Excess 40Ar in metamorphic rocks from Broken Hill, New South Wales: implications for 40Ar/39Ar age spectra and the thermal history of the region

Details Excess 40Ar in metamorphic rocks from Broken Hill, New South Wales: implications for 40Ar/39Ar age spectra and the thermal history of the region Excess 40Ar in metamorphic rocks from Broken Hill, New South Wales: implications for 40Ar/39Ar age spectra and the thermal history of the region

Sep 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981e%26psl..55..123h&link_type=abstract

Earth and Planetary Science Letters, Volume 55, Issue 1, p. 123-149.

Computer Science

68

Scientific paper

40Ar/39Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly (~3°C Ma-1) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ~1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520+/-40 Ma ago, causing temperatures to rise to ~350°C in some places. During this event, accumulated 40Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the 40Ar/39Ar age spectra gives a local estimate of this partial pressure of ~10-4 atm. The region finally cooled below 100°C about 280 Ma ago.
40Ar/39Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess 40Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess 40Ar with respect to radiogenic 40Ar, appears to be responsible for the saddle-shaped age spectra.

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