Ar ar dating of basalts

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Once the locations of K-bearing phases were determined, the compositions of the phases were measured using quantitative analysis.Some otherwise well-crystallized samples were found to contain small amounts of groundmass glass in interstices between crystals; in these cases, virtually all K was concentrated within the glass.In other samples the groundmass was fully crystallized.In these cases K resided in K-feldspar, either as thin (Ar resistance furnace incremental heating method.The shape of spectra and the accuracy and precision of plateau and isochron ages correspond closely to the ranking assigned based on the microprobe evaluation.Glass-free, K-feldspar-rich samples have generally flat spectra, relatively high radiogenic yields (%Ar*), and ages with precision values ( /-2 σ ) ranging from 1-5%. Samples with increasing amounts of groundmass glass have increasingly disturbed spectra, reduced radiogenic yields, and low precision ( /-5-20%) plateau or isochron ages, which in some cases violate stratigraphic order.

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Their first advantage is a better sensitivity of the new generation of CDD electron multipliers and 10^12-ohm resistor faraday collectors.The electron microprobe offers a means of assessing and quantitatively analyzing basaltic groundmass features that are too small to be assessed optically.X-ray mapping and quantitative analyses accurately distinguish among crystal and glass phases as small as 2 μ m.The ages of high ranking samples from the northern cone all agree within analytical error, suggesting that the entire period of eruptive activity spanned less than 30,000 years.The precision and accuracy of data in this study was much enhanced by using microprobe observations of groundmass glass and K-feldspar to aid in the sample selection process.

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