The age of the earth is irrelevant in this post. I'm simply giving the range that Rh-Os can measure. The context here is radiology not evolution.
We absolutely do know the original amount. It Is the current amount of Rh plus the current amount of Os. Why? Looking back in time, the Os in the sample was once all Rh. That was the amount of the parent element in the material at the start, namely the sum of amounts of Rh + Os. The Os comes from the original Rh. It was once Rh, and now it's Os. It's as simple as that.
Rh - Os is embedded in diamond, not exposed to the air like carbon samples are. If there was Os contamination it could easily be measured because only 1.96% of Osmium-187 isotope is naturally occurring. If there was contamination almost all of it would be from the heavier isotopes, Os-188 through Os-192 which are easily weeded out.
OTOH, Diamonds and old rocks are exposed to open air and much more subject to contamination if C14 is used for dating.
The decay rates depends on fundamental physical forces and constants. Isotopes can undergo beta decay, alpha decay, and gamma decay. If any of these forces were not constant, the stars and planets would have blown up or shriveled up long ago.
Those are my 3 responses. If you disagree with any responses what physical reason do you have.
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