Stack allocated immutable arrays

Question

I'm doing extensive computations in f# on short arrays of uint64; I'd like to stack allocate them to avoid the garbage collector running. In C++, I'd do this:

int search(int n, uint64_t* data) {
    while ( /* something */ ) {
        // ... do some computation on data.
        uint64_t* data2 = (uint64_t*) alloca(n * sizeof(uint64_t));
        for (int i=0; i<n; ++i) { data2[i] = /* compute derived value */; }
        int candidate = search (n, data2);
        // ... determine if candidate is better than best etc.
    }
}

But f# doesn't seem to have a primitive, stack-allocated array type?