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Rare evidence for the existence of a Hadean enriched mantle reservoir
Abstract:
Short lived isotopic systems can help unravel the complex early differentiation history of the Earth’s mantle. Excesses in neodymium-142 (142Nd) measured in several occurrences of Archean mantle-derived rocks, compared to the modern upper mantle, imply the formation of an early depleted mantle in the Hadean. However, the existence of a complementary enriched reservoir, which should have also stemmed from such early differentiation event, remains equivocal. New data on 3.4 billion year old amphibolites from the São José do Campestre Massif, NE Brazil, show well resolved 142Nd deficits compared to the modern upper mantle, down to −14.1 ppm. This provides the first clear evidence for an early enriched mantle source, which may represent the missing concomitant reservoir complementary to Earth’s early depleted mantle. |
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Tungsten isotopes in Baffin Island lavas: Evidence of Iceland plume evolution
Abstract:
Tungsten and helium isotope ratios in lavas derived from deeply rooted mantle plumes are tracers of lower mantle compositional heterogeneity or core–mantle exchange. We measured the tungsten isotopic compositions of lavas with exceptionally high-3He/4He ratios that erupted above the head of the Iceland plume on Baffin Island. These lavas have 182W/184W ratios that are indistinguishable from the convecting upper mantle, unlike younger lavas in Iceland that have lower 182W/184W ratios. This implies that only the Iceland plume tail was infused with low-182W/184W material, likely from the core. If high-3He/4He helium also comes from the core, then diffusion across the core–mantle boundary may stratify plume-source mantle domains, with elevated 3He/4He travelling farther into the lower mantle than 182W/184W anomalies. Over Earth history, tungsten diffusion from the core can explain the decline of 182W/184W in the convecting mantle. We speculate that the uneven pace of this decline corresponds with evolving lower mantle dynamics. |
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Calcium isotope fractionation during melt immiscibility and carbonatite petrogenesis
Abstract:
Stable calcium isotopes have been used to suggest that subducted marine carbonates are frequently involved in the formation of carbonatites. Significant Ca isotope fractionations during carbonatite petrogenesis, however, could lead to a dramatically different picture. We present Ca isotope data for (i) coexisting (immiscible) carbonatite and silicate melts from high temperature centrifuging piston cylinder experiments, (ii) primary apatite and calcite/dolomite from natural carbonatites, and (iii) ab initio estimates for equilibrium Ca isotope partitioning in calcite, dolomite, and ankerite. Carbonatitic melts have lower δ44Ca than their conjugate silicate melts, with an equilibrium fractionation factor [1000lnα(1000K)] of −0.21 ± 0.06 (tSE). We develop a quantitative four stage model for carbonatite petrogenesis (partial melting followed by fractional crystallisation, silicate-carbonatite melt immiscibility, and calcite/apatite accumulation) that fully explains our natural data (average δ44CaBSE of −0.30 ± 0.03 ‰) and those from recent studies, without requiring isotopic contributions from recycled marine carbonates. Our results suggest that lighter isotopes of similarly bound cations (e.g., Mg, Fe, Sr, Ba, Zn) should be preferentially incorporated into carbonatitic melts and that calciocarbonatite formation involves melt immiscibility after differentiation of mantle-derived alkaline CO2-bearing silicate melts. |
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