The author reviews various contributions that have been made on the
age of the Earth, starting with Patterson's work with meterorites.
This led to a figure of 4.55 Ga being adopted. Some
meteorites with older dates have been obtained and there has been a
tendency to give additional weight to these and a date of 4.6 Ga is
widely cited. However, numerous researchers have pointed to
evidence that the Earth is younger than the meteorites - although
"disagreement is abundant and the younger age of Earth has not been
Zhang's interesting study uses a model independent approach to
obtaining a mean Xe closure age for the Earth. The conclusion is that
"the accretion of Earth roughly ended at 4.45 Ga, and there was only
much slower mass addition (no giant event that could cause Xe loss)
after this event. · Therefore, all the presently available data are
consistent with a 4.45 +- 0.02 Ga age of Earth".
Perhaps the primary reason for drawing attention to this is because of
its relevance to abiogenesis, the earth's early atmosphere and to the
timescale available. Zhang refers to 4.45 Ga as the probable time of
"the last Martian-size giant impact that stripped Earth's
protoatmosphere and rehomogenised Earth." Thereafter, in Zhang's
view, the Earth can be treated as a closed system - despite subsequent
bolide impacts. Models of abiogenesis need to start at this point
(100 Ma - 150 Ma later than has been widely thought). (Having drawn
attention to reducing time availability in the Archaean, I thought it
relevant also to point out this reduction in time availability).
A secondary reason for drawing attention to this paper is the comment
about concordance that is given towards the end.
"Furthermore, the similarity between the Xe-closure age, the Pb model
and geochron age, and the Sm-Nd age suggests a common cause that
re-initialised all clocks (including Xe accumulation, core-mantle
segregation, and mantle-crust segregation·"
The interesting point for me is that a global catastrophic event is
perceived as re-initialising clocks and providing an explanation for
concordance. I do not want to make too much of this - except to say
that this approach to explaining concordance does not seem to me to
have been given enough emphasis in the literature relating to
David J. Tyler.