Geology Reference
In-Depth Information
And what about the continents the sediments had been derived
from? They must have been even older! It was an exciting prospect.
Holmes pondered these thoughts. He had been extremely
impressed with the quality of Nier's work on the Manitoba peg-
matite and the other twenty-four samples, and entertained the
hope that from these precise data
'it might be possible to fathom
the depths of geological time'
. He recognised that the twenty-
five samples not only gave information about their individual
ages, but that locked up in the data could be evidence pertain-
ing to the initial state of the Earth, its 'primeval' state, if only
he could extract it. It was time to resurrect an old idea.
Fifteen years earlier, in 1932, when working on the possibil-
ity of using another decay scheme, the decay of potassium 41 to
calcium 41, as a new isotopic system for dating rocks, Holmes
had devised the principle of using primeval (or initial) isotope
ratios to calculate
'the time when the separation of the granitic
and basaltic layers took place in the newly-formed earth'
. In
essence the idea was to determine the original ratio of two iso-
topes in any isotopic system as it was when the Earth first
formed. By knowing the rate of decay of that system, the time
taken for the initial ratio to evolve to its present day value would
essentially be the true age of the Earth.
Similarly, it would be possible to determine, for example,
whether a granite had been derived from a basalt or from melt-
ing of the crust, simply by knowing the isotope ratio of the gran-
ite today, and back-calculating to see what the initial ratio had
been when the melt first formed. A granite derived from melt-
ing the crust would contain far more potassium than one derived
by differentiation from a basalt, so the ratios of potassium to
calcium isotopes, after a significant period of time had elapsed,
would be demonstrably different and could be used to indicate
the source material. Holmes recognised that the idea had
tremendous potential not just for dating the age of the Earth,
but for solving many problems related to the origin of igneous
rocks and termed it
'A New Key to Petrogenesis'
.
