Disturbed 40Ar/39Ar spectra from hornblendes: Thermal loss or contamination?
- Rex, D. C.
- Guise, P. G.
- Wartho, Jo-Anne
DOIAbstract
Accurately weighed aliquots of the ∼ 500-Ma-old interlaboratory standard hornblende MMHb-1 were mixed with known quantities of a ∼ 100 Ma-old Himalayan biotite and subjected to normal Full-size image (<1 K) incremental heating analysis. The addition of the biotite created monotonically increasing apparent ages, similar to profiles interpreted as showing Ar diffusive loss in the literature. The release of Ar from biotite dominates the low-temperature portion of the spectra and lowers the observed age. Lower Ca/K ratios in the first increments are consistent with contamination, but do not distinguish the composition of the contaminant phase. A large number of published amphibole “diffusive loss” profiles may, in fact, be due to contamination ...
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