[R] is there a function to find the quantile of the mean of a vector?

[David LeBauer]

Hello,

I am interested in finding the quantile of the mean of a vector,
something analogous to using the pnorm(), but for an mcmc chain
instead of a distribution with known parameters.

One approach would be to write a function that finds the index of x_i
that minimizes (x-mean(x))^2

I suspect there is a function available to do this, but I can't find it?

Thank you,

David