This is a comment on a post of Glen Morton (14 Sept) in which he
critiques the Rb/Sr dating of Grand Canyon rocks by Steve Austin.
He suggest's that Steve's methodology is flawed, because whole
rock samples were used rather than specific minerals within a
rock. I do not want to discuss Steve Austin's interpretations,
but I am mystified by Glen's remarks about methodology. This
post explains why.
GM: " ... Steve has taken several rocks and used them to
construct an isochron. This is not the way the method works.
Using multiple rocks creates problems. You are supposed to create
your isochron by using several minerals in the same rock."
..... Steve created his date by using several rocks. Steve
violates the assumptions of the method."
A bit of theory to start: Rb and Sr are trace elements in the
earth's rocks. Several isotopes of these elements exist, and one
of them (87Rb) is subject to radioactive decay to produce 87Sr.
Thus, with time, 87Rb/86Sr ratios decrease and 87Sr/86Sr ratios
increase.
As magmas crystallise, different minerals take up different
amounts of the trace elements. Thus, processes of fractional
crystallisation result in changes in the 87Rb/86Sr ratio (when
compared with the source magma) whereas the Sr-isotope ratios
will be unchanged. By measuring these ratios, either from
different minerals, or from a suite of rocks derived from the
same primary magma, an isochron is obtained which allows
calculation of both the time of formation and the initial Sr-
isotope ratio.
The 87Rb/86Sr ratios will vary in different minerals because of
the differing partition coefficients for these elements. (The
partition coefficient relates the concentration of the element
in the crystals to the concentration in the liquid). So
separating out different minerals and obtaining the appropriate
trace element data should produce an isochron.
Theoretically, the whole rock isochrons and the mineral isochrons
should give the same age determination. However, more ages are
discordant than concordant - but the tendency has been TO GIVE
GREATER WEIGHT TO THE WHOLE ROCK DATA. More on this later.
It should be pointed out that for many rocks, particularly fine-
grained rocks like basalt and rhyolite, separation of the
minerals is unrealistic: they are far too small! So, working
with whole rock isochrons is the only possible method. Samples
are taken from different localities, but all traceable to the
same primary magma. The expectation is that these samples will
represent different stages of fractional crystallisation and so
will contain different 87Rb/86Sr ratios. This procedure is, as
I understand it, the norm, and the majority of Rb/Sr age
determinations have been carried out using whole rock samples.
Minerals may be studied separately from the host rock when there
is a suggestion that they are not the same age as their host
rock. This occurs, for example, in metamorphism. This is the
reason, as I understand it, for preferring the whole rock age to
the mineral age - and for the growing awareness that metamorphic
"resetting" events are more widespread than was first thought.