Shown below is a schematic plot of \(\varepsilon_{Nd}\) (deviation of \(^{143}Nd/^{144}Nd\) in a sample relative to CHUR) versus time. The two solid lines represent the evolution curves for the depleted mantle reservoir and the continental crust. Which one of the four dashed lines, marked 1, 2, 3, and 4, represents the evolution of the CHUR?
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In isotopic studies, the CHUR line typically represents the baseline isotopic composition of the Earth’s primitive mantle, showing a steady or slightly increasing trend over time.
Step 1: Understanding \(\varepsilon_{Nd}\) and its significance.
\(\varepsilon_{Nd}\) represents the deviation of the isotopic ratio \(^{143}Nd/^{144}Nd\) from a standard, the CHUR (Chondritic Uniform Reservoir). The evolution of \(\varepsilon_{Nd}\) over time provides insights into the differentiation of Earth’s mantle and crust.
Step 2: Analyzing the evolution of the CHUR.
The CHUR line should show a relatively stable or slightly increasing trend with age, as it represents the isotopic composition of the primitive mantle. This line typically shows a gradual increase in \(\varepsilon_{Nd}\) as the mantle evolves over time due to the decay of \(^{147}Sm\) to \(^{143}Nd\).
Step 3: Interpreting the lines.
- Line 1: Likely represents the evolution of the continental crust, which often shows decreasing \(\varepsilon_{Nd}\) values due to lower Sm/Nd ratios and recycling of older material.
- Line 2: This line exhibits a trend that closely follows the expected behavior for CHUR, showing a smooth, gradual increase in \(\varepsilon_{Nd}\) over time. This makes it the best match for the CHUR evolution.
- Line 3 and Line 4: These lines show greater deviations, likely representing isotopically depleted or enriched mantle reservoirs rather than the average chondritic composition.
Conclusion: Line 2 most accurately represents the CHUR.
