How to parallelize a loop in Unity using System.Threading?

Question

C++ / C#

The essence of the problem is simple. I have a giant three-dimensional float4 array (from the Unity.Mathematics library), and there is a loop that performs calculations over each element of this array, and puts the results in order into two sheets. I just could not figure out how to use System.Threading to:
1) Divide the execution of the loop equally between threads
2) Keep the order of the results in the sheets