Composition of rocks and presence of chemicals over time

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A homework question for oceanography asks me to calculate the age of a rock sample given the amount of thorium and radium present. I did so and came up with 1.3 billion years old. The follow-up asks me to state whether it's oceanic crust, why or why not. Oceanic crust is typically about 200 m.y. old (~300 in extreme cases), so it seems like the sample is far older and therefore not oceanic crust. However, does he same thorium persist over that 1.3 b.y., or could it just be older chemicals incorporated into younger rock? I don't totally understand the process.

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Posted: 2 years ago
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