Facilities include a VG 3600 high-sensitivity mass spectrometer that can be operated in fully automatic mode, and two argon extraction systems: i) an internal tantalum resistance furnace of the double-vacuum type used to carry out conventional ⁴⁰Ar/³⁹Ar step-heating analyses of mineral separates, and ii) a laserprobe system employing a Quantronix Nd-YAG laser that can work at either IR or UV wavelengths. The UV laser ablation pits (with dimensions ~50-100 microns) are much better defined than the spots produced by an IR laser enabling high spatial resolution (see images below). Hence the latter normally is used to extract total gas from single separated mineral grains, and the former to carry out intragrain analysis.

Image of a single muscovite grain showing on the left a spot made with IR laser, and on the right ones made with the UV laser. The largest UV spots near the centre have side lengths ~150 µm and depths ~400 µm; scale is the same across whole image. Note large halo surrounding central hot zone in the IR image

Recent studies have focused on applications of laser spot dating, a method which allows comparison of argon contained in different locations in a mineral grain. Conventional step-heating analysis is a complementary method which measures the gas released at different laboratory extraction temperatures.

• Graduate student, Ray Fallon (Fallon et al. 2001), showed that the primary age of the Port Mouton pluton (as recorded by U-Pb techniques) can be obtained from the analysis of muscovite by the spot-dating method. Also recorded in these data, principally in the rims of grains, are the combined effects of subsequent tectonothermal events. On the other hand, conventional age spectrum analysis records average grain ages that have no chronological significance. See accompanying diagram.
  
  



• The spot-dating method was used by Reynolds et al. (2004a) to study a suite of zoned micas from Nova Scotia's South Mountain batholith. Individual grains of these micas contain oscillatory zones of two mica phases, both good targets for this dating method. The maximum ages recorded, ca. 385-382 Ma, principally from one of the phases, are in accord with other geochronological data and interpreted to be the primary age. The younger apparent ages likely reflect post-emplacement loss of argon resulting from pervasive ca. 370-360 Ma magmatic or hydrothermal activity. Conventional age spectrum analysis of bulk concentrates of these micas yielded well defined plateau ages of 370 ± 3 Ma. See accompanying diagram.
  
  



• The differing nature of data sets obtained by conventional step-heating and by spot-dating methods is apparent in our recent study (see also Reynolds et al. 2004b) of the clastic sedimentary rocks of the latest Devonian to Early Carboniferous (Tournaisian) Horton Group in Nova Scotia. The sample with the best developed cleavage (124 in accompanying plots) yields a well-defined plateau age which is interpreted as the time of metamorphism. There is some indication here of a minor resetting event at ca.190 Ma. Spot ages obtained over a cut section of this sample are variable with an upper limit equal to the metamorphic age. The lower ages may reflect the resetting event inferred above. The implication is that the latter cannot be spatially resolved in this rock, but it might be resolved thermally. Sample 20 was collected from one of the weakly-cleaved formations. The spectrum has a well-defined plateau interpreted as a detrital signature because its age is closely similar to ones obtained on detrital muscovite grains separated from a nearby sandstone unit (see figure). There is no clear indication of the above metamorphic age in this spectrum. The apparent spot ages are bracketed between detrital and metamorphic values, and do not appear correlated with any spatial parameters in the rock. From comparison of step-heating and spot-dating data, it appears that the metamorphic age has been better resolved spatially than thermally. Note that there is no evidence of a late-stage resetting in sample 20 and no evidence of a detrital signature in sample 124.
  
  
  
  
  
  



• Laser single grain and spot-dating projects currently in progress involve follow-ups to work begun by graduate student J.-P. Gobeil on Cretaceous (and perhaps by implication, Carboniferous) rocks on land and offshore in Nova Scotia. Gobeil obtained total fusion ages of detrital muscovite grains separated from three horizons of the Early Cretaceous Chaswood Formation that outcrops in central Nova Scotia. From a total of approximately 80 grains, he found ages in the range ca. 385-360 Ma, values typical of ones from the South Mountain batholith. It seems likely that these muscovites had been eroded from the batholith when it was first unroofed and subsequently stored in the rocks of the Horton Group before reaching their final home in the Chaswood Formation. Alternatively, they may not have been eroded from the batholith until Cretaceous times. Recently obtained single-grain ages from the section of the Chaswood Formation that is available via boreholes in the Elmsvale Basin tend to corroborate Gobeil's results. However, apparent ages of detrital grains from the Missisauga and Logan Canyon Formations, the equivalent rocks deposited offshore, have quite different distributions (see accompanying plots of representative data). In a complementary study, we are obtaining ages for detrital muscovite from Horton Group rocks, in particular from the type section in the Windsor area of Nova Scotia for comparison with the data from Cretaceous rocks. These studies have potential economic significance in that ages in the ca. 370-360 Ma range are attributed to late-stage hydrothermal activity in the batholith typically associated with mineralization. It is of interest to confirm the existence of a mineralization signature in the earliest detritus from the batholith and to try to chart this signature at different times in the geologic record. [Collaborators: G. Pe-Piper, St. Mary's University; David J.W. Piper, GSC Atlantic; A.M. Grist, Dalhousie University]
  
  



• ⁴⁰Ar/³⁹Ar ages recorded in detrital muscovite grains have implications for the provenance of sedimentary rocks, the thermal histories of source terranes, and the times of amalgamation of terranes. Information provided by detrital muscovite is complementary to that from detrital zircon. For example, a narrow zircon age distribution accompanied by a broad muscovite distribution that has its upper limit coincident with the zircon peak age suggests that the source terrane initially cooled rapidly, but later underwent a heating event that overprinted the muscovite ages.Total fusion of whole grains may not produce a valid detrital signature because grains can be variably overprinted by later heating events. Ideally, timing of the latter along with the true detrital signature can be extracted from intra-grain age distribution data, age patterns most reliably determined by measuring a large number of spot ages using the UV laser. Some age data on detrital zircons have been reported for both Avalonia and Meguma, two peri-Gondwanan terranes of Atlantic Canada.The distribution patterns are all fairly similar except that Neoproterozoic-Cambrian units in the Meguma lack the characteristic 1900-700 Ma zircons seen elsewhere. Detrital muscovite dating provides an additional method for comparing Neoproterozoic-Cambrian units in these two terranes. [Collaborators: S.M.Barr, Acadia University; C.E. White, NS Natural Resources]