3.3 Dating metamorphic rocks
3.3.1 Open mineral systems
Mineral and whole-rock Rb)Sr systems may respond differently
to metamorphic events. 87Sr generated by Rb decay occupies unstable
lattice sites in Rb-rich minerals and tends to migrate out of the crystal if
subjected to a thermal pulse, even of a magnitude well below the melting
temperature. However, if fluids in the rock remain static, Sr released from
Rb-rich minerals such as mica and K-feldspar will tend to be taken up by the
nearest Sr sink such as plagioclase or apatite. Hence, the whole-rock system
may remain closed, even though mineral systems are open.
The
idea of using whole-rock analysis to see back through a metamorphic event which
disturbs mineral systems was first conceived by Compston and Jeffery (1959).
The model was illustrated graphically by Fairbairn et al. (1961) on a plot of isotope ratio against time (Fig. 3.12).
After the formation of the rock at time t0,
different minerals move along different growth lines, whose steepness corresponds
to their Rb/Sr ratio. Isotopic evolution continues until the minerals are
homogenized by a thermal event at time tM.
Thereafter, isotopic evolution continues along different growth lines to the
present day (tP). Individual
minerals in this model are open systems during the metamorphism. Therefore, a
mineral isochron yields the age of cooling from the thermal event, when each
mineral again became a closed system. However, a whole-rock domain of a certain
minimum size remains as an effectively closed system during the thermal event,
and can be used to date the initial crystallisation of the rock.
Fig. 3.12. Plot of Sr isotope ratio against
time to model the effect of a metamorphic event which opens Rb)Sr mineral systems, but not the
whole-rock system. t0 = age of rock; tM
= age of metamorphism; tP
= present. After Fairbairn et al.
(1961).
The
effects of metamorphism on mineral and whole-rock systems can also be
demonstrated on the isochron diagram, Fig. 3.13 (Lanphere et al., 1964). All systems start on a horizontal cord. Isotopic
evolution then occurs along near-vertical parallel paths (due to the extreme
amplification of the y axis). During
the thermal event, isotope ratios are homogenised to the whole-rock value. If
this only involved 87Sr then vertical vectors would be produced.
However, a possible complication illustrated in Fig. 3.13 involves limited Rb
re-mobilisation. Rb-rich minerals tend to suffer some Rb loss, while Rb-poor
phases may be contaminated by growth of Rb-rich alteration products, leading to
somewhat unpredictable vectors (R). After the event, whole-rock evolution
continues undeflected, while mineral systems define an isochron whose slope
yields the age of metamorphism.
Fig. 3.13. Hypothetical behaviour of a
partially disturbed mineral)whole-rock isochron. Evolution lines: 1 = period from igneous
crystallisation to metamorphism; R = metamorphic re-homogenisation; 2 = period
from metamorphism to present day.
A
practical example of dating plutonism and metamorphism by whole-rock and
mineral analysis of the same body was provided by the work of Wetherill et al. (1968) on the
Fig. 3.14. Rb)Sr isochrons for the
3.3.2 Blocking temperatures
After Rb)Sr mineral systems have been opened
in the thermal pulse of a regional metamorphic event, there must come a time
when mineral systems are again closed to element mobility. By dating the
closure or ‘blocking’ of different mineral systems, Rb)Sr ages give information about the
cooling history of metamorphic terranes. This was first demonstrated by Jager et
al. (1967) and Jager (1973), working on the central European Alps.
Jager
et al. found that in rocks of low
metamorphic grade round the exterior of the
The
blocking temperature of white mica (muscovite and phengite) was similarly
constrained to 500 " 50 oC by the first resetting of the white mica Rb)Sr ages ‘somewhat outside the
staurolite)chloritoid boundary’ (Purdy and Jager, 1976). However, unlike biotite,
white micas can undergo primary crystallisation below the Rb)Sr blocking temperature, so that
ages as low as 35 ) 40 Myr have been obtained even from the outer zones of low-grade Alpine
metamorphism. These ages are argued to date new mica growth at the peak of
metamorphism (Hunziker, 1974). This makes the muscovite Rb)Sr system a more problematical tool
than biotite for studying post-orogenic cooling processes.
Jager
et al. (1967) obtained biotite ages
of ca. 12 ) 16 Myr from the Simplon and Gotthard areas of the
Purdy
and Jager (1976) recognised that the 300 " 50 oC blocking
temperature for biotite might need to be revised if new experimental data for
the thermal stability of stilpnomelane were obtained. Most workers continue to
use a value of 300 oC; however, experimental work (e.g. Brown, 1971)
points to an upper stilpnomelane stability limit of 440 ) 480 oC at ca. 4 kb,
implying a biotite Rb)Sr blocking temperature of over 400 oC. This would be
consistent with evidence from SW Norway, where biotites subjected to
temperatures of over 400 oC in the Caledonian orogeny nevertheless
preserve (800 Myr) Sveco-Norwegian ages (Verschure et al., 1980).
A
more direct method of determining blocking temperatures is to measure mineral
ages in deep boreholes. Del Moro et al.
(1982) determined biotite)whole-rock Rb)Sr ages at depths of up to 3.8 km in the Sasso 22 well in the Larderello
geothermal field, Italy. All of the biotites show almost complete retention of 87Sr
at directly measured in-hole temperatures up to nearly 380 oC,
supporting a biotite closure temperature of ca. 400 oC. However,
Cliff (1985) has argued that in active geothermal systems, convective heat
transport could generate localised thermal pulses whose duration is too short
to allow significant diffusional Sr loss, thus yielding an anomalously high
blocking temperature.
Blocking
temperatures can also be determined theoretically, based on calculations of the
temperature-dependence of volume diffusion processes (Dodson, 1973; 1979).
Ideally, closure of the Rb)Sr system represents an instantaneous transition from a time when Rb and
Sr were completely mobile to when they were completely immobile. In a
fast-cooling igneous body the moment of crystallisation is a good approximation
to this ideal. However, in a slow-cooling regional metamorphic terrane there is
a continuous transition from a high-temperature regime, when radiogenic 87Sr
escapes from crystal lattices by diffusion as fast as it is produced, to
low-temperature conditions when there is negligible 87Sr escape (Fig
3.15). In such a system, the apparent age of a mineral such as biotite
corresponds to a linear extrapolation of the low-temperature 87Sr
growth line back into the x axis. The
temperature prevailing in the system at the time of the apparent age of the
mineral is then defined as the blocking temperature of the mineral in question
(Dodson, 1973). This blocking temperature is dependent on cooling rate, since
the slower the cooling, the longer will be the time during which partial loss
of daughter product may occur, and the lower will be the apparent age (Fig.
3.15).
Fig. 3.15. Schematic diagram to show variation
of temperature and Sr isotope ratio with time in a mineral cooling from a
regional metamorphic event. T0
= peak metamorphic temperature; TC
= closure or ‘blocking’ temperature; tC
= apparent closure age. After Dodson (1973).
If
a mineral is in contact with a fluid phase which can remove radiogenic Sr from
its surface, then the rate of loss of 87Sr depends on the rate of
volume diffusion across a certain size of lattice. In the case of biotite, this
diffusion will be predominantly parallel to cleavage planes rather than across
them. Assuming that the Arrhenius Law is obeyed, Dodson (1979) calculated
blocking temperatures (at a cooling rate of 30 oC/Myr) of 300 oC
for the Rb)Sr system in biotites of 0.7 mm diameter. This was based on experimental
work for argon diffusion in biotite (Hofmann and Giletti, 1970), because the
two elements are thought to have similar diffusional behaviour in crystal
lattices.
A
problem with the volume diffusional control of blocking temperature is that
large (30 cm) fissure-filling biotites in the Central Alps have the same ages,
and hence apparent blocking temperatures, as small (<1 mm) ground-mass
biotites in adjacent gneisses. Dodson (1979) suggested three possible
explanations:
1) Diffusion geometry is independent of grain
size. This could be due to the effects of stress on the crystal lattice.
2) Sr loss is controlled by the rate at which
radiogenic atoms leave the site where they were formed.
3) Blocking temperature is not kinetically
controlled, but depends on a change in the biotite lattice at the blocking
temperature.
The susceptibility of Sr to mobilisation by
fluids increases complexity in the interpretation of Sr blocking temperatures.
Such problems do not arise for argon, because it is an inert gas. Therefore the
latter element is a more reliable tool for studies of ‘thermochronology’. This
subject is discussed in detail in section 10.5.
3.3.3 Open whole-rock systems
The Rb)Sr whole-rock method was widely used as a
dating tool for igneous crystallisation during the 1960s and 1970s, but lost
credibility during the 1980s as evidence of whole-rock open-system behaviour
mounted. For example, Rb)Sr isochrons in metamorphic terranes can yield good linear arrays whose
slope is nevertheless a meaningless value between the protolith and metamorphic
ages. This problem is probably caused by the need to sample over a relatively
large geographical area in order to maximise the range of Rb/Sr ratios. A good
example is provided by the Arendal charnockites of south
Eight
whole-rock sample suites were collected from individual outcrops of Arendal
charnockite over an area of several km2. They yielded ages which
were dominantly in two groups, of ca. 1540 Myr and 1060 Myr. Field and Raheim
(1979a) interpreted the older age as the time of formation of the high-grade
charnockite mineralogy, and the younger as dating a subsequent low-grade event.
This was manifested as slight mineralogical alteration, probably associated
with irregularly spaced narrow fractures which traverse the area. The younger
re-setting event also fell within error of the 1063 " 20 Myr age of undeformed granite
sheets in the area.
In
order to test the effect of making a regional sample collection from an area of
slightly disturbed gneisses, Field and Raheim (1979b) collected a suite of
eight samples over an area of 1 km2. The data (Fig. 3.16) define a
good linear array with an apparent age of 1259 " 26 Myr. The MSWD value of 1.58
implies that the scatter of data about the line could probably be accounted for
by analytical error, but there is no geological evidence for an event at this
time. Therefore, Field and Raheim attributed the linear array to a series of
closely spaced en echelon arrays with
slopes corresponding to the age of re-setting, defined by a 1035 Myr mineral
isochron. Because the range in Rb/Sr at each locality is small (e.g. ‘locality
4', Fig. 3.16), samples lying on each sub-isochron do not deviate much from the
fictitious composite ‘isochron’. It is therefore concluded that in areas where
Rb)Sr systems may
have been disturbed, detailed sampling is necessary to measure the mobility of
the species before regional geochronological interpretations are made.
Fig. 3.16. Rb)Sr ‘isochron’ diagram for Arendal
charnockites showing fictitious 1259 Myr regional isochron composed of a series
of en echelon local isochrons with
the same slope as separated minerals. After Field and Raheim (1979b).
Whole-rock
open-system behaviour can occur at even lower grades of metamorphism in
fine-grained acid volcanic rocks. Such units are attractive for absolute
calibration of the stratigraphic column because they are conformable with
sedimentary strata. They tend to have large and variable Rb/Sr ratios, thus
yielding good isochrons. However, experience has shown that they are
particularly susceptible to radiogenic Sr loss. A good example is provided by
the Stockdale rhyolite of northern
The
Stockdale rhyolite is a fine-grained, flow-banded lava, included in the
uppermost Ordovician succession, and is argued to have a bio-stratigraphic
uncertainty of less than 0.5 Myr. Gale et
al. (1979) determined a 16 point whole-rock isochron, which yielded an age
of 421 " 3 Myr (2F) with MSWD = 1.92. They argued that
because of the relatively small number of data points, this MSWD value could be
attributed to experimental errors (section 2.6.3), and hence that the 421 Myr
age probably represented the time of eruption of the lava. However, if this age
was correct, it would require substantial revision of the Ordovician time-scale
determined by other methods.
McKerrow
et al. (1980) argued that because a
section of the Stockdale rhyolite which lay inside the Shap granite aureole
gave the same age (424 " 18 Myr) as the rest of the lava (421 Myr), the whole unit was probably
disturbed by some kind of hydrothermal event after eruption and subsequent
burial. Compston et al. (1982) sought
to explain the excess scatter over analytical error by a re-setting event
post-dating the eruption of the lava (estimated at 440 Myr from McKerrow et al., 1980). Consistent with this,
re-examination of the probability table of Brooks et al. (section 2.6.3) indicates that for 16 data points, an MSWD
value of 1.92 indicates up to a 95% probability that the result is not an
isochron.
A
perfect isochron would imply complete re-setting, but apparently this did not
occur. Plotting isotope ratios at 395 Myr ago (the date of intrusion of the
Shap granite) on a pseudo-isochron diagram (Fig 3.17) allows an assessment to
be made of scatter introduced by an event after eruption. Compston et al. found that if the four samples
with highest Rb/Sr ratios were removed, along with one sample (no. 5) with an
anomalously high Sr content, then all of the other samples lie close to a 440
Myr reference line. In fact a regression through ten of these points yields a
‘minimum’ age of 430 " 7 Myr. Compston et al. also
noted that isochrons calculated individually for each of the four sampling
localities yield lower MSWD values than the combined data set. This evidence warns
that the combined data set is unsuitable for constructing a single isochron,
despite the attractively precise result. Compston et al. calculated a weighted mean of 412 " 7 Myr for the four local isochrons
and interpreted this as the time of hydrothermal alteration of the rhyolite. In
so far as the Rb)Sr evidence ‘marks a real event’ then 412 Myr may be the date of this
event.
Fig. 3.17. Rb)Sr pseudo-isochron diagram for the
Stockdale rhyolite at the time of Shap granite emplacement (395 Myr ago) to
show possible open- system behaviour of Sr in samples outside the hatched zone.
Four sampling sites are distinguished by different symbols. After Compston et al. (1982).
The
evidence for open-system Rb)Sr systematics in numerous environments, coupled with the availability of high precision U–Pb
and Ar–Ar ages, means that these other methods have now superceded the Rb)Sr method as dating tools for
igneous crystallisation. However, Rb-Sr isochrons still find uses in certain
specialised applications. One such application is the direct dating of metallic
ore deposits, where phases suitable for U–Pb or Ar–Ar analysis are not always
available. We will therefore examine some applications in this area.
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