Tungsten-182 evidence for an ancient kimberlite source.

Area of Science:

• Geochemistry
• Isotope Geology
• Planetary Science

Background:

• Globally distributed kimberlites often exhibit chondritic initial Neodymium-143/Samarium-144 (¹⁴³Nd/¹⁴⁴Nd) and Hafnium-176/Lutetium-176 (¹⁷⁶Hf/¹⁷⁷Hf) isotopic ratios.
• These isotopic signatures suggest derivation from a chemically homogeneous, primitive mantle source that remained isolated from the convecting mantle throughout much of Earth's history.
• Understanding the nature of this reservoir is key to deciphering early Earth processes and mantle evolution.

Purpose of the Study:

• To investigate whether ancient mantle reservoirs, inferred from kimberlite compositions, preserve evidence of early Earth processes.
• To analyze Tungsten-182/Tungsten-184 (¹⁸²W/¹⁸⁴W) and Neodymium-142/Neodymium-144 (¹⁴²Nd/¹⁴⁴Nd) isotopic data in primitive kimberlites.
• To constrain the origin and evolution of long-lived, isolated mantle reservoirs.

Main Methods:

• Analysis of ¹⁸²W/¹⁸⁴W and ¹⁴²Nd/¹⁴⁴Nd isotopic ratios in primitive kimberlites from 10 global localities.
• Kimberlite samples range in age from 1,153 to 89 million years ago (Ma).
• Comparison of isotopic data with chondritic values and modern upper mantle compositions.

Main Results:

• Most analyzed kimberlites (1,153–89 Ma) display homogeneous μ¹⁸²W and μ¹⁴²Nd values, averaging -5.9 ± 3.6 ppm and +2.7 ± 2.9 ppm, respectively.
• Uniform, modestly negative μ¹⁸²W values, alongside chondritic to suprachondritic initial ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios, support derivation from early-formed, isolated mantle reservoirs.
• Two younger kimberlites (72 and 52 Ma) show subchondritic initial ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁴³Nd/¹⁴⁴Nd, with μ¹⁸²W consistent with the modern upper mantle, potentially indicating crustal contamination.

Conclusions:

• The isotopic data provide permissive evidence for the existence of long-lived, early-formed mantle reservoirs.
• Negative μ¹⁸²W values in primitive kimberlites may result from core-to-mantle W transfer, early silicate fractionation, or late accretionary input.
• Younger kimberlites suggest potential contamination of ancient sources by recycled crustal materials.