My point exactly!!! Why exactly 4.5B? Hello??? I love you scientists, I really do.
Do you actually have a question here? Or is the inclusion of the daytime tv show phrase "Hello????" intended to assure us you actually
care about an answer?
In case you actually do care to learn the simple version is:
Primary rocks have been found in N. America, Greenland Australia, Africa and Asia (that's at least 4 continents) that can be radiometrically dated (by independent means) to
at least 3.5 billion years old (scientists sometimes shorten this to 3.5GA which stands for "giga-annum", billion years).
But since we know this represents a
minimum age and we don't know for sure exactly how old we keep looking around to see if anything older shows up.
A few grains in sedimentary rocks reflect an older age of around 4.1 to 4.2GA. So we know we still haven't found the oldest.
But we are reasonably sure the earth and other objects in our solar system are formed from a
single accretionary disk rotating around the sun, which, if we can measure stuff out there then we might see older rocks
from the same "soup" from which we were born.
And we get these occasionally in the form of
meteorites. The good thing about meteorites is that since they are not "geologically active" (ie they aren't melting or have plate tectonics or other processes that will erase the oldest materials, we might be even more sure of getting older dates.
Here's a table showing some meteorite dates and the various different isotopes used.
Type, Number Dated, Method, .................................Age (billionsof years)
Chondrites (CM, CV, H, L, LL, E)13Sm-Nd ......................4.21 +/- 0.76
Carbonaceous chondrites4Rb-Sr ....................................4.37 +/- 0.34
Chondrites (undisturbed H, LL, E)38Rb-Sr .......................4.50 +/- 0.02
Chondrites (H, L, LL, E)50Rb-Sr .....................................4.43 +/- 0.04
H Chondrites (undisturbed)17Rb-Sr ................................4.52 +/- 0.04
H Chondrites15Rb-Sr ....................................................4.59 +/- 0.06
L Chondrites (relatively undisturbed)6Rb-Sr ....................4.44 +/- 0.12
L Chondrites5Rb-Sr .....................................................4.38 +/- 0.12
LL Chondrites (undisturbed)13Rb-Sr .............................4.49 +/- 0.02
LL Chondrites10Rb-Sr ..................................................4.46 +/- 0.06
E Chondrites (undisturbed)8Rb-Sr .................................4.51 +/- 0.04
E Chondrites8Rb-Sr .....................................................4.44 +/- 0.13
Eucrites (polymict)23Rb-Sr ..........................................4.53 +/- 0.19
Eucrites11Rb-Sr ..........................................................4.44 +/- 0.30
Eucrites13Lu-Hf .........................................................4.57 +/- 0.19
Diogenites5Rb-Sr .......................................................4.45 +/- 0.18
Iron (plus iron from St. Severin)8Re-Os .......................4.57 +/- 0.21
After
Dalrymple (1991, p. 291); duplicate studies on identical meteorite types omitted.
So, ya see, we are really narrowing it down. 4.55GA seems to be the zone.