3.5 Dating sedimentary rocks
Absolute dating of the time of deposition of
sedimentary rocks is an important problem, but one that is very difficult to
solve. Accurate dates depend on thorough re-setting of isotopic clocks. In the
case of Rb)Sr dating of sediments, this rests on the assumption that Sr isotope
systematics in the rock were homogenized during deposition or early diagenesis,
and thereafter remained as a closed system until the present day. However, we
will see that these two requirements may be mutually exclusive.
In
principle, sedimentary rocks may be divided into two groups according to the
nature of the Rb-bearing phase present. Allogenic (detrital) minerals are
moderately resistant to open-system behaviour during burial metamorphism, but
problems arise from inherited isotopic signatures. Authigenic minerals are
deposited directly from seawater and hence display good initial Sr isotope
homogeneity. However, they are highly susceptible to recrystallisation after
burial and do not necessarily remain closed systems.
In
practice, the two distinct dating approaches associated with these sediment
types have tended to converge. Analysis of detrital sediments has moved towards
the analysis of fine-grained, almost authigenic, minerals such as illite, in
order to escape the effects of the detrital component. In contrast, analysis of
authigenic minerals has been focussed on the sub-authigenic mineral glauconite,
since the truly authigenic Rb-bearing evaporite minerals are too susceptible to
burial metamorphism to be viable geochronometers.
3.5.1 Shales
Detrital Rb-bearing minerals (mica, K-feldspar,
clay minerals etc.) can be expected to contain inherited old radiogenic Sr.
Therefore, dating of such material should give an average of the provenance ages
of the sedimentary constituents. However, if sufficiently fine-grained shales
are sampled, it appears that the constituent minerals (mainly illite) often
suffer substantial Sr exchange during post-depositional diagenesis. In this
case they may develop an almost homogeneous initial Sr isotope composition soon
after deposition, thereafter remaining effectively closed systems until the
present day.
Compston
and Pidgeon (1962) pioneered whole-rock Rb)Sr dating of shales, and found that
in some circumstances (e.g. the
Subsequent
work on the dating of shales sought to avoid problems of contamination with
detrital micas and feldspars by analysing separated clay-mineral fractions,
whose purity is checked by x-ray diffraction (XRD). XRD analysis of illites can
also yield information about the nature and origin of clay minerals in a shale
which is to be dated.
The
‘illite crystallinity index’ (Kubler, 1966) is defined as the width of the
(001) XRD peak at half its height. A well-crystallised illite, characteristic
of a relatively high-temperature history, has sharp peaks, and therefore a low
index, while low-temperature illites are more disordered, and have irregular
peaks with large indices. In addition to this discriminant, illite has
high-temperature (2M) and low-temperature (1M) polymorphs which can also be
distinguished by XRD (Dunoyer de Segonzac, 1969). ‘1M’ illites with a large
crystallinity index are characteristic of low-temperature growth and
recrystallisation in the sedimentary)diagenetic regime, whereas ‘2M’ illites with a
small index are indicative of temperatures of zeolite-facies metamorphism or
above. The latter reflect a detrital component, or post-diagenetic
metamorphism.
A
comparison of Rb)Sr whole-rock and clay mineral analysis of a Precambrian shale from
Fig. 3.21. Rb)Sr isochron diagram for whole-rock
shales ( > );
separated illites ( ! ); and a carbonate sample ( Î ) from
In
Table 3.1. Age
data (in Myr) for shales from the Yangtse gorge, based on whole-rock or coarse
clay (left column) and fine clay separates (right).
)))))))))))))))))))))))))))))))))))))))))))))))))))))))))
Coarse fraction or whole-rock Fine
fraction
)))))))))))))))))))))))))))))))))))))))))))))))))))))))))
573 " 7 (Rb)Sr > 1.5 :m)
613 " 23 (Rb)Sr)
568 " 12 (U)Pb whole-rock)
572 " 14 (Rb)Sr whole-rock)
435 ) 415 (Rb)Sr < 1 :m)
570 " 4 (Rb)Sr)
574 " 20 (Rb)Sr) 565 ) 490 (Rb)Sr)
602 " 15 (Rb)Sr ca. 1.5 :m)
460 " 9 (Rb)Sr < 1 :m)
Stratigraphic Cambrian ) Precambrian boundary
614 " 18 (Rb)Sr > 1.5 :m)
700 " 5 (Rb)Sr > 1.5 :m)
580 " 25 (Rb)Sr < 1 :m)
691 " 29 (Rb)Sr)
580 ) 420 (Rb)Sr > 1.5 :m)
727 " 9 (Rb)Sr > 1.5 :m)
460 ) 340 (Rb)Sr < 1 :m)
728 " 27 (Rb)Sr) 500 ) 360 (Rb)Sr < 1 :m)
608 " 15 (Rb)Sr > 1.5 :m)
)))))))))))))))))))))))))))))))))))))))))))))))))))))))))
This
interpretation is supported by recent U)Pb zircon dates on tuff and
bentonite from near the base of the Cambrian in Morocco, China and Siberia,
which confirm a young boundary age near 540 Myr (Compston et al., 1990; Bowring et al.,
1993). Therefore, Rb)Sr dating of shales cannot be considered a reliable technique for dating
sedimentary deposition.
3.5.2 Glauconite
The mineral glauconite offers an attractive
possibility for dating sedimentary rocks directly, due to its high Rb content,
easy identification and widespread stratigraphic distribution. Glauconite is a
micaceous mineral similar to illite which is best developed in macroscopic
pellets (called ‘glauconies’ by Odin and Dodson, 1982). These are probably
formed by the alteration of a very fine-grained clay precursor intermixed with
organic matter in a faecal pellet. Glauconies form near the sediment)water interface in the marine
environment. However, by studying pellets on the present day ocean floor, Odin
and Dodson (1982) have shown that ‘glauconitisation’ is a slow process which
may take hundreds of thousands of years to reach completion. During this
process, the potassium content of the pellet increases, and this can therefore
be used to monitor the maturation of the pellet.
Rb)Sr analysis of Holocene glauconies
(Clauer et al., 1992) shows that Sr
isotope equilibrium with seawater is achieved only slowly as the potassium
content increases. The Rb)Sr data can be used to calculate a model Sr age for the pellet by making
the initial ratio equal to the isotopic composition of seawater Sr at the
estimated time of sedimentation (see below). A zero-age pellet starts with a
high apparent model age due to a large content of Sr in detrital mineral
phases. However, as it matures, the pellet homogenises with seawater so that
the model age falls to zero in a fully equilibrated pellet (Fig. 3.22).
Analysis of the potassium content of glauconies therefore provides an essential
screening procedure, in order to select only fully mature material for dating.
Fig. 3.22. Rb)Sr model ages of Holocene (zero-age)
glauconies as a function of potassium content. Open symbol indicates clay
fraction. After Clauer et al. (1992).
Cretaceous
and younger glauconies often yield ages concordant with other dating methods
(e.g. Harris, 1976), but Paleozoic glauconies commonly give ages that are 10)20% younger than expected. Early
workers (e.g. Hurley et al., 1960)
attributed this to post-depositional uptake of K and Rb during diagenesis.
However, Morton and Long (1980) attributed the young ages to 87Sr
loss from the expandable layers of the clay lattice, by some form of ion
exchange with circulating brines.
Morton
and Long calculated model ages for a series of glauconite separates, using
initial ratios based on the 87Sr/86Sr ratio of seawater
at the time of deposition (see section 3.6.1). They showed that in some cases
erroneous glauconite model ages could be increased to near the stratigraphic
age by leaching with ammonium acetate, which is thought to remove excess
loosely-bound Rb from the expandable layers of the lattice. In contrast,
leaching with acetic acid or HCl had unpredictable effects on the glauconite
age, probably due to removal of some tightly bound Sr.
Similar
experiments were performed on glauconites from the 525 Myr-old Bonneterre
Formation (Missouri) by Grant et al.
(1984). Eight un-leached glauconite pellets gave model ages in the range 413)440 Myr. However, the most
radiogenic sample (model age = 426 Myr) converged only slightly on the true age
when subjected to ammonium acetate leaching (437 Myr). Therefore, more rigorous
criteria are needed to determine whether old glauconites have suffered
open-system behaviour, prior to a dating attempt. Until such criteria are
developed, glauconite dating in the Paleozoic must be regarded as a monitor of
diagenetic processes rather than a viable dating tool for stratigraphic
correlation.
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