12
Peter Stifle and Graham Shields
2.1 Weathering of the Whole Rock
Dasch (1969) carried out the first comprehensive study of the behaviour of the
Rb-Sr isotopic system of the whole rock during weathering. Among other things
he investigated the course of weathering at various granite plutons including the
Elburton granite of Piedmont and Georgia (USA), which can be found weathered
to great depths, and tried not only to throw light on the mechanisms that take
place during weathering but also the influence of weathering on the isotopic age.
He described the various stages of weathering on the way to kaolinite. Sample
material was collected from a profile where both fresh and weathered material was
available. The geochemical profile demonstrated that Sr was preferentially
dissolved from the original rock. The fresh granite contained 290 ppm Sr, whereas
the most strongly weathered material contained only 84 ppm Sr. Rb content also
decreased with increasing weathering but to a lesser extent than with St. It follows
therefore that Rb/Sr ratios increase markedly with increasing weathering.
Thus, only few weathered samples lie on or near the isochron for unweathered
samples that would date the age of intrusion of the granite pluton (Fig. 2.1).
Strongly weathered rocks lie to the right of the age-reference line. As a
censequence, weathering results in a younging of the Rb-Sr age, as the gradient of
the reference line decreases. Another consequence of weathering, apart from the
rise in Rb/Sr ratio, can be that the STSr/gSSr ratio will rise. too. The strongly
weathered samples B-5 and B-6 also show the highest sTSr/S6Sr ratios (Fig. 2.1 ~.
This behaviour allows us to suppose that the mineral phases with low Rb/Sr and
87Sr/S6Sr ratios preferentially go into solution and get transported away, as only by
such a process could components with higher Rb/Sr and SrSr/S6Sr ratios remain
behind.
Investigations have shown that plagioclase feldspar is most important in this
respect. No study has been able to show that the Sr isotope system can be
completely rehomogenized on a large scale in a soil profile. Rb-Sr isotope
determinations on whole rock from a weathered soil profile are therefore unlikely
to yield information about the age of an ancient weathering profile. An exception
can be found in the study of Worden and Compston (1973). Worden and
Compston carried out an isotopic study on the weathering profile of a granite body
from Australia. The investigated samples were at various stages of weathering
between fresh granite and strongly decomposed material showing quartz-kaolinite
mineral paragenesis. The age of the granite was estimated at 2600 to 2700 million
years. However, laterite formation was initially assigned a Cenozoic age.
As can be seen from the isochron in Fig. 2.2, the strongly weathered samples
show no deviation from the isochron and, together with the fresh samples, define a
reasonable age of 2580+/-16 Ma. Even the weathered material alone would give
an age practically identical to the above one within margins of error. How can we
interpret this isochron? The authors produce good arguments that would tend to
exclude this as the effect of recent weathering.
Peter Stifle and Graham Shields
2.1 Weathering of the Whole Rock
Dasch (1969) carried out the first comprehensive study of the behaviour of the
Rb-Sr isotopic system of the whole rock during weathering. Among other things
he investigated the course of weathering at various granite plutons including the
Elburton granite of Piedmont and Georgia (USA), which can be found weathered
to great depths, and tried not only to throw light on the mechanisms that take
place during weathering but also the influence of weathering on the isotopic age.
He described the various stages of weathering on the way to kaolinite. Sample
material was collected from a profile where both fresh and weathered material was
available. The geochemical profile demonstrated that Sr was preferentially
dissolved from the original rock. The fresh granite contained 290 ppm Sr, whereas
the most strongly weathered material contained only 84 ppm Sr. Rb content also
decreased with increasing weathering but to a lesser extent than with St. It follows
therefore that Rb/Sr ratios increase markedly with increasing weathering.
Thus, only few weathered samples lie on or near the isochron for unweathered
samples that would date the age of intrusion of the granite pluton (Fig. 2.1).
Strongly weathered rocks lie to the right of the age-reference line. As a
censequence, weathering results in a younging of the Rb-Sr age, as the gradient of
the reference line decreases. Another consequence of weathering, apart from the
rise in Rb/Sr ratio, can be that the STSr/gSSr ratio will rise. too. The strongly
weathered samples B-5 and B-6 also show the highest sTSr/S6Sr ratios (Fig. 2.1 ~.
This behaviour allows us to suppose that the mineral phases with low Rb/Sr and
87Sr/S6Sr ratios preferentially go into solution and get transported away, as only by
such a process could components with higher Rb/Sr and SrSr/S6Sr ratios remain
behind.
Investigations have shown that plagioclase feldspar is most important in this
respect. No study has been able to show that the Sr isotope system can be
completely rehomogenized on a large scale in a soil profile. Rb-Sr isotope
determinations on whole rock from a weathered soil profile are therefore unlikely
to yield information about the age of an ancient weathering profile. An exception
can be found in the study of Worden and Compston (1973). Worden and
Compston carried out an isotopic study on the weathering profile of a granite body
from Australia. The investigated samples were at various stages of weathering
between fresh granite and strongly decomposed material showing quartz-kaolinite
mineral paragenesis. The age of the granite was estimated at 2600 to 2700 million
years. However, laterite formation was initially assigned a Cenozoic age.
As can be seen from the isochron in Fig. 2.2, the strongly weathered samples
show no deviation from the isochron and, together with the fresh samples, define a
reasonable age of 2580+/-16 Ma. Even the weathered material alone would give
an age practically identical to the above one within margins of error. How can we
interpret this isochron? The authors produce good arguments that would tend to
exclude this as the effect of recent weathering.