Are ethnic "minorities" given preference in PhD admissions in the US?
There is a slight preference in the US for what are known as "underrepresented minorities"—that is, people who belong to groups who are not adequately represented in college enrollment relative to their proportion in the general population. That currently includes groups such as Hispanics, African-Americans, and Native Americans, but not groups such as Asian-Americans. So long as someone is a citizen, it does not really matter if one was naturalized or was a "born" citizen.

This does not mean that quotas are used, but it can be used as a "plus" quality in terms of admissions and hiring decisions.