer) or some chemical no matter the plant. Is this tree near an open body of water? A pond or canal for instance?
Just saw your newest picture of a weeping cherry? Not willow but weeping!! Not near the lawn but near utilities. I can't for the life of me see any connection to the utilities, not to cause this tip and margin burn.
If this is a brand new picture I am not seeing the same problem throughout that tree. Your tree looks healthy. Where did you find this branch and leaves on this tree? Have you had high winds? Are there anymore of this same problem on your tree? Any pattern such as just one side? I feel much better. Your tree looks healthy and perhaps this is just a tiny anomaly. (I am sorry again to have brought up courts and stuff!).
A close up of the base of your tree after you've pulled the mulch away down to the soil would be helpful. Mulch and soil on the bark of the tree is a bad thing but wouldn't cause the leaves to look like this. But heck, we are here so good thing to check. Soil, rocks, mulch on the bark of a tree promotes bacteria that will compromise the vascular system of your tree and will eventually girdle and kill your tree.