The vast majority (99.6%) of argon in Earth's atmosphere is 40Ar however the argon in the Sun is almost entirely 36Ar and 38Ar.  The preponderance of 40Ar on Earth is attributed to its formation from 40K via beta emission.  40K has a long half-life of 1.27×109 years and makes up about 0.012% of the potassium on Earth.  When molten rock cools, the argon accumulates in the rock.  When a piece of rock is analyzed, it is found to contain 1.38 mmol of 40K and 1.15 mmol of 40Ar.  How many billions of years ago did the rock form?   Answers must be in billions of years and correct to +/- 0.2 billion years.

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The vast majority (99.6%) of argon in Earth's atmosphere is 40Ar however the argon in the Sun is almost entirely 36Ar and 38Ar.  The preponderance of 40Ar on Earth is attributed to its formation from 40K via beta emission.  40K has a long half-life of 1.27×109 years and makes up about 0.012% of the potassium on Earth.  When molten rock cools, the argon accumulates in the rock.  When a piece of rock is analyzed, it is found to contain 1.38 mmol of 40K and 1.15 mmol of 40Ar.  How many billions of years ago did the rock form?  

Answers must be in billions of years and correct to +/- 0.2 billion years.

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