Question:
Which of the following statement(s) is/are correct for a plot of (\(^{87}Sr/^{86}Sr\)) versus (\(^{87}Rb/^{86}Sr\)) of a rock which has evolved in a closed system?
Which of the following statement(s) is/are correct for a plot of (\(^{87}Sr/^{86}Sr\)) versus (\(^{87}Rb/^{86}Sr\)) of a rock which has evolved in a closed system?
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In radiometric dating using the strontium-rubidium method, the isochron provides a way to determine the age of a rock sample based on its isotopic evolution.
Updated On: Dec 11, 2025
- The slopes of the lines of evolution for the minerals in the rock are all equal and positive
- The slopes of the lines of evolution for the minerals in the rock are all equal and negative
- The slope of the isochron is identical in direction to the slope of the lines of evolution for the minerals in the rock
- The slope of the isochron is opposite in direction to the slope of the lines of evolution for the minerals in the rock
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The Correct Option is B, D
Solution and Explanation
Step 1: Understanding the Isotope Plot.
In a plot of \(^{87}Sr/^{86}Sr\) versus \(^{87}Rb/^{86}Sr\), the isochron represents the line that reflects the evolution of the mineral in the rock over time. The slope of the isochron is the inverse of the slope of the lines of evolution for the individual minerals.
Step 2: Analyzing the options.
(A) The slopes of the lines of evolution for the minerals in the rock are all equal and positive: Incorrect — The slopes are not always positive for all minerals in the system.
(B) The slopes of the lines of evolution for the minerals in the rock are all equal and negative: Incorrect — The slopes can vary, and are not necessarily all negative.
(C) The slope of the isochron is identical in direction to the slope of the lines of evolution for the minerals in the rock: Incorrect — The slope of the isochron is opposite to the slope of the lines of evolution.
(D) The slope of the isochron is opposite in direction to the slope of the lines of evolution for the minerals in the rock: Correct — The slope of the isochron is indeed opposite to the slope of the individual mineral lines of evolution.
Step 3: Conclusion.
The correct answer is (D) The slope of the isochron is opposite in direction to the slope of the lines of evolution for the minerals in the rock.
In a plot of \(^{87}Sr/^{86}Sr\) versus \(^{87}Rb/^{86}Sr\), the isochron represents the line that reflects the evolution of the mineral in the rock over time. The slope of the isochron is the inverse of the slope of the lines of evolution for the individual minerals.
Step 2: Analyzing the options.
(A) The slopes of the lines of evolution for the minerals in the rock are all equal and positive: Incorrect — The slopes are not always positive for all minerals in the system.
(B) The slopes of the lines of evolution for the minerals in the rock are all equal and negative: Incorrect — The slopes can vary, and are not necessarily all negative.
(C) The slope of the isochron is identical in direction to the slope of the lines of evolution for the minerals in the rock: Incorrect — The slope of the isochron is opposite to the slope of the lines of evolution.
(D) The slope of the isochron is opposite in direction to the slope of the lines of evolution for the minerals in the rock: Correct — The slope of the isochron is indeed opposite to the slope of the individual mineral lines of evolution.
Step 3: Conclusion.
The correct answer is (D) The slope of the isochron is opposite in direction to the slope of the lines of evolution for the minerals in the rock.
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