Continuous eclogite melting and variable refertilisation in upwelling heterogeneous mantle

Abstract:

Subduction and recycling of oceanic crust entrained in adiabatic upwelling mantle causes complex heterogeneities, chemically and physically [1-3]. Yet, the creation of such heterogeneites, and their impact on magmatism remain poorly constrained.

The model presented here assumes the presence of widely dispersed bodies of residual eclogites with varying bulk CaO/Na₂O ratios, stirred by convection in the mantle. We examine the effects of such heterogeneities in eclogites on melting and phase relations, and on density and seismic velocity relations in mantle adiabatically upwelling from ~160 to ~90 km depth.

Res2 [1] is the melting residue of a model altered MORB GA2 [2] at 5 GPa, following loss of a siliceous melt fraction during upwelling. Res3 is similar to Res2 (CaO/Na₂O=4) but has a higher CaO/Na₂O ratio (12).

The subsolidus phases are garnet, clinopyroxene and minor quartz/coesite. The Res2 solidus is at 1210±15°C at 3 GPa, 1375±25°C at 4 GPa, and 1410±15°C at 5 GPa [1]. The Res3 solidus is similar, but slightly higher in temperature than that of Res2 at 3 GPa, 1260±15°C.

Along a near-adiabatic path of T_p≈1360°C, the eclogites start to melt at higher pressure than ambient ‘dry’ mantle owing to the lower solidus of the former. The relative slopes of the adiabat and eclogitic solidus ensure self-fluxing continuous melting, caused by continuous exsolution of SiO₂ out of clinopyroxene during adiabatic ascent.

At 5 GPa, near-solidus andesitic Res3 melts (~10%) are much less siliceous and sodic, more calcic and have higher Mg# than Res2 incipient dacitic melts (<5%). During further upwelling to 3 GPa, as eclogitic melt fractions increase, they become basaltic, and cotectics control melt compositions.

Siliceous eclogitic melts formed will react out of existence with peridotitic mantle, effectively refertilising it and producing hybrid, pyroxene and garnet-rich rocks [3]. As eclogitic melts differ, a variety of refertilisied, hybrid mantle rocks are formed along adiabatic paths [1, this study]. Densities and seismic velocities of eclogitic residues also differ during upwelling.

Subsequent melting of these sources may lead to compositional diversity in volcanic rocks.

[1] Rosenthal et al. Sci Rep 4, 6099 (2014) [2] Spandler et al. J Petrol 49, 771-795 (2008) [3] Yaxley & Green Schweiz Mineral Petr Mitt 78, 243-255 (1998)