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
Résumé
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.