I'm very sorry to hear of the trouble you're having with your marriage. It must be very disheartening to hear your wife say she no longer wants to be married. This could shake anyone to the core.
You've received a lot of advice already, which I won't reiterate, so I'd like to take a different tack here. Your wife certainly sounds confused and like she's considering taking a different path in life. Often, people feel they need time on their own to help them know whether the person they're apart from (in this case, you) is the right person for them. This can be hard, but if it is her decision, you really have no choice.
However, there are things you DO have choices about - your own behaviours, your relationship with your kids, and with family and friends. These are the things you can choose to focus on. You say that you and your wife no social life, no friends or family close by. This might be the time to change that. Perhaps you can make efforts to reach out to others, to make new friends, possibly people you've met through work, through your local church, through a support group, or through pursuing a new hobby.
There is no way to know if these actions will have any effect on your wife, or on her decision to stay. Either way, you're making yourself a more independent (and therefore attractive) partner (hopefully for your wife, but if not, then for someone else in the future). Essentially, what you're saying is, "I love you and I support you, but I won't stop living my life waiting for you to make up your mind."
Yes, your wife is depressed, and this may or may not have much to do with her decision, and you should certainly continue to support and love her through her depression. At the same time, you need to respect and take care of yourself as well as your family. Depressed people naturally turn inward, and to some extent that is to be expected. However, your wife is not the only person in this relationship, not the only one hurting, or with feelings. You certainly love her and want her to stay with you, but you won't be able to tolerate an unending amount of pain yourself with her wavering back-and-forth regarding the fate of your marriage. To do so for long is indulgent at best, and selfish at worst - and this is true regardless of whether someone is depressed or not. (And yes, I'm a woman myself, and I've been depressed and mistreated others, so I know of what I speak). If this persists, you may want to let your wife gently know you have limits. You owe both of you this. Otherwise, you risk becoming yourself depressed.
Just because someone is depressed doesn't mean that that's the reason behind all their decisions. You say that you want to find the answer to why your wife feels this way - whether it's you or her depression. Remember - it's likely that your wife doesn't know herself, and at any rate, it's more likely neither of these things so much as other things happening within her of which you, and likely she herself, are unaware. Even if she believes that her depression is what motivates her desire to leave, there is a very real possibility that your marriage was already in trouble, quite apart from your wife's depression, and your own comments reveal some good insight into this. Thus, to chalk all this up as a symptom of her depression may be a real mistake, and a missed opportunity to gain some important insight into yourself as well.
Although it's difficult to consider, it may be that you and your wife have simply grown apart and in different ways. Personal counseling (and couples counseling, if your wife agrees to go) is an excellent way to re-discover yourself and where you're at and how you've ended up here. If you decide to try and save your marriage, then all you have control over is your own behaviour. Remember this: when you change, the relationship changes. It's really that simple. And it may well be that your changing will itself bring about changes in your wife.
I can't reassure you as others have that your wife will change her mind; I know that is well-meaning, but I don't know that to be true. What I do know is that you sound like a compassionate and caring man, capable of great love. Try to focus as much on yourself as on your wife and her problems. We can never know what the future holds, but we can act in ways that sway things in our favour. At the end of the day, this marriage may not last, but I've no doubt that someone who values love and companionship as much as you say, will find a deep and enduring love with a partner equally committed and capable of commitment.
Please know that I am not suggesting you abandon your wife here, especially in her time of need. What I am suggesting is that you not forget to focus on yourself - it is all we really can control, and there is comfort in feeling we have control over something at times like this.
I sincerely hope that your wife finds her way through depression and discovers what it is she wants. For you, I wish peace and courage through these difficult times. They will pass, and when the dust settles, you will no doubt handle what remains - whatever that may be - with a newfound strength and wisdom.