Problems with ar ar dating

Dating of movement on fault systems is also possible with the Ar method.Different minerals have different closure temperatures; biotite is ~300°C, muscovite is about 400°C and hornblende has a closure temperature of ~550°C.Dalrymple, referring to metamorphism and melting of rocks in the crust, has commented: "If the rock is heated or melted at some later time, then some or all the In a recent study 128 Ar isotopic analyses were obtained from ten profiles across biotite grains in high-grade metamorphic rocks, and apparent Ar-Ar "ages" within individual grains ranged from known to cause grave problems in regional geochronology studies.This is approximately 2,500 times as much Ar as is found in natural muscovite.Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize. envisage noble gases from the mantle (and the atmosphere) migrating and circulating through the crust, so there should be evidence of excess in crustal rocks and their constituent minerals could well be the norm rather than the exception.represents primordial Ar carried from source areas in the earth's mantle by the parent magmas, is inherited by the resultant volcanic rocks, and thus has no age significance.

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In these materials, the decay product Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes).

As part of his seminal work on excess argon, Dalrymple (1969) dated 26 historical lava flows with K-Ar to determine whether excess argon was present.

Of the 26 lava flows that were sampled and analyzed, 18 of them gave expected results. Eight rocks yielded unrealistic dates, which were either too old because of the presence of excess 40Ar (5 of them) or too young (negative ages) because of the presence of excess 36Ar (3 of them).

The abundance of Ar is unlikely to provide the age of intrusions of granite as the age typically reflects the time when a mineral cooled through its closure temperature.

However, in a metamorphic rock that has not exceeded its closure temperature the age likely dates the crystallization of the mineral.