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Primary Na contents of kimberlite melts from experimentally determined olivine/melt partition coefficients

Year: 2026
Type: article
Abstract: Kimberlite rocks are rich in Ca, yet almost Na-free (<0.3 wt% Na2O), raising the question of whether they tap a unique Na-poor but clinopyroxene-saturated mantle source or whether Na is lost en route to the surface. To reconstruct primary Na contents of kimberlitic melts, we quantified Na partitioning between olivine and volatile-rich kimberlitic melts (DNaol/melt) at 1–7 GPa, 1100–1480 °C, and oxygen fugacity (fO2) ≈ ΔCCO-1 buffer. DNaol/melt increases from 0.0014 at 1 GPa to 0.013 at 7 GPa, best described by log(DNaol/melt) = –0.007(1) P 2 + 0.16(2) P – 2.75(5), (R2 = 0.85). Applying this calibration to magmatic olivine from 22 kimberlites yields reconstructed Na2Omelt of 1.9–8.6 wt% (typically ±0.25 wt%) at 1 GPa, showing that magmatic Na contents are much higher than indicated by bulk rock measurements. The lower range of these reconstructed Na2O values aligns with the predicted 3.5 ± 0.5 wt% of asthenospheric carbonated silicate melts generated from typical lherzolitic mantle at 7 GPa. The higher values correspond to the same 3.5 ± 0.5 wt% if recalculated for olivine crystallization at 2–4 GPa. Because Na2O loss through fluid exsolution during kimberlite ascent cannot exceed 0.5 wt%, the extreme Na loss leading to the observed low bulk-kimberlite values must occur upon solidification when copious amounts of Na-rich fluids are expelled. These findings suggest that kimberlitic melts do not require a peculiar mantle source, but rather carbon-bearing fertile lherzolites that melt at adiabatic conditions, placing kimberlites into a broad continuum with primitive alkaline ocean island melts.
Source: Geology
DOI: 10.1130/g54051.1
Language: English
Subfield: Geophysics
Open Access status: bronze
OpenAlex ID: W7128435902
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