Q1) Is K compatible or incompatible in mantle minerals? Do you expect to find K concentrated in the mantle or crust over time? What about Ar? Ar has three stable isotopes: naturally occurring 36Ar and 38Ar, and the radiogenic daughter product of 40K, 40Ar. While differentiation and mantle melting in early Earth history would have transferred 36Ar and 38Ar into the early atmosphere, 40Ar forms almost entirely from decay of 40K and would not have been transferred into the early atmosphere. Since the initial abundance of 40Ar in the early Earth was negligibly small, almost all the 40Ar now in existence has grown since the Earth formed, half of it in the last 2-3 Gyr (the half-life of 40K is 1.26 billion years). Q2) How many stable isotopes does Argon have? What are they? Which isotope is the decay product of 40K? We can see that a simple model of K and Ar transport could help us understand the outgassing history of the Earth. We'll model changes in the ratio of 40Ar/36Ar in the mantle and atmosphere, and find the transport and differentiation history that best fits the Earth's present-day ratios. Note it is easier to measure the isotopic ratio of a reservoir than to measure its actual Ar – isotope concentration. All available samples from the mantle, such as basalt lavas, are likely to have become enriched in Ar during melting, and may have lost volatile elements such as Ar during magma ascent, crystallization and degassing. These processes change Ar concentrations, but have no effect on Ar isotopic ratios. This is because isotopes of the same element are chemical identical, meaning chemical processes will affect the isotopes in the same way, and will not change their ratio. We will discover that the 40Ar/36Ar ratios of the mantle and atmosphere constrain the degassing history of the Earth and help determine how much of its total K has been concentrated in the crust. Q3) Why do we measure isotopic ratios rather than the isotopic concentration? K Ar Transport Model –
Q1) Is K compatible or incompatible in mantle minerals? Do you expect to find K concentrated in the mantle or crust over time? What about Ar? Ar has three stable isotopes: naturally occurring 36Ar and 38Ar, and the radiogenic daughter product of 40K, 40Ar. While differentiation and mantle melting in early Earth history would have transferred 36Ar and 38Ar into the early atmosphere, 40Ar forms almost entirely from decay of 40K and would not have been transferred into the early atmosphere. Since the initial abundance of 40Ar in the early Earth was negligibly small, almost all the 40Ar now in existence has grown since the Earth formed, half of it in the last 2-3 Gyr (the half-life of 40K is 1.26 billion years). Q2) How many stable isotopes does Argon have? What are they? Which isotope is the decay product of 40K? We can see that a simple model of K and Ar transport could help us understand the outgassing history of the Earth. We'll model changes in the ratio of 40Ar/36Ar in the mantle and atmosphere, and find the transport and differentiation history that best fits the Earth's present-day ratios. Note – it is easier to measure the isotopic ratio of a reservoir than to measure its actual Ar isotope concentration. All available samples from the mantle, such as basalt lavas, are likely to have become enriched in Ar during melting, and may have lost volatile elements such as Ar during magma ascent, crystallization and degassing. These processes change Ar concentrations, but have no effect on Ar isotopic ratios. This is because isotopes of the same element are chemical identical, meaning chemical processes will affect the isotopes in the same way, and will not change their ratio. We will discover that the 40Ar/36Ar ratios of the mantle and atmosphere constrain the degassing history of the Earth and help determine how much of its total K has been concentrated in the crust. Q3) Why do we measure isotopic ratios rather than the isotopic concentration? K – Ar Transport Model
ess_312_lab_5_K-Ar.docx