Phase relations and melting of nominally ‘dry’ residual eclogites with variable CaO/Na2O from 3 to 5 GPa and 1250 to 1500 °C; implications for refertilisation of upwelling heterogeneous mantle

Abstract : This study investigates the phase and melting relations of nominally ‘dry’ residual eclogites (Res2 and Res3),with varying bulk CaO/Na2O ratios (4 and 12, respectively), from ~160 (5 GPa) to ~90 km (3 GPa) depth. Garnet, clinopyroxene and minor quartz/coesite are subsolidus phases in both compositions. In contrast to Res2, in Res3, the proportions of garnet always exceeding those of clinopyroxene. This also leads to higher modal quartz/coesite in Res3 relative to Res2. In modelling melting along a near-adiabatic upwelling path with a mantle potential temperature of ~1360 °C, at 5 GPa, near-solidus andesitic Res3 partial melts are much less siliceous and sodic, and are more calcic and magnesian than the incipient dacitic melts of Res2. Continuously self-fluxed melting increases considerably from 4 to 3 GPa due to the increased breakdown of Ca-Eskolaite solid solution component in clinopyroxene along the adiabat. This causes a steepening of the solidus, but more-so for Res2 than for Res3. At 3 GPa, the near exhaustion of residual clinopyroxene causes higher melt productivity for Res3 (~60%) than for Res2 (~30%), despite both melts being of basaltic-andesite composition. Resulting Res3 melts are therefore significantly more calcic and magnesian, and less sodic than those of Res2 melts. As Res3 undergoes a higher degree of melting relative to Res2 during adiabatic ascent, Res3 eclogitic residues become significantly more refractory;with relatively higherMg# and grossular in garnet, higherMg# and Ca-tschermaks, and lower jadeite components of clinopyroxene, and higher garnet/clinopyroxene ratios than eclogitic Res2 residuals. In upwelling heterogenous mantle domains, the siliceous eclogitic melts formed within a body of eclogite will react with encapsulating mantle peridotite, effectively refertilising it and producing hybrid pyroxene- and garnet-rich rocks. Subsequent melting of these sources may lead to compositionaly diverse primitive mantlederived magmas,with high Ca/Al and lowNa/Ca signatures indicators of preferential melting of a heterogeneous mantle, previously refertilised by recycled Ca-rich oceanic crustal material, and primitive magmaswith lowCa/Al and high Na/Ca derived frommelting of mantlewith a ‘normal recycled crustal material signature’. Thus, compositional magma diversity may directly reflect precursor compositions of the mantle source region.
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Submitted on : Friday, June 14, 2019 - 12:55:27 PM
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Anja Rosenthal, Gregory Yaxley, Wilson Crichton, Istvan Kovacs, Carl Spandler, et al.. Phase relations and melting of nominally ‘dry’ residual eclogites with variable CaO/Na2O from 3 to 5 GPa and 1250 to 1500 °C; implications for refertilisation of upwelling heterogeneous mantle. Lithos, Elsevier, 2018, 314-315, pp.506-519. ⟨10.1016/j.lithos.2018.05.025⟩. ⟨hal-02156394⟩

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