Argon 40 dating

Helens dacite causes the more fundamental question to be asked—how accurate are K-Ar 'ages' from the many other phenocryst-containing lava flows world-wide?

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Sometimes, when a large amount of argon has been lost, this is not possible.

In these cases, the age of the first batch of argon released is the age of the reheating event, and the age of the last argon released is the minimum age of initial crystallization.

Sur les radiations invisibles mises par les sels d'uranium.

After three of these explosive eruptions (June 12, August 7 and October 17), near-surface magma had low enough steam pressures so that viscous lava flows formed three consecutive, dome-shaped structures within the crater.

The approximate intercept age of the hornblende in Granite Wash Mountains is likewise ~80 Ma, indicating that this grain saw a pulse of reheating at that time. The argon loss profiles within the other phases suggest a Miocene overprint on previously-cooled samples. Knapp and Heizler (1990) provide further evidence for rejuvenated Miocene unroofing from the complexities of K-feldspar closure temperatures.

They show that there are two different domains within the K-feldspars that have different diffusivities.

At Mesquite Mountain, the plateau ages for the hornblende indicate closure at around 80 Ma, the initial phase of thrusting and uplift.

Biotites and potassium feldspar have 0% Ar intercept ages around 20-30 Ma, indicative of early Miocene cooling during extension and unroofing in the Colorado River Extensional Corridor.

Ar step-heating results from Mesquite Mountain and the Northern Granite Wash Mountains.

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