Argon geochronology
... in the Structure Tectonics Team @ ANU
Decussate recrystallization of white mica with
biotite newly grown
in the
resultant microdilation sites,
from the Dunstan Range,
in the
Otago Schist, New Zealand.
Multi-deformed tectonites can retain ages from
different events:
this schistose fabric formed
in the ~110 Ma
Northburn Shear Zone and has
been folded,
and subsequently subjected to a static recrystallization.
Three different ages were retained in the microstructure:
a) in relicts of the folded fabric defined by white mica;
b) in relicts of recrystallised white mica in the hinge zone;
c) in biotite grown in micro-dilation sites interstitial to the white mica.
These different gas reservoirs would not be distinguished using laser
spot analyses. However, furnace step-heating analysis at ANU
has differentiated the ages of these three microstructures.
A key element in our strategy is fabric and microstructural analysis in conjunction with microstructurally-focussed geochronology to read rocks to determine the history of their evolution in space and time.
Argon geochronology is the only technique that allows direct dating of fabric forming minerals. In the Argon Partial Retention Zone information can be obtained as to the timing, as well as the duration of events - GEOSPEEDOMETRY - allowing a focus on rates in geological process, as well as determining how long these processes operated.
The Argon Lab @ ANU is run by Dr Marnie Forster, who is an externally funded ARC supported Australian Research Fellow. The facility consists of a VG1200, with an accurate-temperature-controlled step-heating furnace, large efficient getters, and a new 30W diode laser. The facility is not a production facility - it runs in "research mode" allowing long-duration diffusion experiments, and precise and detailed (quality) age determinations. See FACILITY DESCRIPTION.
During 2008-2009 the VG1200 has been used for step-heating K-feldspar diffusion experiments (each experiment 1-2 weeks duration) with excellent results. It has also been used for numerous analyses of white mica, biotite and hornblende from deformed and metamorphosed rocks. Rocks as young as 7.5 Ma have been dated with precision (one sigma <0.5%). See RESEARCH THEMES.
Software available for data analysis include KArDate and Noble, both programs now being replaced by MacOSX compatible software - eAr for data analysis including the method of asymptotes and limits - and MacArgon for modelling and simulation of the results.
Research collaboration - recent publications - current projects
The Argon Lab @ ANU is part of the Thermochronology and Noble Gas Geochemistry and Geochronology Organization (TANG3O) National Virtual Facility.